Local Flame Structure Research Articles

The stability and morphology of laminar premixed hydrogen/air flames under various gravity conditions

In the present work, direct numerical simulations (DNS) were employed to understand the stability and morphology of laminar lean premixed hydrogen/air flames under various gravity conditions. The combined effect of Darrieus–Landau (DL) instability, thermal-diffusive (TD) instability and Rayleigh–Taylor (RT) instability was studied quantitatively for both the initial linear regime and the long-term nonlinear regime, with a particular focus on the effect of RT instability. In the linear regime, it was found that the RT instability has a negligible effect on the linear growth rate of flames with an equivalence ratio of 0.4, but the effect is significant when the equivalence ratio decreases to 0.3. The performance of various models for the dispersion relation was discussed, including a theoretical model considering the effects of all instabilities, which captures well the trend of the simulated results in the cases with unity Lewis numbers. The hydrogen/air flames with an equivalence ratio of 0.3 was investigated comprehensively in the nonlinear regime. For the cases with unity Lewis numbers, the RT-unstable flame exhibits a similar single-cusp structure as the RT-neutral flame, but with a deeper cusp and larger consumption speed as a result of the destabilizing effect of RT instability. The RT-stable flame gradually returns to a planar flame shape owing to the stabilizing effect of RT instability. For the cases with non-unity Lewis numbers, the effect of domain size on the flame evolution was investigated, showing a domain width of 200δT is enough to obtain a domain-independent result, where δT is the laminar flame thickness. It was found that the cases initialized with multimode perturbations reach the quasi-steady state faster, while the flame statistics are consistent with those of the cases initialized with single mode perturbations. The effect of RT instability was explored by comparing the results from various gravity conditions. The flame consumption speed is the largest in RT-unstable flames, and the smallest in RT-stable flames. The variation in flame consumption speed is due to the difference in the flame surface area, while the local reactivity and flame structure are not influenced by the RT instability. Further fractal analysis reveals that although the RT instability causes significant differences in flame surface area, the characteristic fractal dimension remains constant for flames under different gravity conditions.Novelty and significanceIn the present work, direct numerical simulations were employed to understand the stability and morphology of RT-unstable, RT-neutral and RT-stable laminar lean premixed hydrogen/air flames. For the first time, the effect of DL, TD and RT instabilities was quantitatively examined for both the linear regime and the nonlinear regime. A theoretical model considering various instabilities was compared with the simulated dispersion relations in the linear regime. In the non-linear regime, the flame surface area and corresponding flame consumption speed are influenced by the RT instability, while the flame stretch factor remains unaffected. A fractal analysis was conducted in laminar premixed hydrogen/air flames under the influence of various instabilities. Remarkably, it was discovered that the characteristic fractal dimension remains constant for the cases with and without the RT instability.

A Neural Network-Based Flame Structure Feature Extraction Method for the Lean Blowout Recognition

A flame’s structural feature is a crucial parameter required to comprehensively understand the interaction between turbulence and flames. The generation and evolution processes of the structure feature have rarely been investigated in lean blowout (LBO) flame instability states. Hence, to understand the precursor features of the LBO flame, this work employed high-speed OH-PLIF measurements to acquire time-series LBO flame images and developed a novel feature extraction method based on a deep neural network to quantify the LBO features in real time. Meanwhile, we proposed a deep neural network segmentation method based on a tri-map called the Fire-MatteFormer, and conducted a statistical analysis on flame surface features, primarily holes. The statistical analysis results determined the relationship between the life cycle of holes (from generation to disappearance) and their area, perimeter, and total number. The trained Fire-MatteFormer model was found to represent a viable method for determining flame features in the detection of incipient LBO instability conditions. Overall, the model shows significant promise in ascertaining local flame structure features.

Mechanisms Leading to Stabilization and Incomplete Combustion in Lean CH4/H2 Swirling Wall-Impinging Flames

Abstract In gas turbines, confined highly turbulent flames unavoidably propagate in the vicinity of a relatively cool combustor liner, affecting both the local flame structure and global operation of the combustion system. In our recent work, we demonstrated, using simultaneous [OH] × [CH2O] planar laser-induced fluorescence (PLIF) and stereo-particle image velocimetry (stereo-PIV), that lean CH4/H2 flames at a high Karlovitz number can present a highly broken structure near wall, highlighted by a diffuse CH2O cloud, which suggests local quenching and incomplete oxidation. Such high Karlovitz numbers were achieved using an inclined plate, which substantially extended the lean flammability of the low swirl flames. Yet, how a cooled wall acting as a heat sink played a conducive role in stabilizing high Ka flames remains unanswered. In addition, the origin of the CH2O cloud is also unclear. Hence, in this work, we look to better understand the stabilization mechanisms for lean and ultralean flames on the same configuration, and how they may change with a parametric variation of plate incident angle, plate-nozzle distance, and bulk velocity up to the critical values that lead to flame blow off. The results show that the impinging swirling flow creates a low speed region that helps hold the flame, while the wall prevents mixing with ambient cold air. The production of diffuse CH2O, which indicates the occurrence of local quenching, is associated with a mean strain rate K beyond the extinction strain rate (ESR) Ke. For CH4 flames, most of the reaction zones reside within |K|/Ke<1; for 70% H2 flames at ϕ=0.4, the reaction zones are highly broken and scattered in a large area, where |K|/Ke<8, the interspace of which is fully filled by CH2O. In other words, high H2 fraction flames appear to be more robust to persistent strain rate, thus extending their stability envelope. However, these flames can subsist as highly broken flames featuring strong incomplete combustion.

On the subgrid dissipation concept for large eddy simulation of turbulent combustion

A new subgrid turbulent combustion model for LES is derived based on the eddy dissipation concept (EDC). To be consistent with the LES framework, the relationship between the subgrid quantities and the fine structure ones is built from the unresolved part of the energy cascade, hence the subgrid dissipation concept (SDC). The fine structure quantities are directly computed using the subgrid scale ones. Resolving local laminar flame structures is allowed if the grid resolution permits. The model is examined using premixed piloted jet flames and non-premixed swirl flame. Satisfactory predictions from the SDC model are obtained in the test cases, showing improvements with respect to the standard EDC model. The new SDC model is relatively insensitive to the grid resolution.

Direct numerical simulations of methane, ammonia-hydrogen and hydrogen turbulent premixed flames

Three turbulent premixed flames with the same unstretched laminar flame speeds and thicknesses are analyzed and compared using 3D Direct Numerical Simulation (DNS) in a slot burner configuration at atmospheric conditions: a CH4/air flame at ϕ=1, an NH3−H2/air (46% vol. H2) at ϕ=1 and an H2/air flame at ϕ=0.45.While both stoichiometric methane and ammonia-hydrogen flames behave similarly, the lean hydrogen flame brush is twice as short with less flame surface, and exhibits significant alteration of its local flame structure: for the H2 flame, the thickness of flamelets decreases significantly while burning rates increase drastically. This is observed for flame elements convexly curved with respect to fresh reactants (positive curvature) because of preferential diffusion, where thermo-diffusive instabilities generate long-tail structures that continuously head toward the fresh gases, and also for near-flat flame elements because of local strain effects. Opposite behaviors are observed for flame elements concavely curved (negative curvature) where fuel depletion caused by unfocusing of hydrogen and strain effects are unable to increase the consumption, even leading to near-extinction of the flame. The turbulent methane and ammonia-hydrogen flames studied in this work do not exhibit the instabilities seen in the pure hydrogen flame. However, local flame-flame interactions are observed in the cusp regions of all three flames, which significantly increase the displacement speed as well as flame surface destruction.A 1D-3D comparison indicates that strain effects in regions of low curvature and preferential diffusion effects in regions of positive curvature can be modeled using counterflow flamelets to account for stretch effects on the turbulent hydrogen flame propagation in addition to turbulent wrinkling.

Large eddy simulation of OME3 and OME4 spray combustion under heavy-duty conditions

The synthetic diesel fuel (Poly-)oxymethylene ether (OME) has various interesting properties for the transport sector. OME reduces emissions in direct injection (DI) diesel engines, e.g., soot and NOx, and is considered CO2 neutral. While most studies have examined a mixture of OMEs with different chain lengths, recent studies have focused on neat OME3 and OME4. In the literature similar chemical ignition delay times were shown under homogeneous conditions and significant differences were reported in their thermophysical properties. Furthermore, differences between OME3 and OME4 in terms of the mixture formation were shown on a heavy-duty injector under non-reactive conditions. The question of how this mixture formation affects the ignition and flame characteristics of neat OME3 and OME4 spray flames remains unresolved. To address this topic, this study conducts a combined experimental and numerical investigation under inert and reactive spray conditions for OME3 and OME4 on a heavy-duty injector. Large Eddy Simulations (LESs) and high-pressure, high-temperature spray chamber measurements are conducted. The distinct differences between neat OME3 and OME4 are analyzed along the cause–effect chain comprising the mixture formation, ignition behavior and its effects on the overall heat release rate. Based on the differences in the thermophysical properties, e.g., density and vapor pressure, the investigations in inert conditions reveal differences in the liquid penetration and mixture formation. The reactive spray flame measurements and simulations show a significant offset in the ignition delay time for OME3 and OME4. An explanation is found in the significantly different distribution of the scalar dissipation rate due to the differences in the mixture formation. Despite the differing ignition delay times, similar local flame structures are found for both fuels.

Numerical study of combustion mode and diffusive transport underlying fuel ignitions in lean stratified dimethyl ether-air premixture at various turbulence intensities

This paper presents a systematic numerical investigation of the auto-ignition phenomenon in stratified dimethyl ether (DME)/air turbulent mixtures. The study was conducted at an initial pressure of 30 atm, mean temperature of 650 K, and equivalence ratio of 0.45, with varying Damköhler numbers (Da) ranging from 0.01 to 1.1. The simulations indicated that the transition point between two combustion modes, deflagration wave and spontaneous ignition, occurred at a Da value of 0.06. The chemical explosive mode analysis identified the leading role of diffusive transport in both modes. A diffusion index (DI) was defined to quantify contributions of each diffusive sub-process. The DI results revealed that heat conduction and oxygen diffusion are the most important processes for the low-temperature ignition, followed by DME, CH2O, and HOCH2OCO with decreasing importance. Further DI analysis of the detailed local flame structure demonstrated that heat, DME, O2, and H2O2 diffusive transports have moderate importance for depressing turbulent propagation at local extinction modes. In conclusion, the proposed DI method has the potential to be a more efficient and rigorous substitution for sensitivity analysis and transport budget analysis.

Large eddy simulation of a dual-mode scramjet with direct injection: Flame dynamics under flow separation induced by heat-release

The combustion process in the scramjet engines generally takes place in unsteady conditions. The unsteadiness of the flame can be present locally or globally in supersonic combustion, for instance, reactant mixing, ignition, flame stabilization, and blow-off. Thus, unsteady supersonic combustion is an urgent problem that needs to be solved for real scramjet engine applications. This work helps to advance the knowledge of dual-mode scramjet combustion operating with different equivalence ratios by conducting quantitative analyses that characterize flame dynamics under flow separation. A hydrogen-fueled direct-injection scheme was computed using blastFoam code via large-eddy simulation of a scramjet test rig at a stagnation temperature of 950 K and a pressure of 0.82 MPa. The computational results were in good agreement with the experimental data, and two typical flame structures (cavity shear-layer and cavity assisted jet-wake stabilized flame) corresponding to the scram/ram operating modes were reproduced. The local flame structure and flow regime were investigated using a modified flame index and filter functions. A particular focus was set on the continuous flame flashback during mode transition, which could be divided into three stages involving jet/shear-layer interaction, unsteady heat release, and the establishment of a large-area separation zone, with the decay of the cavity recirculation zone. Additionally, the unsteady jet-wake ram combustion presented three oscillation modes, giving rise to intermittent flameout of varying levels. The coherent-structures motion caused by the combined effects of low-speed separated flow and the fuel-jet was the main cause for this kind of unsteady combustion.

Effect of H2 addition on the local extinction, flame structure, and flow field hydrodynamics in non-premixed bluff body stabilized flames

We examined the effect of hydrogen (H2) enrichment on the primary fuel methane (CH4) in a canonical non-premixed bluff-body stabilized burner operating under typical central jet-dominated flame mode. In the chosen mode of operation, globally, the flow field and flame feature three important successive spatial zones: the recirculation zone, the neck zone, and the jet-like flame zone. The flame is exposed to a higher stretch rate in the neck zone in such a configuration and eventually undergoes local extinction. Such local extinction and subsequent re-ignition/reconnection of broken flame branches have substantial implications for the hydrodynamic instability of the coaxial annular air shear layer. It is well known that H2 addition increases the flame extinction strain rate (κext) and thus alters the local extinction phenomenon. To understand this, we performed experiments at 0%, 10%, 20%, 30%, 50%, 80%, and 100% hydrogen proportion in the H2-CH4 blend. High repetition rate (5 kHz) Particle Image Velocimetry and OH Planar Laser Induced Fluorescence (PLIF) measurements are simultaneously implemented to gain quantitative insight into the flow field and flame structure. A detailed analysis performed over the instantaneous OH–PLIF datasets reveals the absence of local extinctions in flames with H2 enrichment >30% due to an increased extinction strain rate (κext). Furthermore, it is found that H2 enrichment plays a significant role in the reconnection/re-ignition of the broken flame branches formed during the local extinction. For instance, a high reconnection probability is observed in flames with an H2 addition of ≥20%. Consequently, variations in the mean reaction zone height are witnessed for different H2 enrichment levels. Further analysis of the influence of variation in reaction zone height on flow field hydrodynamics is explored using Proper Orthogonal Decomposition (POD) and Continuous Wavelet Transform (CWT). The results obtained from POD and CWT indicated the suppression of vortex shedding at the annular air shear layer for H2 addition greater than 20% and irregular wrinkling of flame fronts. Thus, they quantified the beneficial effect of H2 addition in turbulent flame stabilization.

Experimental and analytical study on inclined turbulent fire in still air. Part 2. Round fire

This paper reports an experimental and analytical study on non-vertical round turbulent jet fire. A series of propane jet fire tests are performed under different initial flame angles (θ0) and initial fuel velocities. The two-dimensional velocity and temperature fields of the flame are measured, and the flame geometry is determined from video images. Experimental results show that the flame entrainment is enhanced as θ0 decreases. The flame deflection-induced buoyancy flow plays an essential role in the local flame structure and turbulence. A model of round fire combined with a kinetic energy equation is established, in which the flame entrainment coefficient is analytically correlated with the buoyancy and streamline curvature. Model calculations reproduce the flame entrainment and flame trajectory, consistent with the experimental indications. Under the same burner area and heat release rate, the entrainment coefficient of round fire is greater than that of line fire, closely associated with the difference in integral buoyancy triggered by different burner geometries. The results of this paper may help understand the flame motion and predict the flame radiation of turbulent jet fires.

The Interaction Between In-Cylinder Flow And Flame Propagation In An Optical SI Engine Measured By High-Speed PIV

The relationship between the flow field and flame propagation is essential in determining the dynamics and effects of turbulent flow in an optical SI engine. In this study, the high-speed PIV technique was applied to evaluate the incylinder velocity and turbulence distribution under firing condition. The simultaneous measurement technique of incylinder flow and flame propagation characterized the relationship between flow field and flame propagation in the same cycle. A 500 cc single-cylinder optical engine was used for this experiment. The bore diameter and stroke length were 86 mm and 86 mm respectively, and compression ratio was 10.4. The flame front configuration was extracted from the PIV image based on planar laser tomography method. The strong contrast between the intensity of burned and unburned region in the captured images due to low gas and seed density of the former was utilized to derive this flame front. In order to evaluate the interaction between flame and flow with high resolution, the interrogation area was set to 16 × 16 pixels. This corresponds to 0.75 × 0.75 mm. The spatial filter of 6 mm was used to separate the instantaneous flow velocity into a low frequency component and a high frequency component. The engine speed was 1200 rpm and absolute intake pressure was 60 kPa. The equivalence ratio was stoichiometry condition. The ignition timing were set to 19 deg.BTDC. The characteristic cycles in the same test condition were extracted and discussed. The bulk flow with the large scale as large as the size of the combustion chamber influences the overall shape of the flame propagation, and the state of the flame propagation greatly fluctuates from cycle to cycle. The flame propagates with enhancing the large scale existing tumble flow that exists in earlier crank angle locations. Focusing on the local flame structure, the high-frequency velocity component was strong near the local flame peak and weak near the local flame valley. The flame at the local peak region propagates while pulling the valley region.

Flame folding and conditioned concentration profiles in moderately intense turbulence

While the flamelet paradigm offers the opportunity to simplify computations of mean species concentrations in turbulent flames, a widely accepted criterion of the validity of this paradigm has not yet been elaborated. In this regard, different physical mechanisms are discussed, and flame folding is one of them. The present work aims at exploring the eventual influence of flame folding on the local flame structure in a turbulent flow. For this purpose, a new diagnostic technique was applied to processing complex-chemistry direct numerical simulation data obtained earlier from a lean hydrogen-air turbulent flame [Dave and Chaudhuri, J. Fluid Mech. 884, A46 (2020)]. The technique consists of counting crossing numbers Nf for a cold boundary of the local reaction zone and a ray normal to the mean flame brush, followed by analyzing statistics sampled from rays characterized by Nf≥3. More specifically, profiles of species mole fractions, temperature, heat release rate, and species production rates, conditioned to combustion progress variable and either Nf or axial distance Δx between two neighboring reaction zones, are sampled and compared with the counterpart profiles obtained from the laminar flame. Results show that these doubly conditioned profiles are close to each other for various crossing numbers or for various axial distances even if the distance is as small as half laminar flame thickness. The lack of a substantial effect of the crossing number or the axial distance on the doubly conditioned profiles implies that small-scale flame folding does not limit the validity of the flamelet paradigm.

Observation study on the flame morphology of outwardly propagating turbulent HCNG-30 premixed flames

The present work reported observation studies on the flame structure of outwardly propagating HCNG-30 (adding H2 into CH4 with a volumetric ratio of 30%) premixed flames, the effects of turbulent intensities (from 0 to 1.31 m/s) and equivalence ratios (from 0.6 to 1.2) were discussed. First, the effects of equivalence ratios on laminar HCNG-30 premixed flames were analyzed and discussed upon the flame morphology and the macro indices to flame structure (critical radius and wrinkling ratio), with the decrease of equivalence ratio from rich to lean, the instability was dominated by both D-T instability and D-L instability. Then, with the presence of turbulence, the flame structure became more wrinkled for both the turbulence effects and the interactions to intrinsic instabilities. Upon the analysis about the spatial oscillation on the flame-front with same sizes, the relationship between the amplitude of the flame-front and the equivalence ratio in intense turbulence conditions is not regular, and this phenomenon could be attributed to the dominant influence of turbulence on the flame structure under intense turbulent conditions. Upon the wavelet analysis about the temporal oscillation on same local flame structure, the effects of turbulence would decline when the flame developed to certain size, and such phenomena could be attributed to the dissipation of turbulence.

Numerical modeling for local flame structure and pollutant formation in biodiesel and n-Dodecane spray jet flames

The present work performs unsteady Reynolds-averaged Navier-Stokes simulation to study the combustion characteristics of soybean methyl esters biodiesel and n-dodecane spray jet flames. To realistically account for the turbulence-chemistry interaction in computationally efficient manner, the Multi-Environment PDF model coupled with unsteady flamelet library are considered in the present work. The present comprehensive spray combustion model is validated against experimental data in terms of liquid and vapor penetration, ignition delay time and lift-off length. It was found that the biodiesel spray flame creates hot reaction zone at the relatively narrow central region with small flame volume. On the other hand, n-dodecane spray flame forms the hot reaction zone at the spreading flame field with large flame volume. These distinctive differences in local flame structures yield different NO distribution of the two spray flames due to its strong dependence on temperature. The biodiesel spray flame produces significantly less C2H2 at the fuel-rich flame zone than the n-dodecane spray flame. The present work clearly shows that the MEPDF model has capability to predict central partially premixed flame of biodiesel, diffusion flame of n-dodecane, and reduction of soot when oxygenate fuel is used in Diesel engine-like conditions.

Assessing diffusion model impacts on enstrophy and flame structure in turbulent lean premixed flames

Diffusive transport of mass occurs at small scales in turbulent premixed flames. As a result, multicomponent mass diffusion, which is often neglected in direct numerical simulations (DNS) of premixed combustion, has the potential to impact both turbulence and flame characteristics at small scales. In this study, we evaluate these impacts by examining enstrophy dynamics and the internal structure of the flame for lean premixed hydrogen-air combustion, neglecting secondary Soret and Dufour effects. We performed three-dimensional DNS of these flames by implementing the Stefan–Maxwell equations in the code NGA to represent multicomponent mass transport, and we simulated statistically planar lean premixed hydrogen-air flames using both mixture-averaged and multicomponent models. The mixture-averaged model underpredicts the peak enstrophy in the multicomponent simulation by up to 13% in the flame front. Comparing the enstrophy budgets of these flames, the multicomponent simulation yields larger peak magnitudes compared to the mixture-averaged simulation in the reaction zone, showing differences of 17% and 14% in the normalised vortex stretching and viscous effects terms. In the super-adiabatic regions of the flame, the mixture-averaged model overpredicts the viscous effects by up to 13%. To assess the effect of these differences on flame structure, we reconstructed the average local internal structure of the turbulent flame through statistical analysis of the scalar gradient field. Based on this analysis, we show that large differences in viscous effects contribute to significant differences in the average local flame structure between the two models.

An Investigation of Axisymmetric Disk Stabilized Propane-Air Flames Operating under Inlet Mixture Preheat and Stratification

ABSTRACT Turbulent, recirculating, lean propane-air flames with inlet mixture stratification and preheat have been simulated under stable and limiting burning conditions. The modeled burner setup comprises a supply tube enclosing three sequential disks producing two consecutive premixing cavities. Fuel is injected in the first cavity and is partially premixed with primary air flowing through this cavity system, resulting in a radially stratified equivalence ratio profile at the inlet of the afterbody disk flame stabilizer. Detailed velocity, turbulence, fuel-air mixing, and imaging data have been previously reported for inlet preheats from 300 to 573 K and for a range of fuel flow rates. The simulations were carried out with a finite-volume-based method, using the dynamic Smagorinsky subgrid model coupled with two combustion methodologies, a quasi-laminar reaction rate approach and the Thickened Flame Model approach. Propane oxidation was modeled with a 22-species skeletal scheme. OH* chemiluminescence distributions were also computed by post-processing quasi-steady state derived algebraic expressions, exploiting directly simulated species thus enabling comparisons with experimental images. The simulations were evaluated against velocity, turbulence, and temperature measurements as well as chemiluminescence images. These comparisons allowed for an assessment of the methodologies’ capability to reproduce important trends such as the notable extension of stable operation to ultra-lean mixtures, the appreciable effect on the near flame aerodynamic stretch and the variations in the local flame structure.

Flame structure and burning velocity of ammonia/air turbulent premixed flames at high Karlovitz number conditions

This paper presents experimental studies of the structures and burning velocities of premixed ammonia/air jet flames at high Karlovitz (Ka) number conditions. Simultaneous planar laser-induced fluorescence (PLIF) imaging of imidogen (NH) and hydroxyl (OH) radicals was performed to investigate the local flame structure and Laser Doppler Anemometry (LDA) measurements were employed for extracting complement relevant turbulent quantities from the flow field. All the selected cases are located in the regime of distributed reaction zones (DRZ) in the Borghi-Peters diagram, with a maximum Karlovitz (Ka) number and turbulent intensity (u′/SL) up to 1008 and 240, respectively. The OH- and NH-PLIF data were used to determine the flame surface density, flame-surface area ratio, and turbulent burning velocity (ST). The main findings include: (a) The NH layer remains thin and continuous over the investigated range of turbulent intensity and Karlovitz number, and the thickness keeps constant statistically without any significant broadening by turbulent eddies; (b) Spatial correlations of the NH and OH radicals show that overlap of NH and OH layers always exists in a thin region where OH has a weaker signal intensity; (c) The ratio of turbulent to laminar burning velocity (ST/SL) shows a nearly linear increase with turbulent intensity, while the ratio of wrinkled flame surface area to that of ensemble-averaged flame surface area increases only slightly with turbulent intensities. The slower increase of wrinkled flame surface area with turbulent intensity can be attributed to under-resolution in the current state-of-the-art PLIF experiments, the neglection of 3D flame wrinkles in 2D experiments, and the increase in flame stretch factor at high turbulent intensities.

Numerical Modeling for Local Flame Structure and Pollutant Formation in Biodiesel and N-Dodecane Spray Jet Flames

Numerical Modeling for Local Flame Structure and Pollutant Formation in Biodiesel and N-Dodecane Spray Jet Flames

Evaluation of the unsteady flamelet progress variable approach in Large Eddy Simulations of the ECN Spray A

Within the Unsteady Flamelet Progress Variable – Large Eddy Simulation (UFPV-LES) approach the local scalar dissipation rate represents one key parameter, significantly affecting the ignition behaviour. In this study, the UFPV-LES approach is evaluated for ECN Spray A baseline conditions, relevant for diesel engines. After confirming its general applicability, using experimental data under non-reacting and reacting conditions, special attention is paid to the distribution of the local scalar dissipation rate. Based on the findings of this analysis, a reduced modeling approach, considering only igniting flamelets starting from the adiabatic mixing line between the fuel and oxidizer, is investigated. The performance of this reduced approach is assessed systematically, using the UFPV-LES results as a reference. Based on an a-priori analysis, regions affected by the model reduction are identified and evaluated. A subsequent evaluation in an a-posteriori analysis, i.e. a coupled LES, reveals similar results in terms of local flame structure as well as global ignition characteristics and confirms the applicability of the reduced model under the ECN Spray A baseline conditions.

Local flame displacement speeds of hydrogen-air premixed flames in moderate to intense turbulence

Comprehensive knowledge of local flame displacement speed, Sd, in turbulent premixed flames is crucial towards the design and development of hydrogen fuelled next-generation engines. Premixed hydrogen-air flames are characterized by significantly higher laminar flame speed compared to other conventional fuels. Furthermore, in the presence of turbulence, Sd is enhanced much beyond its corresponding unstretched, planar laminar value SL. In this study, the effect of high Karlovitz number (Ka) turbulence on density-weighted flame displacement speed, Sd˜, in a H2-air flame is investigated. Recently, it has been identified that flame-flame interactions in regions of large negative curvature govern large deviations of Sd˜ from SL, for moderately turbulent flames. An interaction model for the same has also been proposed. In this work, we seek to test the interaction model’s applicability to intensely turbulent flames characterized by large Ka. To that end, we investigate the local flame structures: thermal, chemical structure, the effect of curvature, along the direction that is normal to the chosen isothermal surfaces. Furthermore, relative contributions of the transport and chemistry terms to Sd˜ are also analyzed. It is found that, unlike the moderately turbulent premixed flames, where enhanced Sd˜ is driven by interactions among complete flame structures, Sd˜ enhancement in high Ret and high Ka flame is predominantly governed by local interactions of the isotherms. It is found that enhancement in Sd˜ in regions of large negative curvature occurs as a result of these interactions, evincing that the interaction model is useful for high Ka turbulent premixed flames as well.