bluuebunny/crossref_metadata_2025 huggingface dataset:
Low NOx Combustion Concepts in Support of the NASA Environmentally Responsible Aircraft Program
Authors: Prociw Alex , Ryon Jason , Goeke Jerry
Published: July 2013

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

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

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

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

This paper will discuss elements of prior designs compared to current designs and discuss the status of Computational Fluid Dynamic (CFD) simulations completed in the first phase of this program.
A Flow Network Combustor Model Applying Reduced Mechanisms
Authors: Maidhof Sebastian
Published: July 2013

A computational tool that combines a flow network solver with both 1D wall heat transfer and with chemical reactor models applying reduced mechanisms is presented. The model is applied to the combustion chamber of a 75kW gas turbine and wall temperatures and emissions of CO are compared with experimental values.
Effect of Aromatic Concentration of a Fischer-Tropsch Fuel on Thermal Stability
Authors: Klettlinger Jennifer Lindsey Suder
Published: July 2013

Fischer-Tropsch (F-T) jet fuel composition differs from petroleum-based, conventional commercial jet fuel because of differences in feedstock and production methodology. Fischer-Tropsch fuel typically has a lower aromatic and sulfur content and consists primarily of iso and normal parafins. The ASTM D3241 specification for Jet Fuel Thermal Oxidation Test (JFTOT) break point testing method was used to test the breakpoint of a baseline commercial grade F-T jet fuel, and various blends of this F-T fuel with an aromatic solution. The goal of this research is to determine the effect of aromatic content on the thermal stability of Fischer-Tropsch fuel. The testing completed in this report was supported by the NASA Fundamental Aeronautics Subsonics Fixed Wing Project.
Dynamics of Thermoacoustic Oscillations Leading to Lean Flame Blowout
Authors: Kabiraj Lipika , Sujith R. I.
Published: July 2013

Lean flame blowout induced by thermoacoustic oscillations is a serious problem faced by the power and propulsion industry. We analyze a prototypical thermoacoustic system through systematic bifurcation analysis and find that starting from a steady state, this system exhibits successive bifurcations resulting in complex nonlinear oscillation states, eventually leading to flame blowout. To understand the observed bifurcations, we analyze the oscillation states using nonlinear time series analysis, particularly through the representation of pressure oscillations on a reconstructed phase space. Prior to flame blowout, a bursting phenomenon is observed in pressure oscillations. These burst oscillations are found to exhibit similarities with the phenomenon known as intermittency in the dynamical systems theory. This investigation based on nonlinear analysis of experimentally acquired data from a thermoacoustic system sheds light on how thermoacoustic oscillations lead to flame blowout.
Low-Order Modelling of the Response of Ducted Flames in Annular Geometries
Authors: Graham Owen S. , Dowling Ann P.
Published: July 2013

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

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

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

In many gas turbine designs, particularly aeroengines, the geometries are composed of a ring of premix ducts linking a plenum and an annular combustor. The most unstable modes are often circumferential modes. The network model is used to characterise the flow response of the geometry to heat fluctuations at an appropriate location, such as the fuel injectors. The heat release at each flame holder is determined in the time domain using the kinematic flame model derived, as a function of the flow perturbations in the premix duct. This approach is demonstrated for an annular ring of burners on a in a simple geometry. The approach is then extended to an industrial type gas turbine combustor, and used to predict the limit cycle amplitudes.
Emission Measurements and OH-PLIF of Reacting Hydrogen Jets in Vitiated Crossflow for Stationary Gas Turbines
Authors: Roa Mario , Lamont Warren G. , Meyer Scott E. , Szedlacsek Peter , Lucht Robert P.
Published: July 2013

An optically accessible gas turbine combustor test rig was constructed to study the combustion characteristics of a coaxial hydrogen/air jet injected into a vitiated swirl crossflow. The test rig has two combustion zones. The main combustion zone (MCZ) is a swirl stabilized dump combustor, and the secondary combustion zone (SCZ) is a reacting crossflow jet, referred to as the jet-in-crossflow (JIC). The SCZ is located downstream of the MCZ. The JIC is a coaxial hydrogen/air jet that penetrates radially into the vitiated stream. The combustor was designed to study the effects of JIC conditions on the SCZ combustion process and in particular on NOx production. The jet velocity and equivalence ratio were systematically varied. A water-cooled sampling probe was used to extract exhaust gases downstream of the SCZ for emission measurements. The JIC flame structure was captured by OH-PLIF images which show the extent of the flame front and the depth of penetration into the vitiated stream. The OH-PLIF images were averaged to determine the JIC reaction zone and were compared to the Holdeman correlation.
An Improved Core Reaction Mechanism for C0-C4 Unsaturated Fuels and C0-C4 Fuel Blends
Authors: Naik Chitralkumar V. , Puduppakkam Karthik V. , Meeks Ellen
Published: July 2013

Simulation of the combustion of fuels used in transportation and energy applications requires accurate chemistry representation of the fuel. Surrogate fuels are typically used to represent liquid fuels, such as gasoline, diesel or jet fuel, where the surrogate contains a handful of components. For gaseous fuels, surrogates are effectively used as well, where methane may be used to represent natural gas, for example. An accurate chemistry model of a surrogate fuel means a detailed reaction mechanism that contains the kinetics of all the molecular components of the fuel model. Since large hydrocarbons break down to smaller molecules during combustion, the core chemistry of C0 to C4 carbon number is critical to all such fuel models, whether gaseous or liquid. The usual method of assessing how accurate the fuel chemistry is involves modeling of fundamental combustion experiments, where the experimental conditions are well enough defined and well enough represented by the reacting-flow model to isolate the kinetics in comparisons between predictions and data. In the work reported here, we have been focused on developing a more comprehensive and accurate core (C0−C4) mechanism. Recently, we revisited the core mechanism to improve predictions of the pure saturated components (J Eng. Gas Turbines Power (2012) 134; doi:10.1115/1.4004388). In the current work, we focused on combustion of unsaturated C0−C4 fuel components and on the blends of C0−C4 fuels, including saturated components. The aim has been to improve predictions for the widest range of fundamental experiments as possible, while maintaining the accuracy achieved by the existing mechanism and the previous study of saturated components. In the validation, we considered experimental measurements of ignition delay, flame speed and extinction strain rate, as well as species composition in stirred reactors, flames and flow reactors. These experiments cover a wide range of temperatures, fuel-air ratios, and pressures. As in the previous work for saturated compounds, we examined uncertainties in the core reaction mechanism; including thermochemical parameters derived from a wide variety of sources, including experimental measurements, ab initio calculations, estimation methods and systematic optimization studies. Using sensitivity analysis, reaction-path analysis, consideration of recent focused studies of individual reactions, and an enforcement of data consistency, we have identified key updates required for the core mechanism. These updates resulted in improvements to predictions of results, as validated through comparison with experiments, for all the fuels considered, while maintaining the accuracy previously reported for the saturated C0−C4 components. Rate constants that were modified to improve predictions for a small number of reactions remain within expected uncertainty bounds.
Nonlinear Phenomena in Thermoacoustic Systems With Premixed Flames
Authors: Kashinath Karthik , Hemchandra Santosh , Juniper Matthew P.
Published: July 2013

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

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

Time domain simulations of the coupled thermoacoustic system are performed with a Galerkin discretization for acoustic pressure and velocity. Limit cycle calculations using a single mode, as well as twenty modes, are compared against predictions from the CIRCE diagram. For the single mode system, the time domain calculations agree well with the frequency domain predictions. The heat release rate is highly nonlinear but, because there is only a single acoustic mode, this does not affect the limit cycle amplitude. For the twenty-mode system, however, the higher harmonics of the heat release rate and acoustic velocity interact resulting in a larger limit cycle amplitude. Multi-mode simulations show that in some situations the contribution from higher harmonics to the nonlinear dynamics can be significant and must be considered for an accurate and comprehensive analysis of thermoacoustic systems.
A Numerical Investigation on the Effect of Hydrogen and Syngas Addition on the Ignition of JP-8 Surrogates
Authors: Helma Thomas , Aggarwal S. K.
Published: July 2013

A numerical study is carried out investigating the effect of hydrogen and syngas addition on the ignition of two JP-8 surrogates, a two-component surrogate and a six-component surrogate. This six-component surrogate has previously been found to accurately simulate the smoke point, volatility, flame temperature profiles, and extinction limits of JP-8, while the two component surrogates has been shown to reproduce the flame structure predicted with the six-component surrogate. CHEMKIN 10101 is used to simulate ignition in a closed homogenous reactor under adiabatic and isobaric conditions. The parameters include temperature ranging from 850–1250 K, pressure of 20 atm, and equivalence ratio ϕ = 1.0. The CRECK-0810 kinetic mechanism, involving 341 species and 9173 reactions, is used to model the ignition chemistry. For the conditions studied, the addition of H2 or syngas in small quantities has no effect on the ignition behavior of either the surrogates or their individual components. Addition of H2 or syngas in larger quantities increases and decreases the ignition delay at low and high temperatures, respectively. For the conditions investigated, the ignition behavior of both the surrogates is predominantly determined by the ignition chemistry of n-dodecane.
Design and Performance Analysis of a Gas Turbine Flameless Combustor Using CFD Simulations
Authors: Levy Y. , Christo F. C. , Gaissinski I. , Erenburg V. , Sherbaum V.
Published: July 2013

This study investigates the performance and the conditions under which flameless oxidation can be achieved for a given annular adiabatic combustor. Numerical modelling of velocity, temperature and species fields are performed for different flow configurations of air and methane streams injected into a proposed design of a gas-turbine combustor. Parametric analysis was performed by systematically varying several parameters: radius of a recirculation zone, radius of the combustor, location of air and fuel ports, air and fuel velocities magnitudes and injection angles. The analysis was performed initially using a three-step global chemistry model to identify a design (geometry and operating conditions) that yield flameless combustion regime. The selected design was then modelled using a skeletal (46 reactions) and a detailed (309 reactions) chemical kinetics mechanism. The k–ε turbulence model was used in the most calculations. Overall, similar qualitative flow, temperature, and species patterns were predicted by both kinetics models; however the detailed mechanism provides quantitatively more realistic predictions. An optimal flow configuration was achieved with exhaust NOx emissions of < 7.5 ppm, CO < 35ppm, and a pressure-drop < 5%, hence meeting the design criteria for gas turbine engines. This study demonstrates the feasibility of achieving ultra-low NOx and CO emissions utilising a flameless oxidation regime.
LES for the Evaluation of Acoustic Damping of Effusion Plates
Authors: Andreini A. , Bianchini C. , Facchini B. , Peschiulli A. , Vitale I.
Published: July 2013

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

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

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

Details of the computational methods implemented in the general purpose open-source unstructured CFD code OpenFOAM® exploited to conduct this analysis are reported together with an analysis of the results obtained from the acoustic computations both regarding the flow field generated and the absorption and energy dissipation coefficient.
LES-Based Study of the Influence of Thermal Boundary Condition and Combustor Confinement on Premix Flame Transfer Functions
Authors: Tay-Wo-Chong Luis , Polifke Wolfgang
Published: July 2013

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

Two configurations are compared against a reference case with 90 mm × 90 mm combustor cross section and nonadiabatic walls: 1) combustor cross section similar to the reference case with adiabatic combustor walls, and 2) different confinement (160 mm × 160 mm) with nonadiabatic walls. It is found that combustor confinement and thermal boundary conditions have a noticeable influence on the flame response due to differences in flame shape and flow field. In particular the FTF computed with adiabatic wall boundary condition, which produces a flame with significant heat release in both shear layers, differs significantly from the FTF with nonadiabatic walls, where the flame stabilizes only in the inner shear layer. The observed differences in flow field and flame shape are discussed in relation to the unit impulse response of the flame. The impact of the differences in FTF on stability limits is analyzed with a low-order thermoacoustic model.
Extension of Fuel Flexibility in the Siemens Dry Low Emissions SGT-300-1S to Cover a Wobbe Index Range of 15 to 49 MJ/m3
Authors: Liu Kexin , Alexander Varkey , Sanderson Victoria , Bulat Ghenadie
Published: July 2013

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

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

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

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

The SITL DLE combustion system is able to cover a WI range of 15 to 49 MJ/m3 in two configurations. The results of testing showed a lowering in WI, from diluting with CO2 and/or N2, a benefit in NOx reduction is observed. This decrease in WI may lead to an increased requirement in fuel supply pressure.
A Novel Damping Device for Broadband Attenuation of Low-Frequency Combustion Pulsations in Gas Turbines
Authors: Bothien Mirko R. , Noiray Nicolas , Schuermans Bruno
Published: July 2013

Damping of thermoacoustically induced pressure pulsations in combustion chambers is a major focus of gas turbine operation. Conventional Helmholtz resonators are an excellent means to attenuate thermoacoustic instabilities in gas turbines. Usually, however, the damping optimum is in a narrow frequency band at one operating condition. The work presented here deals with a modification of the basic Helmholtz resonator design over-coming this drawback. It consists of a damper body housing separated volumes that are connected to each other. Adequate adjustment of the governing parameters results in a broadband damping characteristic for low frequencies. In this way, changes in operating conditions and engine-to-engine variations involving shifts in the combustion pulsation frequency can conveniently be addressed. Genetic algorithms and optimization strategies are used to derive these parameters in a multi-dimensional parameter space. The novel damper concept is described in more detail and compared with cold-flow experiments. In order to validate the performance under realistic conditions, the new broadband dampers were implemented in a full-scale test engine. Pulsation amplitudes could be reduced by more than 80%. In addition, it is shown that due to sophisticated damper placement in the engine two unstable modes can be addressed simultaneously. Application of the damper concept allowed to considerably increase the engine operating regime and finally to reduce NOx emissions by 55%. Predictions obtained with the physics-based model excellently agree with experimental results for all tested damper geometries, bias flows, excitation amplitudes, and most important with the measurements in the engine.
The Behavior of Liquid Fuel Sprays in Acoustically-Forced Air Swirler Flows
Authors: Kim Wookyung , Zhang Shiling , Palies Paul , Cohen Jeffrey , Liljenberg Scott , Hautman Donald
Published: July 2013

The effects of air flow forcing on fuel spray characteristics in a premixing swirler were assessed using ambient-pressure experiments and CFD (LES) analyses. Experimental measurements were performed using phase-locked Phase-Doppler Interferometry on two different swirler/mixer designs. The CFD analyses employed an advanced spray modeling technique to track the surface of the liquid fuel. The swirler designs chosen were representative of advanced low-emissions combustor concepts that emphasize thorough fuel/air mixing for Jet-A fuel. Significant post-processing of the results was performed in order to extract the response of the fuel spray mass flow rate fluctuations and fuel/air ratio to acoustic forcing. The results demonstrated that i) acoustic air forcing did not significantly change the atomization process, but did influence the unsteady transport of fuel droplets within the swirler flow field, ii) the level of fuel mass flow fluctuation was higher for one swirler and the level of fuel/air ratio fluctuations was higher for the other swirler and iii) the different behaviors between the two swirlers are primarily caused by the discrepant alignment of fuel and air distribution and the dissimilar droplet Stokes number which governs the unsteady transport. CFD results were interrogated to help understand the root causes of the observed phenomena. These showed that, for the swirler in which fuel mass flow fluctuations were observed, the swirl number was modulated by the acoustic forcing.
EZEE®: A High Power Density Modulating FLOX® Combustor
Authors: Schütz H. , Lammel O. , Schmitz G. , Rödiger T. , Aigner M.
Published: July 2013

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

The idea of EZEE® is to manipulate the flame position by radial staging of two independent fuel supplies in order to prevent flame extinction with increasing λ. The combustor is developed with the aid of computational tools and accordingly manufactured and experimentally investigated at the DLR test facilities. It is demonstrated that the combustion is complete and stable and that the pollutant emission is low. Presented are first results of our investigations that show the usefulness and the potential of the concept. However, it is also shown that additional fine-tuning is still necessary to further reduce the pollutant emissions.
CO Prediction in LES of Turbulent Flames With Additional Modeling of the Chemical Source Term
Authors: Ketelheun Anja , Aschmoneit Kai , Janicka Johannes
Published: July 2013

The prediction of combustion processes using Large Eddy Simulation (LES) combined with tabulated chemistry has proven to be very successful and become very popular during the last years in both academia and industry. Technical combustion systems feature a wide range of time and length scales which need to be resolved. The LES describes the rather slow, but turbulent and unsteady flow field very well, while the fast chemical reactions can be represented by tabulated chemistry models like Flamelet Generated Manifolds. Pollutants, being only present at lower concentrations and developing slowly are not easy to capture with the standard manifold defined by the fast major combustion products. Therefore, additional modeling in order to predict the carbon monoxide emissions is presented in this paper. The choice of the reaction progress variable and the solution of an additional transport equation with and without extra modeling for the post flame zone was investigated. These models are applied to a standard test case and compared to experimental data and the standard tabulation approach.
Acoustic Response of a Helmholtz Resonator Exposed to Hot-Gas Penetration and High Amplitude Oscillations
Authors: Ćosić Bernhard , Reichel Thoralf G. , Paschereit Christian Oliver
Published: July 2013

Helmholtz resonators are often used in the gas turbine industry for the damping of thermoacoustic instabilities. To prevent thermal destruction, these devices are usually cooled by a purging flow. Since the acoustic velocity inside the neck of the resonator becomes very high already at moderate pressure oscillation levels, hot-gas penetration cannot always be fully avoided. This study extends a well-known nonlinear impedance model to include the influence of hot-gas intrusion into the Helmholtz resonator neck. A time dependent but spatially averaged density function of the volume flow in the neck is developed. The steady component of this density function is implemented into the nonlinear impedance model to account for the effect of hot-gas intrusion. The proposed model predicts a significant shift in the resonance frequency of the damper towards higher frequencies depending on the amplitude of the acoustic velocity in the neck and the temperature of the penetrating hot gas. Subsequently, the model is verified by the experimental investigation of two resonance frequencies (86 Hz and 128 Hz) for two hot gas temperatures (1470 K and 570 K) and various pressure oscillation amplitudes. The multi-microphone-method, in combination with a microphone flush-mounted in the resonator volume, is used to determine the impedance of the Helmholtz damper. Additionally, a movable ultra-thin thermocouple was used to determine the degree of hot-gas penetration and the change of the mean temperature at various axial positions in the neck. A very good agreement between the model and the experimental data is obtained for all levels of pressure amplitudes and of hot-gas penetration depths. The mean air temperatures in the neck were accurately predicted, too.
Transformation of Transverse Acoustic Velocity of the Burner Approach Flow Into Flame Dynamics
Authors: Hauser Martin , Wagner Michael , Sattelmayer Thomas
Published: July 2013

Modern, large gas turbines for power generation have multiple burners, which are distributed around the circumference of the engine and which generate flames in combustors of either annular or can-annular geometry. In both cases considering only the axial modes has proven to be insufficient for the assessment of the thermoacoustic stability. An adequate analysis requires consideration of the circumferential acoustic coupling, generated by the acoustic field in the upstream and downstream annuli and the open passages between the cans, respectively. As in annular combustors the particularly critical eigenmodes with low frequencies are predominantly of circumferential nature, the stability of annular combustors is often governed by the onset of circumferential acoustic oscillations. In this study one single radial swirl burner is exposed to a transverse velocity fluctuation comparable to a circumferential oscillation in the plenum annulus. The transverse velocity fluctuation is transformed into a rotational flow oscillation through a convective process depending on excitation frequency and mass flow rate. The characteristics of this process are determined and the resulting dynamic flow structure in the burner nozzle is analyzed. Phase plots show that the rotational flow oscillation is transported into the flame causing a rotational flame pulsation. The influence of transverse velocity fluctuation on the global dynamic flame behavior is determined through FTF measurements. It is concluded from the increased FTF amplitude observed for transverse velocity excitation that the modification of the acoustic field at the burner exit due to circumferential acoustic modes has to be taken into account for a reliable prediction of the stability limits of annular gas turbines.
Advanced Combustor Concepts for Low Emissions Supersonic Propulsion
Authors: Smith Lance L. , Dai Zhongtao , Lee Jeremiah C. , Fotache Catalin G. , Cohen Jeffrey M. , Hautman Donald J.
Published: July 2013

Novel lean-burn combustor concepts were designed and evaluated for supersonic aircraft propulsion, with a focus on cruise NOx emissions. Premixing to lean conditions is especially challenging at supersonic cruise because combustor inlet temperatures are high and autoignition times are short. However, combustor pressure is significantly lower than at takeoff, so at cruise this allows heated jet fuel to be vaporized before injection as an aid to mixing. Two concepts — differentiated by swirler aerodynamics, swirler size, and staging method — were evaluated in the work reported here, both using injection of vaporized jet fuel. CFD calculations of mixing and combustion were used to design hardware for each concept. Injectors for each were fabricated using stereolithography (SLA) for cold-flow mixing tests, and using metal fabrication for subsequent combustion tests. Combustion test results show that EINOx < 5 was achieved for both concepts in single-sector tests at supersonic cruise combustor conditions.
Combustion Characteristics and NOX Emission of Hydrogen-Rich Fuel Gases at Gas Turbine Relevant Conditions
Authors: Lin Y.-C. , Daniele S. , Jansohn P. , Boulouchos K.
Published: July 2013

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

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

The level of NOx emission for the hydrogen-rich fuel mixtures is generally above that of syngas mixtures, which exhibit already higher NOx emission values than natural gas. Distinct chemical kinetic features are found specifically at elevated pressure. While the pressure effects are weak for syngas, a non-monotonic behavior is observed for the hydrogen-rich fuels. Reaction path analysis is performed to complement and provide more insight to the findings from the measurements. From chemical kinetic calculations a distinct shift in NOx formation pathways (thermal NOx vs. NOx through N2O/NNH reaction channels) can be observed for the different fuel mixtures at different pressure levels.
A Novel Design Method for Robust Acoustic Dampers With Perforated Plates Backed by a Cavity Operating at Low and High Strouhal Numbers
Authors: Scarpato A. , Ducruix S. , Schuller T.
Published: July 2013

It is known that the frequency of thermo-acoustic instabilities may vary according to various parameters during operation. The design of passive acoustic dampers tuned to damp specific unstable frequencies must then include this aspect to offer robust properties. This problem is tackled here for perforated plates backed by a resonant cavity in the absence of grazing flow. Their current design relies on a relatively complex optimization procedure with a large number of parameters to examine. A new methodology is proposed to reduce this number by finding the optimal parameters maximizing absorption in two limit regimes, where the choice of the optimal bias flow velocity and size of the back cavity can be decoupled. The former is only controlled by the plate porosity while the latter fixes the peak absorption frequency. The analysis also includes effects of the plate thickness. In both regimes, the optimal bias flow velocity is analytically determined. A Helmholtz resonance and a narrow absorption peak in the frequency space characterize the first absorption regime reached at high Strouhal numbers. This regime minimizes the size of the resonant back cavity, but the absorption frequency bandwidth narrows with increasing Strouhal numbers. The second absorption regime reached at low Strouhal numbers operates with a quarter-wave resonator. This regime requires larger cavity depths but offers a wider absorption bandwidth around the peak absorption frequency well suited for low frequency dampers when the bias flow velocity or the unstable frequency may vary within the system. Theoretical predictions are validated against experimental data obtained in the two regimes identified. The expressions derived in this study can be used to improve the design of robust acoustic dampers.
Dynamic Characterization of a Laboratory-Scale Thermal Pulse Combustor
Authors: Mondal Sirshendu , Mukhopadhyay Achintya , Sen Swarnendu
Published: July 2013

Pulse combustors are widely applied for heating, drying and even propulsion applications because of their higher efficiency, higher heat transfer rates and lower emission than steady combustors. However, fundamentals of this pulse combustor remain till date largely unexplored. Experiments are conducted on a laboratory-scale thermal pulse combustor. The set-up consists of an upstream section, the combustor and the tailpipe. The optical signal from the flame is measured with a photomultiplier tube and pressure fluctuations are measured using a dynamic pressure transducer. The time series data reconstructed with SSA (Singular Spectrum Analysis) reveals that at a given air flow rate as the fuel flow rate is reduced, three distinct regimes are observed: strongly pulsating, weakly pulsating and non-pulsating. Nonlinear analysis suggests the existence of quasiperiodic orbits for the pulsating cases. The phase difference between pressure and heat release rate fluctuations confirm sustained instability for the pulsating cases. The characteristic frequency is found to decrease with decrease in fuel flow rate and increase in tailpipe length for a given air flow rate. Different orientation of fuel inlet has been implemented to achieve pulsating combustion under lean fuel conditions.
A Numerical Study on the Turbulent Schmidt Numbers in a Jet in Crossflow
Authors: Ivanova Elizaveta , Noll Berthold , Aigner Manfred
Published: July 2013

This work presents a numerical study on the turbulent Schmidt numbers in jets in crossflow. This study contains two main parts. In the first part the problem of the proper choice of the turbulent Schmidt number in the Reynolds-Averaged Navier-Stokes (RANS) jet in crossflow mixing simulations is outlined. The results of RANS employing the shear-stress transport (SST) model of Menter and its curvature correction modification and different turbulent Schmidt number values are validated against experimental data. The dependence of the “optimal” value of the turbulent Schmidt number on the dynamic RANS model is studied. Furthermore a comparison is made with the large-eddy simulation (LES) results obtained using the WALE (Wall-Adapted Local Eddy Viscosity) model. The accuracy given by LES is superior in comparison to RANS results. This leads to the second part of the current study, in which the time-averaged mean and fluctuating velocity and scalar fields from LES are used for the evaluation of the turbulent viscosities, turbulent scalar diffusivities, and the turbulent Schmidt numbers in a jet in crossflow configuration. The values obtained from the LES data are compared with those given by the RANS modeling. The deviations are discussed and the possible ways for the RANS model improvements are outlined.
Studying the Stabilization Dynamics of Swirling Partially Premixed Flames by Proper Orthogonal Decomposition
Authors: Duwig Christophe , Ducruix Sébastien , Veynante Denis
Published: July 2013

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

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

In this paper, the problem is addressed using large eddy simulation (LES) and planar laser induced fluorescence. The LES data are used to follow the fuel air/mixing as well as the fuel combustion evidencing large-scale dynamics. These dynamics are further investigated using Proper Orthogonal Decomposition to identify the role of the premixing stage and of the Precessing Vortex Core in the flame behaviour.
Internal Gas Analysis and CFD Predictions of Double Contra-Rotating Axial Swirlers With Fuel Injection From Between the Two Outlets
Authors: King Phil T. , Mat Lazim T. B. , Andrews Gordon E. , Pourkashanian Mohamed M. , McIntosh Andy C.
Published: July 2013

A double contra-rotating axial swirler was investigated at 0.38 equivalence ratio and 600K inlet temperature at 1 bar, both experimentally and using CFD. Natural gas fuel was injected from between the two contra-rotating flows. The experimental results showed that whilst the NOx emissions were relatively high, the system has excellent flame stability characteristics meaning the flame could be maintained at very low fuel-air ratios, down to an equivalence ratio of around 0.28. CFD predictions were carried out using the equilibrium pdf combustion model alongside the partially premixed combustion model using default model constants, and the NOx model was applied as a post-processing application. The results showed that whilst the location and shape of the flame and the combustor fuel-air distribution could not be precisely captured, the peak temperature, CO, UHC and NOx levels were correctly predicted. The influence of the turbulent Schmidt number, Sct, was therefore investigated in an attempt to improve the predictions. Lower values offered an improvement in the fuel-air mixing predictions, but not in the combustion predictions, where the peak flame temperature could only be predicted correctly when using the default value of Sct = 0.85. This indicates that the use of a universal turbulent Schmidt number for both the fuel-air mixing and combustion models may not be suitable for the present swirler and combustor configuration, therefore future work will look to use a separate value of Sct for the mixture fraction than for the pdf combustion models.
Evaluation of the Level of Gaseous Fuel-Bound Sulfur on Fine Particulate Emission From a Low Emission Gas Turbine Engine
Authors: Spang Brent , Yoshimura Sayaka , Hack Richard , McDonell Vincent , Samuelsen Scott
Published: July 2013

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