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ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V04AT00A001. doi:10.1115/GT2016-NS4A.
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This online compilation of papers from the ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition (GT2016) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

2016;():V04AT04A001. doi:10.1115/GT2016-56023.

At the “Institut für Thermische Strömungsmaschinen” (ITS) a numerical method based on the the meshfree “Smoothed Particle Hydrodynamics” (SPH) approach has been developed with the objective of computing primary breakup in the vicinity of fuel spray nozzles [1, 2]. In recent publications the successful application of the code to different flow problems is demonstrated [3, 4].

In this paper we present the first application of the method to investigate a simplified, but applied fuel spray nozzle geometry of the swirl cup design in 2D. The atomization process of Jet-A1 at ambient and at high pressure conditions is compared in terms of film flow development, mixing and spray characteristics. The influence of pressure is pointed out and quantified.

The study demonstrates that the SPH method is a suitable toolbox for the analysis and the design of fuel spray nozzles. Unique analysis tools that are not available in grid-based CFD methods are presented and applied. Droplet distributions are extracted, which can be considered as possible input in subsequent Euler-Lagrange computations.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A002. doi:10.1115/GT2016-56046.

A novel gas turbine combustor which features a helical arrangement of the burners around the turbine shaft has been subject of a detailed flow analysis. A fundamental investigation of the combustor concept has been conducted in the authors previous work [1]. The main design parameters for such a combustor were identified based on kinematic assessments of the flow fields predicted by CFD.

In particular, it has been shown in the previous work that the swirl rotational direction of adjacent burners determines the overall flow pattern in such a staggered design of the combustor dome. However, for the optimal configuration the exit mean flow angle was lower than the half of its initial value at the combustor inlet. The reason for this unwanted decay of the initial high angular momentum flux was not clear.

In the present work a comprehensive global flow analysis of such a short helical combustor is performed. The underlying physics of large changes of the flow pattern and exit flow angle are elucidated by the analysis of the different terms (momentum, pressure and friction) in the integral balance equation of angular momentum. The term “dynamic flow analysis” is used in contrast to the “kinematic flow analysis” in our previous work and does not refer to transient flow phenomena.

It is shown that the flow in the vicinity of the burners is governed by inertial forces associated to an asymmetric pressure distribution on the sidewall and the combustor dome. Downstream the sidewalls, the swirl rotational direction of circumferentially adjacent burners determines the structure of vortex-breakdown and the flow pattern in the primary combustion zone. It is shown that the turbulent mixing phenomena have minor effects on the flow structure at the combustor exit.

To compare mean flow quantities of different combustor designs, a consistent averaging method is introduced which is based on the work of the Pinako and Wazelt [2].

This analysis can also be applied to conventional combustors to assess different swirl configurations regarding the resulting flow pattern, mixing performance and total pressure loss.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A003. doi:10.1115/GT2016-56100.

An overview of research efforts at NASA Glenn Research Center (GRC) in low-emission combustion technology that have made a significant impact on the nitrogen oxides (NOx) emission reduction in aircraft propulsion is presented. The technology advancements and their impact on aircraft emissions are discussed in the context of NASA’s Aeronautics Research Mission Directorate (ARMD) high-level goals in fuel burn, noise and emission reductions. The highlights of the research presented here show how the past and current efforts laid the foundation for the engines that are flying today as well as how the continued technology advancements will significantly influence the next generation of aviation propulsion system designs.

Topics: Combustion , Emissions , NASA
Commentary by Dr. Valentin Fuster
2016;():V04AT04A004. doi:10.1115/GT2016-56124.

Combustion noise in the laboratory scale PRECCINSTA burner is simulated with a new, robust and highly efficient approach for combustion noise prediction. The applied hybrid method FRPM-CN (Fast Random Particle Method for Combustion Noise prediction) relies on a stochastic, particle based sound source reconstruction approach. Turbulence statistics from reacting CFD-RANS simulations are used as input for the stochastic method, where turbulence is synthesized based on a first order Langevin ansatz. Sound propagation is modeled in the time domain with a modified set of linearized Euler equations and monopole sound sources are incorporated as right hand side forcing of the pressure equation at every timestep of the acoustics simulations. First, reacting steady state CFD simulations are compared to experimental data, showing very good agreement. Subsequently, the computational combustion acoustics setup is introduced, followed by comparisons of numerical with experimental pressure spectra. It is shown that FRPM-CN accurately captures absolute combustion noise levels without any artificial correction. Benchmark runs show that the computational costs of FRPM-CN are much lower than that of direct simulation approaches. The robustness and reliability of the method is demonstrated with parametric studies regarding source grid refinement, the choice of either RANS or URANS statistics and the employment of different global reaction mechanisms.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A005. doi:10.1115/GT2016-56146.

Performed experiments have studied the effect of coke deposition on the characteristics of flow distribution of aviation kerosene RP-3 at supercritical pressure. The whole experiment is divided into two steps: 1) making a coke tube; 2) paralleling the coked tube and a regular one with the same scale and observing the flow distribution status in different system pressure and total mass flow rate. The experimental results indicated that the deposition of coke made a great difference on the flow distribution of fuel.

Based on experimental results, it’s demonstrated that the percentage of total mass flow rate in coke-free tube increases to 68.5%. Further analyses reveal the fact that the total mass flow rate has nearly no impact on flow distribution and the system pressure also influence the distribution very little. What’s more, the amount of coke vs axial position and total amount of coke in coked tube are mentioned in this paper, which is benefit in analysis of flow distribution.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A006. doi:10.1115/GT2016-56155.

Premixed combustion is a common technology applied in modern gas turbine combustors to minimize nitrogen oxide emissions. However, early mixing of fuel and oxidizer opens up the possibility of flame flashback into the premixing section upstream of the combustion chamber. Especially for highly reactive fuels boundary layer flashback is a serious challenge. For high preheating and burner surface temperatures, boundary layer flashback limits for burner stabilized flames converge to those of so-called confined flames, where the flame is stabilized inside the burner duct. Hence, the prediction of confined flashback limits is a highly technically relevant task.

In this study, a predictive model for flashback limits of confined flames is developed for premixed hydrogen-air mixtures. As shown in earlier studies, confined flashback is initiated by boundary layer separation upstream of the flame tip. Hence, the flashback limit can be predicted identifying the minimum pressure rise upstream of a confined flame causing boundary layer separation. For this purpose, the criterion of Stratford is chosen which was originally developed for boundary layer separation in mere aerodynamic phenomena. It is shown in this paper that it can also be applied to near wall combustion processes if the pressure rise upstream of the flame tip is modeled correctly. In order to determine the pressure rise, an expression for the turbulent burning velocity is derived including the effects of flame stretch and turbulence. A comparison of the predicted flashback limits and experimental data shows high prediction accuracy and wide applicability of the developed model.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A007. doi:10.1115/GT2016-56156.

Alternative fuels like hydrogen are presently discussed as one possibility to reduce carbon dioxide emissions from gas turbines. Premixed lean combustion is a current standard to minimize nitrogen oxide emissions. However, the early mixing of fuel and oxidizer upstream of the combustion chamber opens up the possibility of upstream flame propagation, referred to as flame flashback. Flame flashback in gas turbines has to be prevented as it leads to immediate engine shut down or even structural damage. Due to high burning velocities and low quenching distances, flashback is especially critical in premixed hydrogen flames. In particular, the low velocity region near the burner wall promotes flashback. Diluting the mixture near the wall by fluid injection is one approach to counteract this phenomenon and to enhance the safe operating range of a gas turbine burner.

This article presents an experimental study on the effect of air injection on flame flashback investigated at a channel burner configuration. Different injection mass flow rates, positions and angles are compared to a reference case without injection regarding their flashback limits. The effectiveness of the injection increases with the injected mass flow rate. The resulting flashback limits can be correlated with the equivalence ratio at the wall by concepts taken from film cooling. However, the fluid injector is a source of boundary layer disturbances leading to an initial penalty regarding flashback resistance. This penalty increases the closer the injector is located to the burner exit. Furthermore, the penalty increases for lower injection angles as the area of the injector and the corresponding boundary layer disturbances increase.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A008. doi:10.1115/GT2016-56158.

Water injection is often used to control NOx emissions or to increase power output from non-premixed combustion of gaseous and liquid fuels. Since the emission level in premixed natural gas combustion is significantly lower than for non-premixed combustion, water injection for emission reduction is usually not an issue. However, the increasing share of fluctuating power output from renewables motivates research activities on the improvement of the operational flexibility of combined cycle power plants. One aspect in that context is power augmentation by injection of liquid water in premixed combustors without drawbacks regarding emissions and flame stability. For research purposes, water injection technology has therefore recently been transferred to premixed combustors burning natural gas. In order to investigate the influence of water injection on premixed combustion of natural gas, an atmospheric single burner test rig has been set up at Lehrstuhl für Thermodynamik, TU München. The test rig is equipped with a highly flexible water injection system to study the influence of water atomization behavior on flame shape, position and stabilization. Presented investigations are conducted at gas turbine like preheating temperatures (673K) and flame temperatures (1800–1950K) to ensure high technical relevance. In this paper, the interaction between water injection, atomization and macroscopic flame behavior is outlined. Favorable and non-favorable operating conditions of the water injection system are presented in order to clarify the influence of water atomization and vaporization on flame stability and the emission behavior of the test rig. Water spray quality is assessed externally with a Malvern laser diffraction spectrometer whereas spray distribution in the test rig is determined by means of Mie scattering images at reacting conditions. The flame shape is analyzed using time-averaged OH* chemiluminescence images while the efficiency of water injection at various operating points is evaluated using global emission concentration measurements. Finally, the important influence of the water injection system design on the combustor performance will be shown using combined Mie scattering and OH* chemiluminescence images.

At constant adiabatic flame temperatures, a stable flame could be established for water-to-fuel ratios of up to 2.25. While only minor changes could be detected for the heat release distribution in the combustion chamber, the water distribution changes significantly while increasing the amount of water. Finaly, changes in NOx emission concentrations can directly be related to water droplet sizes and the global water distribution in the combustion chamber.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A009. doi:10.1115/GT2016-56163.

The effects of jet fuel composition on ignition probability have been studied in a flowfield that is relevant to turbine engine combustors, but also fundamental and conducive to modeling. In the experiments, a spark kernel is ejected from a wall and propagates transversely into a crossflow. The kernel first encounters an air-only stream before transiting into a second, flammable (premixed) stream. The two streams have matched velocities, as verified by hot-wire measurements. The liquid fuels span a range of physical and chemical kinetic properties. To focus on their chemical differences, the fuels are prevaporized in a carrier air flow before being injected into the experimental facility. Ignition probabilities at atmospheric pressure and elevated crossflow temperature were determined from optical measurements of a large number of spark events, and high speed imaging was used to characterize the kernel evolution. Eight fuel blends were tested experimentally; all exhibited increasing ignition probability as equivalence ratio increased, at least up to 1.5. Statistically significant differences between fuels were measured that have some correlation with fuel properties. To elucidate these trends, the forced ignition process was also studied with a reduced order numerical model of an entraining kernel. The simulations suggest ignition is successful if sufficient heat release occurs before entrainment of colder crossflow fluid quenches the exothermic oxidation reactions. As the kernel is initialized in air, it remains lean during the initial entrainment of the fuel-air mixture; thus richer crossflows lead to quicker and higher exothermicity.

Topics: Fuels , Ignition
Commentary by Dr. Valentin Fuster
2016;():V04AT04A010. doi:10.1115/GT2016-56177.

Alternative production pathways for liquid fuels provide the opportunity to adjust the chemical composition of the product in order to improve combustion performance. In this study, flame characteristics of selected single-component fuels were investigated to provide a basis for a better understanding of the influence of specific fuel components on the combustion behaviour. The measurements were performed in a redesigned gas turbine model combustor for swirl-stabilised spray flames under atmospheric pressure. The combustor features a dual-swirl geometry and a prefilming airblast atomiser. The combustion chamber provides good optical access and yields well-defined boundary conditions. As part of different projects in the field of alternative fuels, two liquid single-component fuels (n-hexane, n-dodecane) and kerosene Jet A-1 were investigated. Flow fields of the nonreacting and reacting flow were measured using stereo particle image velocimetry. The flame structure and spray distribution were derived from CH* chemiluminescence and Mie scattering respectively. Lean blowout limits were measured. Results show noticeable differences in combustion behaviour of the chosen fuels at comparable flow conditions. Furthermore, the results provide a detailed data base for the validation of numerical models.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A011. doi:10.1115/GT2016-56187.

A novel parallelized, automated, predictive imprint cooling model (PAPRICO) was developed for modeling and simulation of combustor liners using a Reynolds averaged Navier-Stokes (RANS) approach. The methodology involves removing the film and effusion cooling jet geometry from the liner while retaining the cooling hole imprints on the liner. The PAPRICO can operate under two modalities, viz., two-sided and one-sided. For the two-sided PAPRICO model, the imprints are kept on the plenum and combustor sides of the liner. For the one-sided PAPRICO model, the imprints are retained only on the combustor side of the liner and there is no need for a plenum. Consequently, the one-sided PAPRICO significantly reduces the size of the mesh when compared with a mesh that resolves the film and effusion cooling holes. The PAPRICO model neither needs a priori knowledge of the cooling flow rates through various combustor liner regions nor specific mesh partitioning. The PAPRICO model uses the one-dimensional adiabatic, calorifically perfect, total energy equation. The total temperature, total pressure, jet angle, jet orientation, and discharge coefficient are needed to determine the imprint mass flow rate, momentum, enthalpy, turbulent kinetic energy, and eddy dissipation rate. These physical quantities are included in the governing equations as volumetric source terms in cells adjacent to the liner on the combustor side. Additionally, the two-sided PAPRICO model integrates the volumetric sources to calculate their corresponding volumetric sinks in the cells adjacent to the liner on the plenum side. The PAPRICO model user-defined subroutines were written in C programming language and linked to the ANSYS Fluent. A Fluent graphical user interface panel was also developed in Scheme language to effectively and conveniently form effusion cooling regions based on jet angle, jet orientation, pattern, and discharge coefficient. The PAPRICO algorithm automatically identifies and computes the jet area, jet diameter, jet centroid, and jet count per cooling region from an arbitrarily partitioned mesh. Jets with concentric patterns, containing multiple jet orientations, can be conveniently grouped into a single imprint zone. A referee combustor liner was simulated using PAPRICO under non-reacting flow conditions. The PAPRICO results were compared with the non-reacting flow results of a resolved geometry containing 1504 cooling jets (with multiple jet sizes, orientations and angles) and 7 dilution jets. The PAPRICO results were also compared with the non-reacting numerical results of the referee combustor liner with prescribed mass and enthalpy source terms. The numerical results were also compared with experimental measurements of mass flow rates through the referee combustor liner. The numerical results clearly conclude that PAPRICO can qualitatively and quantitatively emulate the local turbulent flow field with only one third of the mesh of that which resolves the effusion cooling jets. The simulations with prescribed mass and enthalpy sources fail to emulate the local turbulent flow field. The PAPRICO model can predict the relative flow rates through the various regions in the liner based on comparisons with measurements.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A012. doi:10.1115/GT2016-56189.

This paper describes a model to predict the nitric oxides (NOx) emissions for a dry non-premixed flame gas turbine combustor at full operation conditions. The NOx correlation considered the combustor pressure, the combustor outlet temperature and the fuel composition from natural gas (NG) to pure hydrogen (H2) fuel.

The test data for parametrizing the model was acquired with a high pressure combustion test rig for industrial 10 MWth reverse-flow gas turbine combustors. The experimental results confirm the typical dependencies of NOx emissions. As expected, higher NOx emissions occur with increasing combustor pressure, combustor outlet temperature and hydrogen content of the fuel.

The reference NOx model has been derived on the basis of physical approaches for the pressure and temperature effects. The substitution of natural gas with hydrogen is taken into account by a variable pressure exponent and a variable factor in the exponent of the exponential temperature correlation. As a result, the pressure exponent increases with increasing hydrogen. The temperature exponent factor decreases with increasing hydrogen. The model can describe the data set with limitations at high pressure and high hydrogen content fuel operation conditions.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A013. doi:10.1115/GT2016-56215.

Ultra-Compact combustion presents a novel solution to address the demand for increasingly compact, efficient, and low weight aircraft gas turbine engine propulsion systems. An Ultra-Compact Combustor (UCC) operates by diverting a portion of the compressor exit flow into a cavity about the engine outer diameter. Injection into the cavity can be done at an angle in order to induce bulk circumferential swirl. Swirl velocities in the cavity then impart a centrifugal load of approximately 1000g0. This high-g UCC concept has been investigated by The Air Force Institute of Technology with the goal of incorporating a common upstream flow source to distribute the simulated compressor exit flow into separate core and combustion cavity flow paths.

Experimental results from this test rig are presented, with particular emphasis on establishing the design flow split through the diffuser into the circumferential cavity. The implementation of a core channel plate was instrumental in control of the mass flow splits. Computational Fluid Dynamics (CFD) supplement the experiments and enable a more detailed understanding of the interactions within the diffuser and the interactions between the air injection jets and the fuel jets. A range of cavity equivalence ratios was studied and combustion within the cavity was shown to be a strong function of cavity loading, which was in turn a function of the total mass flow. Varying the orientation of the channel plate with respect to guide-vane leading edges caused a change in the core flow development which then had a secondary effect of aiding the combustion process within the cavity.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A014. doi:10.1115/GT2016-56216.

Combustion at high G-loading offers the promise of higher flame speeds and shorter residence times. Ultra-Compact Combustors (UCC) make use of this phenomenon by injecting air and fuel into a circumferential cavity around the main core flow. Air is injected tangentially into the combustion cavity to induce bulk circumferential swirl. Swirl velocities in the cavity produce a centrifugal load on the flow that is typically expressed in terms of gravitational acceleration, or g-loading. The Air Force Institute of Technology (AFIT) has developed an experimental facility in which g-loads up to 2000 times the earth’s gravitational field (“2000 g’s”) can be established.

This paper investigates the flow within the combustion cavity to determine conditions that lead to the generation of higher g-loads and longer residence times. This is coupled with the desire to completely combust the fuel — ideally within the combustion cavity. These objectives have led to changes within the AFIT test setup to enable optical access into the primary combustion cavity. Particle Image Velocimetry (PIV), complemented by traditional high-speed video imagery, provided high-fidelity measurements of the velocity fields within the cavity. These experimental measurements were compared to a set of Computational Fluid Dynamics (CFD) solutions. Improved cavity air and fuel injection schemes were evaluated over a range of air flows and equivalence ratios. Increased combustion stability was attained by providing uniform distribution of air drivers. Lean cavity equivalence ratios at a high total airflow resulted in higher g-loads and complete combustion showing promise for utilizing the UCC as a main combustor.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A015. doi:10.1115/GT2016-56222.

The characterization of unstable combustion regimes is often performed in the light of the Rayleigh Criterion, in the frequency domain, employing the Power Spectral Analysis of pressure (p′) and heat-release (q′) fluctuations. Equally often, it is assumed a priori that the thermo-acoustic oscillations are periodic, with a dominant frequency and a fixed amplitude (Period-1Llimit Cycle Oscillations). However one has to consider that: 1) p′ and q′, involved in the Rayleigh instability index, are governed by the Linearized Acoustic Energy Perturbation Balance Equations; 2) in the frequency domain any interdependence is measured by the coherence function, based on cross spectral densities, or Fourier spectra of cross-correlations, that in turn suppose a linear interdependence between sampled quantities. Conversely, recent experiments reveal that even simple thermo-acoustic systems exhibit nonlinear behaviour, far more elaborate than period-1 limit cycle oscillations. Therefore, in addition to the conventional linear analysis, a new approach based on Nonlinear Dynamics will be required to characterize the unstable regimes in lean gas-turbine combustors. With such approach, one may avoid the risk of misunderstanding the Deterministic Chaos, underlying in the measured signals also during stable combustion regimes, as stochastic noise. The preserved information will be thus available to analytically formulate an index acting as the earliest warning signal of an impending oscillatory combustion instability.

In the light of this, we have applied the Interdependence Index E, based on chaotic synchronization theory, to pressure and radiant energy signals sampled from an industrial combustor. The index was found: 1) low computationally demanding, since based on quantities already calculated for the phase space reconstruction; 2) really effective in the early detection of self sustained (chaotic or not) thermo-acoustic oscillations; 3) valid for a range of coupling strength, and thus smoothly increasing at the instability onset, as requested by the control system time response; 4) unaffected by the non linear relationship between heat release an chemiluminescence, that may make invalid the pseudo-Rayleigh index, computed from pressure and radiant energy fluctuations; 5) asymmetric and thus able to identify the driven and driver (sub)systems, as in combustion instabilities with no thermo-acoustic feed-back.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A016. doi:10.1115/GT2016-56245.

This study investigates the influence of the fuel injection strategy on safety against flashback in a gas turbine model combustor with premixing of H2-air-mixtures. The flashback propensity is quantified and the flashback mechanism is identified experimentally.

The A2EV swirler concept exhibits a hollow, thick walled conical structure with four tangential slots. Four fuel injector geometries were tested. One of them injects the fuel orthogonal to the air flow in the slots (jet-in-crossflow-injector, JICI). Three injector types introduce the fuel almost isokinetic to the air flow at the trailing edge of the swirler slots (trailing edge injector, TEI). Velocity and mixing fields in mixing zone and combustion chamber in isothermal water flow were measured with High-speed-Particle-Image-Velocimetry (PIV) and Highspeed-Laser-Induced-Fluorescence (LIF). The flashback limit was determined under atmospheric pressure for three air mass flows and 673 K preheat temperature for H2-air-mixtures. Flashback mechanism and trajectory of the flame tip during flashback were identified with two stereoscopically oriented intensified high-speed cameras observing the OH* radiation.

We notice flashback in the core flow due to Combustion Induced Vortex Breakdown (CIVB) and Turbulent upstream Flame Propagation (TFP) near the wall dependent on the injector type. The Flashback Resistance (FBR) defined as the ratio between a characteristic flow speed and a characteristic flame speed measures the direction of propagation of a turbulent flame in the flow field. Although CIVB cannot be predicted solely based on the FBR, its distribution gives evidence for CIVB-prone states.

The fuel should be injected preferably isokinetic to the air flow along the entire trailing edge in oder to reduce the RMS fluctuation of velocity and fuel concentration. The characteristic velocity in the entire cross section of the combustion chamber inlet should be at least twice the characteristic flame speed. The position of the stagnation point should be tuned to be located in the combustion chamber by adjusting the axial momentum. Those measures lead to safe operation with highly reactive fuels at high equivalence ratios.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A017. doi:10.1115/GT2016-56246.

Flashback and self-ignition in the premixing zone of typical gas turbine swirl combustors in lean premixed operation are immanent risks and can lead to damage and failure of components. Thus, steady combustion in the premixing zone must be avoided under all circumstances. This study experimentally investigates the flame holding propensity of fuel injectors in the swirler of a gas turbine model combustor with premixing of H2-NG-air-mixtures under atmospheric pressure and proposes a model to predict the limit for safe operation.

The A2EV swirler concept exhibits a hollow, thick walled conical structure with four tangential slots. Four fuel injector geometries were tested. One of them injects the fuel orthogonal to the air flow in the slots (jet-in-crossflow-injector, JICI). Three injector types introduce the fuel almost isokinetic to the air flow at the trailing edge of the swirler slots (trailing edge injector, TEI). A cylindrical duct and a window in the swirler made of quartz glass allow the application of optical diagnostics (OH* chemiluminescence and Planar Laser Induced Fluorescence of the OH radical (OH-PLIF)) inside the swirler. The fuel-air-mixture was ignited with a focused single laser pulse during steady operation. The position of ignition was located inside the swirler in proximity to a fuel injection hole. If the flame was washed out of the premixing zone not later than four seconds after the ignition the operation point was defined as safe. Operation points were investigated at three air mass flows, three air ratios, two air preheat temperatures (573 K, 673 K) and 40 to 100 percent per volume hydrogen in the fuel composed of hydrogen and natural gas.

The determined safety limit for atmospheric pressure yields a similarity rule based on a critical Damköhler number. Application of the proposed rule at conditions typical for gas turbines leads to these safety limits for the A2EV burner: With the TEIs the swirler can safely operate with up to 80 percent per volume hydrogen content in the fuel at an air ratio of two. With the JIC injector safe operation at stoichiometric conditions and 95 percent per volume hydrogen is possible.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A018. doi:10.1115/GT2016-56309.

Annular combustors of aero-engines and gas turbine are often affected by thermo-acoustic combustion instabilities coupled by azimuthal modes. Previous experiments as well as theoretical and numerical investigations indicate that the coupling modes involved in this process may be standing or spinning but they provide diverse interpretations of the occurrence of these two types of oscillations. The present article reports a numerical analysis of instability coupled by a spinning mode in an annular combustor. This corresponds to experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global experimental flame describing function (FDF) and it is considered that the flames are sufficiently compact to interact with the mode without mutual interactions with adjacent burning regions. A harmonic balance nonlinear stability analysis is carried out by combining the FDF with a Helmholtz solver to determine the system dynamics trajectories in a frequency-growth rate plane. The influence of the distribution of the volumetric heat release corresponding to each burner is investigated in a first stage. Even though the 16 burners are all compact with respect to the acoustic wavelength considered and occupy the same volume, simulations reveal an influence of this volumetric distribution on frequencies and growth rates. This study emphasizes the importance of providing a suitable description of the flame zone geometrical extension and correspondingly an adequate representation of the level of heat release rate fluctuation per unit volume. It is found that these two items can be deduced from a knowledge of the heat release distribution under steady state operating conditions. Once the distribution of the heat release fluctuations is unequivocally defined, limit cycle simulations are performed. For the conditions explored, simulations retrieve the spinning nature of the self-sustained mode that was identified in the experiments both in the plenum and in the combustion chamber.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A019. doi:10.1115/GT2016-56340.

A novel airblast injector is designed for gas turbine combustors. Unlike standard pressure swirl and prefilming/non-prefilming air blast atomizers, the novel injector is designed to improve the fuel injection delivery to the injector and improve atomization of the fuel by using a porous stainless steel tube. There are three swirling air streams in the injector. The liquid fuel is injected through the porous tube, with 7 micron porosity, between the swirling air streams, viz. an inner swirling air through the tube and the other two swirling air streams merging downstream of the tube. The swirl vane angles and the air split ratio are selected to increase the amount of air through the injector and facilitate the atomization process. The liquid fuel is injected through the outer surface of the porous tube, due to the permeability of the tube, produces a thin liquid sheet on the inner surface of the tube. The atomization occurs by surface stripping of the liquid sheet. The advantage of such an injector is that it produces a liquid sheet with uniform thickness around the circumference of the tube under all liquid loading. The porous tube also increases the surface area of contact between the fuel and air and produces a fine spray at engine idle conditions. An experimental approach is adopted in the present study to characterize the spray and aerodynamics of the injector for Jet-A and Gas-To-Liquid (GTL) fuels at atmospheric conditions. The effect of flare height on the Sauter Mean Diameter (SMD) is also studied. Spray characterization, droplet size and volume flux are investigated with PDI measurements. The effect of pressure drop and fuel properties on SMD distribution is analyzed. Velocity profiles at downstream of the injector are obtained from LDV measurements, and the velocity profile at the exit of the injector is also analyzed. A central toroidal recirculation zone (CTRZ) is observed at the exit of the injector. The effect of different configurations of the injector on spray characteristics is studied. A correlation for SMD is obtained.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A020. doi:10.1115/GT2016-56345.

This paper describes stereo-PIV, OH-PLIF and fuel-PLIE (planar laser induced emission) measurements in a pressurized, liquid fueled, swirl combustor. Data were obtained at globally fuel lean conditions, combustor pressures of 2–5 bar, and an inlet air temperature of 450 K. The experiments were performed to characterize the flowfield, heat release and fuel spray distribution. Several challenges are associated with OH-PLIF in pressurized, liquid fuel systems at sustained high repetition rates. For example, in addition to the significantly lower pulse energies of high repetition rate systems relative to low repetition rate ones, the ultraviolet laser used to excite OH also causes the fuel to emit, with the brighter liquid fuel signal overlapping the OH fluorescence spectrum. To overcome these challenges, two intensified high-speed cameras were used to maximize signal separation during data collection and perform signal subtraction in post-processing. The first camera used narrow band spectral filtering, and the intensifier was gated to miss much of the slower decaying fuel signal. As a result, it satisfactorily captures the OH fluorescence along with some of the stronger fuel fluorescence signal. The second camera detected primarily fuel emission with the intensifier gate delayed to capture the tail of the longer-lived fuel phosphorescence, and a long-pass spectral filter capturing all the fuel emission. This paper presents illustrative results showing the instantaneous flow field, flame position as indicated by OH-PLIF, and spray distribution from the fuel PLIE. Multiple flame topologies are observed — specifically, flames stabilized in the outer shear layer occur for all the cases studied, but inner shear layer stabilized flames are also seen in the higher pressure cases.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A021. doi:10.1115/GT2016-56346.

Blowoff sets important operational limits on a combustor system. While blowoff is intrinsically a system-dependent phenomenon, it is also dependent on the chemical and physical properties of the fuel. This paper describes an experimental study of the lean blowout limits of eight liquid fuels in a swirl-stabilized combustor, with data for both a pressure atomizer and an airblast atomizer. Three of the fuels were traditional jet fuels (an average Jet-A, JP-5, and JP-8) and the remaining five fuels spanned a range of physical and kinetic properties. These experiments were performed at a combustor pressure of 345 kPa and an air temperature of 450 K. In addition to some sensitivity of blowoff conditions to the thermal state of the combustor, results also clearly show sensitivities to fuel composition. Strong correlations were observed for pressure atomizer blowoff with fuel physical properties, particularly for boiling point temperature, indicating that fuels less easily atomized and vaporized are harder to blow off. These results are consistent with the idea that delaying atomization and/vaporization, and therefore reducing the level of premixing that drives the local fuel-air ratio towards the very lean global fuel/air ratio, is advantageous in order to promote regions of locally elevated flame temperatures. We suggest that this behavior occurs when the air preheat temperature is above the fuel flashpoint, as a similarly good correlation with boiling point temperature but with the opposite trend, has been previously reported in a study obtained for preheat temperatures below the fuel flashpoint. In contrast, the airblast results do not show strong correlations with fuel physical properties. Rather, the best correlation of the airblast atomizer results is with the percentage of iso-paraffins in the fuel. We speculate that this reflects a sensitivity to kinetic properties of the fuel, as the superior atomization characteristics of the airblast atomizer may de-emphasize the importance of physical properties.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A022. doi:10.1115/GT2016-56368.

The current work focuses on the Large Eddy Simulation of a combustion instability in a laboratory-scale swirl burner. Air and fuel are injected at ambient conditions. Heat conduction from the combustion chamber to the plenums results in a preheating of the air and fuel flows above ambient conditions. The paper compares two computations with different modeling strategies. In the first computation, the temperature of the injected reactantsis 300 K (equivalent to the experiment) and the combustor walls are treated as adiabatic. The frequency of the unstable mode (≈ 635 Hz) deviates significantly from the measured frequency (≈ 750 Hz). In the second computation, the preheating effect observed in the experiment and the heat losses at the combustion chamber walls are taken into account. The frequency (≈ 725 Hz) of the unstable mode agrees well with the experiment. These results illustrate the importance of accounting for heat transfer/ losses when applying LES for the prediction of combustion instabilities. Uncertainties caused by unsuitable modeling strategies when using CFD for the prediction of combustion instabilities can lead to an improper design of passive control methods (such as Helmholtz resonators), as these are often only effective in a limited frequency range.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A023. doi:10.1115/GT2016-56405.

A diffusion swirling flame under external forcing and self-excitation within a single swirler combustor have been studied in this paper with the large-eddy simulation and linear acoustic method. The combustor features pre-vaporized kerosene as the fuel, a single radial air swirler for flame stabilization and a square cross section chamber with adjustable length. Firstly, self-sustained pressure oscillation has been achieved by using of a chocked nozzle on the chamber outlet with large-eddy simulation. Dynamic pressure oscillations are analyzed in frequency domain through Fast Fourier Transform. The major pressure oscillation is identified as the 1st order longitudinal mode of the chamber. Further, the same frequency in the form of harmonic velocity oscillation is imposed on the inlet of the combustor while the chamber length has been changed. Based on this approach, a comparative study of the flame response with different excitation method but same frequency is carried out.

In both self-excited and forced cases, global and local flame responses as well as Rayleigh index have been analyzed and compared. With the flame response function, the excited acoustic modes under the influence of dynamic heat release have been predicted with linear acoustic method and compared with the results obtained from large-eddy simulation. Results show that the flame response presents a great difference in the spacial distribution with different excitation approaches. Thermo-acoustic interaction distributes along the flame front with the expansion of the flame under self-excitation while it damps with the acoustic propagating downstream under forcing condition. As the ratio of flame length to acoustic wave length could not be neglected for the diffusion swirling flame, the global flame response under forcing cannot represent the local response feature of the flame accurately, thus influencing the instability prediction.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A024. doi:10.1115/GT2016-56430.

The Dry-Low-NOx (DLN) Micromix combustion technology has been developed as low emission combustion principle for industrial gas turbines fueled with hydrogen or syngas. The combustion process is based on the phenomenon of jet-in-crossflow-mixing. Fuel is injected perpendicular into the air-cross-flow and burned in a multitude of miniaturized, diffusion-like flames. The miniaturization of the flames leads to a significant reduction of NOx emissions due to the very short residence time of reactants in the flame.

In the Micromix research approach, CFD analyses are validated towards experimental results. The combination of numerical and experimental methods allows an efficient design and optimization of DLN Micromix combustors concerning combustion stability and low NOx emissions.

The paper presents a comparison of several numerical combustion models for hydrogen and hydrogen-rich syngas. They differ in the complexity of the underlying reaction mechanism and the associated computational effort.

For pure hydrogen combustion a one-step global reaction is applied using a hybrid Eddy-Break-up model that incorporates finite rate kinetics. The model is evaluated and compared to a detailed hydrogen combustion mechanism derived by Li et al. including 9 species and 19 reversible elementary reactions. Based on this mechanism, reduction of the computational effort is achieved by applying the Flamelet Generated Manifolds (FGM) method while the accuracy of the detailed reaction scheme is maintained.

For hydrogen-rich syngas combustion (H2-CO) numerical analyses based on a skeletal H2/CO reaction mechanism derived by Hawkes et al. and a detailed reaction mechanism provided by Ranzi et al. are performed.

The comparison between combustion models and the validation of numerical results is based on exhaust gas compositions available from experimental investigation on DLN Micromix combustors.

The conducted evaluation confirms that the applied detailed combustion mechanisms are able to predict the general physics of the DLN-Micromix combustion process accurately. The Flamelet Generated Manifolds method proved to be generally suitable to reduce the computational effort while maintaining the accuracy of detailed chemistry.

Especially for reaction mechanisms with a high number of species accuracy and computational effort can be balanced using the FGM model.

Topics: Combustion , Syngas , Hydrogen
Commentary by Dr. Valentin Fuster
2016;():V04AT04A025. doi:10.1115/GT2016-56450.

The impact of quarter wave tube (QWT) arrangements in terms of axial location, circumferential distribution and number of cavities on damping of azimuthal modes is investigated both experimentally and numerically in an atmospheric annular test rig under isothermal conditions. Well-established measurement techniques are applied to characterize the damping potential for azimuthal modes of different quarter wave tube (QWT) arrangements. For additional insight into the considered configurations eigenfrequency studies are conducted using linearized Euler equations (LEE). Measured boundary conditions are used for inlet and outlet. The reflection coefficient of a single quarter wave tube (QWT) is measured on an impedance rig using longitudinal wave excitation. It is shown that this reflection coefficient can be used for the eigenfrequency analysis of the annular rig in which the QWT is mounted in radial direction.

The effects of different quarter wave tube configurations on the spatial mode shape of the first azimuthal mode and the corresponding change in modal dynamics are analyzed. This provides guidance for the circumferential and axial arrangement of damping devices to most effectively attenuate an annular acoustic system. It is illustrated that the complete acoustic system including the resonators has to be considered to properly dimension the acoustic characteristics of a damping device.

Topics: Waves , Damping
Commentary by Dr. Valentin Fuster
2016;():V04AT04A026. doi:10.1115/GT2016-56460.

Vorticity fluctuations have been identified as an important coupling mechanism during velocity-coupled combustion instability in swirl-stabilized flames. Acoustic oscillations in the combustor can cause all components of vorticity to oscillate, particularly the cross-stream, or azimuthal, vorticity that is excited in shear layer roll-up, and streamwise, or axial, vorticity that is excited during swirl fluctuations. These fluctuations can be induced by longitudinal acoustic fluctuations that oscillate across the swirler and dump plane upstream of the flame. While these fluctuations have been identified in a number of configurations, the sensitivity of this mechanism to flow configuration and boundary conditions has not been studied parametrically. In this study, we investigate the impact of time-averaged swirl level, confinement, and forcing frequency and amplitude on vorticity fluctuation dynamics in the azimuthal direction of a non-reacting swirling jet. The goal of this work is to better understand the dependence of vorticity fluctuations on these parameters as well as the vorticity conversion processes that occur in the flow. We have shown that vorticity fluctuation levels vary with time-averaged swirl number, particularly in the presence of a self-excited precessing vortex core, which dampens most acoustically-driven motion. Additionally, variations in forcing frequency excite flow response in different portions of the flow, particularly for different swirl numbers. Finally, confinement drastically changes the flow topology and unforced dynamics, resulting in significantly different response to forcing and generation of vortical fluctuations.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A027. doi:10.1115/GT2016-56499.

An experimental investigation was undertaken to develop a lean direct injection (LDI) combustor concept. The LDI concept consisted of a 9-point fuel injection system setup in a 3×3 array, where each point is made of a fuel nozzle fitted into a counter-rotating radial-radial swirler. Each swirler consisted of an inner, primary swirler, and an outer, secondary swirler, with opposite flow rotation. The experimental investigation consisted of atmospheric combustion and lean blow-out (LBO) tests, and high pressure combustion. All tests utilized a pressure drop of 4% across the swirlers. Parameters tested included variable swirler strengths and fuel nozzle injection depth. Fuel staging was employed on all configurations.

Two swirlers, with swirl numbers (SN) of 0.64 and 1.07, were used in the swirler array, with the baseline configuration consisting entirely of the low-SN swirlers, and configurations 2 and 3 utilizing 1 and 3, respectively, of the high-SN swirlers. Configuration 2 has the high-SN swirler at the center, whereas Configuration 3 uses three high-SN swirlers in the center row. From the atmospheric tests, flame stability and lean blowout occurred at lower equivalence ratios for configuration 2 and 3, indicating that the presence of the high-swirl swirler improved flame anchoring. Two different nozzle insertion depths were tested, with a deep insertion depth providing partial prefilming on the venturi, and a shallow insertion depth with no pre-filming. Partial pre-filming greatly aided in fuel/air mixing and produced a shorter, blue flame, which was stable, and had lower LBO limit as compared to full direct injection (shallow insertion), which resulted in more unstable, longer, yellow flames.

Emission measurements were conducted at high pressures using water cooled sampling probe, and a gas analyzer system to measure the emission indices of NOx and CO. The results showed that this LDI design produced emissions values comparable to those produced by current lean premixed prevaporized (LPP) combustor designs, well below current ICAO standards.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A028. doi:10.1115/GT2016-56528.

The design and model simulation of a can combustor has been made for future syngas (mainly H2/CO mixtures) combustion application in a micro gas turbine. In previous modeling studies with methane as the fuel, the analysis indicated the design of the combustor is quite satisfactory for the 60-kW gas turbine; however, the cooling may be the primary concerns as several hot spots were found at the combustor exit. When the combustor is fueled with methane/syngas mixtures, the flames would be pushed to the sides of the combustor with the same fuel injection strategy. In order to sustain the power load, the exit temperature became too high for the turbine blades, which deteriorated the cooling issue of the compact combustor. Therefore, the designs of the fuel injection are modified, and film cooling is employed. Consequently, the simulation of the modified combustor is conducted by the commercial CFD software Fluent. The computational model consists of the three-dimensional, compressible k-ε model for turbulent flows and PPDF (Presumed Probability Density Function) model for combustion process between methane/syngas and air invoking a laminar flamelet assumption. The flamelet is generated by detailed chemical kinetics from GRI 3.0. Thermal and prompt NOx mechanisms are adopted to predict the NO formation.

At the designed operation conditions, the modeling results show that the high temperature flames are stabilized in the center of the primary zone where a recirculation zone is generated for methane combustion. The average exit temperature of the modified can combustor is 1293 K, which is close to the target temperature of 1200 K. Besides, the exit temperatures exhibit a more uniform distribution by coupling film cooling, resulting in a low pattern factor of 0.22. The NO emission is also low with the increased number of the dilution holes. Comparing to the results for the previous combustor, where the chemical equilibrium was assumed for the combustion process, the flame temperatures are predicted lower with laminar flamelet model. The combination of laminar flamelet and detailed chemistry produced more reasonable simulation results. When methane/syngas fuels are applied, the high temperature flames could also be stabilized in the core region of the primary zone by radially injecting the fuel inward instead of outward through the multiple fuel injectors. The cooling issues are also resolved through altering the air holes and the film cooling. The combustion characteristics were then investigated and discussed for future application of methane/syngas fuels in the micro gas turbine. Although further experimental testing is still needed to employ the syngas fuels for the micro gas turbine, the model simulation paves an important step to understand the combustion performance and the satisfactory design of the combustor.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A029. doi:10.1115/GT2016-56530.

In order to reduce NOx emissions, modern gas turbines are often equipped with lean-burn combustion systems, where the high-velocity fuel-lean conditions that limit NOx formation in combustors also inhibit the ability of ignition, high altitude relight, and lean combustion stability. To face these issues, internally staged scheme of fuel injection is proposed. Primary and main fuel staging enable fuel distribution control, multi-injections of main fuel leads to a fast and efficient mixing. A fuel-staged low emission combustor in the framework of lean-burn combustion is developed in the present study, the central pilot stage for low power conditions is swirl-cup prefilming atomization, main stage is jet-in-crossflow multi-injection, a combination of primary and main stage injection is provided for higher power output conditions. In lean-burn combustors, the swirling main air naturally tends to entrain the pilot flame and quench it at low power conditions, which is adverse to the operability specifications, such as ignition, lean blow-out, and high-altitude relight. In order to investigate the effects of the main swirl angle on combustion performances, the ignition and lean blow-out performances were evaluated in a single dome rectangular combustor. Furthermore, the spray patterns and flow field are characterized by kerosene-planar laser induced fluorescence and particle image velocimetry to provide insight into spray and combustion performances. Flow-flow interactions between pilot and main air streams, spray-flow interactions between pilot spray and main air streams, and flame-flow interactions between pilot flame and main air streams are comprehensively analyzed. The entrainment of recirculating main air streams on pilot air streams enhances with increase of main swirl angle, because of the upward motion and increasing width of main recirculation zone. A small part of droplets are entrained by the recirculating main air streams at periphery of combustor and a majority of droplets concentrate near the centerline of combustor, making that entrainment of recirculating main air streams on pilot spray and quenching effects of recirculating main air streams on pilot flame is slight, and the quenching effects can be ignored. A follow-on paper will study the effects of venturi angle on the combustion performances.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A030. doi:10.1115/GT2016-56565.

Standard aeronautic fuels have a lower limit of aromatics of 8% (by volume) with about 18% for regular Jet A1. It has been shown that aromatics contained in Jet fuel have an impact on the fine particle emissions. In order to reduce these emissions, alternative fuels with lower aromatic content have been identified as a promising solution. Change Jet fuel composition can have several effects on spray and combustion behaviors, among others: atomization process, droplet evaporation, flame structure, pollutant and particle emissions. Then, it is necessary to evaluate the impact of this change on gas turbine performance and operability.

The present study is focused on the spray behavior investigation with different aromatic content. Four Jet fuels are investigated including conventional Jet A1 kerosene, drop-in fuel with a mixture of half conventional Jet fuel and synthetic paraffinic kerosene (SPK), SPK with 8% of aromatics and pure SPK. The tests are performed at atmospheric conditions on the MERCATO testbed located at ONERA (FR). Phase Doppler Anemometry (PDA) measurements are carried out for the four fuels on an injection system composed of a pressure swirl atomizer and an air swirler.

In this paper, a spray analysis of liquid velocity and droplet diameter measurements is described and linked to the variations of fuel properties. In the range of parameters covered by the four different fuels, it is shown that the spray behavior of each fuel is similar to the conventional Jet A1.

Topics: Pressure , Jet fuels , Sprays
Commentary by Dr. Valentin Fuster
2016;():V04AT04A031. doi:10.1115/GT2016-56571.

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame-acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the detection of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For validation of this new method an experimental setup was developed capable of generating well defined entropy waves. As a reference temperature measurement technique a hot-wire anemometer is employed. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are detected for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adheres well to a Strouhal number scaling for different mean flow velocities. In addition, simultaneously to the entropy wave measurement, the equivalence ratio fluctuations in the mixing tube are detected using the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A032. doi:10.1115/GT2016-56585.

Unsteady temporal fluctuations of the equivalence ratio in lean premixed gas turbine combustors are one of the most important driving mechanisms for thermoacoustic instabilities. In this work, high-amplitude equivalence ratio fluctuations in the mixing section of a swirl-stabilized burner are assessed for the first time. The applied non-intrusive sensor is based on fixed-wavelength modulation spectroscopy of methane at 1653 nm using a near-infrared tunable diode laser. The measurements are performed at isothermal operating conditions without the presence of a flame at 25°C and at atmospheric pressure. The equivalence ratio fluctuations are generated by acoustic forcing of the air flow while the fuel injection flow rate is kept constant. Acoustic forcing amplitudes up to 220% of the mean flow velocity are assessed. Measurements are conducted at different axial distances from the fuel injection point to study the spatio-temporal evolution of the equivalence ratio fluctuations. The results show a frequency-dependent saturation of temporal equivalence ratio fluctuations with increasing forcing amplitude, which can not be described through the available model. These results are in good agreement with preceding studies and indicate the saturation of the flame response due to a saturation of equivalence ratio fluctuations. Furthermore, a decreased attenuation of temporal mixture inhomogeneities for small forcing amplitudes is found.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A033. doi:10.1115/GT2016-56588.

Swirling jets undergoing vortex breakdown are widely used in combustion applications, due to their ability to provide aerodynamic flame stabilization. It is well known that vortex breakdown is accompanied by a dominant coherent structure, the so called precessing vortex core (PVC). Reports on the impact of the PVC on the combustion process range from beneficial to detrimental. In any event, efficient methods for the analysis of the PVC help to increase the benefit or reduce the penalty resulting from it. This study uses Particle Image Velocimetry (PIV) measurements of a generic non-isothermal swirling jet to demonstrate the use of advanced data analysis techniques. In particular, the Finite Time Lyapunov Exponent (FTLE) and local linear stability analysis (LSA) are shown to reveal deep insight into the physical mechanisms that drive the PVC. Particularly, it is demonstrated that the PVC amplitude is strongly reduced, if heating is applied at the wavemaker of the flow. These techniques are complemented by the traditionally used Proper Orthogonal Decomposition (POD) and spatial correlation techniques. It is demonstrated how these methods complement each other and lead to a comprehensive understanding of the PVC that lays out the path to efficient control strategies.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A034. doi:10.1115/GT2016-56591.

We present an application of a newly introduced method to analyze the time-resolved experimental data from the flow field of a swirl-stabilized combustor. This method is based on classic proper orthogonal decomposition (POD) extended by a temporal constraint. The filter operation embedded in this method allows for continuous fading from the classic POD to the Fourier mode decomposition. This new method — called spectral proper orthogonal decomposition (SPOD) — allows for a clearer separation of the dominant mechanisms due to a clean spectral separation of phenomena. In this paper, the fundamentals of SPOD are shortly introduced. The actual focus is put on the application to a combustor flow. We analyze high-speed PIV measurements from flow fields in a combustor at different operation conditions. In these measurements, we consider externally actuated, as well as natural dynamics and reveal how the natural and actuated modes interact with each other. As shown in the paper, SPOD provides detailed insight into coherent structures in swirl flames. Two distinct PVC structures are found that are very differently affected by acoustic actuation. The coherent structures are related to heat release fluctuations, which are derived from simultaneously acquired OH* chemiluminescence measurements. Besides the actuated modes, a low frequency mode was found that significantly contribute to the global heat release fluctuations.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A035. doi:10.1115/GT2016-56622.

An experimental investigation was conducted to study the effect of chevrons on the dynamic behavior of the swirling flow generated by a counter-rotating radial-radial swirler. 3X models of a low swirl number swirler (SN ≈ 0.6) were used to achieve lower velocities for the same Reynolds number (Re) and enhanced visibility of the flow characteristics by enabling high spatial and temporal resolutions. Three swirler configurations were used, including the baseline with no chevrons. Configuration 2 features chevrons on the trailing edge of the primary swirler, and configuration 3 has chevrons on the trailing edge of both primary and secondary swirlers. The swirlers were tested in water flow at Reynolds number (Re) = 51,500 which corresponds to the typical operational pressure drop of 4% of atmospheric pressure for the corresponding 1X model of the swirler at ambient conditions. Water testing was used since it allows additional slowing down of the flow dynamic features so that they can be captured and analyzed. Measurements were conducted in a vertical plane passing through the swirler centerline, and two horizontal (cross-sectional) planes using a High-Speed, Two Dimensional, Particle Image Velocimetry (2D PIV) system to obtain the mean, turbulent and dynamic behavior of the flow. Results of this study introduce the concept of chevrons on swirlers as a promising approach to change the flow dynamic behavior and thus, affect combustion dynamics. The results show that the presence of chevrons break down the region of high modal energy into several smaller regions. However, configuration 2 has few regions of the highest modal energy among the configurations, whereas the modal energy values for configurations 3 has the lowest magnitudes. Thus, the secondary chevrons in configuration 3 play an important role to eliminate these high-energy local spots as well as meet the requirement to break down the large scale structures.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2016;():V04AT04A036. doi:10.1115/GT2016-56629.

A liquid fuel combustor based on the FLOX® burner concept has been developed for application in a Micro Gas Turbine (MGT) Range Extender (REX) for next generation cars. The characterization of this combustor was performed at the High Pressure Optical Test rig (HIPOT) at DLR Stuttgart. The operability limits of the burner were mapped out for full load conditions at 3.5 bars by varying global lambda (λG) from 1.25–2.00 and bulk jet velocity (vBulk) from 80–140 m/s. Exhaust gas measurements show NOx and CO levels below 5 and 10 ppm respectively (corrected for reference 15% O2) at λG = 1.89.

Optical and laser diagnostic measurement techniques have been employed to characterize the spray flames. The flames at stable burner operation points (BOPs) show a predominantly jet like flame shape irrespective of λG and vBulk. Droplets in the size range 2–40 μm have been measured close to the nozzle exit plane. Velocities conditioned on the droplet size show large droplets d > 15 μm transitioning from negative slip velocity at the exit plane to positive slip velocity at downstream location. The positive slip velocities and slow evaporation of large droplets lead to droplets travelling further into the combustion chamber and hence resulting in long flames. A comprehensive data set for the spray characteristic of the new liquid FLOX® burner is made available.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A037. doi:10.1115/GT2016-56658.

Existing kinetic mechanisms for natural gas combustion are not validated under supercritical oxy-fuel conditions because of the lack of experimental validation data. Our studies show that different mechanisms have different predictions under supercritical oxy-fuel conditions. Therefore, preliminary designers may experience difficulties when selecting a mechanism for a numerical model. This paper evaluates the performance of existing chemical kinetic mechanisms and produces a reduced mechanism for preliminary designers based on the results of the evaluation.

Specifically, the mechanisms considered were GRI-Mech 3.0, USC-II, San Diego 204-10-04, NUIG-I, and NUIG-III. The set of mechanisms was modeled in Cantera and compared against the literature data closest to the application range. The high pressure data set included autoignition delay time in nitrogen and argon diluents up to 85 atm and laminar flame speed in helium diluent up to 60 atm. The high carbon dioxide data set included laminar flame speed with 70% carbon dioxide diluent and the carbon monoxide species profile in an isothermal reactor with up to 95% carbon dioxide diluent.

All mechanisms performed adequately against at least one dataset. Among the evaluated mechanisms, USC-II has the best overall performance and is preferred over the other mechanisms for use in the preliminary design of supercritical oxy-combustors. This is a significant distinction; USC-II predicts slower kinetics than GRI-Mech 3.0 and San Diego 2014 at the combustor conditions expected in a recompression cycle. Finally, the global pathway selection method was used to reduce the USC-II model from 111 species, 784 reactions to a 27 species, 150 reactions mechanism. Performance of the reduced mechanism was verified against USC-II over the range relevant for high inlet temperature supercritical oxy-combustion.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A038. doi:10.1115/GT2016-56671.

Thermoacoustic instabilities have the potential to restrict the operability window of annular combustion systems, primarily as a result of azimuthal modes. Azimuthal acoustic modes are composed of counter-rotating wave pairs, which form traveling modes, standing modes, or combinations thereof. In this work, a monitoring strategy is proposed for annular combustors that accounts for azimuthal mode shapes. Output-only modal identification has been adapted to retrieve azimuthal eigenmodes from surrogate data, resembling acoustic measurements on an industrial gas turbine. Online monitoring of decay rate estimates can serve as a thermoacoustic stability margin, while the recovered mode shapes contain information that can be useful for control strategies. A low-order thermoacoustic model is described, requiring multiple sensors around the circumference of the combustor annulus to assess the dynamics. This model leads to a second order state space representation with stochastic forcing, which is used as the model structure for the identification process. Four different identification approaches are evaluated under different assumptions, concerning noise characteristics and preprocessing of the signals. Additionally, recursive algorithms for online parameter identification are tested.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A039. doi:10.1115/GT2016-56676.

Two pyrometric tools for measuring soot temperature response in fuel-rich flames under unsteady inlet airflow conditions are developed. High-speed pyrometry using a high-speed color camera is used in producing soot temperature distributions, with its results compared with those of global soot temperature response measured using a multi-wavelength pyrometer. For the former, the pixel RGB values pertaining to respective bandwidths of red, green and blue filters are used to calculate temperature and for the latter, the emission from whole flame at 660 nm, 730 nm and 800 nm is used to measure temperature. The combustor, running on Jet-A fuel, achieves unsteady inlet airflow using a siren running at frequencies of 150 and 250 Hz and with modulation levels (RMS) 20–50% of mean velocity. Spatiotemporal response of flame temperature measured by the high speed camera is presented by phase-averaged with average subtracted images and by fast Fourier transform at the modulation frequencies of inlet velocity. Simultaneous measurement of combustor inlet air velocity and flame soot temperature using the multi-wavelength pyrometer is used in calculating the flame transfer function of flame temperature response to unsteady inlet airflow. The results of global temperature and temperature fluctuation from the 3-color pyrometer show qualitative agreement with the local temperature response measured by the high speed camera. Over the range of operating conditions employed, the overall flame temperature fluctuation increases linearly with respect to the inlet velocity fluctuation. The two-dimensional map of flame temperature under unsteady combustion determined using a high-speed digital color camera shows that the local temperature fluctuation during unsteady combustion occurs over relatively small region of flame and its level is greater (∼10–20%) than that of overall temperature fluctuation (∼1%).

Commentary by Dr. Valentin Fuster
2016;():V04AT04A040. doi:10.1115/GT2016-56696.

Flexibility is key to the future success of natural gas fired power generation. As renewable energy becomes more widely used, the need for reliable, flexible generation will increase. As such, gas turbines capable of operating efficiently and in emissions compliance from extended low load to full load will have a significant advantage. A wider range of gas fuels, including shale gas and refinery/industrial byproduct gas, is becoming increasingly available, with the opportunity to further reduce the cost of electricity. A combustion system capable of operating with wider ranges of heavy hydrocarbons, hydrogen and inerts will have an advantage to accommodate the future fuel gas trends and provide value to gas turbine operators. The FlameSheet™ combustor incorporates a novel dual zone burn system to address operational and fuel flexibility. It provides low emissions, extended turndown and fuel flexibility. FlameSheetTM is simply retrofittable into existing installed E/F-class heavy duty gas turbines and is designed to meet the energy market drivers set forth above. The operating principle of the new combustor is described, and details of a full scale high pressure rig test and engine validation program are discussed, providing insight on rig and engine emissions, as well as combustion dynamics performance. The FlameSheetTM implementation and validation results on a General Electric 7FA heavy duty gas turbine operating in a combined cycle power plant is discussed with emphasis on operational profile optimization to accommodate the heat recovery steam generator (HRSG), while substantially increasing the gas turbine normal operating load range.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A041. doi:10.1115/GT2016-56713.

Shock focusing ignition techniques can avoid deflagration-to-detonation transition (DDT), which make pulse detonation engine (PDE) more efficient. Numerical simulations of an idealized pulse detonation engine consisting of axial inlet and circumferential inlet are presented in this paper. Using detailed hydrogen-air mixture chemical kinetic model, investigation on detonation direct initiation by shock focusing is done. Studies indicate that in initial static flow field, the regions of high temperature and pressure created by shock focusing can produce detonation at the condition of circumferential inlet Mach 2.4. The temperature and pressure of the focusing region is nearly 3000K and 6.3MPa. But in dynamic flow field, the high temperature and pressure created by shock wave focusing for an incident Mach number of 2.4 decrease to 1027K and 4.5MPa which cannot produce detonation. When the incident Mach number increases to 3.5, the transient temperature and pressure of the focusing region is nearly 3000K and 30MPa, which capable of initiating a detonation wave.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A042. doi:10.1115/GT2016-56714.

This paper presents experimental study on self-excited combustion instability characteristics of premixed low-swirl flames in a multi-nozzle can combustor with counter-swirl and co-swirl arrays. Experiments were carried out over a wide range of inlet velocity from 4 m/s to 15.5 m/s and equivalence ratio from 0.5 to 0.85. Phase-locked OH planar laser-induced fluorescence was employed to measure flame shape and identify heat release rate. Four operation regions: stable combustion region, unstable combustion region, flashback region and extinguish region are observed for both array burners. The stable operating window for counter-swirl array is wider compared to the co-swirl array. Pressure fluctuation amplitude for co-swirl burner is larger than the counter-swirl arrangement at the same operating condition. In the unstable combustion region, the counter-swirl flames trigger the 2L mode of the combustion system while co-swirl flames incite three longitudinal modes with the highest amplitude near 3L. Rayleigh index distribution reveals neighboring flame interaction results in thermoacoustic coupling for multi-nozzle flames. Additionally, for counter-swirl array, thermoacoustic couplings in flame base region and shear region are also the main reasons for inducing self-excited combustion instabilities. For co-swirl array, the instability driving zones also locate at the lip region and the tail of center flame.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A043. doi:10.1115/GT2016-56722.

In the present investigation, a novel fuel injection concept is developed for Dry Low NOx combustors. A multi-injector block is used for fuel-air mixing and flame stabilization. The injectors in the multi-injector block are equally spaced in a rectangular grid of 3×3. Each injector of the multi-injector block has a porous concentric tube through which fuel is injected into the annular space around the porous tube. The porous tube is made up of stainless steel with 30 μm porosity. Air enters the annular space around the porous tube through eight tubes that surround the porous tube. The fuel and air mix in the annular space between the injector wall and the porous tube.

A CO2 mixing technique is adopted to investigate the fuel-air mixing quality under non-reacting atmospheric conditions. CO2 is used to simulate the fuel. The CO2 concentrations are converted to fuel mass fractions for comparison. The experiments are carried out at pressure drop of 4% across the injector. The fuel mass fraction contours show an annular ring of low fuel concentration downstream of the individual injectors with a high concentration in the Central Toroidal Recirculation zone (CTRZ). Furthermore, PIV measurements are conducted to study the injector aerodynamics under the same operating conditions. The PIV measurements also show CTRZ downstream of the individual injector. The jet emanating from the injectors expands gradually downstream. An outer weak recirculation zone (ORZ) is also observed between the jets. Preliminary combustion experiments are carried out to observe the flame shape and heat release distribution. The flame shape follows the velocity contours. The combustion studies show that the flame length is longer. The long flame length is due to the high velocities at the exit of the injector. A probability density function (pdf) is generated for the fuel mixture fraction. The pdf is based on the mixture fraction and velocity data obtained at the exit of the injector block. Chemkin analysis is carried out to estimate the emissions based on the experimentally obtained pdfs.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A044. doi:10.1115/GT2016-56762.

The reduction of full and part load emissions and the increase of the turndown ratio are important goals for gas turbine combustor development. Combustion techniques, which generate lower NOx emissions than unstaged premixed combustion in the full load range, and which have the potential of reducing minimum load while complying with emission legislation, are of high technical interest. Therefore axial staged combustion systems have been designed, either with or without expansion in a turbine stage between both stages. In its simpler form without intermediate expansion stage a flow of hot combustion products is generated in the first stage of the premixed combustor, which interacts with the jets of premixed gas injected into the second stage.

The level of NOx formation during combustion of the premixed jets in the hot cross flow determines the advantage of axially staged combustion regarding full load NOx emission reduction. Employing Large Eddy Simulation in OpenFOAM, a tool has been developed, which allows to investigate staged combustion systems including not only temperature distribution but also NOx emissions under engine conditions. To be able to compute NOx formation correctly the combustion process has to be captured with sufficient level of accuracy. This is achieved by the partially stirred reactor model. It is combined with a newly developed NOx model, which is a combination of a tabulation technique for the NOx source term based on mixture fraction and progress variable and a partial equilibrium approach. The NOx model is successfully validated with generic burner stabilized flame data and with measurements from a large scale reacting jet in cross flow experiment. The new NOx model is finally used to compute a reacting jet in cross flow under engine conditions to investigate the NOx formation of staged combustion in detail. The comparison between the atmospheric and the pressurized configuration gives valuable insight in the NOx formation process. It can be shown that the NOx formation within a reacting jet in cross flow configuration is reduced and not only diluted.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A045. doi:10.1115/GT2016-56764.

In order to balance the low emission and wide stabilization for lean premixed prevaporized combustion, the centrally staged layout is preferred in advanced aero-engine combustors. However, compared with the conventional combustor, it is more difficult for the centrally staged combustor to light up as the main stage air layer will prevent the pilot fuel droplets arriving at igniter tip. The goal of the present paper is to study the effect of the main stage air on the ignition of the centrally staged combustor. Two cases of the main swirler vane angle of the TeLESS-II combustor, 20° and 30° are researched. The ignition results at room inlet temperature and pressure show that the ignition performance of the 30° vane angle case is better than that of the 20° vane angle case. High speed camera, PLIF and CFD are used to better understand the ignition results. The high-speed camera has recorded the ignition process, indicated that an initial kernel forms just adjacent the liner wall after the igniter is turned on, the kernel propagates along the radial direction to the combustor center and begins to grow into a big flame, and then it spreads to the exit of the pilot stage, and eventually stabilizes the flame. CFD of the cold flow field coupled with spray field is conducted. A verification of the CFD method has been applied with PLIF measurement, and the simulation results can qualitatively represent the experimental data in terms of fuel distribution. The CFD results show that the radial dimensions of the primary recirculation zone of the two cases are very similar, and the dominant cause of the different ignition results is the vapor distribution of the fuel. The concentration of kerosene vapor of the 30° vane angle case is much larger than that of the 20° vane angle case close to the igniter tip and along the propagation route of the kernel, therefore, the 30° vane angle case has a better ignition performance. For the consideration of the ignition performance, a larger main swirler vane angle of 30° is suggested for the better fuel distribution when designing a centrally staged combustor.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A046. doi:10.1115/GT2016-56791.

Volatile oil price and environmental impact of conventional jet fuel are key motivators towards the proposing of alternative jet fuels. This article introduces and establishes a relationship between jet fuel properties/composition and smoke emission. It is an important and comprehensive task as it underlines the base references and scientific reasoning on fuel compositions / properties; very few, if any, studies have investigated the effects of each of the properties/ compositions on smoke emissions. Two sets of fuels were tested on small re-commissioned Honeywell GTCP85 APU gas turbine Engine. The first set was consisted of 8 novel fuels, while the second was a blend of varied percentages of Jet A-1 and other alternative fuel. This is to provide a wide range of properties and compositions. The results were compared to those of Jet A-1on the same platform (Honeywell GTCP85 APU). It was observed that not all fuel compositions/properties have the same effects on the smoke number. Some of them such as: Specific Energy, Kinematic, viscosity, Biphenyls, monocycloparaffin, AlkylBenzene, Fluorenes, Distillation temp (90%), Carbon (%mass), Naphthalene, Composite Density, Benzocycloparaffin, Density at 15C°, Aromatics (%Vol) and Net heat of Combustion have a clear direct effect on the smoke number, while others such as iso-paraffin and flashpoint have a reduced impact on smoke number. This data shall be used to predict the effect of certain composition/ property on the smoke emission, thus it could be avoided or to be taking into considerations when producing or using new alternative fuels.

Topics: Fuels , Smoke , Emissions
Commentary by Dr. Valentin Fuster
2016;():V04AT04A047. doi:10.1115/GT2016-56826.

The emissions characteristics of a model gas turbine combustor characterized by the enhancement of the reactions of the secondary mixtures of lean to ultra-lean compositions by the burned gas from the primary stage were investigated at atmospheric pressure. A cylindrical quartz tube with the bottom end closed was used as the combustion chamber and the mixture injector was extending co-axially into the combustion chamber to the bottom wall. The stagnation point reverse flow combustion was used for the primary stage where the primary mixture was injected toward the end wall. The secondary mixture was injected radially from the multiple holes in the outer wall of the injector into the burned gas from the primary stage. The effects of the positions for injection of the primary and secondary mixtures as well as mixture temperatures on NOx, CO and HC emissions were investigated over a range of the primary and secondary mixture equivalence ratios at atmospheric pressure.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A048. doi:10.1115/GT2016-56953.

Stabilization of nearly stoichiometric CO2- and N2-diluted premixed methane/oxygen swirling flames is investigated in an atmospheric test rig equipped with an axial-plus-tangential swirler exhausting in a cylindrical injection tube eventually ended by a diverging quarl. The investigated flames are stabilized aerodynamically away from the solid elements of the combustor without the help of any central bluff body in the injector. The flowrates through the axial and tangential slits of the swirler can be adjusted separately. Effects of swirl and quarl angle on the flowfield and flame shape are analyzed. Laser tomography on small oil particles reveals that fuel, oxidizer and diluents injected in separated channels are well mixed at the injector outlet. Velocimetry measurements and flame images show that the axial-plus-tangential swirler allows a flexible control of the flame leading edge position with respect to the injector outlet. For a fixed injector geometry with a given quarl angle and swirl number, it is found that N2- and CO2-diluted flames feature the same topology if the injected combustible mixtures feature the same adiabatic flame temperature, while they may feature different bulk injection velocities and laminar burning velocities. The operability range of well stabilized CO2-diluted flames is however reduced.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A049. doi:10.1115/GT2016-57026.

It is understood that so-called “entropy waves” can contribute to combustion noise and play a role in thermoacoustic instabilities in combustion chambers. The prevalent description of entropy waves generation regards the flame front as a source of heat at rest. Such a model leads — in its simplest form — to an entropy source term that depends exclusively on the unsteady response of the heat release rate and upstream velocity perturbations. However, in the case of a perfectly premixed flame, which has a constant and homogeneous fuel / air ratio and thus constant temperature of combustion products, generation of entropy waves (i.e. temperature inhomogeneities) across the flame is not expected. The present study analyzes and resolves this inconsistency, and proposes a modified version of the quasi 1-D jump relations, which regards the flame as a moving discontinuity, instead of a source at rest. It is shown that by giving up the hypothesis of a flame at rest, the entropy source term is related upto leading order in Mach number to changes in equivalence ratio only.

To supplement the analytical results, numerical simulations of a Bunsen-type 2D premixed flame are analysed, with a focus on the correlations between surface area, heat release and position of the flame on the one hand, and entropy fluctuations downstream of the flame on the other. Both perfectly premixed as well as flames with fluctuating equivalence ratio are considered.

Topics: Entropy , Waves , Flames
Commentary by Dr. Valentin Fuster
2016;():V04AT04A050. doi:10.1115/GT2016-57043.

Based on the flow reactor with rectangle cross-section, this paper studies the spray autoignition characteristics of liquid kerosene injected into air crossflow under high temperature and high pressure conditions. Millisecond-level kerosene injection, millisecond-level photoelectric detection, and high speed photography record experiment techniques are adopted in this research. The operating conditions of this research are as follows: 2.3MPa inlet pressure, 917K inlet temperature, fuel/ air momentum ratio of 52, and Weber number of 355. Photoelectric sensor and photomultiplier equipped with CH filter are used to get the autoignition delay time (ADT). A total of 320 experiments are conducted under the same operating conditions in order to obtain the random ADT probability distribution. The high speed photography is utilized to observe and record the developing process of spray autoignition of kerosene. The results show that the ADT varies from 2.5–5.5millisecond (ms) in the above operating conditions, and confirm the existence of the random behavior of kerosene spray autoignition in the crossflow. These random behaviors of ADT can be correlated well with Gauss distribution. The primary analysis shows that the random behavior stems from the random distributions in the diameter and dispersion due to intrinsic turbulence breakup and transportation which dominate the characteristics of spray autoignition.

Topics: Sprays
Commentary by Dr. Valentin Fuster
2016;():V04AT04A051. doi:10.1115/GT2016-57046.

Combustion instability is one of the major challenges in Lean Premixed Pre-vaporized (LPP) combustion technology. The effect of inlet conditions on combustion instability has been widely studied. But the diffuser, located between the compressor outlet and combustor inlet, has received little attention regarding its effect on combustion instability. This paper experimentally investigated the combustion instability in a single sector LPP combustor with and without a diffuser. Pressure data has been recorded and used to indicate the oscillation of the system. The results show that with the diffuser the combustor showed no prominent oscillation, and strong thermoacoustic oscillation at around 400 Hz was observed without the diffuser. Further analysis has been conducted by using acoustic net model. The resonant mode frequency of the entire combustor system and each section separately were calculated. It was found that in the case of the diffuser, the resonant frequency shifted from 400 Hz to 453 Hz, which avoiding interacting with heat release within the flame tube, thus help to stabilize the combustor system.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A052. doi:10.1115/GT2016-57061.

Unsteady heat release of stratified swirling flames in a partially premixed pre-vaporized combustor at elevated pressure and temperature is experimentally investigated. Stratified flames produced by a pilot swirler and a main one which are fueled by different flow rates of aviation kerosene. Three fuel staging ratios (SR) of 25%, 30%, and 35% are tested to study its effect on the heat release of stratified flame. CH* chemiluminescence emissions are recorded by a high speed ICCD camera with a 430±10 nm band-pass filter. The Proper Orthogonal Decomposition (POD) analysis is employed to identify the most prominent energetic modes of flame fluctuations. Test results show that the low frequency fluctuation of the flame global heat release is weakened as the SR decreases while the high frequency fluctuation is enhanced. According to the analysis of average heat release of the flame, SR affects the structure of flame by changing the flame angle. The stratified flame dynamics mainly performs the bulk motion, asymmetric motion, and flame shedding. It is found that the flame bulk motion modes contain higher energy than those of other modes. While the other two lower energy modes would transit from asymmetric motion to flame shedding mode as the SR decreases. From the Fast Fourier Transform (FFT) analysis, the low frequency fluctuation of global heat release is dominated by modes of higher energy, while the high frequency fluctuation is related to all the modes.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A053. doi:10.1115/GT2016-57089.

Novel advances in gas turbine combustor technology, led by endeavors into fuel efficiency and demanding environmental regulations, have been fraught with performance and safety concerns. While the majority of low emissions gas turbine engine combustor technology has been necessary for power-generation applications, the push for ultra-low NOx combustion in aircraft jet engines has been ever present. Recent state-of-the-art combustor designs notably tackle historic emissions challenges by operating at fuel-lean conditions, which are characterized by an increase in the amount of air flow sent to the primary combustion zone. While beneficial in reducing NOx emissions, the fuel-lean mechanisms that characterize these combustor designs rely heavily upon high-energy and high-velocity air flows to sufficiently mix and atomize fuel droplets, ultimately leading to flame stability concerns during low-power operation. When operating at high-altitude conditions, these issues are further exacerbated by the presence of low ambient air pressures and temperatures, which can lead to engine flame-out situations and hamper engine relight attempts.

To aid academic and commercial research ventures into improving the high-altitude lean blow-out (LBO) and relight performance of modern aero turbine combustor technologies, the High-Altitude Relight Test Facility (HARTF) was designed and constructed at the University of Cincinnati Combustion & Fire Research Laboratory (CFRL). This paper presents an overview of its design and an experimental evaluation of its abilities to facilitate optically-accessible combustion and spray testing for aero engine combustor hardware at simulated high-altitude conditions. Extensive testing of its vacuum and cryogenic air-chilling capabilities was performed with regard to end-user control — the creation and the maintenance of a realistic high-altitude simulation — providing a performance limit reference when utilizing the modularity of the facility to implement different aero turbine combustor hardware. Ignition testing was conducted at challenging high-altitude windmilling conditions with a linearly-arranged five fuel-air swirler array to replicate the implementation of a multi-cup gas turbine combustor sector and to evaluate suitable diagnostic tools for the facility. High-speed imaging, for example, was executed during the ignition process to observe flame kernel generation and propagation throughout the primary, or near-field, combustion zones.

In the evaluation performed, the HARTF was found to successfully simulate the atmospheric environments of altitudes ranging from sea level to beyond 10,700 m for the employed combustor sector. Diagnostic methods found compatible with the facility include high-speed flame imaging, combustion emission analysis, laser light sheet spray visualization, phase Doppler particle analysis (PDPA), and high-speed particle image velocimetry (HSPIV). Herein discussed are correlations drawn — linking altitude simulation capability to the size of the implemented combustor hardware — and challenges found — vacuum sealing, low pressure fuel injection, fuel vapor autoignition, and frost formation.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A054. doi:10.1115/GT2016-57111.

The effect of ozone (O3) in a turbulent, swirl-stabilized natural gas/air flame was experimentally investigated at atmospheric pressure conditions using planar laser-induced fluorescence imaging of formaldehyde (CH2O PLIF) and dynamic pressure monitoring. The experiment was performed using a dry low emission (DLE) gas turbine burner used in both SGT-700 and SGT-800 industrial gas turbines from Siemens. The burner was mounted in an atmospheric combustion test rig at Siemens with optical access in the flame region. CH2O PLIF imaging was carried out for four different seeding gas compositions and seeding injection channel configurations. Two seeding injection-channels were located around the burner tip while the other two were located along the center axis of the burner at different distances upstream the burner outlet. Four different seeding gas compositions were used: nitrogen (N2), oxygen (O2) and two ozone/oxygen (O3/O2) mixtures with different O3 concentration. The results show that the O3 clearly affects the combustion chemistry. The natural gas/air mixture is preheated before combustion which is shown to kick-start the cold combustion chemistry where O3 is highly involved. The CH2O PLIF signal increases with O3 seeded into the flame which indicates that the pre-combustion activity increases and that the cold chemistry starts to develop further upstream. The small increase of the pressure drop over the burner shows that the flame moves upstream when O3 is seeded into the flame, which confirms the increase in pre-combustion activity.

Topics: Combustion
Commentary by Dr. Valentin Fuster
2016;():V04AT04A055. doi:10.1115/GT2016-57129.

Thermoacoustic phenomena observed in gas turbine combustion chambers are usually related to a dependence of the heat release fluctuations to the velocity fluctuations at the injection point. Linear models for this dependence give information about stability, while nonlinear models can predict also limit cycles amplitude and frequency. This nonlinear dependence can be obtained by means of experiments or numerical tools by measuring the flame describing function (FDF). Analytical expressions have been proposed in the years to obtain nonlinear flame models to be introduced into simplified tools. Through these nonlinearities, information about limit cycle amplitudes, bifurcation diagrams and hysteresis can be achieved. The importance of this information is related to the possibility of managing the variation of certain operating parameters to keep the combustor in a safe operating zone. Additionally, from the bistable zone information about the operational margin can be obtained in order to stay far from the unstable condition.

The aim of this work is to describe a simplified procedure to track combustors bifurcation diagrams using nonlinear flame models implemented in a 3D thermoacoustic tool based on the finite element methods (FEM) in the frequency domain. First flame describing functions (FDF) for not swirled and swirled burners from the literature are considered and applied to simplified configurations. Then, FDFs are introduced into an industrial configuration and the bifurcation diagrams are tracked. For both cases, the paper describes the influence of various FDFs parameters and shapes on the predictions of combustors stability boundaries.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A056. doi:10.1115/GT2016-57142.

A semi-closed oxy-fuel retrofit gas turbine demonstration plant (DEMOXYT) is described and preliminary results of the oxy-fuel burner under relevant conditions are presented. The plant consists of a 100 kWel radial turbine with a pressure ratio of 4.25, featuring a recuperator, to which a specific oxy-fuel burner has been designed and constructed, and a condensing scrubber in the flue gas recirculation loop. An overview and salient thermodynamic data of the plant are provided, as well as important parameters governing its limits of operation. Results from combustion experiments obtained in a pressurized oxyfuel combustor are presented in terms of a burner operation map for 4.2 bar. The combustion testing results describe the stability maps of the burner and the influence of excess oxygen and total oxygen concentration in the reactants on the formation of CO. It was found that excess oxygen values below about 5% could result in excessive CO concentrations in the undiluted product gases of the flame, but that CO formation was relatively independent of the oxygen concentration in the reactants.

Commentary by Dr. Valentin Fuster
2016;():V04AT04A057. doi:10.1115/GT2016-57143.

Incoming standards on NOx emissions are motivating many aero-engines manufacturers to adopt the lean burn combustion concept. However, several technological issues have to be faced in this transition, among which limited availability of air for cooling purpose and thermoacoustics phenomena that should be managed to safely implement this burning mode. In this scenario, standard numerical design tools are not often capable of characterizing such devices. Thus, considering also the difficulties of experimental investigations in a highly pressurized and reactive environment, unsteady scale resolved CFD methods are required to correctly understand the combustor performances. In the last years Large Eddy (LES) and hybrid RANS-LES models such as Scale Adaptive Simulations (SAS) have undergone considerable developments. Such approaches have been already applied for gaseous flames, leading to a strong enhancement in phenomena prediction with respect to standard steady-state simulations. However, huge research efforts are still required when spray flames are considered, since all the numerical models chosen to describe spray dynamics and the related reactive processes can have a strong impact on the accuracy of the whole simulation. In this work a set of scale resolved simulations have been carried out on the DLR Generic Single Sector Combustor spray flame for which measurements both in non-reactive and reactive test conditions are available. Exploiting a two-phase Eulerian-Lagrangian approach combined with a Flamelet Generated Manifold (FGM) combustion model, LES simulations have been performed in order to assess the potential improvements with respect to steady state solutions. Additional comparisons have also been accomplished with SAS calculations based on Eddy Dissipation combustion model (EDM). The comparison with experimental results shows that the chosen unsteady strategies lead to a more physical description of reactive processes with respect to RANS simulations. FGM model showed some limitations in reproducing the partially-premixed nature of the flame, whereas SAS-EDM proved to be a robust modelling strategy within an industrial perspective. A new set of spray boundary conditions for liquid injection is also proposed whose realiability is proved through a detailed comparison against experimental data.

Commentary by Dr. Valentin Fuster

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