ASME Conference Presenter Attendance Policy and Archival Proceedings

2012;():i. doi:10.1115/GT2012-NS2.

This online compilation of papers from ASME Turbo Expo 2012: Turbine Technical Conference and Exposition (GT2012) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Combustion, Fuels and Emissions

2012;():1-9. doi:10.1115/GT2012-68012.

Fuel-cooled thermal management, including endothermic cracking and reforming of hydrocarbon fuels, is an enabling technology for advanced aero engines and offers potential for cycle improvements and pollutant emissions control. The principal engine operability issue that will affect this enabling hydrocarbon fuel cooling technology is coke formation. Furthermore, the extent to which the benefits of high heat sink cooling technology can be realized is directly related to our ability to suppress coke formation. The successful implementation of this enabling technology is, therefore, predicated on coke suppression. In-situ continuous coke deposit removal by catalytic steam gasification is being developed and successfully demonstrated as a means for suppressing pyrolytic coke deposit in fuel-cooled thermal management systems for advanced aero engines. The objective of this research is to investigate the in-situ continuous coke deposit removal by catalytic steam gasification for suppressing pyrolytic coke deposition using a single-tube reactor simulator under representative hypersonic operating conditions. A coke removal system removes coke deposit from the walls of a high temperature passage in which hydrocarbon fuel is present. The system includes a carbon-steam gasification catalyst and a water source. The carbon-steam gasification catalyst is applied to the walls of the high temperature passage. The water reacts with the coke deposit on the walls of the fuel passage side to remove the coke deposit from the walls by carbon-steam gasification in the presence of the carbon-steam gasification catalyst. Experimental data shows the in-situ continuous coke deposit removal by catalytic steam gasification is able to reduce coke deposit rate by more than 10 times.

Commentary by Dr. Valentin Fuster
2012;():11-20. doi:10.1115/GT2012-68019.

A conventional combustion tuning method for a gas turbine needs more than 24 hours with lots of human labor. In addition it is hard to certify whether the plant is optimized because the conventional tuning is based on human decisions and subjective empirical data over a long time. In this study we developed a combustion tuning technology using six sigma tools (CTSS) to effectively meet the increasingly stringent NOx regulations and to save combustion tuning time. CTSS was conducted in five steps—define-identify-design-optimize-verify (DIDOV). First, the NOx reduction target was defined (Step 1, define), the current status of the plant was diagnosed (Step 2, identify), and the vital few control parameters to achieve the defined target were determined by analyzing the correlation between the control parameters and NOx emissions (Step 3, design). For the next step, the optimum condition was derived from one of the six sigma tools (Step 4, optimize), and finally the optimum condition was verified by applying the condition to the gas turbine combustion (Step 5, verify). As a result of CTSS, averaged NOx emissions were reduced by more than 70% and the standard deviation was improved by more than 60%. These results show that CTSS is a potential tool for enhanced reliability of plant operations and scientific method for quick and exact combustion tuning.

Commentary by Dr. Valentin Fuster
2012;():21-30. doi:10.1115/GT2012-68060.

The stability of hydrogen combustion under lean premixed conditions in a back-mixed jet-stirred reactor (JSR), is experimentally and numerically investigated. The goal is to understand the mechanism of flame extinction in this recirculation-stabilized flame environment. Extinction is achieved by holding the air flow rate constant and gradually decreasing the flow rate of the hydrogen fuel until a blowout event occurs. In order to gain insight on the mechanism controlling blowout, two dimensional computational fluid dynamic (CFD) simulations are carried out for the lean premixed combustion (LPM) of hydrogen as the fuel flow rate is reduced. The CFD model illustrates the evolution of the flow-field, temperature profiles, and flame structure within the JSR as blowout is approached. A single element chemical reactor network (CRN) consisting of a plug flow reactor (PFR) with recirculation is constructed based on the results of the CFD simulations, and its prediction of blowout is in good agreement with the experimental results. The chemical mechanism of Li et al. is used in both the CFD and CRN models, and GRI is used in the CRN for comparison. The modeling suggests that lean blowout does not occur with the flame in a spatially homogeneous condition, but rather under a zonal structure. Specifically, the flame is stabilized by the entrainment of combustion products from the re-circulation zone into the base of the reactant jet. The mixture of hot products and incoming premixed reactants proceeds through an ignition induction period followed by an ignition event. As the fuel flow decreases, the induction period increases and the ignition event is pushed further around the recirculation zone. Eventually, the induction period becomes so long that the ignition is incomplete at the point where the recirculating gas is entrained into the jet. This threshold leads to overall flame extinction.

Topics: Flames , Hydrogen
Commentary by Dr. Valentin Fuster
2012;():31-40. doi:10.1115/GT2012-68079.

The intention of this work is to combine Large-Eddy-Simulation (LES) for the prediction of flow and mixture fraction fields with a Reynolds-Averaged-Navier-Stokes (RANS) transported probability density function (TPDF) method for the prediction of turbulent non-premixed flames. The motivation for this work is based upon the property of LES to provide a better description of complex flow fields than most current RANS methods can offer, while TPDF-methods excel in predicting the reacting species fields. However, using the straight forward extension of PDF methods for LES, the filtered density function (FDF) approach requires a large number of PDF particles in each LES cell and is thus computationally expensive. Therefore, a method is proposed to use the time-averaged LES flow field, mixture fraction field and mixture fraction PDF as a turbulence model for a RANS TPDF method operating on a much coarser grid. A projection of the mixture fraction conditioned PDF to evaluate the instantaneous LES density field is proposed as coupling device. The reconstruction of mixture fraction PDF from a LES simulation and the coupling to the TPDF method in postprocessing mode is validated using the TNF Sandia D flame, showing good agreement with experiment.

Commentary by Dr. Valentin Fuster
2012;():41-51. doi:10.1115/GT2012-68117.

This paper describes the use of a spherical combustion bomb to determine the laminar flame speed and Markstein length of a selection of hydrocarbon fuels. The fuels nominally represented Jet A-1 but some were doped with various component compounds which were chosen so as to vary particular jet fuel specification in relative isolation.

Analyses of this kind are typically based on optical measurements and, to simplify the analysis, an approximation of constant pressure is usually achieved by limiting the useable data to the early stages of flame propagation only. The analysis methodology presented in this paper differs inasmuch that calculations were based solely on the recorded pressure data. Moreover, by deducing the response of the flame speed to pressure and temperature, it was possible to utilize the whole combustion pressure record which significantly increased the volume of useful data that could be obtained from each experiment. Other practical difficulties that are often encountered such as flame winkling at large diameters, especially with rich mixtures, were minimized by using a small bomb of only 100mm diameter. The method of analysis via the pressure trace rendered any flame winkling easily discernable wherefrom it could be easily eliminated.

For each fuel, at least six repeat combustion pressure records (about 90 data points each) were obtained for each of six different air-fuel ratios spanning the range from lean to rich and the whole sequence was repeated at a higher initial temperature. This provided a database of over 6000 individual calculations of laminar flame speed from which the relevant parameter coefficients were obtained by means of a regression technique. It was found that the effects of changing the blend composition could be discerned in the various laminar flame speed results and that significant variation in laminar flame speed could possibly be “tailored” into a synthetic jet fuel formulation.

Commentary by Dr. Valentin Fuster
2012;():53-62. doi:10.1115/GT2012-68128.

Precision Combustion, Inc., (PCI) in close collaboration with Solar Turbines, Incorporated, has developed and demonstrated a catalytic combustion system for hydrogen fueled turbines that can reduce oxides of nitrogen (NOx) emissions to low single digit levels while maintaining or improving current levels of efficiency and eliminating emissions of carbon dioxide.

A full scale Rich Catalytic Hydrogen (RCH) injector was developed and successfully tested at Solar Turbines, Incorporated high pressure test facility, demonstrating low single digit NOx emissions for hydrogen fuel in the primary zone temperature range of 1200°C–1500°C (2200°F–2750°F). The testing also demonstrated low combustion noise with stable and robust operation.

A primary benefit of the catalytic hydrogen combustor technology is the capability for significantly-reduced NOx without costly post-combustion controls. This translates into reduced dilution requirements for a target NOx level, substantially improving efficiency and reducing operating costs. In addition, quiet combustor operation increases gas turbine component life. These advantages advance Department of Energy (DOE’s) objectives for achievement of low NOx emissions, improvement in efficiency vs. post-combustion controls, fuel flexibility, a significant net reduction in Integrated Gasification Combined Cycle (IGCC) system net capital and operating costs, and a route to commercialization across the power generation field from micro turbines to industrial and utility turbines.

Commentary by Dr. Valentin Fuster
2012;():63-71. doi:10.1115/GT2012-68153.

The paper presents the findings from a study of the lean blowout (LBO) behaviour of sixteen fuel blends in a heterogeneous laboratory combustor. The LBO results were correlated with fuel blend properties that included the D86 distillation profile, density, viscosity, flash point and ignition delay as represented by derived cetane number (DCN). A spherical bomb was employed to measure laminar flame speed and Markstein length based on pressure measurements. The experiments were conducted with two different starting temperatures and over a range of air fuel ratios from rich to lean. The atomisation behaviour of the fuels was evaluated using a pressure atomised nozzle and a laser diffraction particle sizer. The data allowed the Sauter mean diameter (SMD) values at extinction to be estimated based on the fuel pressure.

Each individual LBO test was conducted at constant air flow rate with the extinction point being attained by reducing the fuel flow rate. The test series for each fuel spanned a range of air flow rates based on combustor liner relative pressure drops from 1% to 6%. These results exhibited three distinct regions (A1, A2 and B) that were evident to varying degrees in the results obtained with all sixteen test fuels. The transition between A1 and A2 was ascribed to combustor flow and was shown to be independent of the fuel being tested. The transition between B and A2 was ascribed to the change from the LBO behaviour being dominated by atomization to it being a mixing / turbulence dominated regime. The individual transitions were found to be dependent on the test fuel blend. In order to accommodate the LBO results in a multivariate analysis the observed trends were represented by three parameters that were determined through curve fitting to the different regions. The three parameters were the SMD and air mass flow rate at the transition between region B and A2 and a projected LBO equivalence ratio at zero air mass flow.

The data was cross correlated between all determined properties and it was shown that the extinction behaviour correlated with chemical reactivity, flame stretch, density and volatility to different degrees in the two regions of operation. It was concluded that there is potential for influencing threshold extinction limits through both chemical and physical jet fuel properties, and the need to take cognisance thereof in fuel formulation, was highlighted.

Commentary by Dr. Valentin Fuster
2012;():73-83. doi:10.1115/GT2012-68164.

Computational fluid dynamics (CFD) simulations to predict and visualize the reacting flow dynamics inside a combustor require fine resolution over the spatial and temporal domain, making them computationally very expensive. The traditional time-serial approach for setting up a parallel combustor CFD simulation is to divide the spatial domain between computing nodes and treat the temporal domain sequentially. However, it is well known that spatial domain decomposition techniques are not very efficient especially when the spatial dimension (or mesh count) of the problem is small and a large number of nodes are used, as the communication costs due to data parallelism becomes significant per iteration. Hence, temporal domain decomposition has some attraction for unsteady simulations, particularly on relatively coarse spatial meshes. The purpose of this study is two-fold: (i), to develop a time-parallel CFD simulation method and apply it to solve the transient reactive flow-field in a combustor using an unsteady Reynolds-averaged Navier Stokes (URANS) formulation in the commercial CFD code FLUENT™ and (ii) to investigate its benefits relative to a time-serial approach and its potential use for combustor design optimization. The results show that the time-parallel simulation method correctly captures the unsteady combustor flow evolution but, with the applied time-parallel formulation, a clear speed-up advantage, in terms of wall-clock time, is not obtained relative to the time-serial approach. However, it is clear that the time-parallel simulation method provides multiple stages of transient combustor flow-field solution data whilst converging towards a final converged state. The availability of this resulting data could be used to seed multiple levels of fidelity within the framework of a multi-fidelity co-Kriging based design optimization strategy. Also, only a single simulation would need to be setup from which multiple fidelities are available.

Commentary by Dr. Valentin Fuster
2012;():85-97. doi:10.1115/GT2012-68181.

This paper contains a numerical examination concerning the ignition behavior of a spray nozzle mounted in a rectangular channel under atmospheric conditions which is run with Jet A-1. On the basis of a comprehensive data set of experimental results, the numerical approach is verified primarily by means of a comparison of the flame growth and position after ignition. In the following several distinct igniter positions and boundary condition settings are simulated. The conditions which prevail at the location of the ignition are investigated with respect to how they influence the ignition process. Due to changes in the fuel placement and flow field characteristics which follow from alternating the boundary conditions such as air and fuel mass flow, ignition is either promoted or impeded. The underlying causes which can lead to a success or failure of the ignition are analyzed. The ignition in the experiment is achieved through a laser-induced breakdown, which is modelled through a Turbulent Flame Speed Closure combustion model with an additional spark ignition extension. A comparison with the ignition statistics from the experiment shows that numerical tools can be used to determine preferential boundary conditions and igniter locations to accomplish a successful ignition in multiphase flow configurations.

Topics: Sprays , Flames
Commentary by Dr. Valentin Fuster
2012;():99-104. doi:10.1115/GT2012-68188.

There are three combustion regimes of individual droplet combustion behavior: the fully enveloped flame, the partially enveloped flame, and the wake flame. From PLIF measurement results, single droplet combustion phenomenon happens in spray flame, as well as lean type gas turbine combustion chamber sometimes. The drag coefficient, evaporation rate, and combustion rate are different according to the burning modes. At present, in Reynolds Averaged Navier Stokes (RANS) method and Large Eddy Simulation (LES) method, the droplets are treated as point source because the grid scale is bigger than the droplet diameter. A two phase combustion model with the consideration of the individual droplet burning mode is proposed before. In this paper, this model is tested by spray flames here again. Furthermore, this model was used in a concept lean premixed pre-vaporized (LPP) combustion case too. In spray flame, the predicted results are close to the experimental data.

Commentary by Dr. Valentin Fuster
2012;():105-112. doi:10.1115/GT2012-68202.

Brazil is the most advanced country in terms of technology, production and use of ethanol as fuel. Alcohol can be obtained from various forms of biomass, being sugarcane a current economic reality. The environmental benefits associated with the use of alcohol are enormous, as approximately 2.3 tons of CO2 are no longer emitted to the atmosphere for each ton of ethanol, without taking into account emissions of other gases such as SO2. Sugarcane is the second largest source of renewable energy in Brazil, with 12.6% of participation in the current energy matrix, considering the ethanol fuel and cogeneration of electricity from sugarcane bagasse.

The aim of this work is an experimental analysis of thermal performance/emissions of a 30 kW regenerative cycle gas turbine engine using ethanol as fuel. The tests were carried out in the laboratories of the Federal University of Itajubá – UNIFEI, Brazil, on steady state condition under different load levels, considering the properties of ethanol in compliance with Brazilian legislations.

The results shown that there were no significant changes in the thermal performance of the gas turbine engine and the emissions ware a slight increase in CO and a reduction in concentrations of NOx.

Commentary by Dr. Valentin Fuster
2012;():113-124. doi:10.1115/GT2012-68216.

This paper reports on a work in progress study measuring flashback, blow out, emissions and turbulent displacement flame speeds in a low swirl injector operated at elevated pressures and inlet temperatures with hydrogen and methane based fuels in an optically accessible combustor rig. The goal is to extend the knowledge of low-swirl flames at conditions relevant to gas turbine engines. Testing was conducted at pressures ranging from 3 to 6 atm, inlet temperatures from 290 to 600K, and inlet bulk velocities from 20 to 60 m/s for natural gas and a 90%/10% by volume hydrogen/methane blend. Blow out limits for natural gas were found to be independent of pressure and inlet temperature but weakly dependent on velocity. Flashback limits for hydrogen were found to be independent of inlet temperature but strongly dependent on velocity and pressure. Local displacement turbulent flame speeds for methane were measured and appear to coincide with atmospheric pressure data in the literature. NOx emissions for both fuels were found to be exponentially dependent upon firing temperature, but emissions for the high hydrogen content flames were consistently higher than natural gas flames.

Commentary by Dr. Valentin Fuster
2012;():125-132. doi:10.1115/GT2012-68222.

Reduction of NOx emission of aircraft gas turbines is moving in the direction of development of direct combustor fuel injection systems providing conditions for rapid mixing and combustion of a uniform lean fuel/air mixture. However, formation of sufficient uniform fuel/air mixture in real combustors fails to be completed. It may result in burning out a considerable portion of fuel in stoichiometric conditions that in turn imposes limits on the emission level minimizing. The research accomplished by a number of authors justifies the necessity of decreasing the extent of stoichiometric zones by means of increasing fuel-air mixing rate on the stoichiometric surface of their contact, to reduce emission. This publication contains the analysis results upon the effect of mixing rate, in terms of a methane-air laminar diffusion combustion. It is proved that changes of mixing rate influence the two main factors governing the emission level: the extent of NO production zone and the efficient rate of its production. If the mixing rate increases explicitly due to the decrease of NOx production scale, the efficient velocity curve will contain a maximum value. Furthermore, the scale effect is all-over stronger than the kinetic one. It is concluded that in case of mixing rate increase, the reduction of NOx emission goes nonlinearly and steadily. The ranges of maximum effect are specified. Herewith, we introduce the relation, which demonstrates that in the diffusion combustion a sufficient reduction of NOx emission can be achieved.

Commentary by Dr. Valentin Fuster
2012;():133-142. doi:10.1115/GT2012-68224.

Dry low emissions premixed combustion systems have the ability to give larger NOx emissions reduction in comparison to diffusion type of combustors. However, these systems are prone to flashback because the fuel and oxidizers are mixed upstream of the combustion chamber. This is particularly true for premixed systems burning high-reactivity fuels due to their higher flame speed. Flashback is undesirable in gas turbines because it leads to overheating and failure of fuel nozzles and premixing sections. Recently, a novel application of Plasma Actuation through non-thermal Dielectric Barrier Discharge has been shown to significantly delay flashback in the core flow along the axis of the premixer. Building on this successful endeavour, efforts were directed to prove the effectiveness of the control method in situations where flashback was triggered in the boundary layer. Results show that the current application delays the occurrence of flashback in the boundary layer of the premixer to higher equivalence ratios. Improvements in the combustor operability margin of 10 to 14% when burning natural gas-air mixtures, and of about 3.5% when replacing the fuel by an equimolar mixture of natural gas and hydrogen, were achieved. It was found that the proposed application of plasma actuation is even more efficient in preventing flashback in the boundary layer than in the core flow.

Commentary by Dr. Valentin Fuster
2012;():143-153. doi:10.1115/GT2012-68234.

Porous burners offer a possible solution to attain higher combustion stability under premixed conditions with ultra low pollutant emissions. To analyze the feasibility of PIM (porous inert media) in energy conversion processes, studies at elevated pressure have been carried out. In the present work, burning velocity of natural gas-air mixtures for lean mixture conditions at elevated pressure is obtained in a conical PIM by determining the flame location using thermocouples. Pressure, thermal power, equivalence ratio and initial temperature were varied in order to study their effect on the flame stability. The pressure was varied from 1.1 to 14.0 bar, and initial temperatures from 300 to 400K. The burning velocity data obtained from present measurements show good agreement with literature data at atmospheric pressure. The results show that the burning velocities measured in PIM decreased non-linearly with increase in pressure. Also, the decrease in the burning velocity in the PIM with pressure is more pronounced for lean mixture conditions. Present results indicate that the PIM produces stable flames for a wide range of operating conditions and generate low pollutant emissions, which show that it is a potential alternative for conventional burners.

Topics: Pressure , Flames
Commentary by Dr. Valentin Fuster
2012;():155-165. doi:10.1115/GT2012-68236.

A study on innovative gas turbine core concepts supported by the NEWAC project (NEW Aero-engine Core concepts, Integrated Project co-funded by the European Commission within the Sixth Framework Programme under contract No. AIP5-CT-2006-030876) focused on the ability of the combustor to maintain combustion during a drastic reduction of the main air, e.g. due to an active control on the core flow to improve the off-design efficiency. A feasibility study was performed at Graz University of Technology including dimensioning, design and validation of a test burner with variable geometry. A low power premixed methane / air burner with swirl-stabilised flame was chosen, on which the outlet surface and the ratio axial to tangential momentum on the mass flow rate could be controlled. During testing at atmospheric conditions, special attention was paid to the extension of the flammability domain and to the flame dynamics (transition attached-detached, flame stability, blowout limits). It was established for instance that based on this technology, a detached flame can be maintained when reducing the design mass flow rate by 40 per cent within a safe stability range. The paper discusses the background of the study, the burner’s design and technology, the measurement techniques and the results of the validation campaign. A discussion on possible advantages of using variable geometry in a combustion chamber versus conventional technologies closes the paper, taking into account the technical challenges to be met.

Topics: Combustion , Design , Geometry
Commentary by Dr. Valentin Fuster
2012;():167-177. doi:10.1115/GT2012-68241.

Linear techniques can predict whether the non-oscillating (steady) state of a thermoacoustic system is stable or unstable. With a sufficiently large impulse, however, a thermoacoustic system can reach a stable oscillating state even when the steady state is also stable. A nonlinear analysis is required to predict the existence of this oscillating state. Continuation methods are often used for this but they are computationally expensive.

In this paper, an acoustic network code called LOTAN is used to obtain the steady and the oscillating solutions for a horizontal Rijke tube. The heat release is modelled as a nonlinear function of the mass flow rate. Several test cases from the literature are analysed in order to investigate the effect of various nonlinear terms in the flame model. The results agree well with the literature, showing that LOTAN can be used to map the steady and oscillating solutions as a function of the control parameters. Furthermore, the nature of the bifurcation between steady and oscillating states can be predicted directly from the nonlinear terms inside the flame model.

Commentary by Dr. Valentin Fuster
2012;():179-188. doi:10.1115/GT2012-68243.

Modern gas turbines equipped with lean premixed dry low emission combustion systems suffer the problem of thermoacoustic combustion instability. The acoustic characteristics of the combustion chamber and of the burners, as well as the response of the flame to the fluctuations of pressure and equivalence ratio, exert a fundamental influence on the conditions in which the instability may occur. A three dimensional finite element code has been developed in order to solve the Helmholtz equation with a source term that models the heat release fluctuations. The code is able to identify the frequencies at which thermoacoustic instabilities are expected and the growth rate of the pressure oscillations at the onset of instability. The code is able to treat complex geometries such as annular combustion chambers equipped with several burners. The adopted acoustic model is based upon the definition of the Flame Response Function (FRF) to acoustic pressure and velocity fluctuations in the burners.

In this paper, data from CFD simulations are used to obtain a distribution of FRF of the κ-τ type as a function of the position within the chamber. The intensity coefficient, κ, is assumed to be proportional to the reaction rate of methane in a two-step mechanism. The time delay τ is estimated on the basis of the trajectories of the fuel particles from the injection point in the burner to the flame front.

The paper shows the results obtained from the application of FRF with spatial distributions of both κ and τ. The present paper also shows the comparison between the application of the proposed model for the FRF and the traditional application of the FRF over a concentrated flame in a narrow area at the entrance to the combustion chamber. The distribution of the intensity coefficient and the time delay proves to have an influence, both on the eigenfrequency values and on the growth rates, in several of the examined modes.

The proposed method is therefore able to establish a theoretical relation of the characteristics of the flame (depending on the burner geometry and operating conditions) to the onset of the thermoacoustic instability.

Topics: Combustion , Flames
Commentary by Dr. Valentin Fuster
2012;():189-198. doi:10.1115/GT2012-68253.

Hydrodynamic instabilities in gas turbine fuel injectors help to mix the fuel and air but can sometimes lock into acoustic oscillations and contribute to thermoacoustic instability. This paper describes a linear stability analysis that predicts the frequencies and strengths of hydrodynamic instabilities and identifies the regions of the flow that cause them. It distinguishes between convective instabilities, which grow in time but are convected away by the flow, and absolute instabilities, which grow in time without being convected away. Convectively unstable flows amplify external perturbations, while absolutely unstable flows also oscillate at intrinsic frequencies. As an input, this analysis requires velocity and density fields, either from a steady but unstable solution to the Navier–Stokes equations, or from time-averaged numerical simulations. In the former case, the analysis is a predictive tool. In the latter case, it is a diagnostic tool. This technique is applied to three flows: a swirling wake at Re = 400, a single stream swirling fuel injector at Re ∼ 106, and a lean premixed gas turbine injector with five swirling streams at Re ∼ 106.

Its application to the swirling wake demonstrates that this technique can correctly predict the frequency, growth rate and dominant wavemaker region of the flow. It also shows that the zone of absolute instability found from the spatio-temporal analysis is a good approximation to the wavemaker region, which is found by overlapping the direct and adjoint global modes. This approximation is used in the other two flows because it is difficult to calculate their adjoint global modes.

Its application to the single stream fuel injector demonstrates that it can identify the regions of the flow that are responsible for generating the hydrodynamic oscillations seen in LES and experimental data. The frequencies predicted by this technique are within a few percent of the measured frequencies. The technique also explains why these oscillations become weaker when a central jet is injected along the centreline. This is because the absolutely unstable region that causes the oscillations becomes convectively unstable.

Its application to the lean premixed gas turbine injector reveals that several regions of the flow are hydrodynamically unstable, each with a different frequency and a different strength. For example, it reveals that the central region of confined swirling flow is strongly absolutely unstable and sets up a precessing vortex core, which is likely to aid mixing throughout the injector. It also reveals that the region between the second and third streams is slightly absolutely unstable at a frequency that is likely to coincide with acoustic modes within the combustion chamber. This technique, coupled with knowledge of the acoustic modes in a combustion chamber, is likely to be a useful design tool for the passive control of mixing and combustion instability.

Commentary by Dr. Valentin Fuster
2012;():199-209. doi:10.1115/GT2012-68256.

Flames in high swirl flow fields with vortex breakdown often stabilize aerodynamically in front of interior flow stagnation points. In contrast to shear layer stabilized flames with a nearly fixed, well defined flame attachment point, the leading edge of aerodynamically stabilized flames can move around substantially, due to both the inherent dynamics of the vortex breakdown region, as well as externally imposed oscillations. Motion of this flame stabilization point relative to the flow field has an important dynamical role during combustion instabilities, as it creates flame front wrinkles and heat release fluctuations. For example, a prior study has shown that nonlinear dynamics of the flame response at high forcing amplitudes were related to these leading edge dynamics. This heat release mechanism exists alongside other flame wrinkling processes, arising from such processes as shear layer rollup and swirl fluctuations.

This paper describes an experimental investigation of acoustic forcing effects on the dynamics of leading edge of a swirl stabilized flame. Vortex breakdown bubble dynamics were characterized using both high-speed particle image velocimetry (PIV) and line-of-sight high-speed CH* chemiluminescence. A wide array of forcing conditions was achieved by varying forcing frequency, amplitude, and acoustic field symmetry. These results show significant differences in instantaneous and time averaged location of the flow stagnation points. They also show motion of the flame leading edge that are of the same order of magnitude as corresponding particle displacement associated with the fluctuating velocity field. This observation suggests that heat release fluctuations associated with leading edge motion may be just as significant in controlling the unsteady flame response as the flame wrinkles excited by velocity fluctuations.

Commentary by Dr. Valentin Fuster
2012;():211-218. doi:10.1115/GT2012-68272.

As the amount of air traffic is rapidly increasing, the local air quality around airports and the global climate change are two major concerns. Under the circumstances, the regulation for NOx emission becomes more stringent year after year. Lean burn technology is one of the key technologies for the next generation civil aircraft engines. Kawasaki Heavy Industries (KHI) has been developing a Lean Pre-mixed Pre-vaporized (LPP) combustor for around 10,000 lb thrust class engine under the project of Environmentally Compatible Aircraft Engines for Small Aircraft (ECO)[1] led by New Energy and Industrial Technology Development Organization (NEDO) and Ministry of Economy, Trade and Industry (METI).

In this paper the results of the LPP combustor development about reducing NOx emissions is presented. The LPP burner main premixed duct is designed to have better mixing fuel and air. KHI have achieved 30%CAEP4 NOx without deterioration of the other combustor performance. In general altitude relight would be one of the weak points for LPP combustion system. Successful lights were confirmed up to 30kft altitude condition in the multi sector rig, which is as good as that of the conventional combustors. Several LPP burners have been developed through CFD results. The burners have been spray-tested and combustion-tested in a single burner test rig in order to improve the burner potential. The burners selected in the single sector tests have been evaluated in a multi sector combustor rig with several combustor configurations. This paper describes the multi sector test results together with the brief introduction on burner development activities through burner tests.

Commentary by Dr. Valentin Fuster
2012;():219-230. doi:10.1115/GT2012-68286.

Given the trend towards leaner combustor primary zones and concurrent increases in injector air mass flow rates for emissions reduction, an automated fuel injector optimisation procedure is proposed for a generic aero-engine combustor. The modelling assumptions and the design of the toolset to be applied for the optimisation study, as well as preliminary results from the computational tools, are presented. The proposed configuration will enable the consideration of the following design parameters: the number of swirlers, the swirl number for each swirler, the air mass flow splits between the swirlers, and the fuel mass flow split for multiple prefilming surfaces. Results from the unsteady RANS spray combustion solver available through the OpenFOAM software package are combined with semi-empirical correlations in order to estimate and capture trends in emissions. Pattern factor and susceptibility to thermoacoustic oscillations are assessed directly through the simulation output. Due to computational costs, only the cruise condition is considered for optimisation, and off-design considerations have been limited to their impact on preliminary combustor sizing and design. A multi-fidelity optimisation strategy incorporating a multi-objective Tabu Search algorithm is also presented in light of the nature of the problem and the complexity of the design spaces constructed from CFD results.

Commentary by Dr. Valentin Fuster
2012;():231-241. doi:10.1115/GT2012-68305.

This paper describes measurements and correlations of turbulent consumption speeds, ST,GC, of hydrogen/carbon monoxide (H2/CO) fuel mixtures, with a focus on recently acquired elevated pressure data. Turbulent consumption speed data were obtained at mean flow velocities and turbulence intensities of 30 < U0 < 50 m/s and 5 < urms/SL,0 < 30, respectively, for H2/CO mixtures ranging from 30–90% H2 by volume at 5 and 10 atm. Experiments were conducted where the mixture equivalence ratio, ϕ was adjusted at each fuel composition to have nominally the same un-stretched laminar flame speed, SL,0. In comparing two blends with the same composition, SL,0 value, and flow conditions, the 5 and 10 atm data have ST,GC values that are consistently about 1.8 and 2.2 times larger than the 1 atm data, respectively. These data are also correlated with a scaling law derived from quasi-steady leading points concepts using detailed kinetics calculations of highly stretched flames. For a given pressure, these scalings do an excellent job in scaling data obtained across the H2/CO fuel composition and fuel/air range. However, the pressure sensitivities are not captured by this scaling. This pressure sensitivity may be more fundamentally a reflection of the non-quasi-steady nature of the flame leading points. In support of this argument, we show that the spread in the data can largely be correlated with the ratio of a chemical time scale to a flow time scale.

Topics: Pressure , Turbulence
Commentary by Dr. Valentin Fuster
2012;():243-254. doi:10.1115/GT2012-68317.

The response of a perfectly premixed, turbulent jet flame at elevated inflow temperature to high frequency flow perturbations is investigated. A generic reheat burner geometry is considered, where the spatial distribution of heat release is controlled by auto-ignition in the jet core on the one hand, and kinematic balance between flow and flame propagation in the shear layers between the jet and the external recirculation zones on the other. To model auto-ignition and heat release in compressible turbulent flow, a progress variable / stochastic fields formulation adapted for the LES context is used. Flow field perturbations corresponding to transverse acoustic modes are imposed by harmonic excitation of velocity at the combustor boundaries. Simulations with single-frequency excitation are carried out in order to study the flame response to transverse fluctuations of velocity. Heat release fluctuations are observed predominantly in the shear layers, where flame propagation is important. The flow-flame coupling in these regions is analysed in detail with a filter-based post-processing approach, invoking a local Rayleigh index and providing insight into the interactions of flame wrinkling by vorticity and convection due to mean and fluctuating velocity.

Commentary by Dr. Valentin Fuster
2012;():255-262. doi:10.1115/GT2012-68345.

The first Alstom GT11N engine was introduced to the market at the end of the eighties. In order to keep our base fleet engines (aged fleet) attractive for dispatch and therefore for operation, one of the key issues of the service business is the development of upgrade packages. For the GT11N fleet an emission reduction package was worked out in recent years with the target of single digit NOx at base load (<10 ppm NOx @15%O2). The purpose of this paper is to present the performed development work, starting from the R&D work with the CFD optimization of the mixing quality, going to the atmospheric combustion tests and finally to the engine validation tests on site.

The first section of the paper focuses on the performed R&D work, mainly on the improvement of the gas/air mixing quality of the EV burner. For the down selection of the most promising configurations the calculated unmixedness at different places of the burner and the hottest flame zones were analyzed and evaluated. The second part of the paper focuses on the results of the atmospheric burner combustion tests. The last part of the paper is focusing on the verification phase and on the achieved results during the engine validation tests on site. The EV-alpha burners were implemented during an A-inspection. Before the implementation a reference measurement with the standard EV burners was carried out. This section gives an overview of the performed engine tests and achieved emissions at base load and part load, about flame stability behaviour and transient operation results.

With the implementation of the EV-alpha burner the target “single digit NOx” could be achieved and the combustion stability kept at comparable levels.

Commentary by Dr. Valentin Fuster
2012;():263-271. doi:10.1115/GT2012-68369.

To describe partially-premixed combustion inside hydrogen-rich combustors, a novel quadrature-based probability density function (PDF) approach is studied here. The PDF approach is comprehensive in describing multiple combustion regimes, and multiple inlet streams. The methodology is implemented in the context of the large eddy simulation (LES) approach. The main bottleneck in utilizing the PDF approach is that the PDF transport equation, which needs to be evolved along with the LES equations, is high-dimensional and intractable using conventional discretization techniques. In order to ensure that the PDF approach is easily transferred to existing industrial flow solvers, a quadrature-based Eulerian method for solving the PDF transport equation is considered here. The corresponding Eulerian equations are implemented in the open source OpenFOAM code using an unstructured grid system. Simulations of an experimental high-pressure combustor demonstrate that the PDF approach significantly changes the reaction structure compared to laminar chemistry assumption.

Commentary by Dr. Valentin Fuster
2012;():273-283. doi:10.1115/GT2012-68399.

Experimental studies of laminar ethanol - air gaseous flames have been undertaken in a large (34 l) cylindrical constant volume combustion bomb to investigate combustion fundamentals at varying ambient conditions. This vessel has been designed to minimise the influence of boundary walls, hence extending the quasi steady pressure region over which meaningful data may be obtained. Gaseous homogeneous mixtures are achieved by injecting liquid ethanol into the bomb which pre-vaporises prior to ignition. Initial pressure and equivalence ratio are predetermined using partial pressure methodology. Flame propagation is recorded utilising high-speed Schlieren photography, and low ignition energies were achieved via a variable discharge system enabling the sensitive early stages of flame propagation and extinction limits to be studied. Data is presented in terms of flame speed against stretch rate from which Markstein lengths and laminar burning velocities are derived for a variety of different initial conditions. The effect of ignition energy, initial pressure (from sub-atmospheric to elevated pressure) along with the effect of increasing initial temperature is studied. Results are discussed in terms of those of previous workers, and compared with predictions from detailed chemical kinetic schemes. Nonlinear trends witnessed during early stage flame propagation are further investigated as a suitable method for deriving extinction stretch rate.

Commentary by Dr. Valentin Fuster
2012;():285-295. doi:10.1115/GT2012-68401.

Previous autoignition studies at conditions relevant to reheat combustor operation have indicated that the presence of relatively small amounts of natural gas (NG) in H2/N2 fuel significantly changes the autoignition behavior. The present study further elucidates the influence of NG on autoignition, kernel propagation, and subsequent flame stabilization at conditions that are relevant for the practical operation of gas turbine reheat combustors (p = 15 bar, Tinlet > 1000 K, hot flue gas, appropriate residence times). The experimental investigation was carried out in a generic, optically accessible reheat combustor. Autoignition events in the mixing zone were recorded by a high-speed camera at frame rates of up to 30,000 fps. This paper describes the autoignition behavior as the H2 volume fraction is increased (decreasing NG) in a H2/NG/N2 fuel mixture for two different jet penetration depths. Additionally, the subsequent flame stabilization phenomena and the structure of the stabilized flame are discussed. The results reveal that autoignition kernels occurred even for the lowest H2 fuel fraction, but they did not initiate a stable flame in the mixing zone. Increasing the H2 volume fraction decreased the distance between the initial position of the autoignition kernels and the fuel injector, finally leading to flame stabilization. The occurrence of autoignition kernels at lower H2 volume fractions (H2/(H2+NG) < 85%) was not found to be significantly influenced by the fluid dynamic and mixing field differences related to the different jet penetration depths. In contrast, autoignition leading to flame stabilization was found to depend on jet penetration; flame stabilization occurred at lower H2 fractions for the higher jet penetration depth (H2/(H2+NG) ≈ 89 compared to H2/(H2+NG) ≈ 95 vol. %).

Commentary by Dr. Valentin Fuster
2012;():297-307. doi:10.1115/GT2012-68426.

NASA’s “Environmentally Responsible Aircraft” (ERA) N+2 advanced, low NOx combustor technologies program is looking at combustion technologies suitable for the 2020 time frame. The objective of this program is to develop fuel-air mixing concepts and associated fuel control valves. The low emissions combustor concept must be capable of meeting or exceeding the N+2 LTO NOx goal of 75% reduction from the ICAO standard adopted by CAEP 6 at engine pressure ratios of at least 55. Goodrich Engine Components is working with NASA to demonstrate concepts with these capabilities.

In the early 2000’s, Goodrich partnered with NASA in demonstrating the ability of a multipoint lean direct injection (LDI) concept to achieve very low NOx emissions index (EI) levels as tested at NASA test facilities. The program was successful in demonstrating the ability of the multipoint concept to deal with NOx at high power conditions but was not optimized to perform equally as well at low power conditions such as start, ground idle, and flight idle conditions.

After review of previous work, Goodrich is investigating a new multipoint combustor design for the N+2 program. The basic multipoint premise of injecting fuel through a large number of injection sites to promote rapid mixing has been retained, but at a much reduced number of nozzles compared to the original work. In the new version, nozzles are arranged in a staggered array pattern and are manifolded in radial stages. The radial stages can be utilized to control radial temperature distribution to the turbine. Radial staging is also being used to maintain sufficient temperature levels around specific nozzles at low power conditions to provide adequate emissions and stability at these conditions.

In addition to the modifications of the general arrangement, injector design features are optimized by stage such that much higher air/fuel mixing rates with lower central recirculation zones dominate high power conditions while more conventional swirl stabilization dominates at low power conditions when inlet temperatures are low. The design work is being guided by CFD analysis as well as qualitative and quantitative rig testing before the final configurations are fabricated and tested at the NASA flame tube rigs.

This paper will discuss elements of prior designs compared to current designs and discuss the status of Computational Fluid Dynamic (CFD) simulations completed in the first phase of this program.

Commentary by Dr. Valentin Fuster
2012;():309-318. doi:10.1115/GT2012-68436.

The low swirl injector (LSI) is a combustion technology being developed for low-emissions fuel-flexible gas turbines. The basic LSI configuration consists of an annulus of swirl vanes centered on a non-swirled channel, both of which allow for the passage of premixed reactants. LSIs are typically designed by following a general guidance of achieving a swirl number between 0.4 and 0.55. This paper aims to develop a more specific guideline by investigating the effects of varying geometry, i.e. vane angle, vane shape, and center channel size, on the LSI performance. A well-studied LSI provides a baseline for this investigation. Nine LSI variations from this baseline design have been evaluated. All LSI are tested with CH4 fuel at bulk flow velocity of 8 to 20 m/s firing into the open atmosphere. Performance metrics are the lean blowoff limit, the pressure drop, flowfield characteristics and emissions. Results show that the lean blow-off limit and NOx and CO emissions are insensitive to LSI geometric variations. The flowfields of seven LSIs exhibit self-similarity implying their turndown ranges are similar. Reducing the center channel size and/or the use of thin vanes instead of thickened vanes can reduce pressure drop across the LSI. Additionally, all ten LSI share a common feature in that 70% to 80% the premixture flows through the vane annulus. These findings are used to develop a more specific engineering guidelines for designing the LSI for gas turbines.

Topics: Ejectors , Geometry
Commentary by Dr. Valentin Fuster
2012;():319-328. doi:10.1115/GT2012-68451.

New regulations regarding NOx emissions are forcing manufacturers to develop advanced research and technology strategies. Ultra-lean combustion is considered as an attractive solution; however, it generally produces combustion instabilities in swirl-stabilized burners. This work provides experimental results for a new burner technology based on two concepts: the trapped vortex combustor (TVC) and the ultra-compact combustor (UCC). Methane/air flame stabilization was achieved by generating hot product recirculation, with a rich pilot flame located in an annular cavity, and by flame holders located in the main flow slightly upstream of the cavity. In addition, azimuthal gyration could be added to the main flow to reproduce the suppression of the last diffuser stage, which increased the velocity and modified the mixing between the cavity and the mainstream due to centrifugal forces. The combustor characterization was performed by coupling several optical diagnostics, pollutant emissions, and pressure measurements (for both cold and reactive conditions) at atmospheric pressure. An understanding of the combustion dynamics was achieved through phase averaged PIV/CH* images. The analysis highlighted the importance of the stabilization process of a double vortex structure inside the cavity and the presence of reactive gas close to the upstream cavity wall. These conditions were improved by a high cavity equivalence ratio and a high main airflow rate. The addition of swirl considerably increased the flame stability.

Commentary by Dr. Valentin Fuster
2012;():329-340. doi:10.1115/GT2012-68458.

A computational tool that combines a flow network solver with both 1D wall heat transfer and with chemical reactor models applying reduced mechanisms is presented. The model is applied to the combustion chamber of a 75kW gas turbine and wall temperatures and emissions of CO are compared with experimental values.

Commentary by Dr. Valentin Fuster
2012;():341-349. doi:10.1115/GT2012-68468.

In the TechCLEAN project of JAXA, experimental research has been conducted to develop a combustor for a small aircraft engine. The combustor was tuned to show the behavior of the Rich-Lean combustion through tests under atmospheric and practical conditions. Finally, through full annular combustion experiments under practical conditions, the combustor was tuned to reduce NOx emissions to almost 40% of the ICAO CAEP4 standard, also sustaining low CO and THC emissions. To investigate the performance of the combustor in detail, parametric experiments were conducted with single-sector combustors under additional test conditions in addition to design conditions of the target engine. Also the performance as a combustor for higher-efficient aircraft engine is examined by increasing inlet air pressure and temperature up to 3MPa and 825K in combustion tests. Obtained results of emission characteristics are discussed in this report.

Commentary by Dr. Valentin Fuster
2012;():351-360. doi:10.1115/GT2012-68471.

Increasing public awareness and more stringent legislation on pollutants drive gas turbine manufacturers to develop combustion systems with low NOx emissions. In combination to this demand the gas turbines have to provide a broad range of operational flexibility to cover variations in gas composition and ambient conditions as well as varying daily and seasonal energy demands and load profiles.

This paper describes the development and implementation of the Alstom AEV (Advanced EnVironmental) burner, an evolution of the EV. Continuous fuel supply to two fuel stages at any engine load simplifies the operation and provides a fast and reliable response of the combustion system during transient operation of the gas turbine. Increased turndown with low emissions is an additional advantage of the combustion system upgrade.

Commentary by Dr. Valentin Fuster
2012;():361-370. doi:10.1115/GT2012-68483.

The standard design process for the Siemens Industrial Turbomachinery, Lincoln, Dry Low Emissions combustion systems has adopted the Eddy Dissipation Model with Finite Rate Chemistry for reacting computational fluid dynamics simulations. The major drawbacks of this model have been the over-prediction of temperature and lack of species data limiting the applicability of the model.

A novel combustion model referred to as the Scalar Dissipation Rate Model has been developed recently based on a flamelet type assumption. Previous attempts to adopt the flamelet philosophy with alternative closure models have failed, with the prediction of unphysical phenomenon. The Scalar Dissipation Rate Model (SDRM) was developed from a physical understanding of scalar dissipation rate, signifying the rate of mixing of hot and cold fluids at scales relevant to sustain combustion, in flames and was validated using direct numerical simulations data and experimental measurements.

This paper reports on the first industrial application of the SDRM to SITL DLE combustion system. Previous applications have considered ideally premixed laboratory scale flames. The industrial application differs significantly in the complexity of the geometry, unmixedness and operating pressures. The model was implemented into ANSYS-CFX using their inbuilt command language. Simulations were run transiently using Scale Adaptive Simulation turbulence model, which switches between Large Eddy Simulation and Unsteady Reynolds Averaged Navier Stokes using a blending function.

The model was validated in a research SITL DLE combustion system prior to being applied to the actual industrial geometry at real operating conditions. This system consists of the SGT-100 burner with a glass square-sectioned combustor allowing for detailed diagnostics. This paper shows the successful validation of the SDRM against time averaged temperature and velocity within measurement errors.

The successful validation allowed application of the SDRM to the SGT-100 twin shaft at the relevant full load conditions. Limited validation data was available due to the complexity of measurement in the real geometry. Comparison of surface temperatures and combustor exit temperature profiles showed an improvement compared to EDM/FRC model. Furthermore, no unphysical phenomena were predicted.

This paper presents the successful application of the SDRM to the industrial combustion system. The model shows a marked improvement in the prediction of temperature over the EDM/FRC model previously used. This is of significant importance in the future applications of combustion CFD for understanding of hardware mechanical integrity, combustion emissions and dynamics of the flame.

Commentary by Dr. Valentin Fuster
2012;():371-381. doi:10.1115/GT2012-68489.

A new meshless Lagrangian particle code has been developed in order to tackle the challenging numerical modeling of primary atomization. In doing so the correct treatment and representation of the interfacial physics are crucial prerequisites. Grid based codes using interface tracking or interface capturing techniques, such as the Volume of Fluid or Level Set method, exhibit some difficulties regarding mass conservation, curvature capturing and interface diffusion. The objective of this work is to overcome these shortcomings of common state-of-the-art grid based FVM approaches. Our multi-dimensional meshless particle code is based on the Smoothed Particle Hydrodynamics method [1] [2]. Various test cases have been conducted, by which the capability of accurately capturing the physics of single and multiphase flows is verified and the future potential of this approach is demonstrated. Compressible as well as incompresssible fluids can be modeled. Surface tension effects are taken into account by two different models, one of them being more suitable for free surface flows and the other for simulating multiphase flows. Solid walls as well as periodic boundary conditions offer a broad variety of numerically modeling technical applications. In a first step, single phase calculations of shear driven liquid flows have been carried out. Furthermore, the disintegration of a gravity driven liquid jet emerging from a generic nozzle has been investigated in free surface simulations. The typical formation of a meniscus due to surface tension is observed. Spray formation is qualitatively in good agreement compared to experiments. Surface tension effects have been taken into account via the cohesive force model. Finally, the results of a two-phase simulation with a fluid density ratio of 1000, which is similar to a fuel-air fluid system as in airblast atomizers, are presented. The surface minimization and pressure jump across the droplet interface due to surface tension can be predicted accurately. The test cases conducted so far demonstrate the accuracy of the existing code and underline the promising potential of this new method for successfully predicting primary atomization.

Commentary by Dr. Valentin Fuster
2012;():383-389. doi:10.1115/GT2012-68570.

Combustion dynamics have detrimental effects on hardware durability as well as combustor performance and emissions. This paper presents a detailed study on the impact of combustion dynamics on NOx and CO emissions generated from a prototype gas turbine combustor operating at a pressure of 180 psia (12.2 bars) with a pre-heat temperature of 720 F (655.3 K) (E-class machine operating conditions). Two unstable modes are discussed. The first is an intermittent mode, at 750 Hz, that emerges at flame temperatures near 2900°F (1866.5 K), resulting in high NOx and CO emissions. With increasing fuel flow, NOx and CO emissions continue to increase until the flame temperature reaches approximately 3250°F (2061 K), at which point the second acoustic mode begins to dominate. Flame images indicate that the intermittent mode is associated with flame motion which induces the high NOx and CO emissions. The second mode is also a 750 Hz, but of constant amplitude (no intermittency). Operation in this second 750 Hz mode results in significantly reduced NOx and CO emissions. At pressures higher than 180 psia (12.2 bars), the intermittent mode intensifies, leading to flashback at flame temperatures above 2850°F (1839 K). In order to mitigate the intermittent mode, a second configuration of the combustor included an exit area restriction. The exit area restriction eliminated the intermittent mode, resulting in stable operation and low emissions over a temperature range of 2700–3200°F (1755–2033 K). A comparison of the NOx emissions, as function of flame temperature, with previous published data for perfectly premixed indicates that, while the low amplitude 750 Hz oscillations have little effect, the intermittent mode significantly increases emissions. Mode shape analysis shows that the 750 Hz instability corresponds to the 1/4 wave axial mode. In the current research a ceramic liner is used while the previous published data was collected with a quartz liner. Typically, quartz is avoided due to reductions in effective flame temperature by radiation losses. Experiments showed that NOx emissions were not affected by the combustor liner type. This agreement between the quartz and ceramic liners data indicates limited effect from the radiation heat losses on NOx emissions.

Commentary by Dr. Valentin Fuster
2012;():391-400. doi:10.1115/GT2012-68590.

The Japan Aerospace Exploration Agency (JAXA) is conducting research and development on aircraft engine technologies to reduce environmental impact for the TechCLEAN project. As a part of the project, combustion technologies have been developed with an aggressive target that is an 80% reduction over the NOx threshold of the ICAO CAEP/4 standard. A staged fuel nozzle with a pilot mixer and a main mixer was developed and tested using a single-sector combustor under the target engine’s LTO cycle conditions with a rated output of 40 kN and an overall pressure ratio of 25.8. The test results showed a 77% reduction over the CAEP/4 NOx standard. A reduction in smoke was found under a higher thrust condition than the 30% MTO condition, and a reduction in CO emission was found under a lower thrust condition than the 85% MTO condition. In the present study, an additional fuel burner was designed and tested with the staged fuel nozzle in a single-sector combustor to control emissions. The test results show that the combustor enables an 82% reduction in NOx emissions relative to the ICAO CAEP/4 standard and a drastic reduction in smoke and CO emissions.

Commentary by Dr. Valentin Fuster
2012;():401-408. doi:10.1115/GT2012-68626.

Fischer-Tropsch (F-T) jet fuel composition differs from petroleum-based, conventional commercial jet fuel because of differences in feedstock and production methodology. Fischer-Tropsch fuel typically has a lower aromatic and sulfur content and consists primarily of iso and normal parafins. The ASTM D3241 specification for Jet Fuel Thermal Oxidation Test (JFTOT) break point testing method was used to test the breakpoint of a baseline commercial grade F-T jet fuel, and various blends of this F-T fuel with an aromatic solution. The goal of this research is to determine the effect of aromatic content on the thermal stability of Fischer-Tropsch fuel. The testing completed in this report was supported by the NASA Fundamental Aeronautics Subsonics Fixed Wing Project.

Commentary by Dr. Valentin Fuster
2012;():409-424. doi:10.1115/GT2012-68642.

A 76mm diameter combustor with single axial flat 8 bladed swirlers of 30°, 45° and 60° vane angle were investigated at a reference Mach number of 0.05 or combustion intensity of 20MW/(m2bar), which represents all the combustion air passing through the swirler. The influence of a swirler outlet shroud or outlet orifice plate was investigated. This increased the pressure loss and the outer dump expansion recirculation zone size, D/d, and these were the main influences on the combustion performance. The swirlers were very large relative to the combustor so that they had a high air flow capacity. They had the same outer diameter, d, of 70mm with a cylindrical combustor diameter, D, of 76mm, D/d = 1.1. These swirlers had a decreasing hub size as the vane angle was increased. There was no significant effect of the outlet shroud on the weak extinction. The presence of the shroud promoted more rapid flame development and this produced a reduction in HC and CO. This also reduced the prompt NOx due to the reduction in HC. The overall NOx was reduced as the axial swirler outlet shroud diameter was reduced. The trends were the same for all three swirler vane angles, but there was little advantage of using higher swirl number large vane angle swirlers when compared at the same pressure loss. The minimum NOx at 15% oxygen for premixed combustion at 1750K with 600K inlet temperature was 2ppm for 30° shrouded axial swirlers, 3ppm for 45° shrouded swirlers and 2.5ppm for 60° shrouded swirlers. However, a 45° unshrouded axial swirler with smaller central hub and large D/d had NOx emissions of 2ppm.

Commentary by Dr. Valentin Fuster
2012;():425-436. doi:10.1115/GT2012-68656.

A summary of the impacts of alternative fuel blends on the gaseous and particulate matter (PM) (mostly soot) emissions of aircraft turbine engines is presented. Six engines were studied under several US Air Force and NASA sponsored programs to assess the impacts of the alternative (non-petroleum) fuels on emissions and/or to support the certification of military aircraft for the use of 50/50 (by volume) alternative fuel/JP-8 blends. One turboshaft (T63) and five turbofan (CFM56-7, CFM56-2, F117, TF33 and PW308) engines were studied. Fuels derived from coal and natural gas produced via Fischer-Tropsch (FT) synthesis, and fuels from animal fats and plant oils produced via hydroprocessing [Hydroprocessed Esters and Fatty Acids (HEFA)] were evaluated. Trends of alternative fuel impacts on emissions compared to conventional fuel for the different engine types are discussed. Results consistently show significant reductions in PM emissions with the alternative fuel blends compared to operation with conventional fuels. These relative reductions were observed to be lower as engine power increased. Engines operated with different alternative fuel blends were found to produce similar slopes of normalized particle number to engine power with only the magnitude of the reductions being a function of the fuel type. These results suggest that it may be plausible to predict particle number emissions from turbine engines operated on alternative fuels based on engine, engine setting, limited PM data and fuel composition. Gaseous emissions measurements show modest reductions of carbon monoxide, unburned hydrocarbons and hazardous air pollutants (HAPs) with the alternative fuels for several engines; however, no clear dependency of fuel impacts based on engine characteristics were observed.

Commentary by Dr. Valentin Fuster
2012;():437-445. doi:10.1115/GT2012-68662.

This paper explores the technical applicability of a low-swirl fuel nozzle designed for use with a liquid-fueled industrial gas turbine combustor. Particle image velocimetry was applied to measure nozzle flow fields with an open methane-air premixed flame configuration. Herein we discuss the effects of the chamfer dimensions of the nozzle tip on flow characteristics. The profiles indicate parallel shifts in axial direction that depend on chamfer dimensions. When velocity is normalized by bulk velocity and plotted against axial distance from the virtual origins, the profiles are consistent. This means that chamfer dimensions primarily affect the axial position of the flame, while keeping other flow characteristics, such as global stretch rate, unchanged. Then, the atmospheric combustion test was conducted with kerosene in a single-can combustor. Lifted flame stabilization was confirmed by observing the flames through a window. Lastly, an engine test was performed to assess the technical applicability of the fuel nozzle under real engine conditions. The engine testbed was a 290 kW simple-cycle liquid-fueled gas turbine engine. The configurations of the fuel nozzle were consistent with the ones used in the PIV and the atmospheric combustion test. Wall temperatures close to the fuel nozzle exit were within the acceptable range, even without the cooling air required with conventional combustors. This is an advantage of the lifted flame stabilization technique. NOx emissions were below maximum levels set under current Japanese regulations (<84 ppm@15% O2). In sum, the proposed fuel nozzle design shows promise for use with liquid-fueled industrial gas turbine engines.

Commentary by Dr. Valentin Fuster
2012;():447-455. doi:10.1115/GT2012-68684.

This paper deals with a detailed thermoacoustic assessment of an annular reheat combustor. Extensive tests have been conducted in the GT26 Test Power Plant in Switzerland. To this end, the combustion chamber has been instrumented with advanced pulsation sensors, an optical probe, strain gauges and accelerometers. A large number of dynamic pressure sensors recorded the acoustic pressure wave propagations in the axial and circumferential directions over a very large frequency range.

A modal analysis technique has been developed to extract the acoustic mode shapes from the experimental data. This technique allows for decomposition in standing and traveling waves, hence revealing the nature of the acoustic field. The extracted mode shapes showed a very good agreement with results from 3-d finite element calculations.

Combustion stability has been quantified by a methodology that extracts pulsation growth rates from experimental data. This novel method relies on advanced statistic processing of the instantaneous pulsation amplitudes. It has the advantage that it is insensitive to probe location, which is of particular advantage for high frequencies (higher than 1 kHz).

Commentary by Dr. Valentin Fuster
2012;():457-467. doi:10.1115/GT2012-68689.

In technically relevant combustion devices, combustion can take place in the vicinity of walls which can significantly affect the reaction and the heat transfer. However, only few studies focus on modelling of flame-wall interaction (FWI) for algebraic combustion models and virtually none consider FWI for algebraic Large Eddy Simulation combustion models. In the present work heat loss models, as previously published in the literature, are employed to extend a LES algebraic combustion model. The performance of the FWI models is evaluated by simulations of a nonadiabatic swirl flame. The simulation results are compared with experimental data of velocity field and heat release. The extent of the quenching zone and heat loss effects are determined in the simulations and compared with data from direct numerical simulations. Comparison of simulation and experimental data shows a significant improvement when heat loss effects are incorporated. Also the characteristic Peclet numbers are correctly predicted by FWI models.

Commentary by Dr. Valentin Fuster
2012;():469-477. doi:10.1115/GT2012-68696.

Lean flame blowout induced by thermoacoustic oscillations is a serious problem faced by the power and propulsion industry. We analyze a prototypical thermoacoustic system through systematic bifurcation analysis and find that starting from a steady state, this system exhibits successive bifurcations resulting in complex nonlinear oscillation states, eventually leading to flame blowout. To understand the observed bifurcations, we analyze the oscillation states using nonlinear time series analysis, particularly through the representation of pressure oscillations on a reconstructed phase space. Prior to flame blowout, a bursting phenomenon is observed in pressure oscillations. These burst oscillations are found to exhibit similarities with the phenomenon known as intermittency in the dynamical systems theory. This investigation based on nonlinear analysis of experimentally acquired data from a thermoacoustic system sheds light on how thermoacoustic oscillations lead to flame blowout.

Commentary by Dr. Valentin Fuster
2012;():479-490. doi:10.1115/GT2012-68698.

The adoption of lean premixed prevaporised combustion systems can reduce NOx emissions from gas turbines, but unfortunately also increases their susceptibility to thermoacoustic instabilities. Initially, acoustic waves can produce heat release fluctuations by a variety of mechanisms, often by perturbing the equivalence ratio. If correctly phased, heat release fluctuations can subsequently generate more acoustic waves, which at high amplitude can result in significant structural damage to the combustor. The prediction of this phenomenon is of great industrial interest.

In previous work, we have coupled a physics based, kinematic model of the flame with a network model to provide the planar acoustic response necessary to close the feedback loop and predict the onset and amplitude of thermoacoustic instabilities in a lab-scale, axisymmetric single burner combustor. The advantage of a time domain approach is that the modal interaction, the influence of harmonics, and flame saturation can be investigated. This paper extends this approach to more realistic, annular geometries, where both planar and circumferential modes must be considered.

In lean premixed prevaporised combustors, fluctuations in equivalence ratio have been shown to be a dominant cause of unsteady combustion. These can occur, for example, due to velocity perturbations in the premix ducts, which can lead to equivalence ratio fluctuations at the fuel injectors, which are subsequently convected downstream to the flame surfaces. Here, they can perturb the heat release by locally altering the flame speed, enthalpy of combustion, and, indirectly, the flame surface area.

In many gas turbine designs, particularly aeroengines, the geometries are composed of a ring of premix ducts linking a plenum and an annular combustor. The most unstable modes are often circumferential modes. The network model is used to characterise the flow response of the geometry to heat fluctuations at an appropriate location, such as the fuel injectors. The heat release at each flame holder is determined in the time domain using the kinematic flame model derived, as a function of the flow perturbations in the premix duct. This approach is demonstrated for an annular ring of burners on a in a simple geometry. The approach is then extended to an industrial type gas turbine combustor, and used to predict the limit cycle amplitudes.

Topics: Modeling , Flames
Commentary by Dr. Valentin Fuster
2012;():491-498. doi:10.1115/GT2012-68711.

An optically accessible gas turbine combustor test rig was constructed to study the combustion characteristics of a coaxial hydrogen/air jet injected into a vitiated swirl crossflow. The test rig has two combustion zones. The main combustion zone (MCZ) is a swirl stabilized dump combustor, and the secondary combustion zone (SCZ) is a reacting crossflow jet, referred to as the jet-in-crossflow (JIC). The SCZ is located downstream of the MCZ. The JIC is a coaxial hydrogen/air jet that penetrates radially into the vitiated stream. The combustor was designed to study the effects of JIC conditions on the SCZ combustion process and in particular on NOx production. The jet velocity and equivalence ratio were systematically varied. A water-cooled sampling probe was used to extract exhaust gases downstream of the SCZ for emission measurements. The JIC flame structure was captured by OH-PLIF images which show the extent of the flame front and the depth of penetration into the vitiated stream. The OH-PLIF images were averaged to determine the JIC reaction zone and were compared to the Holdeman correlation.

Commentary by Dr. Valentin Fuster
2012;():499-521. doi:10.1115/GT2012-68722.

Simulation of the combustion of fuels used in transportation and energy applications requires accurate chemistry representation of the fuel. Surrogate fuels are typically used to represent liquid fuels, such as gasoline, diesel or jet fuel, where the surrogate contains a handful of components. For gaseous fuels, surrogates are effectively used as well, where methane may be used to represent natural gas, for example. An accurate chemistry model of a surrogate fuel means a detailed reaction mechanism that contains the kinetics of all the molecular components of the fuel model. Since large hydrocarbons break down to smaller molecules during combustion, the core chemistry of C0 to C4 carbon number is critical to all such fuel models, whether gaseous or liquid. The usual method of assessing how accurate the fuel chemistry is involves modeling of fundamental combustion experiments, where the experimental conditions are well enough defined and well enough represented by the reacting-flow model to isolate the kinetics in comparisons between predictions and data. In the work reported here, we have been focused on developing a more comprehensive and accurate core (C0−C4) mechanism. Recently, we revisited the core mechanism to improve predictions of the pure saturated components (J Eng. Gas Turbines Power (2012) 134; doi:10.1115/1.4004388). In the current work, we focused on combustion of unsaturated C0−C4 fuel components and on the blends of C0−C4 fuels, including saturated components. The aim has been to improve predictions for the widest range of fundamental experiments as possible, while maintaining the accuracy achieved by the existing mechanism and the previous study of saturated components. In the validation, we considered experimental measurements of ignition delay, flame speed and extinction strain rate, as well as species composition in stirred reactors, flames and flow reactors. These experiments cover a wide range of temperatures, fuel-air ratios, and pressures. As in the previous work for saturated compounds, we examined uncertainties in the core reaction mechanism; including thermochemical parameters derived from a wide variety of sources, including experimental measurements, ab initio calculations, estimation methods and systematic optimization studies. Using sensitivity analysis, reaction-path analysis, consideration of recent focused studies of individual reactions, and an enforcement of data consistency, we have identified key updates required for the core mechanism. These updates resulted in improvements to predictions of results, as validated through comparison with experiments, for all the fuels considered, while maintaining the accuracy previously reported for the saturated C0−C4 components. Rate constants that were modified to improve predictions for a small number of reactions remain within expected uncertainty bounds.

Topics: Fuels
Commentary by Dr. Valentin Fuster
2012;():523-534. doi:10.1115/GT2012-68726.

Nonlinear analysis of thermoacoustic instability is essential for prediction of frequencies, amplitudes and stability of limit cycles. Limit cycles in thermoacoustic systems are reached when the energy input from driving processes and energy losses from damping processes balance each other over a cycle of the oscillation.

In this paper an integral relation for the rate of change of energy of a thermoacoustic system is derived. This relation is analogous to the well-known Rayleigh criterion in thermoacoustics, but can be used to calculate the amplitudes of limit cycles, as well as their stability. The relation is applied to a thermoacoustic system of a ducted slot-stabilized 2-D premixed flame. The flame is modelled using a nonlinear kinematic model based on the G-equation, while the acoustics of planar waves in the tube are governed by linearised momentum and energy equations. Using open-loop forced simulations, the flame describing function (FDF) is calculated. The gain and phase information from the FDF is used with the integral relation to construct a cyclic integral rate of change of energy (CIRCE) diagram that indicates the amplitude and stability of limit cycles. This diagram is also used to identify the types of bifurcation the system exhibits and to find the minimum amplitude of excitation needed to reach a stable limit cycle from another linearly stable state, for single-mode thermoacoustic systems. Furthermore, this diagram shows precisely how the choice of velocity model and the amplitude-dependence of the gain and the phase of the FDF influence the nonlinear dynamics of the system.

Time domain simulations of the coupled thermoacoustic system are performed with a Galerkin discretization for acoustic pressure and velocity. Limit cycle calculations using a single mode, as well as twenty modes, are compared against predictions from the CIRCE diagram. For the single mode system, the time domain calculations agree well with the frequency domain predictions. The heat release rate is highly nonlinear but, because there is only a single acoustic mode, this does not affect the limit cycle amplitude. For the twenty-mode system, however, the higher harmonics of the heat release rate and acoustic velocity interact resulting in a larger limit cycle amplitude. Multi-mode simulations show that in some situations the contribution from higher harmonics to the nonlinear dynamics can be significant and must be considered for an accurate and comprehensive analysis of thermoacoustic systems.

Topics: Flames
Commentary by Dr. Valentin Fuster
2012;():535-541. doi:10.1115/GT2012-68739.

A numerical study is carried out investigating the effect of hydrogen and syngas addition on the ignition of two JP-8 surrogates, a two-component surrogate and a six-component surrogate. This six-component surrogate has previously been found to accurately simulate the smoke point, volatility, flame temperature profiles, and extinction limits of JP-8, while the two component surrogates has been shown to reproduce the flame structure predicted with the six-component surrogate. CHEMKIN 10101 is used to simulate ignition in a closed homogenous reactor under adiabatic and isobaric conditions. The parameters include temperature ranging from 850–1250 K, pressure of 20 atm, and equivalence ratio ϕ = 1.0. The CRECK-0810 kinetic mechanism, involving 341 species and 9173 reactions, is used to model the ignition chemistry. For the conditions studied, the addition of H2 or syngas in small quantities has no effect on the ignition behavior of either the surrogates or their individual components. Addition of H2 or syngas in larger quantities increases and decreases the ignition delay at low and high temperatures, respectively. For the conditions investigated, the ignition behavior of both the surrogates is predominantly determined by the ignition chemistry of n-dodecane.

Topics: Syngas , Hydrogen , Ignition
Commentary by Dr. Valentin Fuster
2012;():543-551. doi:10.1115/GT2012-68761.

This study investigates the performance and the conditions under which flameless oxidation can be achieved for a given annular adiabatic combustor. Numerical modelling of velocity, temperature and species fields are performed for different flow configurations of air and methane streams injected into a proposed design of a gas-turbine combustor. Parametric analysis was performed by systematically varying several parameters: radius of a recirculation zone, radius of the combustor, location of air and fuel ports, air and fuel velocities magnitudes and injection angles. The analysis was performed initially using a three-step global chemistry model to identify a design (geometry and operating conditions) that yield flameless combustion regime. The selected design was then modelled using a skeletal (46 reactions) and a detailed (309 reactions) chemical kinetics mechanism. The k–ε turbulence model was used in the most calculations. Overall, similar qualitative flow, temperature, and species patterns were predicted by both kinetics models; however the detailed mechanism provides quantitatively more realistic predictions. An optimal flow configuration was achieved with exhaust NOx emissions of < 7.5 ppm, CO < 35ppm, and a pressure-drop < 5%, hence meeting the design criteria for gas turbine engines. This study demonstrates the feasibility of achieving ultra-low NOx and CO emissions utilising a flameless oxidation regime.

Commentary by Dr. Valentin Fuster
2012;():553-566. doi:10.1115/GT2012-68775.

In the past decades, several feedback mechanisms for longitudinal acoustic modes in gas turbine combustors have been investigated. These mechanisms are successfully used in predictive tools like acoustic network models to analyze low-frequency instabilities in combustion systems. In contrast, little is known about high-frequency oscillations — fluctuations at several kHz. Most theories are derived from experimental investigations of afterburners in the 1950s and 1960s, indicating an interaction of vortex shedding, fluctuating vorticity and heat release. In this work a different feedback mechanism for high-frequency oscillations in cylindrical flame tubes related to transverse acoustic modes is suggested and analysed: Transverse acoustic pressure fluctuations are linked to an oscillating velocity field. A time-dependent but periodic displacement field can be derived from these velocity fluctuations. The model assumes that the zone of heat release is displaced by the velocity fluctuations. Pressure oscillations and periodically deflected heat release lead to a contribution to the Rayleigh criterion without fluctuations in the global heat release. This effect is studied in a circular cross section presuming a circular zone of heat release. Expressions for the displacement of the flame front are derived from the analytical solution of the wave equation in cylindrical geometries assuming a quiescent medium, constant density and speed of sound. The Rayleigh criterion is integrated and growth rates are evaluated whereas damping effects are neglected as they are not subject to this study. Characteristics of the model are assessed and compared to experimental observations to check the validity and the applicability of the theory.

Commentary by Dr. Valentin Fuster
2012;():567-578. doi:10.1115/GT2012-68792.

Effusion cooled liners, commonly used in gas turbine combustion chambers to reduce wall temperature, may also help reducing the propagation of pressure fluctuations due to thermoacoustic instabilities.

Large Eddy Simulations were conducted to accurately model the flow field and the acoustic response of effusion plates subject to a mean bias flow under external sinusoidal forcing. Even though existing lower order computational models showed good predicting capabilities, it is interesting to verify directly the influence of those parameters such as the staggered arrangement, the hole inclination, the presence of a grazing flow and the level of bias flow, which are not fully included in those models.

A first bi-periodic single hole configuration with normal acoustic forcing was selected to investigate the acousting behavior with varying inclination angle, bias and grazing flow. 90° and 30° perforations were simulated for bias flow Mach number in the range 0.05–0.1 and grazing flow between 0 and 0.08. Those conditions were chosen to expand the knowledge of acoustic properties towards actual liners working conditions. A second more computationally expensive set-up, including 4 inclined holes at 30°, focused on the damping of parallel to the plate waves.

Details of the computational methods implemented in the general purpose open-source unstructured CFD code OpenFOAM® exploited to conduct this analysis are reported together with an analysis of the results obtained from the acoustic computations both regarding the flow field generated and the absorption and energy dissipation coefficient.

Commentary by Dr. Valentin Fuster
2012;():579-588. doi:10.1115/GT2012-68796.

The influence of thermal boundary condition at the combustor wall and combustor confinement on the dynamic flame response of a perfectly premixed axial swirl burner is investigated. Large Eddy Simulations are carried out using the Dynamically Thickened Flame combustion model. Then, system identification methods are used to determine the flame transfer function (FTF) from the computed time series data.

Two configurations are compared against a reference case with 90 mm × 90 mm combustor cross section and nonadiabatic walls: 1) combustor cross section similar to the reference case with adiabatic combustor walls, and 2) different confinement (160 mm × 160 mm) with nonadiabatic walls. It is found that combustor confinement and thermal boundary conditions have a noticeable influence on the flame response due to differences in flame shape and flow field. In particular the FTF computed with adiabatic wall boundary condition, which produces a flame with significant heat release in both shear layers, differs significantly from the FTF with nonadiabatic walls, where the flame stabilizes only in the inner shear layer. The observed differences in flow field and flame shape are discussed in relation to the unit impulse response of the flame. The impact of the differences in FTF on stability limits is analyzed with a low-order thermoacoustic model.

Commentary by Dr. Valentin Fuster
2012;():589-599. doi:10.1115/GT2012-68804.

A practical computational fluid dynamics (CFD) approach to modeling effusion orifices in gas turbine combustor liners is proposed specifically when liner metal geometry is not included and conjugate heat transfer is not invoked. The focus is on eliminating effusion orifices from the model while maintaining the imprint of the orifices on the cold and hot sides of the liner wall. The imprinted boundaries serve as embedded mass flow inlets and outlets on both sides of the wall and maintain the integrity of the wall geometry. An empirical model is then used to extract and inject mass from the cold and hot sides of the liner, respectively. The mass extraction and injection process is performed for each orifice based on local conditions such as pressure, temperature and discharge coefficient. The discharge coefficient is, in turn, dynamically computed for each orifice based on approach angle, approach Mach number, discharge Mach number and orifice length to diameter ratio. With this approach, the fidelity of the liner wall is preserved for better heat transfer predictions and easier near wall meshing. In addition, the discharge coefficient is not assumed but calculated allowing the redeployment of inherently inadequate effusion orifice mesh cells to other critical areas of the combustor. Presented are results of two combustor cases to demonstrate the practicality and accuracy of the proposed method as compared to standard effusion modeling and their comparison with rig data.

Commentary by Dr. Valentin Fuster
2012;():601-609. doi:10.1115/GT2012-68838.

The extension of gas fuel flexibility in the Siemens SGT-300 single shaft (SGT-300-1S) is reported in this paper. A successful development programme has increased the capability of the Siemens Industrial Turbomachinery, Lincoln (SITL) dry low emissions (DLE) burner configuration to a fuel range covering a Wobbe Index (WI) from 15 to 49 MJ/m3.

The standard SGT-300-1S SITL DLE combustion hardware allowed for gas and liquid fuels within a specified range typically associated with natural gas and diesel, respectively. Field operation of the standard production SGT-300-1S has confirmed the reliable operation with an extension to the fuels range to include processed land fill gas (PLG) from 32 to 49 MJ/m3.

The further extension of the fuel range for the SGT-300-1S SITL DLE combustion system was achieved through high pressure testing of a single combustion system at engine operating conditions. The rig facility allowed for the actual fuel type to be tested using a mixing plant. The variations in fuel heating value were achieved by blending natural gas with diluent CO2 and/or N2. Various diagnostics were used to assess the performance of the combustion system including measurement of combustion dynamics, temperature, fuel supply pressure and emissions of NOx, CO and unburned hydrocarbon (UHC).

The results of the testing showed that the standard production burner can operate for a fuel with WI as low as 23 MJ/m3 which corresponds to 35% CO2 (in volume) in the fuel. This range can be extended to 15 MJ/m3 (54.5% CO2 in the fuel) with only minor modification, to control losses through the burner and to maintain similar fuel injection characteristics.

The SITL DLE combustion system is able to cover a WI range of 15 to 49 MJ/m3 in two configurations. The results of testing showed a lowering in WI, from diluting with CO2 and/or N2, a benefit in NOx reduction is observed. This decrease in WI may lead to an increased requirement in fuel supply pressure.

Topics: Fuels , Emissions
Commentary by Dr. Valentin Fuster
2012;():611-621. doi:10.1115/GT2012-68842.

Perforated liners consist of sheet metal perforated with multiple holes with diameters of magnitude in the order of millimeters and regular spacing, backed by an air cavity in front of a rigid wall. This type of liner is very effective at absorbing sound and is used in many applications. At the resonance frequency, the liner shifts the phase of the incident wave by 180° thus providing damping through wave cancellation.

The perforations in the liner convert acoustic energy into flow energy through vortex shedding at the rims of the liner apertures.

Applied to gas turbine combustors they can attenuate thermoacoustic instabilities and as such significantly improve the reliability of the gas turbine with an additional benefit to the emissions. The Siemens SGT-100 to 400 engines exploit this technology in their DLE combustion system in a configuration of two concentric liners separated by an air cavity with the rear liner acting as the rigid wall in the conventional setting.

In this paper the evaluation of double perforated liners in the absorption of normal-incident plane acoustic waves in an impedance tube and in a gas turbine combustor environment is investigated.

A one-dimensional impedance model that embodies the electro-acoustic analogy was used to predict the absorption characteristics of the double perforated liner. The model was validated by comparing the predictions with experimental data obtained from the impedance tube, with excellent agreement. With the confidence in the equations of the model in predicting the acoustic behavior, the model was then applied to predict the damping performance under realistic gas turbine combustor operating conditions. The prediction also shows two distinct peaks in the absorption characteristics of a double-liner.

Geometric parameters such as hole diameters & thicknesses of the two liners, gap between the liners and the overall pressure drop across the liners have been considered for the predictions. A parametric study of these parameters carried out using the ISIGHT software with design investigation tools identified the order of importance of the parameters considered for sound absorption.

The work reported in this paper has successfully validated an impedance model in the prediction of double perforated liners in the absorption of normal-incident plane acoustic waves. Based on the parametric study carried out design guidelines are given for designing a double perforated liner for maximum absorption of normal incident acoustic waves.

Commentary by Dr. Valentin Fuster
2012;():623-632. doi:10.1115/GT2012-68852.

The present study is devoted to the modeling of mean flow effects while computing thermoacoustic modes under the zero Mach number assumption. It is first recalled that the acoustic impedance modelling a compressor or a turbine must be prescribed under an energetical form instead of the classical acoustic variables. Then we demonstrate the feasibility to take into account the coupling between acoustic and entropy waves in a zero Mach number framework to capture a family of low frequency entropic modes. The proposed approach relies on a new Delayed Entropy Coupled Boundary Condition (DECBC) and proves able to capture a family of Low frequency entropic mode even though no mean flow term is included into the fluctuating pressure equation.

Commentary by Dr. Valentin Fuster
2012;():633-638. doi:10.1115/GT2012-68858.

This paper mainly focuses on reliable ignition and flame stabilization of low-BTU gas fuels with non-equilibrium air plasma assist. The results of experimental and numerical analyses indicated that the flame propagation speed was increased while the ignition time was decreased by the effect of non-equilibrium air plasma. Active radicals observed in air plasma made great efforts to the generation of OH radical which can accelerate the fuel oxidation process greatly.

Commentary by Dr. Valentin Fuster
2012;():639-649. doi:10.1115/GT2012-68873.

Damping of thermoacoustically induced pressure pulsations in combustion chambers is a major focus of gas turbine operation. Conventional Helmholtz resonators are an excellent means to attenuate thermoacoustic instabilities in gas turbines. Usually, however, the damping optimum is in a narrow frequency band at one operating condition. The work presented here deals with a modification of the basic Helmholtz resonator design over-coming this drawback. It consists of a damper body housing separated volumes that are connected to each other. Adequate adjustment of the governing parameters results in a broadband damping characteristic for low frequencies. In this way, changes in operating conditions and engine-to-engine variations involving shifts in the combustion pulsation frequency can conveniently be addressed. Genetic algorithms and optimization strategies are used to derive these parameters in a multi-dimensional parameter space. The novel damper concept is described in more detail and compared with cold-flow experiments. In order to validate the performance under realistic conditions, the new broadband dampers were implemented in a full-scale test engine. Pulsation amplitudes could be reduced by more than 80%. In addition, it is shown that due to sophisticated damper placement in the engine two unstable modes can be addressed simultaneously. Application of the damper concept allowed to considerably increase the engine operating regime and finally to reduce NOx emissions by 55%. Predictions obtained with the physics-based model excellently agree with experimental results for all tested damper geometries, bias flows, excitation amplitudes, and most important with the measurements in the engine.

Commentary by Dr. Valentin Fuster
2012;():651-659. doi:10.1115/GT2012-68898.

In most dry low NOx combustor designs the front panel impingement cooling air is directly injected into the combustor primary zone. As this air partially mixes with the swirling flow of premixed reactants from the burner prior to completion of heat release it reduces the effective equivalence ratio in the flame and has a beneficial effect on NOx emissions. However, the fluctuations of the equivalence ratio in the flame potentially increase heat release fluctuations and influence flame stability. Since both effects are not yet fully understood isothermal experiments are made in a water channel where high speed planar laser induced fluorescence (HSPLIF) is applied to study the cooling air distribution and its fluctuations in the primary zone. In addition the flow field is measured with high speed particle image velocimetry (HSPIV). Both, mixing and flow field are also analyzed in numerical studies using isothermal large eddy simulation (LES) and the simulation results are compared with the experimental data. Of particular interest is the influence of the injection configuration and cooling air momentum variation on the cooling air penetration and dispersion. The spatial and temporal quality of mixing is quantified with probability density functions (PDF). Based on the results regarding the equivalence ratio fluctuations regions with potential negative effects on combustion stability are identified. The strongest fluctuations are observed in the outer shear layer of the swirling flow, which exerts a strong suction effect on the cooling air. Interestingly, the cooling air dilutes the recirculation zone of the swirling flow. In the reacting case this effect is expected to lead to a decrease of the temperature in the flame anchoring zone below the adiabatic flame temperature of the premixed reactant, which may have an adverse effect on flame stability.

Topics: Cooling , Combustion
Commentary by Dr. Valentin Fuster
2012;():661-672. doi:10.1115/GT2012-68922.

The effects of air flow forcing on fuel spray characteristics in a premixing swirler were assessed using ambient-pressure experiments and CFD (LES) analyses. Experimental measurements were performed using phase-locked Phase-Doppler Interferometry on two different swirler/mixer designs. The CFD analyses employed an advanced spray modeling technique to track the surface of the liquid fuel. The swirler designs chosen were representative of advanced low-emissions combustor concepts that emphasize thorough fuel/air mixing for Jet-A fuel. Significant post-processing of the results was performed in order to extract the response of the fuel spray mass flow rate fluctuations and fuel/air ratio to acoustic forcing. The results demonstrated that i) acoustic air forcing did not significantly change the atomization process, but did influence the unsteady transport of fuel droplets within the swirler flow field, ii) the level of fuel mass flow fluctuation was higher for one swirler and the level of fuel/air ratio fluctuations was higher for the other swirler and iii) the different behaviors between the two swirlers are primarily caused by the discrepant alignment of fuel and air distribution and the dissimilar droplet Stokes number which governs the unsteady transport. CFD results were interrogated to help understand the root causes of the observed phenomena. These showed that, for the swirler in which fuel mass flow fluctuations were observed, the swirl number was modulated by the acoustic forcing.

Commentary by Dr. Valentin Fuster
2012;():673-682. doi:10.1115/GT2012-68925.

NOx emission reduction is important for developing gas-turbine engines. Predicting the thermal profile and pollutant-emission factor by numerical simulation is effective for reducing the development costs. Here a large eddy simulation coupled with a 2-scalar flamelet approach is applied to the numerical analysis of an industrial gas-turbine combustor. The combustor of an L20A-DLE gas-turbine engine is calculated. Combustor performance under different loads is investigated. NOx production decreases with reducing load, and this tendency agrees well with the experimental results. It is said that NOx production due to a large amount of supplemental burner fuel. NOx production in the simulation is lower than in the experiment. The simulated temperature in the combustor outlet is also lower than the adiabatic temperature. Moreover, the fuel is not burned completely within the combustor region. The difference in the combustion status in a supplemental burner is investigated. For the diffusion flame, a high-temperature region is observed locally owing to the presence of a fuel-rich region. For NOx production, NOx emission reduction is expected using a burner that introduces a premixed flame. From the simulation results, we can estimate NOx production in a gas-turbine combustor. The tendencies in the differences of the loads agreed well with the experimental data, and the superiority of a premixed flame was indicated.

Commentary by Dr. Valentin Fuster
2012;():683-689. doi:10.1115/GT2012-68929.

The development of integrated, coal-gasification combined cycle (IGCC) systems provides cost-effective and environmentally sound options for meeting future coal-utilizing power generation needs in the world. The combustion of gasified coal fuel significantly influences overall performance of IGCC power generation. This study focuses on investigating the nitrogen dilution effects on a double-swirled non-premixed syngas flame. As the references, investigations on the H2 and CO double-swirled flames with N2 dilution are presented. Planar laser-induced fluorescence (PLIF) of OH-radical measurement is adopted to identify main reaction zones and burnt gas regions. Together with temperature and emission measurement during exhaust section, some important characteristics of the syngas flame are overall investigated. Experimental result shows that syngas flame root near the burner exit demonstrates double flame front structure. The existence of N2 expands the flame opening angle and enlarges the main reaction zone, and it may lead to lower NO emission and higher CO emission in exhaust gas.

Topics: Syngas , Nitrogen
Commentary by Dr. Valentin Fuster
2012;():691-700. doi:10.1115/GT2012-68963.

The work presented in this paper intends to deepen our understanding of the mechanisms involved in the spark ignition of liquid fuel sprays. An experimental study is presented regarding the ignition of monodisperse droplet chains of Jet A-1 aviation kerosene in a generic model combustor under well-defined boundary conditions. Breakdowns created by focused laser radiation were used as ignition sparks. They featured rapid spatial expansion, resulting in the formation of spherical blast waves in the surrounding air. The focus of this study lay on the effect of the blast waves on the fuel droplets. Blast wave trajectories were investigated by Schlieren imaging. Their interaction with kerosene droplets was observed by a high-speed camera via a long distance microscope; the droplets were visualized by laser-induced Mie scattering. Droplets within a distance of ten millimetres from the breakdown position were disintegrated by the aerodynamic forces of the post-shock flow field. Different breakup modes were observed, depending on the distance from the breakdown position.

Topics: Lasers , Waves , Drops
Commentary by Dr. Valentin Fuster
2012;():701-712. doi:10.1115/GT2012-68997.

Presented in this paper is a novel gas turbine combustor: EZEE® based upon the FLOX® combustion technology. The specific feature of this combustor is the ability to modulate the power density PA for natural gas (NG) combustion on a high thermal power level between PA = 13.3 MW/m2/bar in the main load and PA = 7.9 MW/m2/bar in the part load operation. The operational range for the global air to fuel excess ratio λ is between λ = 1.6 and λ = 2.7 corresponding to adiabatic flame temperatures between Tad ≈ 2000 K and 1500 K, respectively. The air preheating temperature is 673 K and the pressure level is 8 bar. The inspected operational range satisfies the demands of modern gas turbine combustors.

The idea of EZEE® is to manipulate the flame position by radial staging of two independent fuel supplies in order to prevent flame extinction with increasing λ. The combustor is developed with the aid of computational tools and accordingly manufactured and experimentally investigated at the DLR test facilities. It is demonstrated that the combustion is complete and stable and that the pollutant emission is low. Presented are first results of our investigations that show the usefulness and the potential of the concept. However, it is also shown that additional fine-tuning is still necessary to further reduce the pollutant emissions.

Commentary by Dr. Valentin Fuster
2012;():713-724. doi:10.1115/GT2012-68998.

Nonlinear prediction of combustion instabilities in premixed systems is undertaken on a generic configuration featuring an adjustable feeding manifold length, a multipoint injector composed of a perforated plate and a flame confinement tube. By changing the feeding manifold or flame tube lengths, the system exhibits different types of combustion regimes for the same flow operating conditions. Velocity, pressure and heat release rate measurements are used to examine oscillations during unstable operation. For many operating conditions, a limit cycle is reached at an essentially fixed oscillation frequency and quasi-constant amplitude. In another set of cases, the system features other types of oscillations characterized by multiple frequencies, amplitude modulation and irregular bursts which can be designated by “galloping” limit cycles or GLC. These situations are explored in this article. Imaging during GLCs indicates that the flame is globally oscillating but that the cycle is irregular. Prediction of these special oscillation states is tackled within the Flame Describing Function (FDF) framework. It is shown that it is possible to predict with a reasonable degree of agreement the ranges where a quasi-constant amplitude limit cycle will be established and ranges where the oscillation will be less regular and take the form of a galloping limit cycle. It is found that the FDF analysis also provides indications on the bounding levels of the oscillation envelope in the latter case.

Commentary by Dr. Valentin Fuster
2012;():725-735. doi:10.1115/GT2012-69001.

The prediction of combustion processes using Large Eddy Simulation (LES) combined with tabulated chemistry has proven to be very successful and become very popular during the last years in both academia and industry. Technical combustion systems feature a wide range of time and length scales which need to be resolved. The LES describes the rather slow, but turbulent and unsteady flow field very well, while the fast chemical reactions can be represented by tabulated chemistry models like Flamelet Generated Manifolds. Pollutants, being only present at lower concentrations and developing slowly are not easy to capture with the standard manifold defined by the fast major combustion products. Therefore, additional modeling in order to predict the carbon monoxide emissions is presented in this paper. The choice of the reaction progress variable and the solution of an additional transport equation with and without extra modeling for the post flame zone was investigated. These models are applied to a standard test case and compared to experimental data and the standard tabulation approach.

Topics: Turbulence , Modeling , Flames
Commentary by Dr. Valentin Fuster
2012;():737-748. doi:10.1115/GT2012-69006.

A large operational envelope is a key requirement for modern gas turbines. Fuel staging is used here to improve the part load performance of an enhanced FLOX® type combustor. A swirl-stabilized pilot stage is integrated in the FLOX® burner and the results of high pressure lab-scale experiments at system relevant conditions are presented. The operational envelope of the piloted system could be extended by approximately 10%. Pressure scaling and variations of air preheat temperature and jet velocity describe fundamental characteristics of the piloted system. OH* chemiluminescence imaging is used to investigate flame shapes and the effect of the interacting flames. Emissions and pressure pulsations define limits, and optimum operation conditions of the combustor and show the influence of part load relevant parameters.

Commentary by Dr. Valentin Fuster
2012;():749-758. doi:10.1115/GT2012-69025.

This paper describes the development of an empirical approach that attempts to predict blow-out of bluff body stabilized flames using global flow parameters in systems where liquid fuel injectors are located a short distance upstream of the wake. This approach was created on the hypothesis that flame stability in such a combustion system (referred to as a close-coupled injection) is determined by the strength of the heat source developed in the bluff body recirculation zone and by the availability of sufficient contact time with fresh mixture for its ignition, similar in nature to premixed combustion systems. Based on this concept, global equivalence ratio on the classical DeZubay stability map was replaced by local equivalence ratio in the recirculation zone of the bluff body. This local equivalence ratio was determined experimentally using a chemiluminescence measurement system. Tests were conducted using a single bluff body with a close coupled injection system in a 76×152mm (3×6 inches) combustion tunnel. A wide range of fuel-air ratios and velocities were achieved by variation of the global equivalence ratio, incoming flow velocity, and injector size. The obtained experimental data set was used to develop a transfer function that allowed calculation of the local equivalence ratio in the recirculation zone based on the global flow parameters. Equivalence ratio in the recirculation zone was found to be exponentially dependent upon the square root of the fuel to air momentum flux ratio such that increasing the momentum flux ratio led to a reduction in the recirculation zone equivalence ratio. Additional adjustment of this general trend by the diameter of injector and air flow velocity was necessary to improve the quality of the prediction. The developed approach demonstrated a good prediction of the globally rich blow-out of the flame. In fact, the recirculation zone lean blow-out limit (corresponding with globally rich blow-out) predicted for close coupled injection using the developed transfer function closely coincided with the lean blow-out line of the classical DeZubay envelope and with results obtained with premixed injection using the same bluff body. On the contrary, globally lean (locally rich) blow-out was predicted ∼20% below the DeZubay rich blow-out line, possibly because of the limited range of the fuel flow rates on the experimental rig used.

Topics: Fuels , Flames
Commentary by Dr. Valentin Fuster
2012;():759-766. doi:10.1115/GT2012-69027.

Opportunity gaseous fuels are of great interest for small and medium sized gas turbines. The variety of gaseous fuels that Siemens Industrial Turbomachinery AB (SIT) is requested to make judgments on is continuously expanding. From such requests follows an increasing need for testing new fuels.

The SIT novel approach for fuel flexibility testing, EBIT, has been to combine the single burner rig testing with a full scale engine test to give a cost effective and flexible solution. The combination of the two approaches is accomplished by using a separate feed of testing fuel to one or more burners in a standard gas turbine installation where the other burners use standard fuel from standard fuel system for engine operation. The separate feed of testing fuel can be operated as a slave to engine governor heat demand, but can also be controlled independently.

This paper describes how EBIT has been implemented and tested. Combustion monitoring techniques and measurements to check behavior and predictions for full scale engine tests are presented. Results from testing with a blended natural gas with more than 50% of heat input from pentane, C5H12, in a SGT-700 engine shows that the EBIT concept is possible and powerful.

The SIT 3rd generation DLE burner proves to be very fuel flexible and tolerant to high levels of pentane in the fuel. Less than 20% increase in NOx emissions can be expected when using pentane rich fuels. The burner is used in the SGT-800 47MW engine and the SGT-700 31MW engine.

Commentary by Dr. Valentin Fuster
2012;():767-779. doi:10.1115/GT2012-69033.

Helmholtz resonators are often used in the gas turbine industry for the damping of thermoacoustic instabilities. To prevent thermal destruction, these devices are usually cooled by a purging flow. Since the acoustic velocity inside the neck of the resonator becomes very high already at moderate pressure oscillation levels, hot-gas penetration cannot always be fully avoided. This study extends a well-known nonlinear impedance model to include the influence of hot-gas intrusion into the Helmholtz resonator neck. A time dependent but spatially averaged density function of the volume flow in the neck is developed. The steady component of this density function is implemented into the nonlinear impedance model to account for the effect of hot-gas intrusion. The proposed model predicts a significant shift in the resonance frequency of the damper towards higher frequencies depending on the amplitude of the acoustic velocity in the neck and the temperature of the penetrating hot gas. Subsequently, the model is verified by the experimental investigation of two resonance frequencies (86 Hz and 128 Hz) for two hot gas temperatures (1470 K and 570 K) and various pressure oscillation amplitudes. The multi-microphone-method, in combination with a microphone flush-mounted in the resonator volume, is used to determine the impedance of the Helmholtz damper. Additionally, a movable ultra-thin thermocouple was used to determine the degree of hot-gas penetration and the change of the mean temperature at various axial positions in the neck. A very good agreement between the model and the experimental data is obtained for all levels of pressure amplitudes and of hot-gas penetration depths. The mean air temperatures in the neck were accurately predicted, too.

Commentary by Dr. Valentin Fuster
2012;():781-794. doi:10.1115/GT2012-69034.

The assessment of the stability characteristics of kerosene-fueled, lean direct injection flames is an important issue for the design of low-emission aircraft engine combustion systems. To achieve this task, acoustic network models are widely used. In the present work, this technique is applied to determine the stability behavior of a liquid-fueled, lean direct injection combustor. The required transfer matrices have been measured in an atmospheric combustion test rig. The burner transfer matrix as well as the upstream and downstream reflection coefficients are obtained by using the multi-microphone method. Since the measurement of flame transfer functions for liquid-fueled flames is a complex task, two techniques are applied and compared. First, the flame response to loudspeaker forcing is measured with the multi-microphone technique. Second, a technique based on the simultaneous acquisition of different chemiluminescence signals is applied. The chemiluminescence response at four different wavelengths (310 nm, 407 nm, 431 nm, and 515 nm), corresponding to the species OH*, CH*, CO2* and C2*, respectively, are measured using photomultiplier tubes. With a calibration measurement at different operating conditions, it is possible to calculate the instantaneous heat release rate. Flame transfer functions and matrices are measured in the test rig with the two techniques. Additionally, all acoustically measured transfer matrices and optically measured transfer functions are used to predict possible unstable modes in the test rig. The experimental results and the stability analysis employing the measured flame transfer functions are in good agreement and demonstrate validity of the method.

Commentary by Dr. Valentin Fuster
2012;():795-801. doi:10.1115/GT2012-69036.

Surface-stabilized combustion is credited with high burning rates, extended lean flammability limits, wide modulation range and other advantages. This makes it an attractive technology for compact low-emission combustors.

The experimental gas turbine surface burners reported to this date are produced from compressed and sintered Fe-Cr-Al fiber mats. The authors have developed a new concept of surface burner fabricated by braiding ceramic cords around a ceramic frame. This simple method produces a basket-type surface suitable for stabilizing lean premixed flames over a broad range of operating conditions. The use of ceramics extends possibilities for operation at very high inlet temperatures with reduced risks of material sintering and oxidation.

This paper presents test results with an experimental burner on a pressurized combustion rig with optical access. The experiments were performed under the following conditions: inlet temperatures of 22–740 C, pressures of 1–3 bar, thermal power between 4 kWTh and 32 kWTh and equivalence ratios of 0.28–0.95. Measurements of flue gas composition and pressure drop are also reported in the paper. The operating window for low-NOx and low-CO combustion is analyzed.

With the demonstrated performance, the burner could cover the operating envelope of a 3 kWe recuperated micro turbine [1]–[2] with no pilot and no staging. This would also limit NOx to <40 ppm @ 0% O2 within the micro turbine load range of 100% to 50%.

Topics: Combustion
Commentary by Dr. Valentin Fuster
2012;():803-814. doi:10.1115/GT2012-69051.

Modern, large gas turbines for power generation have multiple burners, which are distributed around the circumference of the engine and which generate flames in combustors of either annular or can-annular geometry. In both cases considering only the axial modes has proven to be insufficient for the assessment of the thermoacoustic stability. An adequate analysis requires consideration of the circumferential acoustic coupling, generated by the acoustic field in the upstream and downstream annuli and the open passages between the cans, respectively. As in annular combustors the particularly critical eigenmodes with low frequencies are predominantly of circumferential nature, the stability of annular combustors is often governed by the onset of circumferential acoustic oscillations. In this study one single radial swirl burner is exposed to a transverse velocity fluctuation comparable to a circumferential oscillation in the plenum annulus. The transverse velocity fluctuation is transformed into a rotational flow oscillation through a convective process depending on excitation frequency and mass flow rate. The characteristics of this process are determined and the resulting dynamic flow structure in the burner nozzle is analyzed. Phase plots show that the rotational flow oscillation is transported into the flame causing a rotational flame pulsation. The influence of transverse velocity fluctuation on the global dynamic flame behavior is determined through FTF measurements. It is concluded from the increased FTF amplitude observed for transverse velocity excitation that the modification of the acoustic field at the burner exit due to circumferential acoustic modes has to be taken into account for a reliable prediction of the stability limits of annular gas turbines.

Commentary by Dr. Valentin Fuster
2012;():815-827. doi:10.1115/GT2012-69078.

Novel lean-burn combustor concepts were designed and evaluated for supersonic aircraft propulsion, with a focus on cruise NOx emissions. Premixing to lean conditions is especially challenging at supersonic cruise because combustor inlet temperatures are high and autoignition times are short. However, combustor pressure is significantly lower than at takeoff, so at cruise this allows heated jet fuel to be vaporized before injection as an aid to mixing. Two concepts — differentiated by swirler aerodynamics, swirler size, and staging method — were evaluated in the work reported here, both using injection of vaporized jet fuel. CFD calculations of mixing and combustion were used to design hardware for each concept. Injectors for each were fabricated using stereolithography (SLA) for cold-flow mixing tests, and using metal fabrication for subsequent combustion tests. Combustion test results show that EINOx < 5 was achieved for both concepts in single-sector tests at supersonic cruise combustor conditions.

Commentary by Dr. Valentin Fuster
2012;():829-835. doi:10.1115/GT2012-69080.

In this paper, characteristics of turbulent combustion and NOx emission for high hydrogen-content fuel gases (H2 > 70 vol. %; “hydrogen-rich”) are addressed. An experimental investigation is performed in a perfectly-premixed axial-dump combustor under gas turbine relevant conditions. Fundamental features of turbulent combustion for these mixtures are evaluated based on OH-PLIF diagnostics. On the other hand, NOx emissions are measured with an exhaust gas sampling probe positioned downstream the combustor outlet.

Compared to syngas mixtures (H2 + CO), the operational limits for hydrogen-rich fuel gases are found to occur at even leaner conditions concerning flashback phenomena. With respect to effects of operating pressure, a strongly reduced operational envelope is observed at elevated pressure. Only with decreasing the preheat temperature a viable approach to further extend the operational range is seen. Evaluation of the averaged turbulent flame shape shows that the profile of the flame front is generally approaching that of an ideal cone. Thus a simplified approach for estimating the turbulent flame speed via the location of the flame tip alone can be applied.

The level of NOx emission for the hydrogen-rich fuel mixtures is generally above that of syngas mixtures, which exhibit already higher NOx emission values than natural gas. Distinct chemical kinetic features are found specifically at elevated pressure. While the pressure effects are weak for syngas, a non-monotonic behavior is observed for the hydrogen-rich fuels. Reaction path analysis is performed to complement and provide more insight to the findings from the measurements. From chemical kinetic calculations a distinct shift in NOx formation pathways (thermal NOx vs. NOx through N2O/NNH reaction channels) can be observed for the different fuel mixtures at different pressure levels.

Commentary by Dr. Valentin Fuster
2012;():837-846. doi:10.1115/GT2012-69164.

In a gas turbine combustor limit cycles of pressure oscillations may occur due to a coupling of combustion dynamics to the acoustic field inside the system. In this case, the engine is subjected to high vibrations and the possibility of structural damage.

Experimental research in this subject was carried out in a laboratory combustor operating in a lean, partially premixed methane/air flame, where the flame stabilizes on a triangular bluff body inside a rectangular combustor duct. Depending on the operating point, the flame shows a stable or unstable behavior. In this last case, amplitudes up to 155 dB (ref 20 μPa) have been recorded. The variation of behavior of the instability with operating conditions is well known. The stable combustion presents a low amplitude broadband noise. The unstable regime is more interesting. It has a main peak with high amplitude and fixed frequency and several secondary peaks at multiple times the frequency of the fundamental one. This peaks can be seen in the pressure and heat release spectrum.

The secondary peaks of the pressure spectrum are due to non-linear effects. Odd numbered peaks came from a change in the acoustic boundary conditions in the burner. The even peaks are the result of frequency doubling of the odd frequencies. The frequency doubling comes from a second order source term of the Ligthill’s analogy.

Commentary by Dr. Valentin Fuster
2012;():847-854. doi:10.1115/GT2012-69165.

This paper describes a novel lean premixed reheat burner technology suitable for Hydrogen-rich fuels. The inlet temperature for such a combustor is very high and reaction of the fuel/oxidant mixture is initiated through auto-ignition, the delay time for which reduces significantly for Hydrogen-rich fuels in comparison to natural gases. Therefore the residence time available for premixing within the burner is reduced. The new reheat burner concept has been optimized to allow rapid fuel/oxidant mixing, to have a high flashback margin and to limit the pressure drop penalty.

The performance of the burner is described, initially in terms of its fluid dynamic properties and then its combustion characteristics. The latter are based upon full-scale high-pressure tests, where results are shown for two variants of the concept, one with a pressure drop comparable to today’s natural gas burners, and the other with a two-fold increase in pressure drop. Both burners indicated that Low NOx emissions, comparable to today’s natural gas burners, were feasible at reheat engine conditions (ca. 20 Bars and ca. 1000C inlet temperature). The higher pressure drop variant allowed a wider operating window. However the achievement of the lower pressure drop burner shows that the targeted Hydrogen-rich fuel (70/30 H2/N2 by volume) can be used within a reheat combustor without any penalty on gas turbine performance.

Commentary by Dr. Valentin Fuster
2012;():855-866. doi:10.1115/GT2012-69189.

An experimental study of lean premixed combustion in a swirl-stabilized combustor is undertaken to characterize the dynamics and time scales close to Lean Blow Out (LBO) conditions. Due to the recent interest in syngas fuels, the effect of hydrogen addition on LBO is studied. In present study, both confined and unconfined turbulent methane air premixed flames have been examined with different hydrogen levels during the extinction transition with high speed imaging of OH* chemiluminescence at 2 KHz. Planar laser induced fluorescence measurement of OH is also performed for studying the flame structure. The blowout conditions are approached by reducing the flow rate of fuel mixture or the equivalence ratio with constant air flow rate. The estimated extinction times from high speed imaging and corresponding flame structures are analyzed and compared between confined and unconfined flames with different hydrogen blends. The extinction time scale and the heat release fluctuations show inverse trends with hydrogen addition for the confined and unconfined flames, and are indicative of different stabilization and blow out mechanisms for the two configurations. These mechanisms which involve heat losses from the flame, inner- and corner recirculation zones and unsteady flame dynamics are described in the paper.

Commentary by Dr. Valentin Fuster
2012;():867-875. doi:10.1115/GT2012-69215.

The most uncertain and challenging part in the design of a gas turbine has long been the combustion chamber. There has been large number of experimentations in industries and universities alike to better understand the dynamic and complex processes that occur inside a combustion chamber. This study concentrates on gas turbine combustors as a whole, and formulates a theoretical design procedure for staged combustors in particular. Not much of literatures available currently in public domain provide intensive study on designing staged combustors. The work covers an extensive study of design methods applied in conventional combustor designs, which includes the reverse flow combustor and the axial flow annular combustors. The knowledge acquired from this study is then applied to develop a theoretical design methodology for double staged (radial and axial) low emission annular combustors. Additionally a model combustor is designed for each type; radial and axial staging using the developed methodology. A prediction of the performance for the model combustors is executed. The main conclusion is that the dimensions of model combustors obtained from the developed design methodology are within the feasibility limits. The comparison between the radially staged and the axially staged combustor has yielded the predicted results such as lower NOx prediction for the latter and shorter combustor length for the former. The NOx emission result of the new combustor models are found to be in the range of 50–60ppm. However the predicted NOx results are only very crude and need further detailed study.

Commentary by Dr. Valentin Fuster
2012;():877-884. doi:10.1115/GT2012-69241.

In the present work, injection of liquid kerosene into a high-pressure subsonic air crossflow was investigated experimentally. Tests were conducted at air pressures up to 2.0 MPa and at air temperatures from normal temperature to elevated temperature. Liquid kerosene was injected at room temperature through a 0.5 mm diameter plain orifice. Schlieren imaging technique was used for jet structure visualization, from which the jet penetration trajectory was determined by the image processing. For the conditions tested, a correlation of jet penetration trajectory was developed, with momentum flux ratio, Weber number and crossflow temperature ratio as independent variables. Upper surface trajectories of kerosene spray under different test conditions were compared. Experimental and analytical results showed that the penetration trajectory of liquid kerosene under higher air temperature was greater than that under normal temperature, while momentum ratios were the same.

Commentary by Dr. Valentin Fuster
2012;():885-895. doi:10.1115/GT2012-69249.

It is known that the frequency of thermo-acoustic instabilities may vary according to various parameters during operation. The design of passive acoustic dampers tuned to damp specific unstable frequencies must then include this aspect to offer robust properties. This problem is tackled here for perforated plates backed by a resonant cavity in the absence of grazing flow. Their current design relies on a relatively complex optimization procedure with a large number of parameters to examine. A new methodology is proposed to reduce this number by finding the optimal parameters maximizing absorption in two limit regimes, where the choice of the optimal bias flow velocity and size of the back cavity can be decoupled. The former is only controlled by the plate porosity while the latter fixes the peak absorption frequency. The analysis also includes effects of the plate thickness. In both regimes, the optimal bias flow velocity is analytically determined. A Helmholtz resonance and a narrow absorption peak in the frequency space characterize the first absorption regime reached at high Strouhal numbers. This regime minimizes the size of the resonant back cavity, but the absorption frequency bandwidth narrows with increasing Strouhal numbers. The second absorption regime reached at low Strouhal numbers operates with a quarter-wave resonator. This regime requires larger cavity depths but offers a wider absorption bandwidth around the peak absorption frequency well suited for low frequency dampers when the bias flow velocity or the unstable frequency may vary within the system. Theoretical predictions are validated against experimental data obtained in the two regimes identified. The expressions derived in this study can be used to improve the design of robust acoustic dampers.

Commentary by Dr. Valentin Fuster
2012;():897-907. doi:10.1115/GT2012-69250.

Thermoacoustic instabilities may occur in every gas turbine combustor and could be hazardous to the flame stability and the structural integrity. It is important to be able to predict how hazardous the instabilities are: at what frequencies will they occur and will they develop into high amplitude limit cycle oscillations? The former question can be answered with the help of the Flame Transfer Function (FTF). The FTF establishes the coupling between burner passage aerodynamics and combustion dynamics and can be used as an input to an acoustic model to predict the eigenfrequencies and their growth rate. In the present research two methods to measure the FTF are used with different signal excitation instruments: a MOOG Valve and a Siren. Both the methods are based on data from pressure transducers only. The FTF is measured here by determining the combustor pressure response of the flame to fluctuations in the fuel mass flow at the burner exit. A siren unit has been developed and mounted at the upstream end of the fuel supply line of a pressurized combustor and is designed to have a harmonic excitation. The experimental method to measure the FTF by means of factorization in known or measurable sub-functions is briefly explained. Subsequently the Siren method is demonstrated by means of extracting the FTF at elevated pressure and as a function of thermal power. The results are compared with the results obtained in previous work of a MOOG valve excitation unit. The experimental investigation of the FTF is carried out in a high pressure combustor rig named DESIRE which is able to perform thermoacoustic measurements up to 500 kW thermal power at 5 bar absolute pressure. The results are compared and discussed. Subsequently a 1-D acoustic network model is presented which predicts the onset of the limit cycle pressure oscillations in the DESIRE combustor, using the FTF as an input. Thermo viscous damping effects and measured reflection coefficients are also included into the network model to improve the model predictions. Finally, the measured and predicted dynamic behavior of the combustor are compared. The results indicate that the network modeling approach is a promising design tool as it gives good agreement between measured and predicted dynamic behavior of the combustor and instability analysis. Well-defined boundary conditions and thermo viscous damping effects are important for the accuracy of the acoustic network models.

Commentary by Dr. Valentin Fuster
2012;():909-919. doi:10.1115/GT2012-69255.

The paper is aimed at evaluating the impact of the combustion model on the accuracy of the results of the numerical simulations of turbulent reactive flows. For this, two numerical simulations of the well known Sandia Flame D case are carried out: a three-dimensional RANS integration of the Navier–Stokes equations using the Eddy Dissipation combustion Model (EDM), and a one-dimensional one, where simplified reaction–diffusion equations are numerically integrated over the radial direction, while the axial convection is modeled by empirical laws. The one-dimensional simulation, however, is based on a more physics related combustion model, the Linear Eddy Mixing model, which also controls the radial turbulent mixing and the large scale radial convection.

The results of the two numerical simulations are compared to experimental data in the literature, showing a significantly better accuracy of the Linear Eddy Mixing (LEM) numerical simulation.

Commentary by Dr. Valentin Fuster
2012;():921-929. doi:10.1115/GT2012-69273.

Pulse combustors are widely applied for heating, drying and even propulsion applications because of their higher efficiency, higher heat transfer rates and lower emission than steady combustors. However, fundamentals of this pulse combustor remain till date largely unexplored. Experiments are conducted on a laboratory-scale thermal pulse combustor. The set-up consists of an upstream section, the combustor and the tailpipe. The optical signal from the flame is measured with a photomultiplier tube and pressure fluctuations are measured using a dynamic pressure transducer. The time series data reconstructed with SSA (Singular Spectrum Analysis) reveals that at a given air flow rate as the fuel flow rate is reduced, three distinct regimes are observed: strongly pulsating, weakly pulsating and non-pulsating. Nonlinear analysis suggests the existence of quasiperiodic orbits for the pulsating cases. The phase difference between pressure and heat release rate fluctuations confirm sustained instability for the pulsating cases. The characteristic frequency is found to decrease with decrease in fuel flow rate and increase in tailpipe length for a given air flow rate. Different orientation of fuel inlet has been implemented to achieve pulsating combustion under lean fuel conditions.

Commentary by Dr. Valentin Fuster
2012;():931-948. doi:10.1115/GT2012-69290.

Laminar flame speeds and ignition delay times have been measured for hydrogen and various compositions of H2/CO (syngas) at elevated pressures and elevated temperatures. Two constant-volume cylindrical vessels were used to visualize the spherical growth of the flame through the use of a schlieren optical setup to measure the laminar flame speed of the mixture. Hydrogen experiments were performed at initial pressures up to 10 atm and initial temperatures up to 443 K. A syngas composition of 50/50 by volume was chosen to demonstrate the effect of carbon monoxide on H2−O2 chemical kinetics at standard temperature and pressures up to 10 atm. All atmospheric mixtures were diluted with standard air, while all elevated-pressure experiments were diluted with a He:O2 of 7:1 to minimize instabilities. The laminar flame speed measurements of hydrogen and syngas are compared to available literature data over a wide range of equivalence ratios where good agreement can be seen with several data sets. Additionally, an improved chemical kinetics model is shown for all conditions within the current study. The model and the data presented herein agree well, which demonstrates the continual, improved accuracy of the chemical kinetic model.

A high-pressure shock tube was used to measure ignition delay times for several baseline compositions of syngas at three pressures across a wide range of temperatures. The compositions of syngas (H2/CO) by volume presented in this study included 80/20, 50/50, 40/60, 20/80, and 10/90, all of which are compared to previously published ignition delay times from a hydrogen-oxygen mixture to demonstrate the effect of carbon monoxide addition. Generally, an increase in carbon monoxide increases the ignition delay time, but there does seem to be a pressure dependency. At low temperatures and pressures higher than about 12 atm, the ignition delay times appear to be indistinguishable with an increase in carbon monoxide. However, at high temperatures the relative composition of H2 and CO has a strong influence on ignition delay times. Model agreement is good across the range of the study, particularly at the elevated pressures. Also, an increase in carbon monoxide causes the activation energy of the mixture to decrease.

Commentary by Dr. Valentin Fuster
2012;():949-960. doi:10.1115/GT2012-69294.

This work presents a numerical study on the turbulent Schmidt numbers in jets in crossflow. This study contains two main parts. In the first part the problem of the proper choice of the turbulent Schmidt number in the Reynolds-Averaged Navier-Stokes (RANS) jet in crossflow mixing simulations is outlined. The results of RANS employing the shear-stress transport (SST) model of Menter and its curvature correction modification and different turbulent Schmidt number values are validated against experimental data. The dependence of the “optimal” value of the turbulent Schmidt number on the dynamic RANS model is studied. Furthermore a comparison is made with the large-eddy simulation (LES) results obtained using the WALE (Wall-Adapted Local Eddy Viscosity) model. The accuracy given by LES is superior in comparison to RANS results. This leads to the second part of the current study, in which the time-averaged mean and fluctuating velocity and scalar fields from LES are used for the evaluation of the turbulent viscosities, turbulent scalar diffusivities, and the turbulent Schmidt numbers in a jet in crossflow configuration. The values obtained from the LES data are compared with those given by the RANS modeling. The deviations are discussed and the possible ways for the RANS model improvements are outlined.

Topics: Turbulence
Commentary by Dr. Valentin Fuster
2012;():961-971. doi:10.1115/GT2012-69297.

Environmental regulations are continuously pushing lower emissions with an impact on the combustion process in gas turbines (GT). As a consequence, GT combustors operate in very lean regimes (i.e. at relatively low temperature) to reduce NOx formation. Unfortunately, stabilization becomes a challenge for these lean premixed flames. The extremely unsteady dynamics of swirl stabilized flames present crucial issues and this investigation aims at understanding the interaction of swirl stabilization with large coherent fluctuations inherent to vortex breakdown.

The investigation utilizes a simplified cylindrical model combustor consisting of a premixing tube discharging in a larger combustion chamber. Fuel and swirling air are injected separately in the mixing tube so that a partially premixed swirling jet encounters vortex breakdown and allows the partially premixed flame to stabilize. The aforementioned extreme sensitivity of lean partially premixed flames challenges any investigation either for measuring, simulating or post-processing the case of interest.

In this paper, the problem is addressed using large eddy simulation (LES) and planar laser induced fluorescence. The LES data are used to follow the fuel air/mixing as well as the fuel combustion evidencing large-scale dynamics. These dynamics are further investigated using Proper Orthogonal Decomposition to identify the role of the premixing stage and of the Precessing Vortex Core in the flame behaviour.

Commentary by Dr. Valentin Fuster
2012;():973-983. doi:10.1115/GT2012-69299.

A radial swirl DLE combustion system was investigated for its gaseous fuel-air mixing performance using various different RANS turbulence models. Two different configurations were investigated; vane passage fuel injection and fuel injection from the wall of an outlet throat directly into the shear layer. The results showed that for vane passage fuel injection, only the standard k-ε and standard Spalart-Allmaras models were able to provide a reasonable prediction for the combustor fuel-air distribution, out of all the RANS models and their variants available in Fluent v6.3, although both models predicted that the fuel and air were better mixed than in the measurements. For outlet throat wall fuel injection no models were able to provide a reasonable prediction. This same issue is also reported by several other researchers and represents a serious problem area in combustion modeling for low NOx applications. Improved fuel jet penetration was achieved by using an extremely low value of 0.1 for the turbulent Schmidt number, therefore future work will concentrate on using a localized value of Sc in the vicinity of the fuel injection hole.

Commentary by Dr. Valentin Fuster
2012;():985-993. doi:10.1115/GT2012-69301.

A radial swirl low NOx combustor was investigated using CFD at 0.5 equivalence ratio and 600K inlet temperature at 1 bar. The equilibrium pdf combustion model using 16 species chemistry was shown to give a slightly improved prediction for the temperature distribution, but a poorer prediction for the CO distribution, over similar work using the flamelet model with 53 species chemistry. The NOx model was applied as a post-processing application and was shown to give a vastly superior result for the equilibrium pdf model using 16 species chemistry over the flamelet model using 53 species chemistry. Various NOx model configurations were tested and it was shown that only the mixture fraction based turbulence chemistry interaction model was able to provide a good result and all other models drastically under-predicted the peak nitric oxide levels. Combustion model predicted O modeling with excluded and combustion model predicted OH modeling were both shown to give a good match against measurements for the peak nitric oxide levels within the combustor when using mixture fraction based turbulence chemistry interaction.

Commentary by Dr. Valentin Fuster
2012;():995-1003. doi:10.1115/GT2012-69303.

A double contra-rotating axial swirler was investigated at 0.38 equivalence ratio and 600K inlet temperature at 1 bar, both experimentally and using CFD. Natural gas fuel was injected from between the two contra-rotating flows. The experimental results showed that whilst the NOx emissions were relatively high, the system has excellent flame stability characteristics meaning the flame could be maintained at very low fuel-air ratios, down to an equivalence ratio of around 0.28. CFD predictions were carried out using the equilibrium pdf combustion model alongside the partially premixed combustion model using default model constants, and the NOx model was applied as a post-processing application. The results showed that whilst the location and shape of the flame and the combustor fuel-air distribution could not be precisely captured, the peak temperature, CO, UHC and NOx levels were correctly predicted. The influence of the turbulent Schmidt number, Sct, was therefore investigated in an attempt to improve the predictions. Lower values offered an improvement in the fuel-air mixing predictions, but not in the combustion predictions, where the peak flame temperature could only be predicted correctly when using the default value of Sct = 0.85. This indicates that the use of a universal turbulent Schmidt number for both the fuel-air mixing and combustion models may not be suitable for the present swirler and combustor configuration, therefore future work will look to use a separate value of Sct for the mixture fraction than for the pdf combustion models.

Commentary by Dr. Valentin Fuster
2012;():1005-1016. doi:10.1115/GT2012-69310.

Applications of natural gas and hydrogen co-firing have received increased attention in the gas turbine market, which aims at higher flexibility due to concerns over the availability of fuels. While much work has been done in the development of a fuels database and corresponding chemical kinetics mechanism for natural gas mixtures, there are nonetheless few if any data for mixtures with high levels of hydrogen at conditions of interest to gas turbines. The focus of the present paper is on gas turbine engines with primary and secondary reaction zones as represented in the Alstom and Rolls Royce product portfolio.

The present effort includes a parametric study, a gas turbine model study, and turbulent flame speed predictions. Using a highly optimized chemical kinetics mechanism, ignition delay times and laminar burning velocities were calculated for fuels from pure methane to pure hydrogen and with natural gas/hydrogen mixtures. A wide range of engine-relevant conditions were studied: pressures from 1 to 30 atm, flame temperatures from 1600 to 2200 K, primary combustor inlet temperature from 300 to 900 K, and secondary combustor inlet temperatures from 900 to 1400 K. Hydrogen addition was found to increase the reactivity of hydrocarbon fuels at all conditions by increasing the laminar flame speed and decreasing the ignition delay time. Predictions of turbulent flame speeds from the laminar flame speeds show that hydrogen addition affects the reactivity more when turbulence is considered. This combined effort of industrial and university partners brings together the know-how of applied, as well as experimental and theoretical disciplines.

Commentary by Dr. Valentin Fuster
2012;():1017-1028. doi:10.1115/GT2012-69333.

An Inter-Turbine Burner (ITB) that is capable of increasing the thrust of a gas turbine engine with minimal effect on SFC has been developed. Gas turbine engines using multistage turbine sections have the inherent disadvantage of temperature loss through the turbine section. This occurs when each successive turbine stage extracts energy from the superheated mass airflow stream. The net result is limited energy potential due to the first stage turbine temperature limits. An Inter-Turbine Burner (ITB) is able to utilize constant temperature burning through the turbine section by adding burners between the turbine stages. The resultant engine is suited for missions requiring large amounts of constant or intermittent power extraction. The Spytek ITB incorporates a modified version of an Ultra-Compact Combustor (UCC) [1] (high-g burner) which was originally developed by the Air Force Research Laboratory in Dayton, Ohio.

The ITB has been incorporated into a gas turbine engine and has been successfully tested operating at a near constant temperature (NCT) cycle. The engine/ITB is specifically configured and packaged for high power density use. Temperature rises across the ITB (T6-7) were tested in ranges from 421K-588K with representative increases in power take-off noted. The burner, positioned directly upstream of the ITB turbine, operates with vitiated air taken directly from the core engine exhaust stream. The engine tested is a two spool turbo-jet (ITB shaft inclusive), in the 1334(N) class. The ITB is cross shaft linked to an axial compressor booster stage, attached to the engine inlet which super-charged the core engine.

The two major areas addressed in development were the ability to provide air into the primary burn zone of the ITB to sustain combustion and second, the ability to successfully entrain the combustion products from the ITB vortex chamber into the main air stream without causing undue restrictions or hot-streak problems which can affect the life of the ITB turbine. Flexibility in ITB testing is further enhanced through the use of two adjustable test features, 1) a variable flow splitter, capable of adjusting the amount of air diverted into the ITB combustor, and 2) a variable nozzle guide vane pack upstream of the ITB turbine. A proprietary entrainment system rapidly mixes the ITB combustor products with the main stream dilute flow products without any undo effects on the ITB turbine. The Mark#1 version of the ITB system exhibits power on demand increases of 16%–22%.

Commentary by Dr. Valentin Fuster
2012;():1029-1038. doi:10.1115/GT2012-69352.

The present work investigates the effect of natural gas fuel sulfur on particulate emissions from stationary gas turbine engines used for electricity generation. Fuel sulfur from standard line gas was scrubbed using a system of fluidized reactor beds containing a specially designed activated carbon purposely built for sulfur absorption. A sulfur injection system using sonic orifices was designed and constructed to inject methyl mercaptan into the scrubbed gas stream at varying concentrations. Using these systems, particulate emissions created by various fuel sulfur levels between 0 and 15 ppmv were investigated. Particulate samples were collected from a Capstone C65 microturbine generator system using a Horiba MDLT-1302TA micro dilution tunnel and analyzed using a Horiba MEXA-1370PM particulate analyzer. In addition, ambient air samples were collected to determine incoming particulate levels in the combustion air. The Capstone C65 engine air filter was also tested for particulate removal efficiency by sampling downstream of the filter. To further differentiate the particulate entering the engine in the combustion air from particulate being emitted from the exhaust stack, two high efficiency HEPA filters were installed to eliminate a large portion of incoming particulate. Variable fuel sulfur testing showed that there was a strong correlation between total particulate emission factor and fuel sulfur concentration. Using eleven variable sulfur tests, it was determined that an increase of 1 ppmv fuel sulfur will produce an increase of approximately 2.8 μg/m3 total particulate. Also, the correlation predicted that, for this particular engine, the total particulate emission factor for zero fuel sulfur was approximately 19.1 μg/m3. With the EC and OC data removed, the correlation became 2.5 μg/m3 of sulfur particulate produced for each ppmv of fuel sulfur. The correlation also predicted that with no fuel sulfur present, 7.8 μg/m3 of particulate will be produced by sulfur passing through the engine air filter.

Commentary by Dr. Valentin Fuster
2012;():1039-1049. doi:10.1115/GT2012-69357.

Flashback is the main operability issue associated with converting lean, premixed combustion systems from operation on natural gas to operation on high hydrogen content fuels. Most syngas fuels contain some amount of hydrogen (15–100%) depending on the fuel processing scheme. With this variability in the composition of syngas, the question of how fuel composition impacts flashback propensity arises. To address this question, a jet burner configuration was used to develop systematic data for a wide range of compositions under turbulent flow conditions. The burner consisted of a quartz burner tube confined by a larger quartz tube. The use of quartz allowed visualization of the flashback processes occurring. Various fuel compositions of hydrogen, carbon monoxide, and natural gas were premixed with air at equivalence ratios corresponding to constant adiabatic flame temperatures (AFT) of 1700 K and 1900 K. Once a flame was stabilized on the burner, the air flow rate would be gradually reduced while holding the AFT constant via the equivalence ratio until flashback occurred. Schlieren and intensified OH* images captured at high speeds during flashback allowed some additional understanding of what is occurring during the highly dynamic process of flashback. Confined and unconfined flashback data were analyzed by comparing data collected in the present study with existing data in the literature. A statistically designed test matrix was used which allows analysis of variance of the results to be carried out, leading to correlation between fuel composition and flame temperature with (1) critical flashback velocity gradient and (2) burner tip temperature. Using the burner tip temperature as the unburned temperature in the laminar flame speed calculations showed increased correlation of the flashback data and laminar flame speed as opposed to when the actual unburned gas temperature was used.

Topics: Fuels , Flames
Commentary by Dr. Valentin Fuster
2012;():1051-1058. doi:10.1115/GT2012-69407.

Syngas with CO/H2/N2 as the primary ingredients is the main fuel for the gas turbine in the IGCC system, but changes of the heating value and CO/H2 ratio frequently cause great impacts on the normal operation of the combustor, which is a challenge to the design of the syngas combustor. In this paper based on the Sandia/ETH-Zurich CO/H2/N2. Flame A, numerical simulations and predictions of the impacts of these two factors on the flame structure and diffusion combustion characteristics are carried out, and the main results are as follows. The heating value and CO/H2 ratio both have important impacts on the diffusion combustion characteristics. Most of the impacts have regularity such as changes of the axis velocity and temperature distribution, the highest temperature of the whole temperature field, the combustion efficiency and the NO formation with the heating value and CO/H2 ratio. However, the most important one is that the relationship between the flame size and the Wobbe Index is linear, for it has established connections between the structure size of the combustor and the characteristics of the fuel and the linear relationship can be used to provide reference for the combustor design. So the Wobbe Index is not only an important parameter to judge whether different fuels can be exchanged or not under the same initial pressure and heat load of a combustion equipment, but also an important parameter for syngas combustor design.

Commentary by Dr. Valentin Fuster
2012;():1059-1067. doi:10.1115/GT2012-69413.

Spray ignition is a complex combination of physical and chemical processes. Therefore there is no way to simulate all processes without significant simplifications. Dynamic and thermal effects of droplets in a two phase flow are respected through averaged properties of mass, momentum and energy. When modeling mixing of fuel vapor with the surrounding gas, the differences between diffusion rates of components as well as the finite penetration of the diffusion fluxes are not taken into account. Last seen in the fact that fuel vapor is uniformly mixed with the gas filling within the calculation cell, whose dimensions are several orders of magnitude larger than the droplet. As a consequence, the chemical reactions in the two-phase medium are represented by volume processes in the computational cell. Meanwhile, it is well known that the surrounding of an individual droplet or a group of droplets is characterized through significant inhomogeneity. Under such circumstances, self-ignition of fuel vapor cannot be considered as a process in a well stirred reactor of a size of the computational cell.

In this work a new approach for spray ignition simulation is presented. The simulation is split into two tasks. A CFD package is used to calculate spray injection and subsequent two-phase flow in the given conditions. From this two-phase flow the droplet track data and the gas phase parameters are extracted to serve as variable boundary conditions for the second task, the one dimensional single droplet ignition simulations, named “Spraylet”, which are fully transient and based on comprehensive chemistry. The obtained ignition delay times can be transformed into spatial distributions of ignition probability. While the CFD part is handling spray formation, turbulence, temperature, pressure and global vaporization, the spraylet calculations take care of droplet related physics and chemistry.

The spraylet model is based on the transient differential energy and mass conservation equations in the liquid and gas phases with variable physical properties. Also the concept of multi-component diffusion is applied. The effects of the presence of neighboring droplets in the flow, usually referred to as “spray” effects, is approximated by taking modifications of the conditions at the outer boundary of the computational domain into account. Thus the model considers the finite rates of diffusion and chemical reactions, as well as spray effects, and allows for spray ignition simulations to predict the most probable instants of ignition of the individually calculated droplet trajectories.

The validation of the spraylet-based simulation is performed by comparison with experimental data obtained in the hot-wind-tunnel. N-heptane as liquid fuel was injected into cross flow of air with a pressure of 5 bar and a temperature of 800 K.

Topics: Simulation , Sprays
Commentary by Dr. Valentin Fuster
2012;():1069-1079. doi:10.1115/GT2012-69421.

The international effort to reduce the environmental impact of electricity generation, especially CO2-emissions requires considerations about alternative energy supply systems. An effective step towards low pollution power generation is the application of hydrogen as a possible alternative gas turbine fuel, if the hydrogen is produced by renewable energy sources, such as wind energy or biomass. The use of hydrogen and hydrogen rich gases as a fuel for industrial applications and power generation combined with the control of polluted emissions, especially NOx, is a major key driver in the design of future gas turbine combustors.

The micromix combustion principle allows a secure and low NOx combustion of hydrogen and air and achieves a significant reduction of NOx-emissions. The combustion principle is based on cross-flow mixing of air and gaseous pure hydrogen and burns in multiple miniaturized diffusion-type flames. For the characterization of the jet in cross-flow mixing process, the momentum flux ratio is used.

The paper presents an experimental analysis of the momentum flux ratio’s impact on flame anchoring and on the resultant formation of the NOx-emissions. Therefore several prototype test burner with different momentum flux ratios are tested under preheated atmospheric conditions. The investigation shows that the resultant positioning and anchoring of the micro flames highly influences the NOx-formation.

Besides the experimental investigations, numerical simulations have been performed by the application of a commercial CFD code. The cold flow simulation results show the mixing of the air and hydrogen after the injection, in particular in the Counter Rotating Vortices (CRV). Furthermore, the hydrogen jet interacts also with another vortex system resulting from a wake flow area behind the combustor geometry. Furthermore, reacting flow simulations have been performed by the application of a Hybrid Eddy Break-Up (EBU) combustion model. The combustion pressure has been varied from atmospheric conditions up to a pressure of 16 bar.

The experimental and numerical results highlight further potential of the micromix combustion principle for low NOx-combustion of hydrogen in industrial gas turbine applications.

Commentary by Dr. Valentin Fuster
2012;():1081-1094. doi:10.1115/GT2012-69446.

Humidified Gas Turbines (HGT) offer the attractive possibility of increasing the plant efficiency without the cost of an additional steam turbine, as is the case for a combined gas-steam cycle. In addition to efficiency gains, adding steam into the combustion process reduces NOx emissions. It increases the specific heat capacity (hence, lowers possible temperature peaks) and reduces the oxygen concentration. Despite the thermo-physical effects, steam alters the kinetics, and thus, reduces NOx formation significantly. In addition, it allows operation using a variety of fuels, including hydrogen and hydrogen-rich fuels. Therefore, ultra-wet gas turbine operation is an attractive solution for industrial applications. The major modification compared to current gas turbines lies in the design of the combustion chamber, which should accommodate a large amount of steam without losing in stability. In the current study, the premixed combustion of pure hydrogen diluted with different steam levels is investigated. The effect of steam on the combustion process is addressed using detailed chemistry. In order to identify an adequate oxidation mechanism, several candidates are identified and compared. The respective performances are assessed based on laminar premixed flame calculations under dry and wet conditions, for which experimentally determined flame speeds are available. Further insight is gained by observing the effect of steam on the flame structure, in particular HO2 and OH* profiles. Moreover, the mechanism is used for the simulation of a turbulent flame in a generic swirl burner fed with hydrogen and humidified air. Large Eddy Simulations (LES) are employed. It is shown that by adding steam, the heat release peak spreads. At high steam content, the flame front is thicker and the flame extends further downstream. The dynamics of the oxidation layer under dry and wet conditions is captured, thus, an accurate prediction of the velocity field, flame shape and position is achieved. The latter is compared with experimental data (PIV and OH* chemiluminescence). The reacting simulations were conducted under atmospheric conditions. The steam-air ratio was varied from 0% to 50%.

Commentary by Dr. Valentin Fuster
2012;():1095-1104. doi:10.1115/GT2012-69449.

Gas turbine combustor design relies strongly on the turbulent flame velocity over the whole turbine operation range. Due to the fact that turbulent flame velocity depends strongly on the laminar one, its characterisation at different thermodynamic conditions is necessary for further optimisation of gas turbines. The Markstein number, which quantifies the response of the flame to the stretch, also has to be considered. Additionally, the Markstein number can be utilised as an indicator for laminar and turbulent flame front stability.

The current attempts to replace conventional fuels, such as kerosene, with alternative ones, obtrude their comparison in order to find the most appropriate substitute. Additionally, significant differences in the flame behaviour, which could be recognised through different combustion characteristics, can lead to modification of currently used gas turbine design. Even so, the experimental data of alternative fuels are scarce, especially at elevated pressure conditions.

So, the combustion characteristics, laminar burning velocity and Markstein number of kerosene Jet A-1 and several alternative fuels (GTL and GTL blends) are investigated experimentally in an explosion vessel. For this purpose an optical laser method is employed based on the Mie-scattering of the laser light by smoke particles. Within this experimental study the influence of three crucial parameters: initial temperature, initial pressure and mixture composition on the burning velocity and Markstein number are investigated. The experiments were performed at three different pressures 1, 2, 4bar; three different temperatures 100°C, 150°C, 200°C; and for a range of equivalence ratio 0.67–1.67. The observed results are compared and discussed in detail.

Topics: Combustion , Fuels
Commentary by Dr. Valentin Fuster
2012;():1105-1111. doi:10.1115/GT2012-69451.

The current work explores the operation of an industrial gas turbine (25 MWe) unit working under variable NOx emission limits (from 75 to 120 mg/Nm3) and variable fuel composition. The latter is typical for flared streams in oil process industry (refineries/well operation/petrochemicals).

In the current case the nitrous oxides (NOx) emission limits are dependent on the mixing ratio of natural gas (NG, 89%vol. methane) and refinery gas (RG, 40% vol. H2, 56%vol. CH4, C4+) that is used as fuel for the engine. This mixture is process dependent and varies in matter of minutes from 100% NG to 100% RG. Siemens has faced this challenge by using a SGT-600 unit (25MWe) operating in non-premixed combustion mode with a novel water injection strategy, the adaptive water injection for the control of NOx.

The adaptive water injection for NOx control has been developed after evaluating different NOx control strategies. It consists of a closed loop, in which NOx emission measurements are used as the main control parameter, linked to a fixed parametric curve based on the compressor discharge pressure. Fast response times and fail–safe strategies have also been developed and tested under operating conditions of the GT. The water injection method could cope with the variation of load, fuel supply mixture and NOx limit without any flame instability problems. This is done while maintaining the optimal amount of water needed to achieve the emissions target. The robustness of the system has been satisfactory over the tested period and seems a viable solution.

Commentary by Dr. Valentin Fuster
2012;():1113-1124. doi:10.1115/GT2012-69477.

Gas turbines offer a high operational flexibility and a good turn down ratio to meet future requirements of power production. In this context stable operation over a wide range and for different blends of fuel is requested. Thermoacoustic stability assessment is crucial for accelerating the development and implementation of new combustion systems.

The results of nonlinear and linear thermoacoustic stability assessments are compared on basis of recent measurements of flame describing functions and thermoacoustic stability of a model swirl combustor operating in the fully turbulent regime. The different assessment methods are outlined. The linear thermoacoustic stability assessment yields growth rates of the thermoacoustic instability whereas the limit cycle amplitude is predicted by the nonlinear stability method.

It could be shown that the predicted limit cycle amplitudes correlate well with the growth rates of excitation obtained from linear modeling. Hence for screening the thermoacoustic stability of different design approaches a linear assessment may be sufficient while for detailed prediction of the dynamic pressure amplitude more efforts have to be spent on the nonlinear assessment including the analysis of the nonlinear flame response.