Experimental study on heat release distribution during supersonic combustion

The present article reports a numerical analysis of instability coupled by a spinning mode in an annular combustor. This corresponds to experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global experimental flame describing function (FDF). A harmonic balance nonlinear stability analysis is carried out by combining the FDF with a Helmholtz solver to determine the system dynamics trajectories in a frequency-growth rate plane. The influence of the distribution of the volumetric heat release corresponding to each burner is investigated in a first stage. Even though each of the 16 burners is compact with respect to the transverse mode wavelength, and the 16 flames occupy the same volume, this distribution of heat release is not compact in the azimuthal direction and simulations reveal an influence of this volumetric distribution on frequencies and growth rates. This study emphasizes the importance of providing a suitable description of the flame zone geometrical extension and correspondingly an adequate representation of the level of heat release rate fluctuation per unit volume. It is found that these two items can be deduced from a knowledge of the heat release distribution under steady-state operating conditions. Once the distribution of the heat release fluctuations is unequivocally defined, limit cycle simulations are performed. For the conditions explored, simulations retrieve the spinning nature of the self-sustained mode that was identified in the experiments both in the plenum and in the combustion chamber.

Effects of experimental parameters on combustion modes were investigated experimentally in a directly connected wind tunnel facility that supplied vitiated airflow with total pressure of 1.0 MPa, total temperature of 1500 to 2400 K, and Mach number of 2.5. A supersonic combustor with constant cross-sectional area was used. No backward-facing steps and diverging section were attached. Fuel hydrogen was injected at an angle of 30 degrees to the airflow. The combustion modes were categorized into four: non-ignition, weak combustion, supersonic combustion, and dual-mode combustion. These combustion modes were organized in terms of total temperature, combustor length, and equivalence ratio. The effect of reaction in recirculation region formed upstream the injection holes became considerable with increasing the equivalence ratio. Under the same condition of combustor length, total temperature, and equivalence ratio, the combustion mode transited in a shorter combustor length in case of normal injection than in case of oblique injection. In case of normal injection, the size of the recirculation region probably had a greater effect on combustion modes than the combustor length. A simple model to predict the condition of the transition to the supersonic or dual-mode combustion was suggested, and agreed relatively well with experimental results in case of low equivalence ratio.

Many electronics cooling applications involve direct air cooling of the components and their heat transfer performance is often estimated, in the design phase, using heat transfer coefficient data from either in-house data or published sources. The agreement between predicted and observed system performance is quite often not acceptable, which means that the cooling system must be developed empirically—an expensive and time-consuming process. The authors believe they have identified one important reason for this state of affairs: the heat transfer coefficient values presented in the literature are being used improperly. Almost all of the published heat transfer coefficient data are from single-active-component experiments, which implicitly define h based on the adiabatic temperature of the component (had), while most users assume h to be defined on the basis of the mean fluid temperature (hm). This misunderstanding leads to underpredictions of the temperature rise of the components by 20–30 percent or more. This paper reviews the options for defining the heat transfer coefficient, shows how the problem arises, and then describes the steps necessary to properly use the existing heat transfer coefficient data base. There are two options for applying the existing data base to a fully powered array: One can either calculate the adiabatic temperatures of the components from the known heat release distribution in the array or calculate the values of hm that can be used with Tm, again using the known heat release distribution. The two options represent different ways to apply the same general method, superposition. The superposition method of Sellars, et al., (1956) is adapted to discrete systems and used as a guide to the forms recommended.

Imaging of OH* or CH* chemiluminescence with intensified cameras is often employed for the determination of heat release in premixed flames. Proportionality is commonly assumed, but in the turbulent case this assumption is not justified. Substantial deviations from proportionality are observed, which are due to turbulence-chemistry interactions. In this study a model based correction method is presented to obtain a better approximation of the spatially resolved heat release rate of lean turbulent flames from OH* measurements. The correction method uses a statistical strain rate model to account for the turbulence influence. The strain rate model is evaluated with time-resolved velocity measurements of the turbulent flow. Additionally, one-dimensional simulations of strained counterflow flames are performed to consider the non-linear effect of turbulence on chemi-luminescence intensities. A detailed reaction mechanism, which includes all relevant chemiluminescence reactions and deactivation processes, is used. The result of the simulations is a lookup table of the ratio between heat release rate and OH* intensity with strain rate as parameter. This lookup table is linked with the statistical strain rate model to obtain a correction factor which accounts for the non-linear relationships between OH* intensity, heat release rate, and strain rate. The factor is then used to correct measured OH* intensities to obtain the local heat release rate. The corrected intensities are compared to heat release distributions which are measured with an alternative method. For all investigated flames in the lean, partially premixed regime the corrected OH* intensities are in very good agreement with the heat release rate distributions of the flames.

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

A Rijke tube with a distributed heat source is investigated. Driven by the widely existing thermoacoustic instability in lean premixed gas turbine combustors, this work aims to explore the physicochemical underpinning and assist in the elucidation and analysis of this problem. The heat release model consists of a row of distributed heat sources with individual heat release rates. The integrated heat release rate is then coupled with the acoustic perturbation for thermoacoustic analysis. A continuation approach is employed to conduct the bifurcation analysis and capture the nonlinear behaviour inherent in the system. Unlike the conventional approach by the Galerkin method, the acoustic equations are originally discretized using the Method of Lines (MOL) to build up a dynamic system. The model is first validated and shown to yield good predictions with available experimental data. Influences of multiple heat sources, time delay, and heat release distribution are then studied to reveal the extensive nonlinear characteristics involved in the case of a distributed heat source. It is found that distributed heat source plays an important role in determining the stability of a thermoacoustic system.

Thermoacoustic instabilities are a cause for concern in combustion applications as diverse as small household burners, gas turbines or rocket engines. In this work, a feedback mechanism is analyzed, which couples combustion chamber acoustics with convectively transported fluctuations of entropy (entropy waves) generated within a premixed flame. Essential elements of this thermo-acoustic feedback loop are fluctuations in fuel concentration, induced by acoustic disturbances at the location of fuel injection, convective transport of fuel inhomogeneities through the premixing section of the burner, modulations in heat release rate and hot gas entropy resulting from the consumption of fuel/air mixture with varying fuel concentration by the flame, and the generation of sound through entropy non-uniformities at the turbine inlet. From a qualitative analysis based on relative phases, it is concluded that depending on the various convective and acoustic time lags involved, entropy waves may couple constructively as well as destructively with combustor acoustics. However, such qualitative analysis does not indicate whether the coupling between entropy and acoustic waves is strong enough to significantly influence thermo-acoustic stability. Therefore, a linear model has been constructed to estimate the effect of entropy waves on the thermo-acoustic response and stability of a combustor with a choked exit nozzle, as it might be found in a gas turbine. Note that phenomena like dispersion of convective waves, distributed heat release, vortical velocities, etc., have not been taken into account, as they would burden the presentation with unnecessary complexity. Results obtained indicate that the interaction between combustor acoustics and entropy waves can be significant, especially for the lowest non-axisymmetric modes, and even at frequencies higher than those usually associated with convective waves. As expected, it was observed that the coupling between pressure and entropy waves at the exit nozzle can enhance as well as reduce the thermo-acoustic stability of a combustor, or the responsiveness to an external or internal fluid-mechanic excitation mechanism. It is concluded that comprehensive thermo-acoustic analysis of a premixed combustor with a choked exit must in general include generation and propagation of entropy waves and coupling with combustor acoustics.

The majority of recent stationary gas turbine combustors employ swirling flows for flame stabilization. The swirling flow undergoes vortex breakdown and exhibits a complex flow field including zones of recirculating fluid and regions of high shear. Often, self-excited helical flow instabilities are found in these flows that may influence the combustion process in beneficial and adverse ways. In the present study we investigate the occurrence and shape of self-excited hydrodynamic instabilities and the related heat-release fluctuations over a wide range of operating conditions. We employ high-speed stereoscopic particle image velocimetry and simultaneous OH*-chemiluminescence imaging to resolve the flow velocities and heat release distribution, respectively. The results reveal four different flame shapes: A detached annular flame, a long trumpet shaped flame, a typical V-flame, and a very short flame anchored near the combustor inlet. The flame shapes were found to closely correlate with the reactivity of the mixture. Highly steam-diluted or very lean flames cause a detachment, whereas hydrogen fuel leads to very short flames. The detached flames feature a helical instability, which in terms of frequency and shape is similar to the isothermal case. A complete suppression of the helical structure is found for the V-flame. Both, the trumpet shaped flame and the very short flame feature helical instabilities of different frequencies and appearances. The phase-averaged OH*-chemiluminescence images show that the helical instabilities cause large scale-heat release fluctuations. The helical structure of the fluctuations is verified using a tomographic reconstruction technique.

ABSTRACTAn experimental study was conducted to characterize the effects of vitiate species (H2O and CO2) on supersonic combustion. These species were added to an initially clean airflow that was supplied by an electrically heated direct-connected facility. Wall pressures were measured by pressure sensors, combined with combustion optical images, and the influence of major vitiate species (H2O and CO2) on supersonic combustion was analyzed. Experimental results showed that the presence of vitiate species decreased combustor pressures by 3% to 30% for the same fuel equivalence ratio. The combustion flame images indicated that the flame-spreading angle decreased with the increase of the molar fraction of H2O and CO2, and when the molar fraction of CO2 was increased to 3.5%, an illogical oscillation, specifically embodying as the flame round switching between the upstream cavity and the downstream cavity, could be observed in the combustion, which might possibly cause the flame extinction.

Experimental observations and numerical simulations were conducted to study the flameholding characteristics and the flame stabilization mechanism in a supersonic combustor with hydrogen injection upstream of a cavity flameholder. OH radical distribution of the combustion flowfiled was obtained using OH spontaneous emission and OH-PLIF (Planar Laser-Induced Fluorescence). The supersonic combustion flowfield with L/D=7 cavity was calculated by large eddy simulations. The combustion model was based on a two scalar partially-premixed flamelet model with a level set approach. The results showed that hydrogen fuels were transported into the cavity shear; lean mixture and rich mixture were produced in internal cavity and declining injected jet respectively. An approximately steady partially premixed flame front exists in the cavity shear layer. The flame front propagates and extends to region around the fuel jet due to the counter-rotating vortices induced by the jet and the cavity shear layer. The flame front sustained in the shear layer likely penetrates through the jet core and ignites the whole jet. Behind the flame front most of the jet beam is ignited and burned as diffusion flames. The physical process of the flame stabilization demonstrated the similarity with a triple flame theory or edge flame concept, which indicted that triple flame theory might be the basic flameholding mechanism of the cavity flameholders.

In an effort to elucidate the fundamental processes controlling bluff body flame stabilization, the dependence of the spatial distribution of the local equivalence ratio and the heat release dynamics upon the mode of fuel injection was studied. Experiments were performed in a single flame holder combustion channel which was supplied with a high-temperature air stream. Jet-A fuel was injected across the incoming air stream from one of two locations: a cylindrical fuel bar installed 0.25 m upstream of the bluff body, or from fuel injectors integrated within the bluff body 2.5 cm upstream of the trailing edge (i.e., close-coupled injection). The time-averaged spatial distributions of the combustion heat release were characterized by CH* and C2* chemiluminescence imaging of the flame, and ratios of the C2* to CH* light emission were used to characterize the local equivalence ratio. The spatial average of the C2*/CH* value in the flame was found to increase linearly with increasing global equivalence ratio for fuel injection upstream of the bluff body, whereas this value was relatively constant for close-coupled injection. This constant value equaled the same average C2*/CH* value obtained for upstream fuel injection at globally stoichiometric conditions, suggesting that combustion resulting from close-coupled fuel injection took place, on average, in stoichiometric flamelets throughout the combustor. The heat release dynamics due to asymmetric (von Ka´rma´n) vortex shedding were also investigated for each operating condition by recording high-speed movies of the flame at 24 kHz. Upon processing of these movies, the amplitudes of heat release fluctuations due to von Ka´rma´n vortex shedding were found to be significantly higher for close-coupled injection than for injection well upstream of the flame holder for all operating conditions. This is attributed to an increase in span-wise fuel-air mixing and near-wake heat release for upstream fuel injection, resulting in a hotter recirculation zone which suppressed the von Ka´rma´n instability more than the close-coupled case.

Analysis and evaluation of Sub-Scale Unit operational and test data has not shown any cause to expect unsuccessful operation of the Demonstration Plant. Major changes to the Demonstration Plant based on the SSU results significantly improve the prospect for a smooth shakedown and startup of the plant. Modification of in-bed heat transfer to compensate for changes in the design heat transfer rates and heat release distribution has been accomplished. Provisions to vary reinjection rates during operation over the range of 0 to 90 percent maximum expected flow rates and control dust loadings have been added. Redesign of the bed air distribution (grid plates), allowing for free thermal expansion, has been included. Improvements to the design of instrumentation probes to provide reliable sensing for control valves were provided. In addition, the operation of the SSU has verified that the fluidized bed combustion process as designed for the Demonstration Plant should achieve the required performance. All major components of the Demonstration Plant have been specified. Fabrication releases have been issued for critical items. The design phase of the project is near 95% completion. Construction is well underway. Concrete work is more than 90% complete. Structural steel erection has started, as well as silo fabrication. The facility should be enclosed with roofing and siding before the arrival of severe winter weather. A test program has been prepared and submitted to DOE. Arrangements are being made for operational manpower and test equipment.

Heat Release Distribution in the LRE Combustor in Stable and Unstable Modes

<div class="htmlview paragraph">A model has been developed for a divided-chamber automotive diesel engine which describes the intake, compression, combustion and expansion, and exhaust processes in sufficient detail to permit calculations of pressure, fuel-air ratio distribution, heat release distribution, NO formation, soot mass loading, and soot oxidation processes. The novel feature of this model is the use of a stochastic mixing approach during the combustion and expansion processes to describe the nonuniform fuel-air ratio distribution within the engine. In this approach, the fuel-air ratio distribution during the combustion and emissions formation processes can be followed as it evolves with time. Experimental data generated on a single-cylinder divided-chamber diesel engine were used to verify the accuracy of the model predictions. Agreement between experimental data and predicted values of engine performance and NO<sub>x</sub> emissions levels was good. The model is used to examine the origin of NO and soot emissions in a divided-chamber diesel engine. The cycle-simulation results show that NO forms primarily in the pre-chamber. The soot studies show that soot oxidation occurs in the pre- and main-chambers, and soot oxidation mechanism is both kinetically and mixing limited.</div>

This study presents a new three-step global chemical mechanism for n-decane/air combustion, specifically optimized for efficient simulation of oblique detonation waves (ODWs). The parameters in the global mechanism were specially formulated to enable predictions of essential features in ODWs, including ignition delays, heat release distribution, and detonation velocity with substantially reduced computational cost while preserving fidelity. Comprehensive validations are conducted through extensive numerical simulations, supported by direct comparisons with experimental shock tube data and a skeletal mechanism (Skel40). The proposed mechanism was found to demonstrate predictive robustness across a wide range of combustor conditions, spanning initial temperatures from 600 to 1600 K, pressures from 10 to 1000 kPa, and equivalence ratios from 0.5 to 1.6. As demonstrated in two-dimensional simulations of ODWs, the proposed three-step mechanism predicts key features consistent with those from Skel40 but with a 56-fold improvement in computational efficiency. This new capability provides a reliable and convenient alternative for system-level detonation modeling.