Mean Flame Research Articles

Optical diagnostics and multi-point pressure sensing on the knocking combustion with multiple spark ignition

Engine knock is an abnormal combustion phenomenon that limits the thermal efficiency and service life of spark-ignition engines. A better understanding of the knock mechanisms and characteristics is beneficial to knock alleviation and engine efficiency improvement. In this study, a metal liner with four evenly-spaced spark plugs in the periphery of the combustion chamber is designed to initiate knock from different positions. Four spark strategies are applied to the single-cylinder optical research engine and six pressure sensors are utilized to analyze the local pressure oscillations in the cylinder. The knocking combustion is investigated by simultaneous 72 kHz high-speed imaging and 6-point pressure sensing. The experimental results indicate that using multiple spark-ignition could promote knock intensity, advance the start of auto-ignition and introduce more acoustic resonance modes. The center pressure sensor is more sensitive to the first radial resonant mode (0, 1) of the knock pressure oscillation, while the side sensors are more sensitive to the first and second circumferential resonant modes (1, 0) and (2, 0). The knock onset judged by natural flame photography is earlier than that by pressure analysis because the auto-ignition event happens first and induces the subsequent pressure fluctuation. Natural flame luminosity analysis demonstrates that the initial auto-ignition sites only cause weak pressure oscillations, and the instantaneous combustion of the remaining end-gas increases the heat release rate significantly and gives rise to more violent pressure oscillations. Statistically, the maximum amplitude of pressure oscillation follows an exponential relationship with the peak mean flame luminosity. The end-gas resides in the gaps among the flame fronts generated by different spark strategies while the first auto-ignition sites are not evenly distributed in the end-gas zone. This fact gives insights into the local temperature non-uniformity of the end gas zone that affects the spatial distributions of the initial auto-ignition sites in the cylinder.

Acoustic-Convective Interference in Transfer Functions of Methane/Hydrogen and Pure Hydrogen Flames

Abstract We investigate the occurrence and source of modulations in the gain and phase of flame transfer functions (FTF) measured in perfectly premixed, bluff body stabilized CH4/H2 and pure H2 flames. The modulations are shown to be caused by flow disturbances originating from the upstream geometry, in particular the grub screws used to center the bluff body, indicative of a more generalized phenomenon of convective wave propagation. Velocity measurements are performed at various locations around the injector dump plane, inside the injector pipe, and in the wake of the bluff body to provide detailed insight into the flow. Peaks corresponding to natural shedding frequencies of the grub screws appear in the unforced velocity spectra and it is found that the magnitude of these convective modes depends on their location. Flame imaging and PIV measurements show that these disturbances do not show up in the mean velocity and flame shape which appear approximately axisymmetric. However, the urms and vrms fields capture a strong asymmetry due to convective disturbances. To further quantify the role of these convective disturbances, hydrodynamic transfer functions are constructed from the forced cold flow, and similar modulations observed in the FTFs are found. A strong correlation is obtained between the two transfer functions, subsequently, the modulations are shown to be centered on the vortex shedding frequency corresponding to the first convective mode. The reason behind the excitation of the first mode is due to a condition that states that for acoustic-convective interaction to be possible, the shedding (convective) frequency needs to be lower than the cutoff frequency of the flame response. This condition is shown to be more relevant for hydrogen flames compared to methane flames due to their shorter flame lengths and thus increased cutoff frequency.

Influence of molecular transport on burning rate and conditioned species concentrations in highly turbulent premixed flames

Apparent inconsistency between (i) experimental and direct numerical simulation (DNS) data that show the significant influence of differential diffusion on the turbulent burning rate and (ii) recent complex-chemistry DNS data that indicate mitigation of the influence of differential diffusion on conditioned profiles of various local flame characteristics at high Karlovitz numbers, is explored by analysing new DNS data obtained from lean hydrogen–air turbulent flames. Both aforementioned effects are observed by analysing the same DNS data provided that the conditioned profiles are sampled from the entire computational domain. On the contrary, the conditioned profiles sampled at the leading edge of the mean flame brush do not indicate the mitigation, but are significantly affected by differential diffusion phenomena, e.g. because reaction zones are highly curved at the leading edge. This observation is consistent with a significant increase in the computed turbulent burning velocity with decreasing Lewis number, with all the results considered jointly being consonant with the leading point concept of premixed turbulent combustion. The concept is further supported by comparing DNS data obtained by allowing for preferential diffusion solely for a single species, either atomic or molecular hydrogen.

Impact of wall heat transfer in Large Eddy Simulation of flame dynamics in a swirled combustion chamber

Large Eddy Simulation (LES) is a fundamental research tool to study gas turbines and aero-engine combustors. In LES, although rarely addressed systematically, it is known that thermal boundary conditions control the heat transfer between the flow and the combustor walls. This work presents a study on the impact of thermal wall boundary conditions for the PRECCINSTA test bench, operated by the German Space Agency (DLR). Two approaches are tested: Heat Resistances Tuning (HRT), where a local resistance is tuned using experimental temperature data, and full Conjugate Heat Transfer (CHT), where the chamber wall-temperature is solved and coupled to the flow computation. Results reveal that the HRT method captures the mean flame correctly but the predicted flame becomes unstable and responds to a thermoacoustic oscillation which is not observed experimentally. On the contrary, using CHT, the flame is correctly predicted and stable as in the experiments. Finally, to understand the differences between the HRT and the CHT simulations, Dynamic Mode Decomposition (DMD) analysis is performed showing that the correct response of the flame branches to the pressure oscillations is recovered only in the CHT simulations for which thermoacoustically stable operations are retrieved.

Assessment of Bray Moss Libby formulation for premixed flame-wall interaction within turbulent boundary layers: Influence of flow configuration

Three-dimensional direct numerical simulations (DNS) of two different flow configurations have been performed for premixed flames interacting with chemically inert isothermal walls at the unburned gas temperature in fully developed turbulent boundary layers. The first configuration is an oblique flame-wall interaction (FWI) of a V-flame in a turbulent channel flow and the second configuration is a head-on quenching planar flame in a turbulent boundary layer. These simulations are representative of stoichiometric methane-air mixture at unity Lewis number under atmospheric conditions. The turbulence in the non-reacting conditions for these simulations is representative of the friction velocity based Reynolds number of Reτ=110. Differences in the statistical behaviours of the mean values of progress variable, temperature, and density during the FWI process have been reported for the two configurations. It is found that the mean flame brush thickens in the near wall region leading to significant departures from the strict Bray Moss Libby (BML) formulation limit during the FWI process and that is reflected in the probability density function (PDF) distributions of c for both flame configurations. The closures from the BML formulation for Reynolds averaged progress variable c¯ and the Favre averaged variance of the progress variable c′′2˜ have also been investigated and it is found that these closures need to be modified to account for the FWI process even when the flame away from the wall represents high Damköhler number premixed turbulent combustion. Furthermore, the statistical behaviours of the quantities required for Flame Surface Density (FSD) based mean reaction rate closure including the flame orientation factor σy, the flamelet length scale Ly and the flame stretch factor I0 have been interrogated from the DNS data for the two flame configurations. The flamelet length scale and the stretch factor extracted from the DNS data are compared with the closures for these quantities proposed in the literature. It is found that σy exhibits significant spatial variation for both cases. The existing closures for Ly and I0 which exhibit the best quantitative agreement with DNS data have been identified. It has been found that the models for Ly and I0 have scopes for further improvement to enable satisfactory predictions of these quantities during the FWI process within turbulent boundary layers.

Effect of Ionic Wind Induced by DC Electric Field on Biogas/Air Turbulent Premixed Flame Structure

ABSTRACT Effect of DC electric field on lean premixed biogas turbulent flame structure at low and medium turbulence intensity was studied experimentally using OH-PLIF. OH signal distribution, turbulent burning velocity and flame structure geometric parameters were derived. Results show that downward electric field could broaden the downstream burned gas and push the flame tip down. The flame base is closer to the burner rim in downward electric field for the modification of thermal diffusion and flow modification caused by ionic wind. The effect on flame tip height reduction is more obvious for biogas flame with a higher CO2 ratio. According to the flame structure analysis, the turbulent burning velocity of biogas is increased slightly due to the modification of flow field by ionic wind generated by downward electric field, and the increment decreases with the increase of turbulence intensity. In addition, there’s a decrease in mean flame surface density and an increase in centerline flame brush thickness. The flame surface curvature PDF distribution is nearly not affected by the application of DC electric field. For turbulent flames, the effect of electric field is non-uniform and inconstant, which induces extra flow perturbation. This perturbation is different from turbulence, and the combined effects lead to the variation of flame structure.

Statistics of local and global flame speed and structure for highly turbulent H[formula omitted]/air premixed flames

A statistical analysis is conducted for turbulent hydrogen-air premixed flames at a range of Karlovitz numbers up to 1,126 by direct numerical simulations (DNS) with detailed chemistry. The local and global burning velocities are evaluated and the deviation from the laminar flame speed is assessed. It is found that the global turbulent flame speed is largely determined by the integral length scale than the turbulent Karlovitz number, due to the flame surface area enhancement. The turbulent flame speed in all examined cases correlates well with the flame surface area, according to Damköhler’s first hypothesis; even at Karlovitz number well above 1,000, reaction zones stay intact and only the preheat zone is broadened by the strong turbulence level. The statistical analysis with the probability density function (PDF) for the displacement speed shows that the highest probability of the local flame speed coincides with the one-dimensional unstretched flame speed. Despite some deviations, the mean flame structures and reaction rate of hydrogen of the higher Ka cases are found to resemble those of the laminar flame, and this further confirms that the turbulent flame brush topology is mainly determined by the large scale turbulence behavior. The results also suggest that the engineering modeling based on the flamelet concept may be valid for a wider range of Ka conditions.

Experimental Investigation Into the Role of Mean Flame Stabilization on the Combustion Dynamics of High-Hydrogen Fuels in a Turbulent Combustor

Abstract A bluff-body turbulent combustor is mapped for its thermo-acoustic stability across variation in airflow rate, nondimensionalized as the Reynolds number (Re) and fuel composition. The combustor stability is evaluated for three fuels, namely, pure hydrogen (PH), synthesis natural gas (SNG), and syngas (SG). The combustion dynamics display markedly different behavior across the fuels, in the extent of the unstable region, as well as the observed dominant Eigenvalues. At low Re, SNG displays stable combustion, while SG exhibits high amplitude oscillations at the fundamental duct acoustic mode. As the Re is increased, SNG displays very high amplitude oscillations at the duct acoustic mode, while SG exhibit relatively low amplitude oscillations at the third harmonic. In the case of PH, high amplitude oscillations observed at higher Re at the first harmonic. These peculiarities are investigated in light of the role of mean flame stabilization. The combustion dynamics of the fuels is influenced by the global equivalence ratio, as well as the jet momentum ratio. These effects significantly demarcate the dynamics of SNG and SG combustion. This is seen manifested in the mean flame structure of flame at high amplitude oscillations, whereby result in SNG flame to be present in the wake, while the SG flame resides in the shear layer. The driving by the flame because of their mean stabilization is quantified by a spatial Rayleigh index. It confirms the presence of large driving regions for SNG compared to that of SG, results in the observed differences in amplitude of the oscillations.

Experimental study on the effect of canyon cross wind yaw angle on airflow and flame characteristics in a tunnel

The effect of canyon cross wind yaw angle on airflow and flame characteristics of ethanol pool fires in a tunnel was investigated. The fire source location and canyon cross wind speed were studied as factors. The wind yaw angle was ranged in 45°–135° with an interval of 15°. The results showed that the flow field in the tunnel presented three different conditions with the boundaries of wind yaw angles 60° and 90°. The mass burning rate and flame tilt characteristics were affected by the flow field near the fire source. The mass burning rate decreased with the increasing wind yaw angle when the wind yaw angle was in 45°–90°, but remained steady when the angle was in 90°–135°. Additionally, a normalized mass burning rate model considering normalized distance to the upstream portal and the wind yaw angle was developed. The flame close to the upstream portal wiggled, rotated and oscillated irregularly due to the influence of turbulence induced by the external canyon cross wind. This made mean flame length and flame tilt angle in turbulence zone different from those in other zones. A prediction correlation between the flame tilt angle and the canyon cross wind speed and yaw angle was established.

Effect of DME addition on turbulent flame structure in lean premixed CH4/DME/air mixtures

An experimental study was conducted on DME addition to lean premixed CH4/air Bunsen flame in order to investigate differential diffusion and turbulence effects on flame structure. Three DME volume fractions of 0, 20% and 50% were adopted in the presence of carefully controlled laminar burning velocity at a near-constant level to highlight the differential diffusion effects. By extracting the flame front from the OH-PLIF images, the flame front curvature, turbulent burning velocity and mean flame surface density were obtained and discussed. Results show that DME addition and turbulence will eventually make the curvature distribution more symmetrical. In addition with the turbulence intensity increasing, the DME addition seems to have minor effect on curvature. The turbulent burning velocity is decreased with DME addition, which is in agreement with earlier research results for Le > 1 mixtures. In this study, we prove that the mean flame surface density at different height can qualitatively represent the local consumption speed (ST,LC). DME addition only changes the ST,LC distribution structure of the flame, while turbulence not only changes the structure, but also increases the maximum ST,LC value. It is also found that the decrease of turbulent burning velocity (ST,GC), caused by DME addition at low turbulence intensity, is mainly manifested by an increase of the flame height. Moreover at thin reaction zone, the interaction between turbulence and differential diffusion effect promotes the change of ST,LC distribution. Therefore, the effect of differential diffusion on ST,GC is enhanced by turbulence as the diffusion layer is broadened.

On the impact of fuel injection angle in Euler–Lagrange large eddy simulations of swirling spray flames exhibiting thermoacoustic instabilities

This study deals with the fundamental problem of combustion dynamics in gas turbine combustors operating with liquid fuel. In this framework the present work proposes the study of an academic liquid fueled combustor sensitive to thermoacoustic instabilities, simulated via high-fidelity Large Eddy Simulations. The experimental setup addressed is SICCA-spray from EM2C laboratory featuring both stable and unstable flames depending on the combustion chamber length. The proposed analysis, based on the Euler-Lagrange modeling approach, studies the impact of the spray injection angle θ on both the stable flame and the triggering of the longitudinal combustor acoustic mode when using a longer quartz tube. For the liquid injection modeling, the FIM-UR semi-empirical model is adopted with three different θ values: θ=35∘,45° and 60°. In stable conditions, the spray angle is proven to have a negligible impact on the flame anchoring point, however, the mean flame length and fuel distribution are found to be slightly modified by the velocity at which droplets enter the combustion chamber. For the thermoacoustically unstable conditions, two well-established stable limit cycles with the same frequency and similar amplitudes are obtained when fuel is injected at θ=45∘ and 60°. Contrarily, the system stabilizes when θ=35∘ pointing to the importance of the dynamics of the liquid film layer formed inside the injector for this setup. Likewise, this liquid film layer dynamics and its modeling appear critical as already suggested by previous studies on the same configuration. The detailed analysis of the thermoacoustically unstable different predictions is then performed through the investigation of the spatial fields of the local Rayleigh index obtained following the novel extension in the frequency domain of the Rayleigh criterion complemented by the application of Dynamic Mode Decomposition. It confirms that the injection angle of the liquid spray has a significant effect on the thermoacoustic response of the system. Indeed the influence of θ on the dynamics of the liquid fuel when entering the combustion chamber is proven to have an impact on the synchronization mechanism governing the liquid phase with respect to acoustics sustaining the observed limit cycles. More specifically, couplings at the liquid phase level are evidenced by introducing two novel indices correlating the fluctuations of liquid fuel volume fraction and evaporation rate with pressure.

Inhibition of turbulent methane/air premixed bunsen flames by dimethyl methylphosphonate

There is little knowledge about the inhibition efficiency of dimethyl methylphosphonate (DMMP) against turbulent flames, in which vortices of different sizes corrugate and stretch the flame front. This study is to investigate the performance and the mechanism of DMMP in inhibiting turbulent methane/air premixed Bunsen flames. Several important parameters are chosen to characterize the combustion of the flames, including flame brush height, flame brush thickness and turbulent burning velocity. It is found that the flame brush expands both horizontally and vertically with the increase of turbulent intensity. For all the flames, the centerline mean flame brush thickness increases linearly with flame height. Turbulent burning velocity decreases monotonically with the increasing of DMMP addition. However, the normalized turbulent burning velocity increases with the normalized turbulent intensity. Compared with the performance in laminar premixed flames, the inhibition efficiency depends on the turbulent intensity. For low-intensity turbulent flames, the inhibition efficiency is higher than that in laminar premixed flames, but the relative inhibition efficiency decreases with DMMP addition. This can be attributed to the relative size of the vortices and the flame front thickness: when the Kolmogorov length scale is less than the preheat zone thickness but greater than the reaction zone thickness, vortices reduce the burning velocity; when the Kolmogorov length scale is less than the reaction zone thickness, vortices facilitate the combustion. The trend is opposite in high-intensity turbulent flames, which is mainly attributed to the generating of the flame pockets in the flame tips.

Modelling combustion in spark ignition engines with special emphasis on near wall flame quenching

A flame front is quenched when approaching a cold wall due to excessive heat loss. Accurate computation of combustion rate in such situations requires accounting for near wall flame quenching. Combustion models, developed without considering wall effects, cannot be used for wall bounded combustion modelling, as it leads to wall flame acceleration problem. In this work, a new model was developed to estimate the near wall combustion rate, accommodating quenching effects. The developed correlation was then applied to predict the combustion in two spark ignition engines in combination with the famous Bray–Moss–Libby (BML) combustion model. BML model normally fails when applied to wall bounded combustion due to flame wall acceleration. Results show that the proposed quenching correlation has significantly improved the performance of BML model in wall bounded combustion. As a second step, in order to further enhance the performance, the BML model was modified with the use of Kolmogorov–Petrovski–Piskunov analysis and fractal theory. In which, a new dynamic formulation is proposed to evaluate the mean flame wrinkling scale, there by accounting for spatial inhomogeneity of turbulence. Results indicate that the combination of the quenching correlation and the modified BML model has been successful in eliminating wall flame acceleration problem, while accurately predicting in-cylinder pressure rise, mass burn rates and heat release rates.

Reynolds number scaling of burning rates in spherical turbulent premixed flames

Abstract

Optical diagnostics on the effects of reverse reactivity stratification on the flame development in dual-fuel combustion

The effects of reverse reactivity stratification, i.e., port-injection (PI) of n-heptane with direct-injection (DI) of iso-octane and PI of iso-octane with DI of n-heptane, on combustion characteristics and flame development were investigated. The volume fractions of port-injected fuel were 30%, 70% and 90%. The DI timings were −25°, −15° and −5° CA ATDC. The optical diagnostic methods of high-speed imaging and flame emission spectra were adopted. Results show when n-heptane is port injected and iso-octane is direct injected, the PI volume fraction plays more important role in controlling combustion phasing, while the DI timing is the dominant factor on flame development structure that can be classified into four classes. When iso-octane is port injected and n-heptane is direct injected, the DI timing dominates the combustion phasing. Flame kernels merge to form larger flame area with reactivity stratification introduction compared with the condition that nearly only one initial flame kernel forms in each downstream spray without reactivity stratification introduction. The initial mean flame speed is around 80 m/s with reactivity stratification introduction, which is nearly twice faster than that without reactivity stratification introduction. Finally, when comparing conditions of reverse reactivity stratification, it provides guidance for dual-fuel combustion engines that the overall reactivity cannot be regarded as the solely important parameter to optimize combustion phasing and other combustion characteristics. At some extent, the effects of overall reactivity on combustion characteristics are lower than the effects of volume fraction, equivalence ratio, injection order or start of injection (SOI) timing of high reactivity fuel.

The localised forced ignition and early stages of flame development in a turbulent planar jet

The localised forced ignition and the early stages of the subsequent flame propagation in a planar turbulent methane/air jet in ambient air have been simulated using Direct Numerical Simulation (DNS) and a two-step chemical mechanism. Sixteen identical energy depositions events were simulated for four independent flow realisations at four different locations. The successful ignition and subsequent flame propagation have been found to be well correlated to the mean mixture fraction and flammability factor values of the energy deposition location. Furthermore, similarly to what has been observed in experiments, the early stages of flame development from the ignition kernel involved initial downstream convection of the kernel, followed by simultaneous radial expansion and downstream propagation and finally the upstream propagation of the flame base indicating the onset of flame stabilisation. The mixture composition and the scalar dissipation rate (SDR) values in the immediate vicinity of the ignitor have been identified to play key roles in determining the outcome of the external energy deposition, while the development of an edge flame structure propagating along the stoichiometric mixture fraction iso-surface was found to be necessary but not sufficient for the flame to propagate upstream. It has also been found that in the case of successful self-sustained burning, the edge flame was developing in low SDR regions, and that the most probable edge flame speed remains close to the theoretical laminar value irrespective of the flame development history. Finally, the mean flame speed of the edge flame elements propagating towards the nozzle exit has been found to be considerably greater than the unstrained laminar burning velocity. Thus, the edge flame, depending on its orientation with respect to the flow, is able to propagate upstream and initiate the onset of flame stabilisation.

Effect of Soot-Inhibitor Additives on the Thermal Structure and Soot Volume Fraction Inside Laminar Diffusion Natural Gas Flames

Abstract Soot study is a fundamental issue for the combustion process of hydrocarbon fuels. Losses in combustion efficiency, health risks, environmental loosestrife, and damage in furnaces may appear as a result of soot existence. This present paper aims at providing an experimental mapping of the changes in the soot volume fraction and axial flame mean temperature associated with the addition of different percentages of soot inhibitor additives (namely, Argon, Nitrogen, and Helium) in a vertical laminar diffusion natural gas flame issuing from a honeycomb circular burner. The soot volume fraction is acquired by the laser extinction technique, while the axial variations of the mean flame temperature are accomplished by a bare 51 µm (Pt-30%Rh versus Pt-6%Rh) thermocouple to render radiation loss insignificant. The concentration of the individual additives is varied from 5% to 25% (step 5%) and the experiments are conducted at a fixed natural gas throughput (350 mL/min) to ensure unvaried thermal input. Measurement traverses along and across (at fixed radial locations) are conducted. The fuel flowrate is measured by a precision digital gas flowmeter (type: Varian intelligent), while the flow of the individual additive is admitted via solenoid valves (handled with labview program) and is injected through mixing pipes located at burner entry. The different regimes of the soot inception (molecular; zone 1), soot growth zone (zone 2), and soot oxidation (zone 3) are accurately defined and assessed in relation to the temperature results for the different cases under investigation.

Solenoidal and potential velocity fields in weakly turbulent premixed flames

Direct Numerical Simulation data obtained earlier from two statistically 1D, planar, fully-developed, weakly turbulent, single-step-chemistry, premixed flames characterized by different (7.53 and 2.50) density ratios σ are analyzed to explore the influence of combustion-induced thermal expansion on the turbulence and the backward influence of such flow perturbations on the reaction zone. For this purpose, the actual velocity fields are decomposed into solenoidal and potential velocity subfields, followed by independently processing each subfield. Results show that the potential and solenoidal velocities are negatively (positively) correlated in unburned reactants (burned products, respectively) provided that σ=7.53. Moreover, not only potential, but also solenoidal velocity subfields contribute to countergradient turbulent scalar transport. Furthermore, correlation between strain rates generated by the solenoidal and potential velocity fields and conditioned to the reaction zone is positive (negative) in the leading (trailing, respectively) halves of the mean flame brushes. The potential strain rate correlates negatively with the flame curvature within the reaction zone, whereas the solenoidal strain rate and the curvature are negatively (positively) correlated in the leading (trailing, respectively) halves of the mean flame brushes. If σ=7.53, the stretch rate conditioned to the reaction zone is negative (positive) in regions characterized by large positive (negative, respectively) potential strain rates, whereas the opposite trend is observed for the solenoidal strain rate. Thus, the potential and solenoidal velocity fields differently affect the reaction zone. In the case of σ=2.5, such differences are significantly less pronounced. Finally, an approximate decomposition of the mean rate of viscous dissipation of flow kinetic energy into solenoidal and potential contributions is suggested and supported by DNS data. The results also show that the dilatation-induced dissipation plays an important role. Therefore, the influence of thermal expansion on the dissipation rate is not reduced to an increase in mixture viscosity in the rotational dissipation rate.

Application of Helmholtz-Hodge decomposition and conditioned structure functions to exploring influence of premixed combustion on turbulence upstream of the flame

In order to explore the influence of combustion-induced thermal expansion on turbulence, a new research method is introduced. The method consists in jointly applying Helmholtz-Hodge decomposition and conditioned structure functions to analyzing turbulent velocity fields. Opportunities offered by the method are demonstrated by using it to process Direct Numerical Simulation data obtained earlier from two statistically 1D, planar, fully-developed, weakly turbulent, single-step-chemistry, premixed flames characterized by two significantly different (7.52 and 2.50) density ratios, with all other things being approximately equal. To emphasize the influence of combustion-induced thermal expansion on turbulent flow of unburned mixture upstream of a premixed flame, the focus of analysis is placed on structure functions conditioned to the unburned mixture in both points. Two decomposition techniques, i.e. (i) a widely used orthogonal Helmholtz-Hodge decomposition and (ii) a recently introduced natural Helmholtz-Hodge decomposition, are probed, with results obtained using them being similar in the largest part of the computational domain with the exception of narrow zones near the inlet and outlet boundaries. Computed results indicate that combustion-induced thermal expansion can significantly change turbulent flow of unburned mixture upstream of a premixed flame by generating anisotropic potential velocity fluctuations whose spatial structure differ substantially from spatial structure of the incoming turbulence. The magnitude of such potential velocity fluctuations is greater than the magnitude of the solenoidal velocity fluctuations in the largest part of the mean flame brush in the case of the high density ratio. In the case of the low density ratio, the latter magnitude is larger everywhere, but the two magnitudes are comparable in the middle of the mean flame brush. Contrary to the potential velocity fluctuations, the influence of the thermal expansion on the solenoidal velocity field in the unburned mixture is of minor importance under conditions of the present study.

Application of the sparse-Lagrangian multiple mapping conditioning approach to a model supersonic combustor

The Multiple Mapping Conditioning/Large Eddy Simulation (MMC-LES) model is extended for the first time to high-speed, compressible flow conditions and validated against non-reacting and reacting experimental data from a model supersonic combustor. The MMC-LES method solves the subgrid joint composition filtered density function through a Monte Carlo approach, and it permits a low-cost numerical implementation using a sparse distribution of stochastic Lagrangian particles. The sensitivity of results to the particle resolution is examined, and similar to past low-speed applications of MMC-LES, that sensitivity is found to be low. In comparison to the model equations for subsonic turbulent combustion conditions, the pressure work and viscous heating effects have been incorporated here to account for the effects of compressibility. As expected, the viscous heating effects are small for this flow case and can be ignored, while the pressure work is not negligible and makes a significant contribution at expansion fans and shock fronts where the magnitude of the pressure derivative term in non-reacting/reacting cases is as much as 23.8%/24.5% and 19.2%/18.6% of the stochastic particle standardized enthalpy, respectively. The MMC-LES predictions show good quantitative agreement with the available experimental data for the mean and root-mean-square of axial velocity, mean temperature, and wall pressure. Good qualitative comparison to the data is also observed for major flow characteristics, including location and size of shocks, expansion fans, and recirculation zone, and combustion characteristics such as flame lift-off distance. Although the effects of the pressure work on the mean flame lift-off distance are negligible, they have a significant influence on the predicted spatial fluctuations of the flame base.