Turbulent Premixed Combustion Research Articles

Fuel-air ratio effect on hydrogen-methane flames in a high pressure burner for gas turbines

This numerical simulation studied the effect of H2-CH4 flame equivalence ratio on turbulent premixed combustion at 50%-50% concentration (by volume). The equivalence ratio was varied from 0.45 to 1.0 in 0.5 increments for 126 kW operating power, matching a 3.3 bar inlet reactant pressure. Tests utilized a gas turbine combustor. The thermal field, flow field, and pollutant emissions (NOx and CO) underwent rigorous analysis. The modelling framework applied steady Reynolds-Averaged Navier-Stokes (RANS) equations coupled to a probability density function (PDF) approach for turbulence-chemistry interactions and a NOx formation model. Results showed increasing equivalence ratio from 0.45 to 1.0 elevated temperature approximately 900 K, significantly promoting NOx up to 1600 ppm and CO beyond 1900 ppm. However, equivalence ratio changes minimally impacted the overall flow field, maintaining stabilized flames. These findings provide new insight on thermochemical effects and flame stability in gas turbine (G.T) combustors across a range of equivalence ratios relevant for clean, high-pressure H2-CH4 combustion. The combined RANS-PDF methodology enables predictive simulation of turbulence, kinetics, emissions, and flame stability to guide optimal fuel-air ratio selection and low-emission combustor design.

Evaluation of the wall heat flux in filtered premixed turbulent combustion

The paper focuses on modelling the heat flux through the isothermal wall boundary in premixed turbulent combustion. In a statistically stationary configuration, the distorted flames, advected by the incoming flow toward the wall, can be either head-on or entrained. The wall heat flux model with respect to the head-on and entrained parts are constructed differently to describe their distinct behaviours. In model validation, to exclude the interference from various inaccuracies, either numerically or experimentally, the filtered three-dimensional direct numerical simulation (DNS) fields are adopted as benchmark. Results justify that the newly proposed model is able to capture the wall heat flux statistics. The predictions are consistently satisfactory at different filter widths, performing clearly better than direct evaluation from the filtered temperature gradient and effective diffusivity. In addition, it is also straightforward to integrate this model together with existing flow solvers.

Experimentally Closing the Balance of Progress of Reaction in Premixed Turbulent Combustion in the Thin Flame Regime

We investigate the possibility of determining the local turbulent flame speed by measuring the individual terms in the balance of a mean progress of reaction variable for the case of a low turbulence methane-air Bunsen flame in the thin flame regime. Velocity distributions and flame edge positions were measured by particle image velocimetry techniques at 3 kHz for a flame stabilized by a surrounding pilot of the same stoichiometry, for a turbulent Reynolds number around 66 and Karlovitz numbers of the order of 4. The conservation equation for mean progress variable was analyzed along different streamlines as a balance of terms expressed as velocities, including terms for convection, turbulent diffusion, mean reaction, and turbulent and molecular diffusion. Each term was estimated from local velocities and flame locations using a thin flame approximation, and their uncertainty was evaluated based on propagation of experimentally measured statistical correlations. The largest terms were the convective and reaction terms, as expected, with smaller roles for turbulent and molecular diffusion across the flame brush. Countergradient diffusion and transition to gradient diffusion were observed across the flame brush. Closure of the balance of terms in the conservation equations using independently measured terms was not consistently achieved across the flame brush within the reckoned uncertainties, arriving at a balance within 20–30% of the absolute value. Testable hypotheses are offered for the possible reasons for the mismatch, including the role of spatial filtering and 3D effects on the reaction rate term. Finally, the experiments identify the inaccuracies in measuring a true local turbulent flame speed, and suggest a consistent methodology to reduce errors in such estimations. This is the first time such a detailed experimental closure is attempted for any configuration. The results suggest that the significant improvements in spatial resolution are necessary for a full closure.

Heat release and flame scale effects on turbulence dynamics in confined premixed flows

As industry transitions to a net-zero carbon future, turbulent premixed combustion will remain an integral process for power generating gas turbines, aviation engines, and high-speed propulsion due to their ability to minimize pollutant emissions. However, accurately predicting the behavior of a turbulent reacting flow field remains a challenge. To better understand the dynamics of premixed reacting flows, this study experimentally investigates the effects of combustion heat release and flame scales on the evolution of turbulence in a high-speed, confined bluff-body combustor. The combustor is operated across a range of equivalence ratios from 0.7 to 1 to isolate the role of chemical heat release, flame speed, and flame thickness on the evolution of turbulence as the flow progresses from reactants to products. High-speed particle image velocimetry and CH* chemiluminescence imaging systems are simultaneously employed to quantify turbulent flame and flow dynamics. The results notably demonstrate that the flame augments turbulence fluctuations as the flow evolves from reactants to products for all cases, which opposes most simulations of premixed turbulent reactions. Notably, turbulence fluctuations increase monotonically with the heat of combustion and corresponding turbulent flame speed. Spatial profiles of turbulence statistics are conditioned on the mean flame front, and nondimensionalizing the turbulence profiles using laminar flame properties is shown to collapse all conditions onto a single curve. The resulting nondimensional profile confirms that turbulence dynamics scales with the heat of combustion and was used to develop a novel correlation to predict the increase in turbulent fluctuations across the premixed flame. A Reynolds averaged Navier Stokes decomposition is also explored to further characterize the effects of combustion heat release on the dominant mechanisms of turbulent energy transport. The cumulative results can guide modeling capabilities to better predict flame and flow dynamics and accelerate design strategies for premixed turbines with carbon-free fuels.

Development of Machine Learning Models for Studying the Premixed Turbulent Combustion of Gas-To-Liquids (GTL) Fuel Blends

AbstractStudying the spatial and temporal evolution in turbulent flames represents one of the most challenging problems in the combustion community. Based on previous 3D numerical analyses, this study aims to develop data-driven machine learning (ML) models for predicting the flame radius evolution and turbulent flame speeds for diesel, gas-to-liquids (GTL), and their 50/50 blend (by volumetric composition) under different thermodynamic and turbulence operating conditions. Two ML models were developed in this study. Model 1 predicts the variations of the flame radius with time, equivalence ratio, and turbulence intensity, whereas model 2 predicts the variations of the turbulence flame speed with the operating parameters. The k-fold cross-validation technique is used for model training, and the developed neural network-based model is used to investigate the effects of operating parameters on the premixed turbulent flames. In addition, the possible minimum and maximum values of responses at the corresponding operating parameters are found using a genetic algorithm (GA) approach. Model 1 could capture the computational fluid dynamics (CFD) outputs with high precision at different flame radiuses and time instants with a maximum absolute error percentage of 5.46%. For model 2, the maximum absolute error percentage was 6.58%. Overall, this study demonstrates the applicability and promising performance of the proposed ML models, which will be used in subsequent research to analyze turbulent flames a posteriori.

Data-based instantaneous conditional progress variable dissipation rate modeling for turbulent premixed combustion

Manifold-based turbulent combustion modeling reduces computational cost considerably by projecting the thermochemical state onto a lower-dimensional space of a few manifold variables and reconstructing the state from a set of manifold equations. In turbulent premixed combustion, a one-dimensional space is defined by the progress variable, and the progress variable dissipation rate appears as a parameter in the resulting one-dimensional manifold equations. Closure of the manifold equations requires a model for the dependence of this dissipation rate on the progress variable. Existing models for these profiles are often ad hoc, derived analytically with crude assumptions, and assumed to be spatially and temporally ‘universal’ — yet the resulting thermochemical state can be sensitive to these profiles. To evaluate the assumptions in traditional models, instantaneous conditional progress variable dissipation rate profiles are extracted from DNS databases of turbulent premixed jet flames. At low Karlovitz number, the instantaneous conditional progress variable dissipation rate profiles exhibit some variance but are relatively well-characterized by a ‘universal’ model. However, a large variance in the instantaneous conditional progress variable dissipation rate profiles is observed at high Karlovitz number, indicating the form of the conditional dissipation rate is certainly not ‘universal’. To capture this variation at high Karlovitz number, a data-based model for the instantaneous conditional progress variable dissipation rate profiles is generated, and the resulting model predictions demonstrate excellent agreement with profiles extracted from DNS and far less error than ‘universal’ models. The sensitivity of the solutions of the one-dimensional premixed manifold equations to the dissipation rate profile is demonstrated a priori, and the error associated with the observed variance in the instantaneous conditional progress variable dissipation rate profiles (i.e., relative to the DNS mean) indicates that an instantaneous modeling approach is necessary. A variable importance study is conducted, and the conditional velocity gradient is identified as the critical quantity that characterizes the observed variance in the instantaneous conditional progress variable dissipation rate profiles, providing further motivation for a data-based modeling approach.

Modeling of non-homogeneous premixed hydrogen-air flame acceleration and deflagration to detonation transition in an obstructed channel

This paper uses Large Eddy Simulation (LES) technology to study the dynamic behavior of flame acceleration (FA) and deflagration-to-detonation transition (DDT) in a duct with different obstacle layouts in non-homogeneous concentration fields. A turbulent premixed combustion model with three additional source terms was established using the OpenFoam code library. It was found that the predicted results of the coupled model were in good agreement with the measured values of the experiment. Numerical results show that turbulent combustion and self-ignition effects are the main factors affecting flame acceleration. Still, self-ignition effects are a necessary and sufficient condition for inducing DDT phenomena. Detonation flames have around-flow and diffraction effects and do not evolve into jet and vortex flames, which differ from deflagration flames. Two initiation mechanisms were found under different obstacle layouts: hotspots were generated in the area swept by the Mach stem at the flame front, triggering detonation near the turbulent flame through the reactive gradient mechanism; focusing of sufficiently strong reflected waves deposited enough energy in a particular area near the flame brush and directly triggered detonation. In addition, the central obstacle layout is most conducive to FA and DDT, while the symmetrical layout is the least favorable.

Probabilistic deep learning of turbulent premixed combustion

A probabilistic data-driven approach that models the filtered reaction rate in large-eddy simulation (LES) is investigated. We propose a novel framework that incorporates a conditional generative adversarial network and a Gaussian mixture model to take into account the statistical fluctuations that are present in LES of turbulent reacting flows due to non-resolved subgrid structures, which cannot be predicted by purely deterministic models and machine learning algorithms. The data from a direct numerical simulation of turbulent premixed combustion are spatially filtered using a wide range of filter widths and employed for the training. We extract physically relevant parameters from the database and reduce the input features to the network to the most influential ones based on the result of feature importance analysis. The trained model is then tested on unseen timesteps and untrained LES filter widths, where it is able to accurately predict the distribution of the filtered reaction rate.

Effects of H2 addition on the instability characteristics of CO/air and CH4/air in a horizontal narrow channel

ABSTRACT This paper investigates the instability characteristics of CO/H2/air and CH4/H2/air mixtures in a narrow channel under standard temperature and pressure conditions. The CO/H2 mixture exhibits a greater overpressure growth rate and maximum explosion overpressure as compared to the CH4/H2 mixture at the same hydrogen content. As the hydrogen content increases, the maximum explosion overpressure (P max) value increases, while the corresponding time (θ) value gradually decreases. The maximum explosion overpressure of CH4/H2 and CO/H2 mixtures is 28.27 kPa and 25.09 kPa, respectively. The flame wrinkles to form cells due to the Darius-Landau instability, and the interaction between the vortex tube and premixed flame affects the structure of turbulent premixed combustion. The oscillation frequency of the overpressure increases with increasing hydrogen content, and the oscillation belongs to the Helmholtz oscillation. The CO/H2 mixtures exhibit greater flame thickness than CH4/H2 mixtures, resulting in lower Peclet numbers and therefore greater relative importance of heat losses. As the hydrogen content increases, the sensitivity coefficient of R99 (OH + CO <=> H + CO2) in the CO/H2 mixture gradually decreases, while the sensitivity coefficients of R38 (H + O2 <=> O + OH) and R84 (OH + H2 <=> H + H2O) increase. The elementary reaction R38 plays a leading role in increasing the laminar burning velocity of the CH4/H2 mixture. The trend of R38 variation in the two mixtures is opposite with increasing hydrogen content.

Quenching modes of local flame–wall interaction for turbulent premixed methane combustion in a constant volume vessel

The quenching mode of local flame–wall interaction (FWI) is investigated for its response to different levels of turbulence intensity as well as its effect on quenching distance, wall heat flux, and near-wall reaction. For that, direct numerical simulations of turbulent premixed methane combustion in a constant volume vessel are carried with initial Karlovitz numbers (Ka) of 1.0, 10.0, and 30.0. Local flame–wall quenching positions are identified based on the local fuel consumption speed during the turbulent combustion process, and the local FWI events have been classified into four quenching modes according to the flame–wall geometric relationships of quenching positions, namely head-on quenching (HOQ), oblique-wall quenching, side-wall quenching (SWQ), and back-on quenching (BOQ). The results show that in the case with higher initial Ka, the flame surface shows a more complicated wrinkled structure due to the flame–turbulence interaction. Meanwhile, the local quenching distance defined based on the identified quenching position is strongly influenced by the near-wall flow, and the range of the local quenching mode extends further to BOQ. However, for all three cases, HOQ and near-HOQ modes account for the majority of local FWI. Wall heat flux and heat release rate (HRR) of near-wall reaction yield high values for the FWI under HOQ or BOQ and are low for SWQ. In addition, there is a discrepancy in the near-wall transportation of some species under different quenching modes, which further leads to the difference in FWI-induced near-wall reaction regarding its total and elementary HRR.

Closure modeling for the conditional pressure gradient in turbulent premixed combustion

In turbulent premixed combustion, turbulence is affected by both the influence of combustion heat release and turbulent shear effects. At low Karlovitz number, heat release effects are more dominant than turbulent shear effects, but, at high Karlovitz number, turbulent shear effects dominate the turbulence dynamics. Closure models for the velocity and scalar fields that only account for one of those effects are incapable of generally describing combustion-turbulence interactions at any finite Karlovitz number. A manifold-based turbulence model that solves the conditional mean momentum transport equation conditioned on a progress variable has the potential to provide such a general description, but closure models for the equation are required. Among the unclosed terms, the conditional mean pressure gradient is significant at both low and high Karlovitz numbers but has fundamentally different behaviors in each regime. In this work, a general closure model is developed to capture the conditional pressure gradient in both Karlovitz number regimes by considering heat release and turbulent shear effects. The deviation of the conditional pressure gradient from the unconditional pressure gradient is modeled with two components. One represents the pressure decrease across the flame due to combustion heat release, and the other represents the hydrodynamic pressure fluctuations due to turbulent shear. The heat release component depends on the heat release rate and the flame-normal vector, and the turbulent shear component depends on the conditional Reynolds stresses and the gradient of the progress variable probability density function. The model is validated against Direct Numerical Simulations of spatially-evolving turbulent premixed hydrogen/air planar jet flames at low and high Karlovitz numbers. The proposed model well captures the conditional pressure gradient at both Karlovitz numbers and shows a natural transition of its dominant term: the heat release component at low Karlovitz number and the turbulent shear component at high Karlovitz number.

Flame Self-interaction and Flow Topology in Turbulent Homogeneous Mixture n-Heptane MILD Combustion

Moderate or Intense Low-oxygen Dilution (MILD) combustion has potential to achieve both high energy efficiency and ultra-low emissions. This analysis adopts the critical point theory to characterise the Flame-Self Interaction (FSI) events and flow topologies in turbulent, homogeneous mixture, n-Heptane MILD combustion using Direct Numerical Simulations (DNS) with reduced chemical mechanism. The local flame geometry has also been categorised using the mean and Gauss curvatures. It was found that the Tunnel Formation (TF) and Tunnel Closure (TC) topologies are the most probable FSI events at all values of the reaction progress variable c, while the Unburned Pocket (UP) and Burned Pocket (BP) topologies were mostly present towards the unburned and burned mixtures of the flame, respectively. Moreover, increasing the turbulence intensity did not result in any significant changes in the distribution of FSI events. Investigation of the flow topology distribution showed that the features associated with non-zero dilatation rate did not exist in the MILD cases considered. This is a consequence of the negligible thermal expansion effect due to the small temperature rise in MILD combustion cases. Increasing the dilution factor caused a reduction in the frequency of FSI events for all c levels. The distributions of flame self-interaction events in homogeneous mixture MILD combustion have been found to be significantly different from previously reported distributions for conventional turbulent premixed combustion.

An improved TRF mechanism for a new turbulent premixed combustion model with application to engine combustion CFD

In the present work, an existing TRF (toluene reference fuel) chemical kinetic mechanism has been improved. In the improvement, the existing TRF mechanism was found to have unphysically higher laminar flame speeds at temperatures greater than 750 K which are typical in engine compression-combustion operating conditions. Eight dominant reactions which mainly control the laminar flame speeds under higher temperatures ( T &gt; 750 K) have been identified. Based on a recently published power-law laminar flame speed correlation which is able to provide good predictions under a wide range of engine conditions, correct reaction rate constants for the eight high-temperature dominant reactions have been determined for physical laminar flame speeds. The identification and improvement of the eight high-temperature dominant reactions are the first novelty of this work. In order to simulate ethanol and gasoline blends, a latest ethanol sub-mechanism has been implemented into the TRF mechanism. The improved TRF mechanism was validated using available experimental data in the literature. Based on the improved TRF mechanism, a new mechanism library has been generated, which can be used for an improved mechanism-dynamic-selection turbulent premixed combustion model in which turbulent diffusivity is constructed as a function of local turbulence/thermodynamics conditions. The dynamic turbulent diffusivity sub-model is the second novelty of this work. The improved TRF mechanism and its combination with the improved mechanism-dynamic-selection turbulent premixed combustion model were successfully applied to the prediction of combustion and emissions of GTDI (gasoline turbocharged direct injection) engines under both warm-up and transient cold start operating conditions, indicating that the improved models in this work are beneficial for predictive engine combustion CFD (computational fluid dynamics).

Modelling Intermediate Species in Partially Premixed Turbulent Combustion Based on the LES-FGM Method

Modelling Intermediate Species in Partially Premixed Turbulent Combustion Based on the LES-FGM Method

Formula omitted]-[formula omitted] and [formula omitted]-[formula omitted] studies of filtered probability density function models and NO formation prediction in turbulent stratified premixed combustion using machine learning

Accurate prediction of NO formation in turbulent stratified premixed combustion using large eddy simulation (LES) is challenging. In the present work, a reaction rate method for NO source modeling in the context of LES was proposed with the focus being placed on accurate modeling of the filtered probability density functions (FPDFs) and joint FPDFs. The NO source and various NO pathways from a direct numerical simulation (DNS) database of turbulent stratified premixed flames were conditionally averaged the progress variable and mixture fraction to provide the lookup table. A random forest (RF) model was developed for the FPDFs/joint FPDFs of the mixture fraction and progress variable. The β-PDF model was also analyzed for comparison. First, the FPDFs and joint FPDFs predicted by the β-PDF and RF models for the progress variable and mixture fraction were compared with those from the DNS data. It was found that the FPDFs and joint FPDFs by the RF model agree well with those from the DNS. In contrast, the β-PDF model failed to accurately capture the joint FPDFs as the correlations of the mixture fraction and progress variable are neglected. Then, the filtered NO source and various NO pathways were modeled a-priori using the lookup table and the joint FPDFs for turbulent stratified premixed flames. The results showed that the RF model reproduces the filtered NO source and NO pathways very well, and performs much better than the β-PDF model. Finally, a-posterior validation of the model performance was carried out in the context of LES. Various quantities from the LES were compared with the DNS filtered quantities. The results showed that the LES/RF model outperforms the LES/β-PDF model in the a-posterior simulation. Overall, the LES/RF model is promising for NO formation modeling using the reaction rate method a-priori and a-posterior in LES of turbulent stratified premixed combustion.

Multiple Mapping Conditioning Mixing Time Scales for Turbulent Premixed Flames

A novel multiple mapping conditioning (MMC) mixing time scale model for turbulent premixed combustion has been developed. It combines time scales for the flamelet and distributed flame regimes with the aid of a blending function. The blending function serves two purposes. Firstly, it helps to identify zones where the premixed flame resides and where the time scale associated with the premixed flame shall be used. Secondly, it uses the Karlovitz number to identify the turbulent premixed combustion regime and to reduce the weighting of the premixed flame time scale if Karlovitz numbers are high and deviations from the flamelet regime are expected. A series of three-dimensional direct numerical simulations (DNS) of statistically one dimensional, freely propagating turbulent methane-air flames provides a wide range of turbulent combustion regimes for the mixing model validation. The new mixing time scale provides correct predictions of the flame speed of freely propagating turbulent flames which could not be matched by most recognized mixing models. The turbulent flame structure predicted by the new model is in good agreement with DNS for all combustion regimes from flamelet to the thickened reaction zone.

The effects of turbulence on the flame structure and NO formation of ammonia turbulent premixed combustion at various equivalence ratios

Ammonia is carbon-free and is regarded as a potential fuel to address global warming issues. In this work, three-dimensional direct numerical simulations (DNS) of ammonia/air turbulent premixed flames were performed to explore the influence of turbulence and equivalence ratio on the flame structure and NO formation characteristics. Two equivalence ratios were considered, i.e.ϕ = 0.9 and ϕ = 1.1. The general flame structures were presented and species distributions were examined. The NO mass fraction was found to be the highest in the product for the lean case and in the reaction zone for the rich case. The conditional mean values of species mass fractions and reaction rates were compared with those of the unstrained and strained laminar premixed flames to explore how well the laminar flame structures can approach those of the turbulent flame. The budget analysis of the species transport equations showed that turbulent diffusion plays an important role in species transport. The turbulent diffusivity DT was estimated using the gradient hypothesis based on the DNS data. Various laminar flame simulations with different diffusivities were carried out. It was shown that the conditional means of the DNS agree well with those of the laminar flames including DT in the transport property calculation. The global and local NO formation characteristics were investigated. It was found that the mean NO production conditioned on the progress variable is lower compared with the corresponding laminar flame in the rich case. However, the relative contributions from various NO pathways are rarely affected by turbulence. The NO mass fraction is higher in negative curvature regions compared with positive curvature regions of the flame surface for the rich case, which is due to the preferential diffusion of H2 and other radicals and the enhanced NO pathways in negatively curved regions.

A new compact active turbulence generator for premixed combustion: Non-reacting flow characteristics

A new compact active turbulence generator is developed, tested, and characterized, which extends the capabilities of such generators used in turbulent premixed combustion research. The generator is composed of two blades that resemble the shape of two bow-ties. Hot-wire anemometry and high-speed imaging are performed to characterize the non-reacting flow produced by the generator and the blades dynamics, respectively. Two mean bulk flow velocities of 5.0 and 7.0 m/s are examined. For comparison purposes, in addition to the developed generator, tests are also performed for a free jet as well as one and two perforated plates. The results show that the centerline root mean square velocity fluctuations can become as large as 1.8 m/s. For the newly developed device, the power-law decay of the one-dimensional kinetic energy is −1.0 and −1.3 for the mean bulk flow velocities of 5.0 and 7.0 m/s, respectively. The normalized energy dissipation rate is relatively small for the newly developed device, while the energy dissipation rate is relatively large. The spectral analysis of the velocity data does not show dominant frequencies equal to the blades rotation frequencies, and the one dimensional kinetic energy and dissipation spectra follow −5/3 and 1/3 power-law relations, respectively. It is shown that the small eddies produced by the newly developed device dissipate the turbulent kinetic energy faster than those corresponding to the rest of the tested turbulence generators.

Numerical simulation of methane-hydrogen-air premixed combustion in turbulence

3D modeling is based on an experimental platform named turbulence constant volume combustion chamber (CVCC). The turbulence model applies the realizable k-ε model. The combustion model applies the premixed combustion model. Simulation of methane-hydrogen-air turbulent premixed combustion is realized using the model. The initial condition is T0 °C = 298 K, P0 = 1 bar and the gas equivalence ratio is φ = 1. The characteristics of flame are analyzed by modifying the turbulence intensity (uRMS = 0 m/s, 0.7 m/s, 1.3 m/s, 2.0 m/s)and hydrogen fraction (R (H2) = 0%, 10%, 30%, 50%). It is calculated that in the process of spherical flame combustion and propagation, the pressure increase rate increases first and then decreases. Turbulence intensity and hydrogen fraction have similar effect on the flame propagation process. If the turbulence intensity is used as an independent variable, the flame propagation rate increases with its increase. When the flame radius is 30 mm, the flame propagation velocity of 2.0 m/s turbulence intensity is about 2.7 times that of laminar flow. Due to which both the maximum combustion pressure and the maximum pressure rise rate are monotonically increasing. As it increases, the combustion pressure rises and falls the fastest. As the R (H2) increases, the flame propagation rate can significantly increase. When R (H2) is 50%, the maximum velocity is 4.82 m/s, which is about 1.5 times that of the maximum of pure methane. When R (H2) is high, the greater the maximum pressure is, the shorter the time for the maximum pressure to occur will be. When R (H2) is 50%, the maximum pressure is about 9 bar, which is 2.25 times of the force of pure methane combustion.

A multidimensional combustion model for oblique, wrinkled premixed flames

A new premixed turbulent combustion model is proposed, which is based on one-dimensional (1D) filtering of density times progress variable and reaction source term of laminar premixed flame profiles using a filter kernel that reflects the variation of the area of planar flame fronts moving through filter volumes. It is shown that these genuinely multidimensional effects qualitatively change the relation between the filtered reaction source term and the Favre-filtered reaction progress variable compared to strictly 1D models, particularly at large filter widths. Analytical results for the filtered quantities are obtained by approximating the laminar flame profiles by suitable ansatz functions. Filtered data from Direct Numerical Simulations (DNS) of statistically planar turbulent premixed flames at a range of turbulence levels of the unburnt mixture and for two different heat release parameters is used to optimize and validate the model. For filter widths smaller than the laminar flame thickness, minor effects from subgrid flame folding are observed. For larger filters, the effect of subgrid flame folding can be represented well through a wrinkling factor, which reduces the effective filter width used in the 1D filtering operation. Two wrinkling factor models are proposed, using fractal dimensions patterned on literature models. The modelled reaction source term written as function of Favre averaged progress variable and filter width shows excellent agreement with filtered DNS data conditioned on the Favre averaged progress variable for all investigated turbulence levels, filter widths and both heat release parameters.