Heat Release Rate Distribution Research Articles

The design of a combustion chamber operated in MILD regime - numerical modeling of hydrogen combustion in oxygen steam mixtures

The goal of the current work is to develop a combustion chamber that can operate on recirculated steam produced by burning hydrogen in oxygen under conditions of Moderate or Intense oxygen Dilution (MILD) in atmospheric and stoichiometric conditions. The study investigates several configurations of combustor with nozzles, taking into account the overall temperature, OH radicals, and heat release rate distribution throughout the combustor’s domain. Thermal power variations of the steam generator (5 to 20 kW) were examined in conjunction with different oxygen dilutions with steam, down to 3% of O2 (by mol.). The outcomes reveal that a rise in dilution degree promotes a drop in the mean temperature across every case and reagents’ recirculation with homogeneous temperature field, suggesting the presence of MILD combustion. The highest temperature values were observed at the stoichiometric mixture fraction. Higher dilution degree revealed more efficient heat release across the domain with low fluctuations from the reference MILD combustion data. Of the two combustion models studied, the Partially Stirred Reactor model did not show flame extinction at the highest dilution degrees, unlike the Eddy Dissipation model. The selected final design of the combustion chamber was used for constructing the actual combustor dedicated for lab-scale operation.

Numerical study of CH* chemiluminescence and heat release rate in methane inverse diffusion flame

Flame optical diagnostic method based on CH* chemiluminescence is often used in the combustion process monitoring. In this work, a numerical study of CH* chemiluminescence was conducted for the laminar methane inverse diffusion flame with a range of global oxygen/fuel equivalence ratios. CH* chemiluminescence is mainly distributed in the upstream of the inverse diffusion flame and the CH* concentration increases with the global oxygen/fuel equivalence ratio. CH* is generated by C2H, which mainly comes from the reactions OH + C2H2<=>C2H + H2O and H + C2H2<=>C2H + H2. In addition, most CH* is quenched by collisions with H2O, CO, CO2 and H2, and the CH* collision quench is enhanced with increasing global oxygen/fuel equivalence ratios. The CH* generated in the peak reaction zone will diffuse to the fuel and oxidizer sides due to the mass diffusion and the thermal diffusion. Moreover, the correlation between CH* and local heat release rate distribution is explored and CH* can characterize the local heat release rate on the fuel side.

Investigation of the correlation between OH*, CH* chemiluminescence and heat release rate in methane inverse diffusion flame

The characterization of the heat release rate is of great importance for studying the combustion process. In this work, the correlation between heat release rate and OH*, CH* chemiluminescence in methane inverse diffusion flame is explored with a numerical simulation over a wide range of oxygen/fuel equivalence ratios and methane flow rates. It is found that the flame heat release rate is mainly related to the formation and consumption of the species OH, C2H2, CH3, CH4, CO, CO2, H, H2O and O. The ground state OH concentration gradient is correlated with the heat release rate distribution, and the peak location of the gradient in the ground state OH concentration aligns with the peak location of the heat release rate. The outline of the OH* distribution is consistent with the profile of the maximum of the OH concentration gradient. CH* is used to indicate the main distribution of the heat release rate, and the outline of the OH* distribution coincides with the outline of the heat release rate.

Data-driven discovery of heat release rate markers for premixed NH[formula omitted]/H[formula omitted]/air flames using physics-informed machine learning

The spatial distribution of heat release rate (HRR) is important for flame front identification. However, direct measurement of HRR is impossible using the current experimental technology. Indirect HRR markers have thus been proposed and used in experiments to approximate the heat release profile. Suitable HRR markers have been identified in previous studies for flames with hydrocarbon fuels. However, study of HRR markers for premixed flames with NH3/H2 blend fuel has rarely been done. Due to the increasing interest in carbon-free fuels, such as NH3/H2, a generally-valid but accurate HRR marker is in demand. The present study involves data-driven discovery of optimal HRR markers for premixed NH3/H2/air flames using physics-informed machine learning. The training data are generated by detailed simulations of turbulent flames with detailed chemistry. Several markers are finally identified. The most accurate combinations that can be measured experimentally read [NH]0.952[OH]0.062 and [NH2]1.039[OH]0.641. They show very high reconstruction quality for both spatial and temporal evolutions of HRR in premixed NH3/H2/air flames for a variety of equivalence ratios (ranging from 0.45 to 1.25), fuel compositions (NH3/H2 ratios ranging from 5/5 to 7/3 by volume), and turbulence intensity (Ret ranging from 16.5 to 145.5). Their validity is not limited to the specific kinetic mechanism employed to generate the training data.

On the impact of H[formula omitted]-enrichment on flame structure and combustion dynamics of a lean partially-premixed turbulent swirling flame

Large Eddy Simulation (LES) with Conjugate Heat Transfer (CHT) is used to analyze the impact of H2-enrichment on the flame structure and combustion dynamics of a lean partially-premixed turbulent CH4/Air swirling flame. Experimentally, the combustor is operated at atmospheric pressure with H2 fuel fractions of up to 50%, by volume. LES-CHT results are compared and validated against time-resolved stereo PIV, OH* chemiluminescence, OH-PLIF imaging and acoustic pressure measurements. In terms of dynamics, for the pure CH4 and 20% of H2 enrichment cases, no thermoacoustic oscillation is observed in either the experimental or numerical data. As the fuel fraction of hydrogen is increased, the flame length reduces due to the increase in laminar flame speed and the heat release rate distribution becomes more compact. CHT simulations reveal that H2-enrichment leads to higher temperatures at the centerbody tip. At 50% H2, in agreement with experiments, LES predicts a bi-modal thermoacoustic oscillation, with two main frequencies corresponding to the quarter and chamber modes of the system. Dynamic Mode Decomposition is performed on the measured OH-PLIF images and LES 3D fields to extract each mode contribution to the overall flame dynamics. It is observed that both modes are characterized by local variations of equivalence ratio, while only the higher frequency (chamber) mode is characterized by vortices periodically detaching from the backplane and the centerbody walls causing a strong periodic wrinkling of the flame front during the thermoacoustic oscillation.

Modulation of wall turbulence by propagating flame of premixed hydrogen–air combustion

The mechanism of modulation of turbulent structures in the vicinity of a combustion engine wall is not yet clarified. Thus, we conducted direct numerical simulations of wall turbulence based on premixed hydrogen–air combustion using a detailed chemical reaction mechanism to clarify the mechanism of modulation of turbulence structures such as quasi-streamwise vortices by a flame propagating toward a wall. In particular, the turbulence modulation can alter the momentum and heat fluxes through the wall surface. In the simulations, we solved the fundamental equations for the compressible flow of multiple chemical species under combustion. The flame propagation speed was affected by the turbulence and was 2.04 times larger than that of the laminar flame. Moreover, upon examining the variation in the flow field, we determined that the turbulence kinetic energy in the flow field increased temporarily owing to the newly produced vortices along the flame surface including the existing turbulence vortices. In contrast, the turbulence vortices along the wall were suppressed as the flame approached the wall, and the turbulent flow tended to become laminar. In terms of vorticity transport equation, the suppression of the turbulence vortices was caused by the thermal expansion of the combustion rather than the increased viscosity. Furthermore, we determined that the vortex structure slightly influenced the chemical reaction process, but it strongly influenced the heat release rate distribution and concentration distribution of the chemical species. The current findings on the stated mechanism will aid in improving the accuracy of practical prediction and control methods for wall turbulence with combustion.

Exhaust gas recirculation effects on flame heat release rate distribution and dynamic characteristics in a micro gas turbine

Exhaust gas recirculation (EGR) is an option proposed to augment the CO2 content in the exhaust gas for the efficient removal of CO2. In the field of micro-gas turbine (MGT), EGR is also a feasible solution to improve the part-load performance and fuel flexibility. This research combined EGR with an adjustable fuel feeding combustor to assess the part-load performance of a 300 kW MGT and the flame spatiotemporal characteristics. The radical chemiluminescence intensity of hydroxyl is selected to represent the heat release rate (HRR) in natural gas flames. The influence of EGR on HRR was investigated experimentally under various load ratios (50%–100%) and EGR ratios (0–20%). In addition, the temperature at the combustion chamber outlet is also measured. The results show that EGR can effectively reduce OTDF when the load ratio is high. Both the spatial distribution non-uniformity and fluctuation amplitude of HRR are suppressed after applying EGR. And EGR can also reduce the influence of mixing on HRR spatiotemporal characteristics. At last, the frequency characteristics of HRR are analyzed. The result shows that the flame frequency has a strong correlation with the characteristic frequency of turbulence.

Influence of hydrogen content and injection scheme on the describing function of swirled flames

The dynamics of V-shape swirled lean premixed methane/air flames enriched with hydrogen is examined for two injection schemes. The response of the flames submitted to harmonic flowrate modulations is compared when hydrogen is premixed with the main methane/air flow and when it is injected pure as a pilot jet, directly at the flame base. Experiments are carried out at constant thermal power. Results obtained for the flame describing function (FDF) show that for a given hydrogen content, the premixed and pilot injection strategies lead to drastically different responses, although the shape of these flames are close. In the fully premixed strategy, the frequency bandwidth over which the flame is very responsive widens as the flame shortens due to the higher reactivity of the hydrogen enriched combustible mixture. Increasing the hydrogen content leads to an increased receptivity of the premixed flame sheet to incident flow perturbations. The opposite effect is seen for the pilot injection strategy due to a rebalancing of the heat release rate distribution along the flame brush with higher reaction rates close to the flame base compared to the reference methane/air case and the fully premixed hydrogen injection strategies. With hydrogen pilot injection, heat release rate fluctuations at the flame base interfere with those further downstream along the reaction layer, leading to an overall reduction of the FDF gain. This mechanism is evidenced with a set of experiments and confirmed by a low order model that considers a non uniform distribution of the heat release along a wrinkled flame sheet. It is also shown to persist when the forcing level is varied. These experiments indicate that the FDF of swirling V-flames can be lowered over a broad frequency range with hydrogen piloting due to a higher reactivity at the flame base.

Improved delayed detached eddy simulation of supersonic combustion fueled by liquid kerosene

The purpose of this study is to quantitatively investigate the influence of diffusion characteristics and equivalence ratios (ERs) of gaseous/liquid kerosene on transient combustions in a three-dimensional cavity-based scramjet combustor using Improved Delayed Detached Eddy Simulation (IDDES) with a 19 species and 54 reactions kerosene/air mechanism. Additionally, the similarities and differences between gaseous and liquid kerosene supersonic combustion are identified based on the pressure, mixture fraction, temperature, and heat release rate distributions. The findings indicated that the injection velocity of liquid kerosene is an order of magnitude lower than that of gaseous kerosene; however, the residence time of liquid kerosene in the cavity was amplified by two orders of magnitude. The results also highlighted the substantial differences in the reaction heat release position between gaseous and liquid kerosene combustion. For a combustion process of liquid kerosene at an ER of 0.215, there is no obvious boundary layer separation in the isolator. The combustion process is controlled by the mixing efficiency of the shear layer, and the mode of combustion is cavity shear-layer stabilized combustion. When the ERs are 0.27 and 0.43, the flame propagates upstream of the cavity and forms boundary layer separation and oblique shock waves. Then, the combustion process is controlled by the fuel transportation in the cavity recirculation zone, and the mode of combustion is the cavity recirculation-zone stabilized combustion.

Reconstruction model for heat release rate based on artificial neural network

Optimizing the distribution of heat release rate (HRR) is the key to improve the performance of various combustors. However, limited by current diagnostic techniques, the spatial measurement of HRR in many realistic combustion devices is often difficult or even impossible. HRR prediction is theoretically possible through establishing correlations between HRR and other quantities (e.g., chemiluminescence intensity) that can be experimentally determined; however, up to now, few universal correlations have been established. A novel artificial neural network (ANN) approach was adopted to build the mapping relationship between the combustion heat release rate and the measurable chemiluminescent species. Proper orthogonal decomposition (POD) technology is used to extract the combustion physics and reduce the data of the spatial-temporally high-resolution combustion field. The correlation between the reduced-order HRR and chemiluminescent species is built using an ANN model. A unique segmentation approach was proposed to improve the training efficiency and accuracy. Validation in a supersonic hydrogen-oxygen nonpremixed flame proves the accuracy and efficiency of the proposed HRR reconstruction model based on the reduced-order POD method and data-driven ANN model.

Experimental analysis of two-period quasi-periodic oscillations in a turbulent hydrogen combustor

The thermo-acoustic instability behavior of non-premixed hydrogen combustion is analyzed in the present work with the inlet Reynolds number, Re as the control parameter. It is observed that the dominant frequency (mode) shifts from the fundamental duct-acoustic mode to the first harmonic, as Re is varied. Continuous wavelet transform of the pressure oscillations reveals that at different Re, the dynamical nature of oscillations is topologically different, although they have identical spectral content. An interesting dynamical behavior identified as two-period quasi-periodic oscillations is observed, which has not been reported in prior literature. The state is seen to be unstable to incremental changes in Re and transitioning to limit cycle oscillations subsequently. Further diagnosis employing time-resolved OH* chemiluminescence and particle image velocimetry reveals the amplitude modulation of the quasi-periodic oscillation as a result of staggered vorticity distribution across the bluff body which results in asymmetric heat release rate distribution. A general framework for such oscillations is then applied, leading to identifying specific conditions that need to be satisfied to enable the existence of these oscillations. This study leads to novel observations on the asymmetric nature of shear layers, which can result in hitherto observed dynamical states.

Direct numerical simulation of DME auto-ignition with temperature and composition stratification under HCCI engine conditions

Two-dimensional (2D) direct numerical simulation (DNS) was used to reveal the effect of temperature and composition stratifications on the ignition characteristics of dimethyl-ether (DME)/air mixture at three initial mean temperatures. The simulation was conducted within a constant volume with an isotropic turbulence condition under 40 bar. Nine 2-D cases with temperature stratification (T′), composition stratification (Ф′) and different T-Ф correlations (negative-correlated, uncorrelated, positive-correlated) were performed under the mean initial temperature of T0 = 780 K, 840 K and 1037 K. The results show that the mean heat release rate (HRR) distribution largely depends on T0 when temperature stratification was applied. When T0 = 780 K, T′ can obviously reduce the maximum of mean HRR and prolong combustion duration compared with 0D case. However, the HRR profile is barely changed for T0 = 940 K and 1037 K. Whereas, it was found that composition stratification can reduce the peak value of mean HRR and spread out HRR distribution regardless of initial mean temperature. For first ignition stage, the temperature stratification has larger effect on reducing HRR than composition stratification. Three cases with different T-Ф correlations all reduce the peak value of HRR at first ignition stage. However, for high temperature stage, UC and PC T-Ф field can largely reduce the peak value of HRR and prolong combustion duration while NC T-Ф field shows no visible effect on HRR. In addition, two different combustion modes i.e. flame propagation mode and volumetric auto-ignition mode were distinguished according to HRR contours, budget term and volume averaged temperature gradient. And the mass fraction of YOH+HO2 and YH are the more reasonable choice to track the flame front.

A numerical study of the effect of nozzle diameter on diesel combustion ignition and flame stabilization

The role of nozzle diameter on diesel combustion is studied by performing computational fluid dynamics calculations of Spray A and Spray D from the Engine Combustion Network. These are well-characterized single-hole sprays in a quiescent environment chamber with thermodynamic conditions representative of modern diesel engines. First, the inert spray evolution is described with the inclusion of the concept of mixing trajectories and local residence time into the analysis. Such concepts enable the quantification of the mixing rate, showing that it decreases with the increase in nozzle diameter. In a second step, the reacting spray evolution is studied focusing on the local heat release rate distribution during the auto-ignition sequence and the quasi-steady state. The capability of a well-mixed-based and a flamelet-based combustion model to predict diesel combustion is also assessed. On one hand, results show that turbulence–chemistry interaction has a profound effect on the description of the reacting spray evolution. On the other hand, the mixing rate, characterized in terms of the local residence time, drives the main changes introduced by the increase of the nozzle diameter when comparing Spray A and Spray D.

Characterization of heat release rate by OH* and CH* chemiluminescence

As single component chemiluminescence cannot accurately measure the distribution of heat release rate, this paper put forward the idea of employing OH* and CH* chemiluminescence for the measurement. However, it is difficult to characterize the relationship between chemiluminescence and heat release rate, so the method of deep learning was applied to process numerical simulation results of methane-air steady premixed flames and a deep neural network model was developed to determine the distribution of heat release rate with OH* and CH* chemiluminescence. The evaluation indexes of the model performed satisfactorily: root mean square error of the normalized heat release rate is less than 0.13, mean relative error of peak and opening distance is below 3%, and mean error of peak position less than 0.01 mm. The validation results showed that OH* and CH* chemiluminescence could properly measure the distribution of heat release rate: relative error of peak ranges from −6% to 6%; on test dataset, correlation coefficient between predictive results and simulation results is above 0.99 in terms of heat release rate peak, peak positions and opening distance.

DNS study of the optimal heat release rate marker in premixed methane flames

The objective of this study is to identify a chemical marker combination that can be measured by Laser-Induced Fluorescence and used for predicting the heat release rate distribution in premixed methane-air flames at various equivalence ratios. Direct Numerical Simulations of turbulent methane flames at different equivalence ratios have been performed to obtain an indirect but accurate estimation of heat release based on all possible chemical markers. Two detailed kinetic mechanisms have been considered to check that a robust estimation is obtained. The simulation results are post-processed and analyzed by a pixel-to-pixel image comparison. At the difference of other studies, the normalized distribution of heat release and the reconstruction based on chemical markers are compared directly, so that it is possible to investigate as well geometric flame parameters, like flame thickness, for specific regions of heat release. Optimal marker combinations for measurable species have been combined with corresponding exponents. Finally, for methane flames with an equivalence ratio ranging from 0.6 to 1.4, it is found that the marker combination ([OH]1.07 × [CH2O]1.17) (where [ ] denotes the normalized concentration) leads to a very high reconstruction quality for heat release, particularly so at high levels of heat release. Alternatively, ([H]1.11 × [CH2O]0.86) leads to an even better approximation of the whole heat release field. However, OH-based markers are easier to measure with a good accuracy.

Heat release rate variations in high hydrogen content premixed syngas flames at elevated pressures: Effect of equivalence ratio

Three-dimensional direct numerical simulations with detailed chemistry were performed to investigate the effect of equivalence ratio on spatial variations of the heat release rate and flame markers of hydrogen/carbon monoxide syngas expanding spherical premixed flames under turbulent conditions at elevated pressures. The flame structures and the heat release rate were analysed and compared between fuel-lean, stoichiometric and fuel-rich centrally ignited spherical flames. The equivalence ratio changes the balance among thermo-diffusive effects, Darrieus–Landau instability and turbulence, leading to different flame dynamics and the heat release rate distribution, despite exhibiting similar cellular and wrinkling flames. The Darrieus–Landau instability is relatively insensitive to the equivalence ratio while the thermo-diffusive process is strongly affected by the equivalence ratio. As the thermo-diffusive effect increases as the equivalence ratio decreases, the fuel-lean flame is more unstable than the fuel-rich flame with the stoichiometric flame in between, under the joint effects of the thermo-diffusive instability and the Darrieus–Landau instability. The local heat release rate and curvature display a positive correlation for the lean flame, no correlation for the stoichiometric flame, and negative correlation for the rich flame. Furthermore, for the fuel-lean flame, the low and high heat release rate values are found in the negative and positive curvature zones, respectively, while for the fuel-rich flame, the opposite trends are found. It is found that heat release rate markers based on species concentrations vary strongly with changing equivalence ratio. The results suggest that the HCO, HO2 concentrations and product of OH and CH2O concentrations show good correlation with the local heat release rate for H2/CO premixed syngas-air stoichiometric flame under turbulent conditions at elevated pressures.

Numerical investigation of the effect of pressure on heat release rate in iso-octane premixed turbulent flames under conditions relevant to SI engines

A series of direct numerical simulations (DNS) of iso-octane/air turbulent premixed flames in the thin reaction zones regime have been performed in order to investigate the effect of pressure on heat release rate under conditions relevant to spark-ignition (SI) engines (up to 20 bar and 800 K in the unburnt gas). Chemistry is represented by a reduced kinetics mechanism containing 74 species and 976 reactions (reduced from CaltechMech). The effect of pressure has been isolated by fixing the Karlovitz numbers, the Lewis numbers, and the ratio of integral length scale to laminar flame thickness. On one hand, the results suggest that pressure has very limited effect on the mean heat release rate, such that turbulent burning velocity is proportional to the turbulent surface area. In addition, while the chemical pathways are strongly affected by pressure in laminar flames, the global effect of turbulence on these pathways is negligible, independent of pressure. On the other hand, the local distribution of heat release rate was found to be significantly affected by pressure through differential diffusion-chemistry effects.

Different Flame Patterns Linked With Swirling Injector Interactions in an Annular Combustor

Experiments in cold and reactive conditions carried out in linear arrays of injectors indicate that the flows established by neighboring injectors exhibit alternating patterns if the distance between two injectors is too small (see, for example, ASME-GT2013-94280, GT2014-25094, and GT2015-42509). This issue is investigated in this article by making use of a recently developed annular combustion chamber (ACC). This device designated as MICCA is equipped with multiple swirling injectors and its side walls are made of quartz providing full optical access to the flame region, thus allowing detailed studies of the combustion region structure and dynamics. Experiments reported in this article rely on direct observations of the flame region through light emission imaging using two standard cameras and an intensified high-speed CMOS camera. The data gathered indicate that interactions between successive injectors give rise to patterns of flames which exhibit an alternate geometry where one flame has a relatively low expansion angle while the next spreads sideways. This pattern is then repeated with a period which corresponds to twice the injector spacing. Such arrangements arise when the angle of the cup used as the end-piece of each injector exceeds a critical value. The effects of mass flow rate, equivalence ratio, and injector offset are also investigated. It is shown that the angle which defines the cup opening is the main control parameter. It is also found that when this angle exceeds a certain value and when the laminar burning velocity is fast enough, the flame pattern switches in an unsteady manner between two possible configurations. It is shown that these alternating flame patterns lead to alternating heat release rate distributions and inhomogeneous heat transfer to the chamber walls featuring a helicoidal pattern. Conditions leading to alternating flame patterns are finally discussed by making use of a recent flow regime diagram.

Computation of Forced Premixed Flames Dynamics

ABSTRACTBluff body stabilized turbulent premixed flames subject to inlet velocity oscillation over a wide range of forcing frequency and amplitude are simulated using a flamelet-based combustion model. Two sets of detailed chemical kinetic schemes are used to model combustion chemistry. It is observed that the computed dynamics of forced flames agree reasonably well with experimental measurements. The flame elongation and shortening at a frequency of 40 Hz and strong flame-vortex interaction at a higher frequency of 160 Hz are captured well in the computations. The global flame describing function extracted from the computational results shows a linear response at 40 Hz and a nonlinear behavior at 160 Hz as observed in the experiments. The nonlinear response is due to vortex roll-up and its subsequent shedding. The quantitative agreement of the computed flame describing function (FDF) with experimental measurement is uniformly good over a wide range of forcing frequency and amplitude. Some influence of chemical kinetics on the FDFs is observed, which mainly stems from the difference in laminar burning velocity and spatial heat release rate distribution.

Some aspects of stabilization and structure of laminar premixed hydrogen-air flames in a microchannel

In the present paper, flame stabilization and structure are investigated numerically for non-adiabatic hydrogen-air flames at different equivalence ratios and inlet velocities. A cylindrical microcombustor in which combustion occurs in the annular region between two concentric tubes is investigated. The inner hollow tube contains static nitrogen gas and this combination acts as a thermal reservoir that stores and recirculates heat to the incoming mixture. Investigations are carried out using detailed numerical model incorporating two-dimensional transport, thermal radiation, multi-step kinetics, and conjugate heat transfer. Flame is sustained by the continuous ignition mechanism activated by an uninterrupted temperature field between gas mixture and wall developing at steady state. Heat losses from flame resulted in a crossover temperature higher than that of the lean-limit and stoichiometric free flames due to slow radical build-up. Thermal diffusion of hydrogen is shown to be responsible for enhancing the burning intensity of leading edge. Diffusion and reaction kinetics at the flame tip result in twin-peaked heat release rate distribution, most prominently for fuel rich flame (ϕ = 1.7).