ASME Conference Presenter Attendance Policy and Archival Proceedings

2018;():V04AT00A001. doi:10.1115/GT2018-NS4A.

This online compilation of papers from ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition (GT2018) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

2018;():V04AT04A001. doi:10.1115/GT2018-75028.

Gas turbine combustion engineers strive to obtain high flame stability at low power conditions (idle); whereas good emissions characteristics are desired at high power conditions. Although lean combustors provide good emissions characteristics, they suffer from the issue of poor flame stability. Often a pilot flame along with a lean main flame is used in order to improve flame stability issue. Positioning of an axial jet concentric to a swirl jet is one of commonly used configurations for a pilot flame. The interaction between both co-annular jets leads to distinct flow patterns in the combustor and subsequently the flame structure is impacted. If the axial jet is able to penetrate the inner recirculation zone (IRZ) generated by the swirled flow of the main jet, then a jet type of flame is obtained. The jet flame in vicinity of co-annular swirled flow is also denoted as type 1 flame, and is known to have good flame stability. On the other hand, if the pilot jet is not able to penetrate the IRZ, then the recirculating type of flame is obtained. A recirculating flame type has good emissions characteristics but suffers from poor flame stability. In this work the numerical predictions have been performed to gain more insight into occurrences of these two different flame structures which have been experimentally recorded. Using ANSYS Fluent CFD software, non-reactive steady state turbulent flow simulations have been performed to understand the flow and mixing field in a 3D combustor. Laser Doppler Anemometry (LDA) and hydroxyl radical (OH*) chemiluminescence have been used to measure the flow and characterize the flame structure, respectively.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A002. doi:10.1115/GT2018-75050.

Micro gas turbines (MGT) used as combined heat and power (CHP) systems present interesting advantages over conventional CHP systems. For low power MGTs (1–3 kWel), a single stage jet-stabilized combustor is well suited to reach low emissions despite high preheat temperatures. Although several jet stabilized combustors in this power range have already been presented in literature, the influence of single design parameters was not described yet. Therefore this paper presents a design study with different combustor geometries. Besides the number of nozzles, also the diameter of air and fuel nozzles as well as the length of the mixing zone is varied. Experiments are performed in an atmospheric combustor test rig with preheat temperatures of 730°C. Additionally RANS-CFD simulations for selected conditions are carried out. The results indicate an inverse correlation between the number of nozzles (diameter adjusted for constant inlet velocities) and the emissions, where an increase in the number of nozzles leads to lower emissions and a shorter reaction zone. Changing the air nozzle diameter and therefore the jet velocity creates a different trend. As the pressure loss decreases with smaller inlet velocities, the CO- and NOx-emission levels increase. An extension of the mixing section as well as of the diameter of the fuel nozzles have only minor effects for the examined geometries. The knowledge acquired in this work helps to evaluate the influence of design parameter changes on the behavior of small scale jet-stabilized combustors and allows an optimization of the combustor design regarding flame length and emissions.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A003. doi:10.1115/GT2018-75064.

Elevated pressure and temperature conditions are widely encountered during gas turbine operation. To avoid unexpected ignition and explosion of mixtures of fuel and air under these conditions, it is imperative to identify the flammability limits of relevant fuel mixtures. Common fuels include process gases such as natural gas, coke oven gas and IGCC syngas fuel. The flammability limits of pure fuels and common gas/air mixtures have been widely reported, however a significant lack of flammability data for fuel mixtures relevant for use in gas turbines as well as data at elevated pressure and temperature conditions is available. The objective of this study is to characterize the flammability limits of fuel/air mixtures and their dependence on initial temperature and pressure. Experimental studies of lean flammability limits (LFLs) for methane, hydrogen, and carbon monoxide, in addition to mixtures of these gases (i.e. CH4/H2, H2/CO, and CH4/CO2) were performed at temperatures up to 200 °C and pressures up to 9 bar. ASTM Standard E918 (1983) provided the framework for tests using a one-liter pressure-rated test cylinder in which the fuel-air mixtures were prepared and then ignited. Flammability is determined using a 7% and 5% pressure rise criterion per the ASTM E918 and European EN 1839 standards, respectively. The LFLs for each gas and gas mixture are found to decrease linearly with increasing temperature for the temperature range tested. The LFLs of hydrogen and mixtures containing hydrogen are observed to increase with an increase in the initial pressure, whereas the LFLs of all other mixtures exhibit a negligible dependence on pressure. For mixtures, predicted LFL values obtained using Le Chatelier’s mixing rule (LC) are fairly consistent with the experimentally determined values near ambient conditions, however it is not recommended for use at elevated pressure and/or temperature. Finally, the experimental data presented in this study are compared with previous experimental studies, flammability limits calculated using numerical methods, and past studies of predicted LFL values for similar fuel/air mixtures. The purpose for characterizing the flammability limits for these gaseous mixtures is to extend the results to developing appropriate procedures for the safe industrial use of renewable gases, such as bio-derived methane, biogas composed mainly of methane and carbon dioxide, and renewably derived syngas which contains large quantities of hydrogen and carbon monoxide gas.

Topics: Pressure , Temperature
Commentary by Dr. Valentin Fuster
2018;():V04AT04A004. doi:10.1115/GT2018-75065.

This study examines the viability of electrochemical solid-state nitric oxide sensors as an alternative to CEM (Continuous Emission Monitoring) instruments for use in monitoring the emissions of a 60 kW gas turbine. While CEM equipment is used regularly at large scale centralized power plants, it is not cost effective for small generators used for distributed power generation. Yet in areas with poor air quality, assessment of the emissions generated by such generators is an important question. A commercial and inexpensive potential alternative to the complex CEM systems used in power plants comes from the automotive industry, which employs solid-state sensors to monitor and control emissions in diesel vehicles. In the present study, two models of commercial solid-state sensors (UniNOx® and NTK) were evaluated in the exhaust stream of the Capstone C-60 gas turbine engine. A referee instrument (Horiba PG-350) using the EPA approved NO measurement method, chemiluminescence, was used in the study as a reference point. Over 8 months of testing, it was found that both solid state sensors followed the general NO trends measured by Horiba PG-350 analyzer. Both solid-state sensors responded to small incremental changes in engine load with high-resolution changes in NO ppm. It was determined that the NTK sensor had more favorable performance over test period than the UniNOx® sensor with regards to Accuracy, Lower Detectable Limit (LDL), and Rise Time and Fall Time. The UniNOx® sensor was found to have higher precision than the NTK sensor.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A005. doi:10.1115/GT2018-75070.

The present study experimentally investigates the effects of different circumferential damper configurations on the instabilities in an annular combustor. The combustor consists of multiple bluff body swirl stabilized flames. It is operated with an ethylene-air premixture at a power of 66 kW. Combinations of Helmholtz resonators are used as dampers circumferentially arranged around the combustion chamber. The tests are performed at operating conditions where the combustor is self-excited and characterized by a strong standing mode and periodic mode switching. For each test, the dynamic pressure is measured at different locations and overhead imaging of OH* of the entire combustor is conducted simultaneously at a high sampling frequency. The measurements are then used to compare the pressure fluctuations of the different cases in order to find the best positioning of the dampers. The azimuthal modes in the chamber are determined and the phase shift between OH* and pressure is analysed. Based on the Rayleigh criterion, these investigations allow us to find out if the dampers only remove energy from the pressure oscillations, or if they also influence the instability margins of the combustor and the flame dynamics. Finally, the results are compared with the theoretical findings in literature and observed discrepancies are discussed.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A006. doi:10.1115/GT2018-75131.

In a gas turbine, the combustor is feeding the turbine with hot gases at a high level of turbulence which in turns strongly enhances the heat transfer in the turbine. It is thus of primary importance to properly characterize the turbulence properties found at the exit of a combustor to design the turbine at its real thermal constraint. This being said, real engine measurements of turbulence are extremely rare if not inexistent because of the harsh environment and difficulty to implement experimental techniques that usually operate at isothermal conditions (e.g. hot wire anemometry). As a counterpart, high fidelity unsteady numerical simulations using Large Eddy Simulations (LES) are now mature enough to simulate combustion processes and turbulence within gas turbine combustors. It is thus proposed here to assess the LES methodology to qualify turbulence within a real helicopter engine combustor operating at take-off conditions. In LES, the development of turbulence is primarily driven by the level of real viscosity in the calculation, which is the sum of three contributions: laminar (temperature linked), turbulent (generated by the sub-grid scale model) and artificial (numerics dependent). In this study, the impact of the two main sources of un-desired viscosity is investigated: the mesh refinement and numerical scheme. To do so, three grids containing 11, 33 and 220 million cells for a periodic sector of the combustor are tested as well as centred second (Lax-Wendroff) and third order (TTGC) in space schemes. The turbulence properties (intensity and integral scales) are evaluated based on highly sampled instantaneous solutions and compared between the available simulations.

Results show first that the duration of the simulation is important to properly capture the level of turbulence. If short simulations (a few combustor through-times) may be sufficient to evaluate the turbulence intensity, a bias up to 14% is introduced for the turbulence length scales. In terms of calculation set-up, the mesh refinement is found to have a limited influence on the turbulence properties. The numerical scheme influence on the quantities studied here is small, highlighting that the employed schemes dissipation properties are already sufficient for turbulence characterization. Finally, spatially averaged values of turbulence intensity and lengthscale at the combustor exit are almost identically predicted in all cases. However, significant variations from hub to tip are reported, which questions the pertinence to use 0-D turbulence boundary conditions for turbines. Based on the set of simulations discussed in the paper, guidelines can be derived to adequately set-up (mesh, scheme) and run (duration, acquisition frequency) a LES when turbulence evaluation is concerned. As no experimental counterpart to this study is available, the conclusions mainly aim at knowing the possible numerical bias rather than commenting on the predictivity of the approach.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A007. doi:10.1115/GT2018-75146.

This article presents numerical prediction of a thermoacoustic limit cycle in an industrial gas turbine combustor. The case corresponds to an experimental high pressure test rig equipped with the full-scale Siemens SGT-100 combustor operated at two mean pressure levels of 3 bar and 6 bar. The Flame Transfer Function (FTF) characterising the global unsteady response of the flame to velocity perturbations is obtained for both operating pressures by means of incompressible Large Eddy Simulations (LES). A linear stability analysis is then performed by coupling the FTFs with a wave-based low order thermoacoustic network solver. All the thermoacoustic modes predicted at 3 bar pressure are stable; whereas one of the modes at 6 bar is found to be unstable at a frequency of 231 Hz, which agrees with the experiments. A weakly nonlinear stability analysis is carried out by combining the Flame Describing Function (FDF) predicted by LES with the low order thermoacoustic network solver. The frequency, mode shape and velocity amplitude corresponding to the predicted limit cycle at 209 Hz are used to compute the absolute pressure fluctuation amplitude in the combustor. The numerically reconstructed amplitude is found to be reasonably close to the measured dynamics.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A008. doi:10.1115/GT2018-75165.

In the near future, combustion engineers will shape the burner according to the flame, and not the opposite way anymore. In this contribution, this idea is explored with the help of additive manufacturing (AM). The focus is put on the design and the production of swirlers using advanced materials with the least possible efforts in terms of manufacturing. The material chosen for this study is Inconel 718. There are three motivations to this project. The first one is to design new shapes and assess these in comparison to conventional ones. The second motivation is to be able to manufacture them using additive manufacturing, and to gather know-how on selective laser melting. The third motivation is to elaborate a methodology involving engineering, research and education to promote — only if and when this is desirable — the production of series of premium parts such as high-end components of gas turbine combustor using AM. First-of-a-kind swirler shapes are explained and designed. These are unlikely to be produced using conventional manufacturing. They are then successfully produced in Inconel 718 using AM. The raw parts are directly submitted for testing with no surface post-processing. The paper states why at current state-of-the-art the raw surface quality still needs improvement, and highlights the benefits of the new swirler shape versus conventional.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A009. doi:10.1115/GT2018-75195.

This paper presents a numerical framework for characterizing fuel injection in modern combustors. The approach utilizes scaling analysis to describe the droplet evaporation in non-dimensional and fluid-independent terms. The results of the model are validated against published experimental data of isolated droplets evaporating at subcritical and near-critical conditions. The model is incorporated in a spray calculation framework and extended to the supercritical regime to assess the impact of different fluid-properties and evaporation models on temperature and fuel vapor distributions.

The results suggest that in a non-convective environment the transient and quasi-steady evaporation rates vary exponentially with Lewis number. Furthermore, the results show fluid-independent behavior of the droplet evaporation, indicating that a single-component fluid can potentially be used as a modeling surrogate for jet fuel. The first-principles analysis demonstrates that classical evaporation models overestimate transient evaporation and underestimate quasi-steady evaporation, with discrepancies up to 70% at supercritical conditions. This is due to limitations in fuel-property description and the lack of non-isothermal droplet characterization at near-critical conditions. The temperature profiles are typically under-predicted and fuel vapor concentrations are over-predicted in standard spray calculations with subcritical evaporation models. As such the proposed framework breaks new ground in modeling of supercritical fuel injection. The improved quality in the predicted fuel concentration and temperature distribution can enable more accurate assessment of flame position, improving the estimation of combustion stability margins and NOx emissions. The model can be incorporated in commercial codes to guide the design of combustors operating at supercritical conditions.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A010. doi:10.1115/GT2018-75201.

The isothermal flow fields of injectors have undergone several computational and experimental investigations using point and planar measurement techniques,. Within the swirl induced vortex breakdown region it is only LES that has been able to predict fully the presence of a three dimensional helical vortex structure for a particular injector, and in certain conditions (no central fuel jet), a precessing vortex core. These structures can be elucidated from point and planar measurements and favorable comparisons of velocity statistics between experiment and LES predictions strengthen these findings. However, volumetric, 3-component measurement of velocity data has not been widely available to provide conclusive evidence of the exact three dimensional nature of the vortex structures that exist.

An experimental setup utilizing time resolved tomographic PIV on a water flow rig is described in this paper. This is used to provide as high-quality aerodynamic study as possible of a single stream radially-fed air swirl gaseous fuel injector. The level of accuracy of the tomographic PIV technique is demonstrated by calculating the divergence of the velocity field as well as validating the results against a comprehensive 2 and 3 component standard PIV velocity database and other measurement techniques and predictions.

Structure identification methods have been employed to visualise and understand the complex flow topology within the near field of the injector. The change in topology with and without the stabilising central jet is further investigated and agrees with findings of planar PIV results. While the velocity error associated with the tomo-PIV results is higher than the planar results the data agree well within the identified uncertainty bounds and are complimentary in understanding the flow field structure. Thus a full volumetric aerodynamic survey is available for the first time on this isothermal flow case.

Topics: Fuel injectors
Commentary by Dr. Valentin Fuster
2018;():V04AT04A011. doi:10.1115/GT2018-75245.

Burning leaner is an effective way to reduce emissions and improve efficiency. However, this increases the instability of the combustion and hence, increases the tendency of the flame to blowout. On the other hand, the ignition delay of a jet fuel is a crucial factor of the instability feedback loop. Shorter ignition delay results in faster feedback loop, and longer ignition delay results in slower feedback loop. This study investigates the potential effect of ignition delay on the lean blowout limit of a gas turbine combustion chamber.

At the Low Carbon Combustion Centre of The University of Sheffield, a range of tests were carried out for a range of jet fuels on a Rolls-Royce Tay combustor rig. The ignition delay for each fuel was tested using Advanced Fuel Ignition Delay Analyser (AFIDA 2805).

Lean blowout tests (LBO) was conducted on various air flows rates. High speed imaging was recorded using a high speed camera to give further details of the flame behavior near blowout limit for various fuels. The instability level was observed using the pressure, vibration and acoustic fluctuation. This paper presents results from an experimental study performed on a small gas turbine combustor, comparing Lean Blowout limit of different conventional, alternative and novel jet fuels with various ignition delay characteristics. It was observed that at higher cetane number, the blowout is improved remarkably. The Ignition plays an important role in determining the average instability level, and as result determines the Lean Blowout limit of a fuel.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A012. doi:10.1115/GT2018-75254.

Large-eddy simulation (LES) of a pressurized, swirl stabilized, well-characterized aero-engine model combustor have been conducted at different operating conditions. The combustor is fueled by ethylene and features a secondary air injection system to mimic the rich-quench-lean concept (RQL) of aircraft engine combustors. This paper discusses the influence of three different equivalence ratios on the sooting behavior by means of numerical simulation. A finite-rate chemistry model (FRC) with assumed probability density function model (APDF) is used to describe the turbulent combustion. Polycyclic aromatic hydrocarbons (PAHs), their radicals and soot are modeled by a recently developed sectional approach. Feedback effects such as consumption of gaseous soot precursors by growth of soot are inherently captured by solving the governing equations simultaneously. Quantitative measurement data obtained by different laser diagnostic techniques are used for validation. Predicted temperatures and velocities are in excellent agreement to the measurements. The influence of different equivalence ratios on the sooting behavior of the combustor is qualitatively captured well by the soot model. Details of the soot evolution are discussed and the remaining differences between the simulation and the measurements are analyzed in detail.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A013. doi:10.1115/GT2018-75304.

A 12-nozzle FLOX® combustor is used to generate a full-premixed methane-air flame at ϕ = 0.86 at atmospheric pressure. Combustion stability is examined using 5-kHz simultaneous stereo Planar Image Velocimetry (PIV), OH Planar Laser-induced Fluorescence (OH PLIF) and OH chemiluminescence imaging. Proper Orthogonal Decomposition (POD) of the PIV results from various measurement planes reveals that slow jet oscillations with a characteristic Strouhal number of 0.012 are the dominant fluctuations in the flow field. Jet impingement on and detachment from the walls during jet oscillations are shown to cause the liftoff heights of the flames to increase and decrease. Such changes in flame lift-off heights are also primarily asymmetric among geometrically-symmetric flame pairs. In addition, direct flame-flame interactions are observed as jets collide during oscillations. Dynamic Mode Decomposition (DMD) of the same flow fields is shown able to capture not only the same low-frequency jet-oscillation mode, but also a series of modes spanning the whole resolvable spectrum, which are potential origins of higher-frequency peaks observed in the power spectra of integrated OH chemiluminescence signals.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A014. doi:10.1115/GT2018-75329.

The National Combustion Code (OpenNCC) was used to perform parametric design and analysis for three iterations of pre-filming injector design for gas-turbine combustors. The CFD analysis had significant impact on the design, integration and fabrication of a third-generation Lean-Direct Injection (LDI-3) flame-tube assembly consisting of nineteen injection elements. The air passages of the three pilot elements and sixteen main injection elements consisted of CFD-optimized compound-angle discrete jets and dual axial-bladed swirl-venturi passages, respectively. The aerodynamic characteristics of the nineteen-element injection array were evaluated by performing non-reacting flow simulation using a Time-Filtered Navier-Stokes (TFNS) method. The pilot and main injection elements were fueled with conventional pressure-atomizers and newly designed pre-filming nozzles, respectively. Fuel-air mixing and combustion performance was evaluated with reacting-flow TFNS computations using a 14-species, 18-step reduced kinetics mechanism for Jet-A fuel, Lagrangian spray modeling and a PDF turbulent-chemistry interaction model. The TFNS reacting-flow simulations provided considerable insight into the correlation between aerodynamics, combustion and emissions performance of the newly-designed pilot and main injection elements for the LDI-3 combustor at simulated cruise conditions.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A015. doi:10.1115/GT2018-75337.

Many sources of uncertainty exist when emissions are modelled for a gas turbine combustion system. They originate from uncertain inputs, boundary conditions, calibration, or lack of sufficient fidelity in the model. In this paper, a non-intrusive polynomial chaos expansion (NIPCE) method is coupled with a chemical reactor network (CRN) model using Python to rigorously quantify uncertainties of NOx emission in a premixed burner. The first objective of the uncertainty quantification (UQ) in this study is development of a global sensitivity analysis method based on NIPCE to capture aleatory uncertainty due to the variation of operating conditions and input parameters. The second objective is uncertainty analysis of Arrhenius parameters in the chemical kinetic mechanism to study the epistemic uncertainty in the modelling of NOx emission. A two-reactor CRN consisting of a perfectly stirred reactor (PSR) and a plug flow reactor (PFR) is constructed in this study using Cantera to model NOx for natural gas at the relevant operating conditions for a benchmark premixed burner. UQ is performed through the use of a number of packages in Python. The results of uncertainty and sensitivity analysis using NIPCE based on point collocation method (PCM) are then compared with the results of advanced Monte Carlo simulation (MCS). Surrogate models are also developed based on the NIPCE approach and compared with the forward model in Cantera to predict NOx emissions. The results show the capability of NIPCE approach for UQ using a limited number of evaluations to develop a UQ-enabled emission prediction tool for gas turbine combustion systems.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A016. doi:10.1115/GT2018-75338.

The Ultra Compact Combustor (UCC) is an innovative combustor system alternative to traditional turbine engine combustors with the potential for engine efficiency improvements with a reduced volume. Historically, the UCC cavity had been configured such that highly centrifugally loaded combustion took place in a recessed circumferential cavity positioned around the outside diameter of the engine. One of the obstacles with this design was that the combustion products had to migrate radially across the span of a vane while being pushed downstream by a central core flow. This configuration proved difficult to produce a uniform temperature distribution at the first turbine rotor. The present study has taken a different spin on the implementation of circumferential combustion. Namely, it aims to combine the combustion and space saving benefits of the highly centrifugally loaded combustion of the UCC in a new combustor orientation that places the combustor axially upstream of the turbine versus radially outboard. An iterative design approach was used to computationally analyze this new geometry configuration with the goal of fitting within the casing of a JetCat P90RXi. This investigation revealed techniques for implementation of this concept including small-scale combustor centrifugal air loading development, maintaining combustor circumferential swirl, combustion stability, and fuel distribution are reported. Furthermore, dramatic improvements in the uniformity of the turbine inlet temperature profiles are revealed over historical UCC concepts.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A017. doi:10.1115/GT2018-75361.

In this theoretical and numerical analysis, a low-order network model is used to investigate a thermoacoustic system with discrete rotational symmetry. Its geometry resembles that of the MICCA combustor; the FDF employed in the analysis is that of a single-burner configuration, and is taken from experimental data reported in the literature. We show how most of the dynamical features observed in the MICCA experiment, including the so-called slanted mode, can be predicted within this framework, when the interaction between a longitudinal and an azimuthal thermoacoustic mode is considered. We show how these solutions relate to the symmetries contained in the equations that model the system. We also discuss how considering situations in which two modes are linearly unstable compromises the applicability of stability criteria available in the literature.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A018. doi:10.1115/GT2018-75366.

This article presents numerical simulations of the Rolls-Royce BR700 combustor operating at various realistic conditions. Emphasis is put on the prediction of soot emissions. Three-dimensional steady Reynolds-averaged Navier-Stokes (RANS) simulations were performed employing the kϵ-model for turbulence closure. Combustion is modelled by finterate chemistry and the turbulence-chemistry interactions are captured by an APDF-approach (assumed probability density function) for temperature fluctuations. The injection of the liquid fuel Jet A-1 is described by a spray model applying Lagrangian methods for spray transport and atomization. The multi-component fuel is modelled as surrogate of n-decane, iso-octane and toluene. Reaction kinetics are described by a detailed mechanism, which is optimised for Jet A-1 oxidation and accurately resolves the reaction paths up to the smallest aromatic soot precursors benzene and toluene. Heavier PAHs (Polycyclic Aromatic Hydrocarbons) are lumped to sections and hence, modelled by a sectional approach, which leads to soot nucleation. The soot particle dynamics are described by a two equation model. Due to the model’s efficiency feasible computational costs are realised. Overall, four operation points are investigated, which are take-off, climb, approach and taxi. The simulation results at the combustor exit are compared to the experimentally determined Smoke Number of the engine exhaust gas by means of empirical correlations. The comparison shows a very good agreement. In particular, the Smoke Number’s trend is predicted well with respect to the thrust of the operation points.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A019. doi:10.1115/GT2018-75406.

Lean premixed (LPM) combustion systems are susceptible to thermoacoustic instability, which occurs when acoustic pressure oscillations are in phase with the unsteady heat release rates. Porous media has inherent acoustic damping properties, and has been shown to mitigate thermoacoustic instability; however, theoretical models for predicting thermoacoustic instability with porous media do not exist. In the present study, a 1-D model has been developed for the linear stability analysis of the longitudinal modes for a series of constant cross-sectional area ducts with porous media using a n-Tau flame transfer function. By studying the linear regime, the prediction of acoustic growth rates and subsequently the stability of the system is possible. A transfer matrix approach is used to solve for acoustic perturbations of pressure and velocity, stability growth rate, and frequency shift without and with porous media. The Galerkin approximation is used to approximate the stability growth rate and frequency shift, and it is compared to the numerical solution of the governing equations. Porous media is modeled using the following properties: porosity, flow resistivity, effective bulk modulus, and structure factor. The properties of porous media are systematically varied to determine the impact on the eigenfrequencies and stability growth rates. Porous media is shown to increase the stability domain for a range of time delays (Tau) compared to similar cases without porous media.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A020. doi:10.1115/GT2018-75407.

In this study, syngas combustion was investigated behind reflected shock waves in CO2 bath gas to measure ignition delay times and to probe the effects of CO2 dilution. New syngas data were taken between pressures of 34.58–45.50 atm and temperatures of 1113–1275K. This study provides experimental data for syngas combustion in CO2 diluted environments: ignition studies in a shock tube (59 data points in 10 datasets). In total, these mixtures covered a range of temperatures T, pressures P, equivalence ratios φ, H2/CO ratio θ, and CO2 diluent concentrations. Multiple syngas combustion mechanisms exist in the literature for modelling ignition delay times and their performance can be assessed against data collected here. In total, twelve mechanisms were tested and presented in this work. All mechanisms need improvements at higher pressures for accurately predicting the measured ignition delay times. At lower pressures, some of the models agreed relatively well with the data. Some mechanisms predicted ignition delay times which were 2 orders of magnitudes different from the measurements. This suggests there is behavior that has not been fully understood on the kinetic models and are inaccurate in predicting CO2 diluted environments for syngas combustion. To the best of our knowledge, current data are the first syngas ignition delay times measurements close to 50 atm under highly CO2 diluted (85% per vol.) conditions.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A021. doi:10.1115/GT2018-75408.

The fundamental impact of the precessing vortex core (PVC) as a dominant coherent flow structure in the flow field of swirl-stabilized gas turbine combustors has still not been investigated in depth. In order to do so, the PVC needs to be actively controlled to be able to set its parameters independently to any other of the combustion system. In this work, open-loop actuation is applied in the mixing section between the swirler and the generic combustion chamber of a non-reacting swirling jet setup to investigate the receptivity of the PVC with regard to its lock-in behavior at different streamwise positions. The mean flow in the mixing section as well as in the combustion chamber is measured by stereoscopic particle image velocimetry and the PVC is extracted from the snapshots using proper orthogonal decomposition. The lock-in experiments reveal the axial position in the mixing section that is most suitable for actuation. Furthermore, a global linear stability analysis is conducted to determine the adjoint mode of the PVC which reveals the regions of highest receptivity to periodic actuation based on mean flow input only. This theoretical receptivity model is compared with the experimentally obtained receptivity data and the applicability of the adjoint-based model for the prediction of optimal actuator designs is discussed.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A022. doi:10.1115/GT2018-75419.

In the world economy more and more attention is paid to the environment protection. This has brought a requirement for reduction of harmful substances emission from the gas turbine engine combustors to the foreground scene. Several concepts of low-emission combustion of liquid fuels have been suggested to solve the problem of nitric oxide emission reduction. The authors consider combustion of lean homogenized (quick-mixed) fuel-air mixtures to be the most promising concept for a multi-mode combustion chamber.

Based on the accumulated experience, the authors have formed some notion with respect to design peculiarities of low-emission combustors. Based on such general notions, an attempt has been made to create a model combustion chamber for decreasing harmful substances emission. A design for compact mixing modules has been worked out, as well as for a perforated flame tube. 3D computations have been carried out for the flow in the combustor compartment with 3 mini-modules, so to compare design and experimental data. In calculations the air entered the flame tube through a channel with a rectangular cross-section and, further, through swirlers of three burners (60% of air flow). Besides, the air came into the gap between the flame tube and casing through two side channels and, further, it got inside the flame tube through cooling system holes (40% of air flow).

In parallel, tests have been carried out in similar combustor compartment, using standard fuels, measuring harmful substances emission at gas temperature (T4) up to 1700 K. Data obtained testifies to essential reduction of nitric oxides in the experimental combustor being considered. Emission index NOx does not exceed value of 1 g/kg f in all the conditions investigated. Fuel efficiency is ≥ 99% for all the measurement regimes, except one, where it is 98%.

Additionally, tests have been conducted, using bio fuel obtained from plant raw material. Research results have revealed problems of changeover to such type of fuel mixtures.

Comparing test data with 3D simulation results, it can be noted that there, where computed value of the fuel combustion efficiency coincides with the measured one, NOx value also coincides. However, the emission index value is higher there, where the fuel combustion efficiency value obtained in computation is higher, i.e. where there are zones with higher temperature. The experimental results obtained have confirmed possibility of organizing low-emission combustion, as well as possibility of achieving the nitric oxide emission index level equal to 1 g/kg f at the combustor inlet temperature of 682K. It is evident that more detailed design study is required for transfer of the experimental technology to the working compartment of the combustion chamber.

The achieved level of harmful substances emission, after improvement and implementation of technology, may allow meeting the strictest ICAO requirements and reducing the airport fees significantly.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A023. doi:10.1115/GT2018-75420.

Simulations for the Cambridge swirl bluff-body spray burner are performed near blow-out conditions. A hybrid stress blended eddy simulation (SBES) model is utilized for sub-grid turbulence closure. SBES blends the RANS-SST model at the boundary layer with large eddy simulation dynamic Smagorinsky model outside the boundary layer. The injected N-heptane spray droplets are tracked using a typical Eulerian-Lagrangian approach. Heat transfer coupling between the bluff-body walls and the near-walls fluid is accounted for by coupling the solid and fluid energy equations at the bluff-body surface. Mixing and chemistry are modeled using the Flamelet Generated Manifold (FGM) model. The study investigates how successful the FGM model is in predicting finite rate effects like local extinction and flame lift-off height. To this end, two near blow-out spray flames, the H1S1 (75% to blow-out) and H1S2 (88% to blow-out) are simulated. Good results are shown matching the spray Sauter mean diameter (SMD) and axial velocity mean and rms experimental data. The results also show that the FGM model captured reasonably well the flame structure and lift-off height as well as the spray pattern. Overall the spray droplets mean D32 and mean axial velocity were under-predicted, while the rms distribution matched reasonably well for the H1S1 flame. The mean flame brush lift-off height is estimated based on the statistically stationary mean flame brush and is estimated to be around 6 mm from the bluff-body base. Instantaneous local flame extinction is observed. The H1S2 flame, however, showed similar but slightly better match with the measurements for the mean spray data compared to the H1S1 flame, with slight under-prediction for D32 at Z = 10 mm and Z = 20 mm. Future work will investigate the sensitivity of the simulation to the spray boundary conditions and grid resolution.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A024. doi:10.1115/GT2018-75423.

Combustion instability in gas turbines is often mitigated using fuel staging, a strategy where the fuel is split unevenly between different nozzles of a multiple-nozzle combustor. This work examines the efficacy of different fuel staging configurations by comparing axisymmetric and non-axisymmetric fuel staging in a four-around-one model gas turbine combustor. Fuel staging is accomplished by increasing the equivalence ratio of the center nozzle (axisymmetric staging) or an outer nozzle (non-axisymmetric staging). When the global equivalence ratio is ϕ = 0.70 and all nozzles are fueled equally, the combustor undergoes longitudinal, self-excited oscillations. These oscillations are suppressed when the center nozzle equivalence ratio is increased above ϕStaging = 0.79. This bifurcation equivalence ratio varies between ϕStaging = 0.86 and ϕStaging = 0.76 for the outer nozzles, and is attributed to minor hardware differences between each nozzle. High speed CH* chemiluminescence images in combination with dynamic pressure measurements are used to determine the instantaneous phase difference between the heat release rate fluctuation and the combustor pressure fluctuation throughout the combustor. This analysis shows that the staged flame has similar phase relationships for all staging configurations. It is found that axisymmetric staging can be as effective as non-axisymmetric staging; however, the aforementioned hardware variations can impact both the bifurcation equivalence ratio and the effectiveness of staging.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A025. doi:10.1115/GT2018-75424.

We provide the first demonstration of an acousto-optically modulated quantum cascade laser (AOM QCL) system as a diagnostic for combustion by measuring nitric oxide (NO), a highly-regulated emission produced in gas turbines. The system provides time-resolved broadband spectral measurements of the present gas species via a single line of sight measurement, offering advantages over widely used narrowband absorption spectroscopy (e.g., the potential for simultaneous multi-species measurements using a single laser) and considerably faster (> 15kHz rates and potentially up to MHz) than sampling techniques which employ FTIR or GC/MS. The developed AOM QCL system yields fast tunable output covering a spectral range of 1725–1930 cm−1 with a linewidth of 10–15 cm−1. For the demonstration experiment, the AOM QCL system has been used to obtain time-resolved spectral measurements of NO formation during the shock heating of mixture of a 10% nitrous oxide (N2O) in a balance of argon over a temperature range of 1245–2517 K and a pressure range of 3.6–5.8 atm. Results were in good agreement with chemical kinetic simulations. The system shows revolutionary promise for making simultaneous time-resolved measurements of multiple species concentrations and temperature with a single line of sight measurement.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A026. doi:10.1115/GT2018-75427.

Low-emissions gas turbine combustion, achieved through the use of lean, premixed fueling strategies, is susceptible to combustion instability. The driving mechanism for this instability arises from fluctuations of pressure, fuel/air flow rate, and heat release rate. If these fluctuations are relatively in-phase, the combustion system will evolve to a self-excited state. The self-excited instability frequency and amplitude depend mainly on the operating condition and the geometry of the combustor. In this study, we consider the onset and decay of self-excited instabilities, resulting from transients in fuel/air ratio, in both single-nozzle and multi-nozzle combustors. In particular, we examine the differences in the instability onset and decay processes between these two flame configurations, as most gas turbine combustors have multiple nozzles, but most gas turbine combustor experiments utilize a single-nozzle. A nonlinear logistic regression analysis is applied to study the timescales of the decay and onset transients. Variations in the equivalence ratio change the heat release rate distribution inside the combustor, which is captured using chemiluminescence imaging. The normalized Rayleigh index, which shows the spatial distribution of the instability driving, is calculated to analyze the driving strength in different regions of the flame. Comparisons between the single- and multi-nozzle flame transients, including both center and outer flames for the multi-nozzle combustor, suggest that both confinement from the wall and flame-flame interaction are crucial to determining flame dynamics as the equivalence ratio transient changes the heat release rate distribution near corner recirculation zone and flame shear layers.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A027. doi:10.1115/GT2018-75460.

Information about heat release can be used to discuss the flame dynamics and stability behavior of turbulent combustion systems. The most common experimental approach to determine heat release fluctuations is the recording of OH*-chemiluminescence. Since there is a strong dependence of chemiluminescence on strain rate and mixture gradients, spatial information must be judged with care. As already shown in previous work, Laser Interferometric Vibrometry (LIV) directly detects the line-of-sight values of density fluctuations along the laser beam axis. Neglecting friction, losses of thermal radiation and conduction and assuming only small fluctuations of pressure in the reaction zone, heat release fluctuations can be calculated directly from density fluctuations. With available LIV techniques only pointwise scanning was feasible, resulting in time-consuming traversing of the flame to cover the whole flame field. A new camera-based full-field-LIV-system, developed at Technische Universität Dresden, is capable to simultaneously determine spatial information of heat release within the whole field with only one measurement, lasting a few minutes. This leads to a dramatical reduction of measurement time and furthermore reduces experimental efforts. Since the system is able to measure the complete flame at once, it is also possible to get information about the transient behavior of the combustion process. In this work the full-field-LIV system was applied for the first time on a swirl stabilized, lean and premixed methane flame. The results of this newly developed technique were checked against those of a commercially available single-beam LIV-system. Finally, the flame transfer function (FTF) was recorded with full-field-LIV and OH*-chemiluminescence and compared against each other.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A028. doi:10.1115/GT2018-75468.

In a test campaign to lower the minimum part-load of a combined cycle plant, a series of turndown tests on two GE Frame 9E gas turbines with DLN1 combustor technology were carried out under premix operation by Stadtwerke München (SWM). It has been found that the load can be reduced significantly compared to the conventional turndown ratio, before either CO emissions or combustion dynamics form the limiting factor of the turndown test. To exploit this potential safely and operate the gas turbines close to these physical limits, emissions and combustion dynamics must be monitored online. The azimuthal thermoacoustic mode that is observed in the can-annular machines is monitored with the IfTA PreCursor, based on the online determination of the modal decay rate. For this method, the acoustic pressure is measured at the cans around the gas turbine circumference to observe the azimuthal acoustic propagation that is enabled by the cross-firing tubes between the cans. Using this strategy to monitor CO emissions and thermoacoustic stability in real-time, a reduction of the minimal part-load limit by approximately 20% is achieved for the considered gas turbines. In must-run situations without demand for electricity generation, the operating costs can be directly reduced by the fuel savings. As an additional benefit, SWM can offer a broader power reserve for grid stabilization on the energy market. This monitoring strategy has been fully implemented in the control system and first experiences of the extended part-load limit are currently being gathered.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A029. doi:10.1115/GT2018-75472.

In this study, we apply periodic flow excitation of the PVC at the centerbody of a generic swirl-stabilized combustor to investigate the impact of the precessing vortex core (PVC) on flame shape and flame dynamics. Previous studies revealed considerable influence of the PVC on combustion properties such as flame dynamics and fuel/air mixing. We employ time-resolved OH*-chemiluminescence and pressure measurements to investigate the influence of the PVC on flame dynamics and flame shape transition. The PVC is typically present in flames which are detached from the burner outlet. This lift-off is observed for increasingly lean mixtures in this study. With the help of the PVC actuation, studied in this work, the transition point between attached and detached flame is shifted towards richer mixtures. Moreover, the dynamics of heat release rate fluctuations that are related to PVC and thermoacoustic instabilities are extracted from the OH*-chemiluminescence data. This reveals a considerable damping of the thermoacoustic oscillations due to the PVC actuation under technically premixed conditions and the rise of additional modes due to the interaction of both dynamics.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A030. doi:10.1115/GT2018-75476.

Non-intrusive polynomial chaos expansion (NIPCE) is used to quantify the impact of uncertainties in operating conditions on the flame transfer function of a premixed laminar flame. NIPCE requires only a small number of system evaluations, so it can be applied in cases where a Monte Carlo simulation is unfeasible. We consider three uncertain operating parameters: inlet velocity, burner plate temperature, and equivalence ratio. The flame transfer function (FTF) is identified in terms of the finite impulse response from CFD simulations with broadband velocity excitation. NIPCE yields uncertainties in the FTF due to the uncertain operating conditions. For the chosen uncertain operating bounds, a second-order expansion is found to be sufficient to represent the resulting uncertainties in the FTF with good accuracy. The effect of each operating parameter on the FTF is studied using Sobol indices, i.e. a variance-based measure of sensitivity, which are computed from the NIPCE. It is observed that in the present case uncertainties in the finite impulse response as well as in the phase of the FTF are dominated by the equivalence-ratio uncertainty. For frequencies below 150 Hz, the uncertainty in the gain of the FTF is also attributable to the uncertainty in equivalence-ratio, but for higher frequencies the uncertainties in velocity and temperature dominate. At last, we adopt the polynomial approximation of the output quantity, provided by the NIPCE method, for further UQ studies with modified input uncertainties.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A031. doi:10.1115/GT2018-75481.

In a recent joint research project, a new FLOX®-combustion system was developed to couple a fixed-bed gasifier with a micro gas turbine. Product gases from biomass gasification exhibit low calorific values and varying compositions of mainly H2, CO, CO2, N2 and CH4. Furthermore, combustion characteristics differ significantly compared to the commonly used natural gas. As the FLOX®-technology is considered as efficient and fuel-flexible featuring low emissions of hazardous pollutants, the design of the LCV-combustor is based on it. It contains a two-staged combustor consisting of a jet-stabilized main stage adapted from the FLOX®-concept combined with a swirl stabilized pilot stage. The combustor was operated in a Turbec T100 test rig using an optically accessible combustion chamber, which allowed OH*-chemiluminescence and OH-PLIF measurements for various fuel compositions. In particular, the hydrogen content in the synthetically mixed fuel gas was varied from 0 % to 30 %. The exhaust gas composition was additionally analysed regarding CO, NOx and unburned hydrocarbons. The results provide a comprehensive insight into the flame behaviour during turbine operation. Efficient combustion and stable operation of the micro gas turbine was observed for all fuel compositions, while the hydrogen showed a strong influence. It is remarkable, that with hydrogen contents higher than 9 % no OH radicals were detected within the inner recirculation zone, while they were increasingly entrained below hydrogen contents of 9 %. Without hydrogen, the inner recirculation zone was completely filled with OH radicals and the highest concentrations were detected there. Therefore, the results indicate a different flame behaviour with low and high hydrogen contents. Although the flame shape and position was affected, pollutant emissions remained consistent below 10 ppm based on 15% O2. Only in case of 0% hydrogen, CO-emissions increased to 43 ppm, which is still meeting the emission limits. Thus, the combustor allows operation with syngases having hydrogen contents from 0% to 30%.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A032. doi:10.1115/GT2018-75483.

Autoignition and flame stabilisation in a gas turbine combustor presents severe challenges for safe and reliable gas turbine operation as soon as they occur in parts of the combustor that are not designed to sustain higher thermal loads. Especially when operating on highly-reactive fuels like hydrogen, higher autoignition and flashback risk associated with these fuels have to be taken into account. In the present study, flame stabilisation initiated by autoignition events is investigated in an optically accessible mixing duct of a generic reheat combustor at typical reheat conditions. The experiments were conducted at pressures of 15 bar, vitiated air temperatures higher than 1100 K and bulk velocity of 200 m/s. The fuel was a hydrogen-nitrogen mixture with up to 70 vol. % hydrogen and was injected by a coflow inline injector along with preheated carrier air of temperatures up to 623 K. The autoignition-driven flame stabilisation process was investigated by recording the luminescence signal with high-speed cameras and by tracking the temporal and spatial development of autoignition kernels in the mixing duct. A detailed and comprehensive data set could be generated providing the basis for an in-depth analysis of the stabilisation process on time scales down to 0.3 milliseconds, which is fast enough to disclose the small timescales at which the autoignition kernels develop in the mixing section. A stabilising sequence was found to lead to the stabilised flames due to a non-interrupted sequence of autoignition kernels. The stabilising sequence was found behave differently in two different temperature regimes where sequence durations and amounts of kernels differed significantly from each other. A state in which the cross section of the mixing section is fully blocked by one or more kernels in vertical direction could be identified for all operating conditions and the development of subsequent autoignition kernels after the section blockage changed clearly once this state was reached.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A033. doi:10.1115/GT2018-75509.

Screech is an acoustic combustion instability that drives pressure oscillations much greater than normal combustion turbulent fluctuations. Discrete Fourier Transforms (FTs) are commonly utilized to analyze high resolution pressure sensor data. This isolated sensor analysis approach provides valuable frequency information, but since sensors are usually not located at peak amplitudes, resultant waves may be rotating and maximum amplitudes may be difficult to determine. Understanding the underlying counter-traveling or counter-rotating resultant wave structure is useful to develop screech mitigation strategies and necessary to quantify the impact of fuel schedule or geometry changes. A screech wave analysis methodology (SWAM) is developed for transverse, longitudinal, and complex modes. Typically, only two or three sensors are located in a transverse plane and in a longitudinal plane. The SWAM approach utilizes all the sensors in an analysis plane to provide an integrated sensor analysis. SWAM results are evaluated based upon exact wave solutions, and demonstrated with test data.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A034. doi:10.1115/GT2018-75517.

This study deals with thermoacoustic instabilities in a generic sequential combustor. The thermoacoustic feedback involves two flames: the perfectly premixed swirled flame anchored in the first stage and the sequential flame established downstream of the mixing section, into which secondary fuel is injected in the vitiated stream from the first stage. It is shown that the large amplitude flapping of the secondary fuel jet in the mixing section plays a key role in the thermoacoustic feedback. This evidence is brought using high-speed Background Oriented Schlieren (BOS). The fuel jet flapping is induced by the intense acoustic field at the fuel injection point. It has two consequences: first, it leads to the advection of equivalence ratio oscillations toward the sequential flame; second, it modulates the residence time of the ignitable mixture in the mixing section, which periodically triggers autoignition kernels developing upstream of the chamber. In addition, the BOS images are processed to quantify the flow velocity in the mixing section and these results are validated using PIV. This study presents a new type of thermoacoustic feedback mechanism which is peculiar to sequential combustion systems. In addition, it demonstrates how BOS can effectively complement other diagnostic techniques that are routinely used for the study of thermoacoustic instabilities.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A035. doi:10.1115/GT2018-75529.

This study assesses and compares two alternative approaches to determine the acoustic scattering matrix of a pre-mixed turbulent swirl combustor:

1) The acoustic scattering matrix coefficients are obtained directly from a compressible Large Eddy Simulation (LES). Specifically, the incoming and outgoing characteristic waves f and g extracted from the LES are used to determine the respective transmission and reflection coefficients via System Identification techniques.

2) The flame transfer function (FTF) is identified from LES time series data of upstream velocity and heat release rate. The transfer matrix of the reactive combustor is then derived by combining the FTF with the Rankine-Hugoniot relations across a compact heat source and a transfer matrix of the cold combustor, which is deduced from a linear network model. Linear algebraic transformation of the transfer matrix consequently yields the combustor scattering matrix.

A cross-comparison study that includes comprehensive experimental data shows that both approaches successfully predict the scattering matrix of the reactive turbulent swirl combustor.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A036. doi:10.1115/GT2018-75531.

Sequential combustion constitutes a major technological step-change for gas turbines applications. This design provides higher operational flexibility, lower emissions and higher efficiency compared to today’s conventional architectures. Like any constant pressure combustion system, sequential combustors can undergo thermoacoustic instabilities. These instabilities potentially lead to high-amplitude acoustic limit cycles, which shorten the engine components’ lifetime and therefore reduce their reliability and availability. In case of a sequential system, the two flames are mutually coupled via acoustic and entropy waves. This additional inter-stages interaction markedly complicates the already challenging problem of thermoacoustic instabilities. As a result, new and unexplored system dynamics are possible. In this work, experimental data from our generic sequential combustor are presented. The system exhibits many different distinctive dynamics, as function of the operation parameters and of the combustor arrangement. This paper investigates a particular bifurcation, where two thermoacoustic modes synchronize their self-sustained oscillations over a range of operating conditions. A low-order model of this thermoacoustic bifurcation is proposed. This consists of two coupled stochastically driven non-linear oscillators, and is able to reproduce the peculiar dynamics associated with this synchronization phenomenon. The model aids in understanding what the physical mechanisms that play a key role in the unsteady combustor physics are. In particular, it highlights the role of entropy waves, which are a significant driver of thermoacoustic instabilities in this sequential setup. This research helps to lay the foundations for understanding the thermoacoustic instabilities in sequential combustion systems.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A037. doi:10.1115/GT2018-75546.

Preventing flame flashback into the premixing section is one of the major challenges in premixed combustion systems. For jet flames, the flame typically propagates upstream inside the low velocity region close to the burner wall, referred to as boundary layer flashback.

The physical mechanism of boundary layer flashback of laminar flames is mainly influenced by flame-wall interaction and flame quenching. Flashback is initiated if the burning velocity at some wall distance is higher than the local flow velocity. Since the burning velocity drops towards the wall due to heat losses, the wall distance of flashback can be defined at the location closest to the wall where the burning velocity still is sufficiently high. The well-established critical gradient concept of Lewis and von Elbe to predict flashback limits of laminar flames represents these assumptions but neglects the important influence of flame stretch on the burning velocity close to the wall. For that reason, a modified prediction model is developed in this work based on similar assumptions as in the critical gradient concept, but including the effect of flame stretch. A validation for hydrogen-air and methane-air flames highlights its advantages compared to the critical gradient concept and shows good prediction accuracy.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A038. doi:10.1115/GT2018-75552.

Thermoacoustic instabilities in gas turbine combustion systems, caused by a feedback loop between acoustic fluctuations and the flame, can be a major factor in determining the durability of the combustor. Of interest here are helical modes caused by a Kelvin-Helmholtz instability emanating from a region of high shear close to the outlet of the fuel injector. A liquid fuelled lean burn fuel injector, containing three air flow passages is studied in the present work using non-reacting compressible unsteady RANS CFD simulations. An acoustic wave is injected at the downstream boundary with excitation frequencies of 300Hz and 450Hz to compare to an unforced case. Analysis of the flow response is carried out using linear stability analysis, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). The linear stability analysis required interpolation of the solution from the unstructured CFD grid onto a uniform cylindrical polar mesh. The analysis found an absolute instability in the shear region between two passages. This m = −2 mode is unstable over frequencies from 400Hz to 1000Hz with wavelengths of 1.08 to 1.41 of the injector outer diameter. For the unforced case the POD identifies the first two modes with azimuthal wave number m = 1 and these are seen to spiral from the splitter plate inwards to disturb the pilot and outwards to the main. The dominant frequency is around 450Hz which is consistent with measurements and close to the linear stability analysis value. For the 300Hz forced case POD identifies the first four modes as being helical but has difficulty determining the dominant azimuthal wave number. There is shown to be a significant interaction between the acoustic and helical modes and double the total resolved kinetic energy as compared to the unforced case. The 450Hz forced case shows the asymmetric m = 1 mode to be damped and the m = 2 helical mode is relatively unchanged. The resolved kinetic energy was only marginally higher than the unforced case and significantly lower than the 300Hz forced case. The DMD analysis showed how, as the forcing increased the flow through the injector, the flow is simultaneously pushed radially inwards and accelerated azimuthally. It also identified the region downstream of the splitter plate with significant fluctuations and is likely to be the wavemaker region responsible for the generation of helical instabilities. This work improves understanding of how helical modes of different azimuthal wave numbers react to acoustic forcing. The ability to manipulate the strength of these modes through alteration of the fuel injector geometry gives designers an additional tool to control thermo-acoustic instabilities.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A039. doi:10.1115/GT2018-75553.

Gas turbine flexibility is key in markets with significant renewable energy resources. Operational load flexibility and fuel flexibility ensure the gas turbine remains competitive in supporting power generation in renewable markets. For applications where heat is required in support of a process industry the gas turbine offers an additional advantage. Alternate fuels such as hydrogen are generated as byproducts from chemical processing plants. Hydrogen also has the ability to be a ‘battery fuel’ as excess energy produced by wind and solar can be used to produce hydrogen through electrolysis. This work focuses on the retrofit and commercial introduction of significant quantities of hydrogen fuel into the gas supply of an existing commercial E-class gas turbine in Europe. The commercially operating plant provides combined heat (in the form of process steam) and power. The gas turbine operates with a lean premixed combustor without the need for diluent injection, and is able to operate flexibly with hydrogen mixed into the base natural gas fuel supply. The fuel mixing allows consumption of a chemical plant process gas resulting in positive economic and environmental benefit. This involves several considerations including combustor control/operation, safe flashback margin, emissions, stability and hardware durability. Enhancements to the control system through an automated combustor tuning package which is able to compensate real time for fluctuations in fuel gas constituents was implemented. Significant testing of the fuel flexible concept in a full scale high pressure combustor test facility was performed to ensure the desired increase in hydrogen consumption could be achieved. The experience, in addition to adaptations for field testing, was used to test and validate a new long term operational limit of 25% hydrogen content in the fuel. The success of the test campaign allows reduction in natural gas fuel consumption and cost, reduction in flaring of product waste gas, with a reduced power plant CO2 footprint. The development program and engine field testing substantiation are described in detail herein.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A040. doi:10.1115/GT2018-75554.

An air-blast fuel injector in an aero-engine plays an important role in the thermoacoustic behaviour of the combustion system. Previous studies have looked at the response of the gas phase to acoustic forcing of fuel injectors but less attention has been paid to the liquid phase. Even if the fuel flow rate is assumed to remain constant, the atomisation process is driven by the relative velocity of the air and fuel. Hence air velocity fluctuations will lead to changes in atomisation which will significantly affect the flame. In this work, we use both experiment and computational fluid dynamics (CFD) to investigate the response of the spray produced by a multi-passage lean-burn aero-engine fuel injector to acoustic forcing. A Phase Doppler anemometry (PDA) system was used to measure the spray size and velocity statistics at points downstream of a fuel injector operating under atmospheric conditions. The plenum-fed injector was subject to acoustic forcing from downstream at a range of frequencies from 50 to 450 Hz. The data were sorted according to phase bins to observe how the spray statistics change during the acoustic forcing cycle. To further understand these results and assess numerical prediction strategies, CFD simulations were performed. A compressible unsteady Reynolds-averaged Navier-Stokes (URANS) method was used to predict the acoustically forced air-flow through the injector. Lagrangian spray particles were introduced into the flow, close to the prefilmer lip, with a time varying size distribution determined using existing empirical breakup correlations based on the instantaneous air velocity field. Constants needed for the breakup correlations were calibrated using the data from the unforced and lowest frequency results. Results were then sampled at the same downstream location as in the experiment. With this approach, better agreement was observed between experiment and CFD when the instantaneous air mass flow rates and velocities are based on the local flow field, close to the fuel-prefilmer lip, rather than the whole passage. This shows the importance of including the details of the response of the separate passages, including phase differences, for accurate predictions of unsteady fuel injector flows. Experimental results also show a phase difference between the peak air velocity and the Sauter Mean Diameter (SMD) of the particles at the measurement plane. At lower frequencies, the phase shift is captured by the CFD results based on a quasi-steady breakup correlation. This indicates that the phase shift at low frequencies can be explained mainly by the particles travelling to the measurement plane at a slower speed than the air. However, as frequency increases, CFD based on the quasi-steady assumption alone is seen to no longer capture the phase shift, indicating that the phase shift in the experiment is not just due to the time of flight of the particles. Introducing a time delay into the breakup model allows the experimental phase shift to be matched. However, if running averaging is used, the variation in the SMD at a high frequency is over attenuated compared to the experimental measurements. More research is needed to fully understand the unsteady atomisation behaviour of this type of fuel injector, particularly its dependency on the time history of the unsteady air flow.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A041. doi:10.1115/GT2018-75559.

Validation test results of a low-order thermoacoustic model of combustion dynamics in a liquid-fueled, gas turbine combustor are presented. A finite element model designed in COMSOL Multiphysics is used as a tool for predicting naturally occurring combustion instabilities. The combustion rig consists of an inlet plenum, nozzle, combustor, and variable length transition tube. The combustor is run at pilot only mode at high pressure (up to 4 atm), resulting in sooty fuel-rich flame. The global flame transfer function is measured using flame emission at OH* band (307±5 nm) from the whole flame with inlet velocity fluctuation which is measured using multi-microphone measurement. The impedances at the choked inlet and exit of combustor are measured using a multi-microphone method, showing that the actual impedances of choked inlet and exit are different from theoretical values. Using the measured flame transfer function and impedances, the model’s capability to predict the instabilities is examined for two different rig configurations: one with a 10”-long transition tube and the other with a 20”-long transition tube. The modelling results are shown to converge to eigenfrequencies close to those measured experimentally. They correctly predict the stability regime of each of the tested conditions and frequencies of oscillations to within a maximum of 4% error. Using measured acoustic boundary conditions at the inlet and exit orifice improves the prediction of instability frequency.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A042. doi:10.1115/GT2018-75564.

Alternative aviation fuels are evaluated for lean-flame NOx formation, lean-flame blowout (LFBO), and rich-flame soot threshold. The alternative aviation fuels evaluated are: hydro-processed renewable jet (HRJ) made from camelina and tallow, Fischer-Tropsch (FT) jet made from coal and natural gas, and alcohol-based jet. Chromatographic analysis (from another lab) shows that these fuels are composed mainly of iso-alkanes. These fuels are burned as received and with 10–20% aromatic added. For comparison, petroleum-based jet fuels are burned: JP5, JP8, Jet-A, and an aromatic blend. Additionally, we burn several pure fuels: n-hexane, cyclo-hexane, n-octane, iso-octane, n-dodecane, 1-3-5 tri-methyl benzene (TMB), and toluene. The NOx and blowout experiments are performed in a jet-stirred reactor (JSR), and the soot threshold experiments are performed on a Meker burner. The fuels are burned pre-vaporized, premixed, and preheated.

NOx is measured by probe sampling and chemiluminescent analysis at a fixed temperature for each fuel. Soot tendency is measured as the fuel-air equivalence ratio (phi) for the first appearance of yellow tips in the flame. This is termed the soot threshold. LFBO is examined by measuring the JSR temperature immediately prior to blowout.

Relatively small changes in NOx, LFBO, and soot threshold are measured for the petroleum-based and alternative jet fuels examined. HRJ-tallow is the cleanest burning jet fuel tested with respect to NOx and soot measurements. It exhibits the lowest NOx and the highest soot threshold among alternative fuels tested.

A possible correlation between decreasing NOx and increasing soot threshold is explored. The impact of adding aromatic content to the alternative jet fuels is also examined. With 20% aromatic content added to the alternative jet fuels, the soot threshold decreases, though the HRJ-tallow with 20% aromatic added continues to maintain a higher soot threshold than petroleum-based jet fuels. The addition of 20% aromatic does not show a clear trend in NOx. Over the four alternative jet fuels tested, the 20% aromatic addition causes a maximum change in the NOx emission of ± 13% (relative); the average change is close to zero.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A043. doi:10.1115/GT2018-75570.

The capability of current turbomachinery-based engines limit the obtainable altitude and flight Mach number of modern aircraft. Maturing hypersonic technologies such as ram and scramjets allow greatly increased flight velocity but are not able to power themselves from the ground and, thus, rely on lift aircraft. Combined-cycle engines incorporate turbomachinery and ramjet technologies to allow both high and low flight Mach numbers. These high Mach capable systems are typically flown using specialty fuels such as JP-7 that are more applicable to hypersonic applications than conventional gas-turbine fuels such as JP-8/Jet A or JP-5. Potential issues exist, however, when operating a combined cycle that employs a legacy main combustor since these platforms were not designed for operation with these fuels. Of particular concern is the re-ignition performance of the turbomachinery core. The relight of the gas-turbine combustor occurs at high altitudes and relatively high vehicle speeds, yielding low pressure and temperature in the combustor as well as high combustor-dome velocities. All of these conditions are unfavorable for ignition. Additionally, heavy fuels require more energy for atomization and vaporization, which increases the probability that ignition will become a problem in a turbine-based combined-cycle (TBCC) system. Successful demonstration of legacy main-combustor technologies in hypersonic combined-cycle applications will eliminate the need for costly design of new main-burner technology. The literature does not provide information on the effects of running specialty fuels such as JP-7 and JP-10 in burners with conventional aerodynamic features. To fill that gap, a three-cup sector of a conventional fighter-class swirl-stabilized combustor configured to provide optical access through the sidewall was used in the study. Two test conditions representative of varying flight Mach number and altitude are evaluated. For each flight condition, the combustor pressure drop is varied to characterize ignition as a function of burner inlet velocity. Data are correlated by loading parameter and equivalence ratio at ignition, and at each test point the conditions are varied until ignition cannot be achieved.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A044. doi:10.1115/GT2018-75574.

The National Jet Fuel Combustion Program (NJFCP) is an initiative, currently being led by the Office of Environment & Energy at the FAA, to streamline the ASTM jet fuels certification process for alternative aviation fuels. In order to accomplish this objective, the program has identified specific applied research tasks in several areas. The National Research Council of Canada (NRC) is contributing to the NJFCP in the areas of sprays and atomization and high altitude engine performance. This paper describes work pertaining to atomization tests using a reference injection system. The work involves characterization of the injection nozzle, comparison of sprays and atomization quality of various conventional and alternative fuels, as well as use of the experimental data to validate spray correlations. The paper also briefly explores the application viability of a new spray diagnostic system that has potential to reduce test time in characterizing sprays. Measurements were made from ambient up to 10 bar pressures in NRC’s High Pressure Spray Facility using optical diagnostics including laser diffraction, phase Doppler anemometry (PDA), LIF/Mie Imaging and laser sheet imaging to assess differences in the atomization characteristics of the test fuels. A total of nine test fluids including six NJFCP fuels and three calibration fluids were used. The experimental data was then used to validate semi-empirical models, developed through years of experience by engine OEMs and modified under NJFCP, for predicting droplet size and distribution. The work offers effective tools for developing advanced fuel injectors, and generating data that can be used to significantly enhance multi-dimensional combustor simulation capabilities.

Topics: Fuels , Sprays , Aviation
Commentary by Dr. Valentin Fuster
2018;():V04AT04A045. doi:10.1115/GT2018-75575.

The study focuses on the emission of pollutants generated during combustion of methanol, including the formation of formaldehyde and acrolein, the two main additional hazardous pollutants formed during the combustion of methanol. The study is based on chemical kinetic analysis using the CHEMKIN code (1D-reactor model) and ANSYS code for numerical computation. A parallel experimental combustion study was performed using a laboratory scaled swirl stabilized combustor model using methanol fuel. The combustor model was built in order to calibrate and test the developed theoretical models. The chemical kinetic analysis, using the CHEMKIN code, showed that under the investigated conditions, the maximum acrolein concentration at the exhaust was 0.12ppmv. Currently, there is no emission standard that we know of that refers to acrolein. However, it may very well be that due to the high toxify of acrolein, even such a low value may be too high. The results from the CFD simulations about the acrolein concentration at the exhaust revealed very low values, less than 0.01ppm, and consequently were not presented. Formaldehyde emission was evaluated by chemical kinetics studies, CFD analyses, and measured experimentally using two different analyzers. It is shown that the current common standard for the allowable limit of formaldehyde, which is based on the absolute amount exhausted, is too low and will limit the use of methanol only to small power units.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A046. doi:10.1115/GT2018-75579.

Increasingly stringent regulations on emissions in the gas turbine industry require novel designs to minimize the environmental impact of oxides of nitrogen (NOx). The development of advanced low-NOx technologies depends on accurate and reliable thermochemical mechanisms to achieve emissions targets. However, current combustion models have high levels of uncertainty in kinetic rates that, when propagated through calculations, yield significant variations in predictions. A recent study identified and optimized nine elementary reactions involved in CH formation to accurately capture its concentration and improve prompt-NO predictions. The current work quantifies the uncertainty on peak CH concentration and NOx emissions generated by these nine reaction rates only, when propagated through the San Diego mechanism. Various non-intrusive spectral methods are used to study atmospheric alkane-air flames. 1st- and 2nd-order total-order expansions and tensor-product expansions are compared against a reference Monte Carlo analysis to assess the ability of the different techniques to accurately quantify the effect of uncertainties on the quantities of interest. Sparse grids, subsets of the full tensor-product expansion, are shown to retain the advantages of tensor formulation compared to total-order expansions while requiring significantly fewer collocation points to develop a surrogate model. The high resolution per dimension can capture complex probability distributions witnessed in radical species concentrations. The uncertainty analysis of lean to rich flames demonstrated a high variability in NOx predictions reaching up to 400 % of nominal predictions. Wider concentration intervals were observed in rich conditions where prompt-NOx is the dominant contributor to emissions. The high variability and scale of uncertainty in NOx emissions originating from these nine elementary reactions demonstrate the need for future experiments and data assimilation to constrain current models to accurately capture CH for robust NOx emissions predictions.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A047. doi:10.1115/GT2018-75601.

To predict the flame front position, we adopt the large eddy simulation (LES) and incorporate into it a combustion model, the hybrid turbulent combustion model (HTC model), applicable to any flame mode. We take previously obtained test results for the axisymmetric jet lean premixed flame in a cylindrical chamber at high pressure and investigate the effects of equivalence ratio variation on flame front positions. Full details of these tests are available in the literature. We find the simulation results of the axial positions of the flame front points, defined by the inflection point of the OH concentration distributions, show good agreement with test results (less than 4% difference between them). Therefore, we conclude that the HTC model is capable of capturing the actually observed tendency when changing the equivalence ratios and that the combination of the HTC model with LES suggests that flame stretch effect is important to predict the flame front position for lean premixed combustion.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A048. doi:10.1115/GT2018-75603.

In this paper, nonlinear instability of an elliptical jet is investigated by considering the impacts of orifice geometry variations using regular perturbation method. In the breakup mechanisms, created disturbances on the jet interfaces will grow owing to the nonlinear dynamics of fluid. In this study, a scrutiny of nonlinear cylindrical jet breakup is done initially. Next, Cosserat equations as a low order form of Navies-Stokes equations are solved on the nonlinear form to exert the impacts of orifice deformation on various aspect ratios. These nonlinear equations, Cosserat equations, are linearly solved in the past papers. As a result, the dispersion equation is derived to find the most unstable wavelength as well as the breakup length. Results reveal that the elliptical jet with low aspect ratio is more unstable rather than cylindrical jet with aspect ratio of one. Furthermore, the nonlinear equations can predict the break up length of elliptical jet more accurate than linear equations. The predicted results are shown to be in good agreement with the experimental results.

Topics: Stability , Jets , Geometry
Commentary by Dr. Valentin Fuster
2018;():V04AT04A049. doi:10.1115/GT2018-75621.

It has been known for a sometime that the compressor exit profile can have a significant effect on the overall performance of the fuel injector. This effect has been increased recently with the advent of larger leaner injection systems. With a modern gas turbine combustion system the fuel injectors are presented with a non-uniform feed generated by the upstream compressor and OGV/pre-diffuser assembly. For generic lean burn combustion systems previous experimental and numerical work highlighted a complex interaction between the compressor efflux and the upstream diffuser. Circumferential non-uniformities in the flow presented to the fuel injector can amount up to ±10% of the mean velocity. Previous investigations examined only the isothermal flow field and the effects of this level of non-uniformity on reacting performance are not known. There are potential impacts on local fuel atomisation, air/fuel mixing and hence emissions performance.

The main aim of this paper is to observe and assess the effect of these upstream conditions using a reacting flow test facility. In the initial design phases reacting flow experiments are generally conducted in simple, single sector plenum fed test facilities. Since this does not capture the effects of non-uniformities modifications were made to the facility to produce an aerodynamically representative feed. CFD was used in the design process to ensure that the aerodynamic features present in engine geometries would be faithfully reproduced by the test rig modifications. The CFD also highlighted changes in the downstream isothermal flow field. This included differences in the overall effective area of the fuel injector and, importantly, a redistribution of mass flow between the various fuel injector passages. Additionally, the cone angle, and the flow structure downstream was observed to change.

Back-to-back tests were then conducted in a reacting flow test facility for various pilot-mains fuel flow splits and air-to-fuel ratios. Visualisation of the flame showed notable qualitative differences in the structure and stability of the flame. Quantitative measurements indicated that, compared to a plenum feed, a representative feed produced changes in the production of carbon monoxide, unburned hydrocarbons and oxides of nitrogen. Emission results were used to calculate the extent of the mass flow redistribution between passages. From this a correction was applied to the estimated AFRs. This correction did not fully collapse the emissions data, suggesting that while the mass flow redistribution contributes to the change in emissions it is not fully responsible. Covered in this paper are initial observations. However, further work is required to fully understand how the changes to the aerodynamic flow field alter the emissions performance, but it is clear that having a representative fuel injector feed is important in low TRL testing.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A050. doi:10.1115/GT2018-75638.

In this work the implementation and validation of a finite-rate-chemistry (FRC) combustion model for ANSYS® CFX® 15.0 is presented. For the solution of the stiff system of species transport equations a splitting scheme is used where transport processes and chemical reactions are solved numerically in separate steps. In this splitting scheme the software Cantera is used for the integration of the chemistry sub-step. It is coupled via user-defined-functions (“USER-Fortran”) to ANSYS® CFX® 15.0. To provide validation data for this model under gas turbine relevant conditions, a down sized version of an industrial burner is investigated experimentally at different operating conditions and with different fuels. The burner is operated in a high-pressure combustion test rig with optical access at technically relevant pressures. Data for emissions of nitric oxide and carbon monoxide are obtained along with OH* chemiluminescence images of the flame. Additionally, investigations are made on the risk of flashback in this burner. The operating points are simulated using the FRC model developed in this work. It is demonstrated that this model approach can predict carbon monoxide and nitric oxide emissions very well, despite the simplistic treatment of turbulence-chemistry interaction. Moreover, it is shown that this model approach can also predict the onset of flashback: the change in flame shape, which is an indicator for flashback, can be well reproduced with this model.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A051. doi:10.1115/GT2018-75644.

The thermoacoustic behavior of a combustion system can be determined numerically via acoustic tools such as Helmholtz solvers or network models coupled with a model for the flame dynamic response. Within such a framework, the flame response to flow perturbations can be described by a Finite Impulse Response (FIR) model, which can be derived from LES time series via system identification. However, the estimated FIR model will inevitably contain uncertainties due to e.g., the statistical nature of the identification process, low signal-to-noise ratio or finite length of time series. Thus, a necessary step towards reliable thermoacoustic stability analysis is to quantify the impact of uncertainties in FIR model on the growth rate of thermoacoustic modes. There are two practical considerations involved in this topic. First, how to efficiently propagate uncertainties from the FIR model to the modal growth rate of the system, considering it is a high dimensional uncertainty quantification (UQ) problem? Second, since longer CFD simulation time generally leads to less uncertain FIR model identification, how to determine the length of the CFD simulation required to obtain satisfactory confidence? To address the two issues, a dimensional reduction UQ methodology called “Active Subspace approach” is employed in the present study. For the first question, Active Subspace approach is applied to exploit a low-dimensional approximation of the original system, which allows accelerated UQ analysis. Good agreement with Monte Carlo analysis demonstrates the accuracy of the method. For the second question, a procedure based on Active Subspace approach is proposed, which can serve as an indicator for terminating CFD simulation. The effectiveness of the procedure is verified in the paper.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A052. doi:10.1115/GT2018-75647.

This paper describes the coupled flow and flame dynamics during blowoff and reattachment events in a combustor consisting of a linear array of five interacting nozzles using 10 kHz repetition-rate OH planar laser induced fluorescence and stereoscopic particle image velocimetry. Steady operating conditions were studied at which the three central flames randomly blew-off and subsequently reattached to the bluff-bodies. Transition of the flame from one nozzle was rapidly followed by transition of the other nozzles, indicating cross-nozzle coupling. Blow-off transitions were preferentially initiated in one of the off-center nozzles, with the transition of subsequent nozzles occurring in a random order. Similarly, the center nozzle tended to be the last nozzle to reattach.

The blowoff process of any individual nozzle was similar to that for a single bluff-body stabilized flame, though with cross-flame interactions providing additional means of re-stabilizing a partially extinguished flame. Subsequent to blowoff of the first nozzle, the other nozzles underwent similar blowoff processes. Flame reattachment was initiated by entrainment of a burning pocket into a recirculation zone, followed by transport to the bluff-body; the other nozzles subsequently underwent similar reattachment processes. Several forms of cross-nozzle interaction that can promote or prevent transition are identified. Furthermore, the velocity measurements indicated that blowoff or reattachment of the first nozzle during a multi-nozzle transition causes significant changes to the flow fields of the other nozzles. It is proposed that a single nozzle transition redistributes the flow to the other nozzles in a manner that promotes their transition.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A053. doi:10.1115/GT2018-75689.

Modern low-emission gas turbine combustion systems often experience thermo-acoustic instabilities at certain operating conditions, which adversely affect the performance of the engine. One way to mitigate the detrimental effect of such instabilities is to place passive damping devices along the wall of the combustion chamber. To achieve greatest overall damping requires good understanding of the acoustic properties of the damping devices at engine conditions and determination of the undesired acoustic mode shapes for optimal placement at the wall. This paper presents an experimental study which characterises the acoustic properties of bias flow liners operating at frequencies in the low kilohertz regime (> 1 kHz). The engine conditions are simulated in the experiment at ambient conditions by maintaining dynamic similarity, i.e. by matching a number of non-dimensional parameters in the experiment which characterise the engine conditions. The present experimental study contributes to the existing measurement database by taking into account the strong gradient in characteristic impedance between grazing and bias flow medium. The acoustic properties of the investigated damper configurations are assessed in terms of the surface impedance at the interface between grazing and bias flow. An impedance model is suggested which accounts for the strong gradient in characteristic impedance between grazing and bias flow medium. The impedance model may serve conveniently as input to an acoustic mode shape prediction in the combustion chamber to identify the optimal placement of the damping devices.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A054. doi:10.1115/GT2018-75692.

In gas turbines, thermoacoustic oscillations grow if moments of high fluctuating heat release rate coincide with moments of high acoustic pressure. The phase between the heat release rate and the acoustic pressure depends strongly on the flame behaviour (specifically the time delay) and on the acoustic period. This makes the growth rate of thermoacoustic oscillations exceedingly sensitive to small changes in the acoustic boundary conditions, geometry changes, and the flame time delay. In this paper, adjoint-based sensitivity analysis is applied to a thermoacoustic network model of an annular combustor. This reveals how each eigenvalue is affected by every parameter of the system. This information is combined with an optimization algorithm in order to stabilize all thermoacoustic modes of the combustor by making only small changes to the geometry. The final configuration has a larger plenum area, a smaller premix duct area and a larger combustion chamber volume. All changes are less than 6% of the original values. The technique is readily scalable to more complex models and geometries and the inclusion of further constraints, such that the combustion chamber itself should not change. This demonstrates why adjoint-based sensitivity analysis and optimization could become an indispensible tool for the design of thermoacoustically-stable combustors.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A055. doi:10.1115/GT2018-75701.

Volatile renewable energy sources induce power supply fluctuations. These need to be compensated by flexible conventional power plants. Gas turbines in combined cycle power plants adjust the power output quickly but their turn-down ratio is limited by the slow reaction kinetics which lead to CO and unburned hydrocarbon (UHC) emissions. To extend the turn-down ratio, part of the fuel can be converted to syngas, which exhibits a higher reactivity. By an increasing fraction of syngas in the fuel, the reactivity of the mixture is increased and total fuel mass-flow and the power output can be reduced. An Autothermal On-board Syngas Generator in combination with two different burner concepts for natural gas/syngas mixtures was presented in a previous study [1]. The study at hand shows a mass-flow variation of the reforming process with mass-flows which allow for pure syngas combustion and further improvements of the two burner concepts which result in a more application-oriented operation. The first of the two burner concepts comprises a generic swirl stage with a central lance for syngas injection. Syngas is injected with swirl to avoid a negative impact on the total swirl intensity and non-swirled. The second concept includes a central swirl stage with an outer ring of jets. For this burner, syngas is injected in both stages to avoid NOx emissions from the swirl stage. Increased NOx emissions produced by natural gas combustion of the swirl pilot was reported in last year’s paper. For both burners, combustion performance is analyzed by OH*-chemiluminescence and gaseous emissions. The lowest possible adiabatic flame temperature without a significant increase of CO emissions was 170 K – 210 K lower for the syngas compared to low load pure natural gas combustion. This corresponds to a decrease of 15–20 % in terms of thermal power.

Topics: Stress , Syngas
Commentary by Dr. Valentin Fuster
2018;():V04AT04A056. doi:10.1115/GT2018-75760.

Clean technology has become a key feature due to increasing environmental concerns. Swirling flows, being directly associated with combustion performance and hence minimized pollutant formation, are encountered in most propulsion and power-generation combustion devices. In this study, the development process of the conceptual swirl burner developed at the Swedish National Centre for Combustion and Technology (CeCOST), is presented. Utilizing extensive computational fluid dynamics (CFD) analysis, both the lead time and cost in manufacturing of the different burner parts were significantly reduced. The performance maps bounded by the flashback and blow-off limits for the current configuration were obtained and studied in detail using advanced experimental measurements and numerical simulations. Utilizing high speed OH-chemiluminescence, OH/CH2O-PLIF and Large Eddy Simulation (LES), details of the combustion process and flame-flow interaction are presented. The main focus is on three different cases, a stable case, a case close to blow-off and flashback condition. We show the influence of the flame on the core flow and how an increase in swirl may extend the stability limit of the anchored flame in swirling flow burners.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A057. doi:10.1115/GT2018-75799.

Can-annular combustors consist of a set of independent cans, connected on the upstream side to the combustor plenum, and on the downstream side to the turbine inlet, where a transition duct links the round geometry of each can with the annular segment of the turbine inlet. Each transition duct is open on the sides towards the adjacent transition ducts, so that neighbouring cans are acoustically connected through a so called cross-talk open area. This theoretical, numerical and experimental work discusses the effect that this communication has on the thermoacoustic frequencies of the combustor. We show how this communication gives rise to axial and azimuthal modes, and that these correspond to particularly synchronised states of axial thermoacoustic oscillations in each individual can. We show that these combustors typically show clusters of thermoacoustic modes with very close frequencies and that a slight loss of rotational symmetry, e.g. a different acoustic response of certain cans, can lead to mode localization. We corroborate the predictions of azimuthal modes, clusters of eigenmodes and mode localization with experimental evidence.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A058. doi:10.1115/GT2018-75804.

Temperature and composition spots in a turbulent flow are detected and time-resolved using Laser Induced Thermal Grating Spectroscopy (LITGS). A 355 nm wavelength PIV laser is operated at 0.5–1 kHz to generate the thermal grating using biacetyl as an absorber in trace amounts. In a open laminar jet, a feasibility study shows that small (≃ 3%) fluctuations in the mean flow properties are well captured with LITGS. However, corrections of the mean flow properties by the presence of the trace biacetyl are necessary to properly capture the fluctuations. The actual density and temperature variation in the flow are determined using a calibration procedure validated using a laminar jet flow. Finally, travelling entropy and composition spots are directly measured at different locations along a quartz tube, obtaining good agreement with expected values. This study demonstrates that LITGS can be used as a technique to obtain instantaneous, unsteady temperature and density variations in a combustion chamber, requiring only limited optical access.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A059. doi:10.1115/GT2018-75818.

This work presents the atomization characteristics and dynamics of water-in-heptane (W/H) emulsions injected into a gaseous crossflow. W/H mixtures were tested while varying momentum flux ratios and aerodynamic Weber numbers. Different injector orifice diameters and orifice length-to-diameter ratios were used to test the generality of the results.

The atomization properties of W/H mixtures were compared with properties of neat water and neat heptane to evaluate the effect of an emulsion on droplet sizing, cross-sectional stability and dispersion, and jet penetration depth. Liquid dynamics were extracted through analyzing instantaneous spray measurements and dynamic mode decomposition (DMD) on high-speed video recordings of the atomization processes. Correlations were proposed to establish preliminary relationships between fundamental spray processes and test conditions. These correlations allowed for emulsion behavior to be compared with neat liquid behavior.

The use of emulsions induces greater spray instability than through using neat liquids, likely due to the difficulty in injecting a stable emulsion. Neat liquid correlations were produced and successfully predicted various spray measurements. These correlations, however, indicate that injector geometry has an effect on spray properties which need to be addressed independently. The emulsions are unable to adhere to the neat liquid correlations suggesting that an increased number of correlation terms are required to adequately predict emulsion behavior.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A060. doi:10.1115/GT2018-75834.

Lean premixed combustion is extensively used in gas turbine industry to reduce pollutant emissions. However, combustion stability still remains as a primary challenge associated with high hydrogen content fuels. Flashback is a crucial concern for designing gas turbine combustors in terms of operability limits. The current experimental study addresses the boundary layer flashback of hydrogen-air premixed jet flames at gas turbine premixer conditions (i.e. elevated pressure and temperature). Flashback propensity of a commercially available injector, originally designed for natural gas, is studied at different operating conditions and corresponding measurements are presented. The pressure dependence of flashback propensity is consistent with previous studies. The previously developed flashback model is successfully applied to the current data, verifying its utilization for various test conditions/setups. In addition, the model is used to predict flashback propensity of the injector at the actual engine preheat temperature. The injector is then modified to increase boundary layer flashback resistance and the corresponding data are collected at the same operating conditions. To avoid the boundary layer flashback, the mixture is leaned out in the near-wall region, where the flame can potentially propagate upstream. The comparison of gathered data shows a clear improvement in flashback resistance. This improvement is further elaborated by numerically studying fuel/air mixing characteristics for the two injectors.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A061. doi:10.1115/GT2018-75837.

Cooling holes in combustion liners are typically circular, as are the holes used for acoustic damping of combustor instabilities. This hole geometry is chosen for several reasons, which include manufacturability, stress concentration considerations, and because the behavior of round holes is well understood. However, developments in both manufacturing techniques and modeling methods now allow other hole geometries to be considered, such as perpendicular arrangements of narrow slots carefully shaped to prevent high stress concentrations (patent pending). The latter have been demonstrated to provide more efficient cooling and lower stress compared to round holes. However, the acoustic properties of such arrangements are not straightforward to model yet since it is not known how some important factors are affected by this geometry change.

In order to study the acoustic behavior of such arrangements of holes, a series of tests were conducted in an impedance tube (in reflection) for various slot shapes, width and spacing, with a backing cavity and purge flow (controlled via pressure drop). Some baseline measurements for circular holes were also obtained in an attempt to quantify any difference resulting from the change in hole shape.

The results have shown that at any pressure drop, the maximum absorption coefficient for narrow perpendicular slots and round holes is nearly equal when the hole area corrected by the discharge coefficient (i.e. the effective area) is also nearly the same, and that narrow perpendicular slots give more broadband absorption on the high frequency side of the absorption peak for a given cavity. It is suggested that since the maximum absorption between round holes and narrow perpendicular slots are nearly equal at an equivalent effective area, the resistance (real part of impedance) of the holes depends on the maximum flow velocity through the hole rather than the average velocity, for the particular combinations of hole dimension and sample plate thickness tested. It is also suggested that the greater broadband absorption at high frequencies for the narrow slots could be due to the fact that their high length to width ratio would result in a characteristic dimension dominated by their small width (similar to the airfoil thickness being the representative dimension of the size of the wake for an airfoil) hence a lower Strouhal number.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A062. doi:10.1115/GT2018-75874.

One method frequently used to reduce NOx emissions is exhaust gas recirculation (EGR), where a portion of the exhaust gases, including NOx, is reintroduced into the combustion chamber. While a significant amount of research has been performed to understand the important fuel/NOx chemistry, more work is still necessary to improve the current understanding on this chemistry and to refine detailed kinetics models. To validate models beyond global kinetics data such as ignition delay time or flame speed, the formation of H2O was recorded using a laser absorption diagnostic during the oxidation of a mixture representing a simplistic natural gas (90% CH4 /10% C2H6 (mol.)). This mixture was studied at a fuel lean condition (equiv. ratio = 0.5) and at atmospheric pressure. Unlike in conventional fuel-air experiments, NO2 was used as the oxidant to better elucidate the important, fundamental chemical kinetics by exaggerating the interaction between NOx and hydrocarbon-based species. Results showed a peculiar water formation profile, compared to a former study performed in similar conditions with O2 as oxidant. In the presence of NO2, the formation of water occurs almost immediately before it reaches more or less rapidly (depending on the temperature) a plateau. Modern, detailed kinetics models predict the data with fair to good accuracy overall, while the GRI 3.0 mechanism is proven inadequate for reproducing CH4 / C2H6 and NO2 interactions.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A063. doi:10.1115/GT2018-75878.

Reliable and robust simulations of detonations in inhomogeneous and turbulent environments are of direct importance in the design of rotating detonation engines (RDEs). In particular, computational models will be especially useful in designing and optimizing discrete injectors that introduce fuel and air separately into the detonation chamber, but ensure appropriate level of mixing to sustain detonations but minimize backflow of detonation products and pressure waves into the feed plenums. Since the structure of detonations itself is non-ideal, models have to include a detailed description of this reacting flow in order to be predictive in nature. Here, a highly-scalable open source based solver has been developed for complex detonating flows such that a) the detonation processes are described using detailed chemical kinetics, b) the method is computationally efficient through the introduction of adaptive mesh refinement, and c) the solver can handle complex geometries of relevance to RDE design. Grid convergence of key metrics for detonations is evaluated using canonical flows. Further, the importance of the use of detailed chemical kinetics is illustrated by extracting the composition structure behind a two-dimensional detonation front. Finally, simulations of a practical RDE configuration are used to demonstrate the applicability of this solver to analyzing geometries. The simulation captures the general trends of the experiment well. It is found that the detonation occurs under partially-premixed conditions. Propagation of pressure waves to the injection system is observed which could influence flow behavior in the oxidizer plenum.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A064. doi:10.1115/GT2018-75891.

Liquefied petroleum gas (LPG) will be suitable for satisfying part of the growing global energy demands. The widespread utilization of LPG as a gas turbine fuel for power generation requires an advanced combustor that achieves dry low nitrogen oxides (NOx) combustion and flashback-resistant combustion. This paper describes the development of a “multi-cluster combustor” as an advanced dry low NOx and flashback-resistant combustion technology for dual gaseous fuels of natural gas and petroleum gas. The dual gaseous fuel capability will contribute to expanding fuel flexibility. The purpose of this paper is to evaluate the feasibility of the dual gaseous fueled combustion with the multi-cluster combustor with the same configuration. The combustor was tested in a single-can combustor test stand at medium pressure with both fuels. In the tests, natural gas consisted mainly of methane with a content of over 90 vol.%, and petroleum gas consisted almost entirely of propane. The test results showed that the combustor achieves dry low NOx combustion of both fuels within their stable ranges without flashback. This paper concluded from the test results that the multi-cluster combustor possesses the potential capability to achieve dry low NOx and flashback-resistant combustion of dual gaseous fuels of natural gas and petroleum gas. As the next step, further tests will be required with petroleum gas including butane and for high pressure conditions.

Commentary by Dr. Valentin Fuster
2018;():V04AT04A065. doi:10.1115/GT2018-75896.

Multiple, interacting flames in DLE systems can increase flame surface area and promote mixing of hot-products into the reactants — leading to an efficient usage of combustion volume and improved injector performance. An optically-accessible, confined, linear array of five swirl nozzles was recently built [1] to investigate flame dynamics and validate computational strategies. The present work focuses on modeling a dataset representative of lean gas turbine conditions, using a flamelet approach. A preheated (500K), premixed fuel-air mixture (ϕ = 0.55, Tflame = 1732K) at atmospheric pressure was injected through the swirlers at 40 m/s into a rectangular chamber. High-speed laser measurements of the flow (3 component velocity field from 10 kHz stereoscopic particle image velocimetry (S-PIV)) and flame (planar laser induced fluorescence of the hydroxyl radical (OH-PLIF)) were used for model validation.

The objectives of this work: (1) Evaluate a flamelet-progress variable method based on flamelet-generated manifolds (FGM) and examine its sensitivity to models for micro (scalar dissipation) and large scale mixing (anisotropic RANS vs LES) and (2) Obtain insight into the velocity field and flame stabilization in an interacting system.

Computations indicate that high-swirl nozzles produce bluff-body flames anchored to shear-layer vortices due to an arrested flow expansion. The anisotropic RANS turbulence model under-predicts the recirculation zone strength but predicts flow development and Reynolds stress profiles fairly well. While LES is more accurate overall, both models over-predict flow fluctuations in the transitional flow at the end of the recirculation bubble where flow becomes axially positive. The flamelet approach predicts the flame-shape and length correctly but over-predicts the reaction rate in-between swirlers. The effect of including a reactive SDR model is to significantly increase flame-flow interaction (higher scalar variance) but does not appear to influence the overall shape or location of the flame.

Commentary by Dr. Valentin Fuster

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