LES of oxy-fuel jet flames using the Eulerian Stochastic Fields method with differential diffusion

The Eulerian Stochastic Fields (ESF) Monte Carlo method to solve the transported PDF (TPDF) equation is extended to account for differential diffusion effects by incorporating species individual molecular diffusivities. The method has been applied in Large Eddy simulation (LES) to non-piloted oxy-fuel jet flames at different Reynolds numbers experimentally investigated by Sevault et al. [1]. Due to the high H₂ content in the fuel stream and CO₂ in the oxidizer these flames pose new challenges to combustion modeling as the flame structures are different compared to CH₄/air flames. The simulations show very good agreement with the experiments in terms of mixture fraction conditional mean values for temperature and mayor species on the fuel lean side and the reaction zone, deviations on the fuel rich side are discussed. The trend and location of localized extinction is reproduced well in the simulations, as well as differential diffusion effects in the near field. Additionally, it is shown that a neglect of differential diffusion in the combustion model leads to a lifted flame.     

Transported PDF simulation of turbulent CH4/H2 flames under MILD conditions with particle-level sensitivity analysis

Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH₄/H₂ flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.     

The Exergy Losses Analysis in Adiabatic Combustion Systems including the Exhaust Gas Exergy

The entropy generation analysis of adiabatic combustion systems was performed to quantify the exergy losses which are mainly the exergy destroyed during combustion inside the chamber and in the exhaust gases. The purpose of the present work was therefore: (a) to extend the exergy destruction analysis by including the exhaust gas exergy while applying the hybrid filtered Eulerian stochastic field (ESF) method coupled with the FGM chemistry tabulation strategy; (b) to introduce a novel method for evaluating the exergy content of exhaust gases; and (c) to highlight a link between exhaust gas exergy and combustion emissions. In this work, the adiabatic Sandia flames E and F were chosen as application combustion systems. First, the numerical results of the flow and scalar fields were validated by comparison with the experimental data. The under-utilization of eight stochastic fields (SFs), the flow field results and the associated scalar fields for the flame E show excellent agreement contrary to flame F. Then, the different exergy losses were calculated and analyzed. The heat transfer and chemical reaction are the main factors responsible for the exergy destruction during combustion. The chemical exergy of the exhaust gases shows a strong relation between the exergy losses and combustion emission as well as the gas exhaust temperature.     

Effects of fuel cetane number on the structure of diesel spray combustion: An accelerated Eulerian stochastic fields method

An Eulerian stochastic fields (ESF) method accelerated with the chemistry coordinate mapping (CCM) approach for modelling spray combustion is formulated, and applied to model diesel combustion in a constant volume vessel. In ESF-CCM, the thermodynamic states of the discretised stochastic fields are mapped into a low-dimensional phase space. Integration of the chemical stiff ODEs is performed in the phase space and the results are mapped back to the physical domain. After validating the ESF-CCM, the method is used to investigate the effects of fuel cetane number on the structure of diesel spray combustion. It is shown that, depending of the fuel cetane number, liftoff length is varied, which can lead to a change in combustion mode from classical diesel spray combustion to fuel-lean premixed burned combustion. Spray combustion with a shorter liftoff length exhibits the characteristics of the classical conceptual diesel combustion model proposed by Dec in 1997 (http://dx.doi.org/10.4271/970873), whereas in a case with a lower cetane number the liftoff length is much larger and the spray combustion probably occurs in a fuel-lean-premixed mode of combustion. Nevertheless, the transport budget at the liftoff location shows that stabilisation at all cetane numbers is governed primarily by the auto-ignition process.     

Studies of the flow and turbulence fields in a turbulent pulsed jet flame using LES/PDF

A turbulent piloted jet flame subject to a rapid velocity pulse in its fuel jet inflow is proposed as a new benchmark case for the study of turbulent combustion models. In this work, we perform modelling studies of this turbulent pulsed jet flame and focus on the predictions of its flow and turbulence fields. An advanced modelling strategy combining the large eddy simulation (LES) and the probability density function (PDF) methods is employed to model the turbulent pulsed jet flame. Characteristics of the velocity measurements are analysed to produce a time-dependent inflow condition that can be fed into the simulations. The effect of the uncertainty in the inflow turbulence intensity is investigated and is found to be very small. A method of specifying the inflow turbulence boundary condition for the simulations of the pulsed jet flame is assessed. The strategies for validating LES of statistically transient flames are discussed, and a new framework is developed consisting of different averaging strategies and a bootstrap method for constructing confidence intervals. Parametric studies are performed to examine the sensitivity of the predictions of the flow and turbulence fields to model and numerical parameters. A direct comparison of the predicted and measured time series of the axial velocity demonstrates a satisfactory prediction of the flow and turbulence fields of the pulsed jet flame by the employed modelling methods.     

Partially premixed flamelet in LES of acetone spray flames

An effective partially premixed flamelet model for large eddy simulation (LES) of turbulent spray combustion is formulated. Different flame regimes are identified with a flame index defined by budget terms in a 2-D multi-phase flamelet formulation, and the application in LES of partially pre-vaporized spray flames shows a favorable agreement with experiments. Simulations demonstrate that, compared to the conventional single-regime flamelets, the present partially premixed flamelet formulation shows its ability in capturing the subgrid regime transitions, yielding a well prediction of peak gas temperature and the downstream flame spreading. A propagating premixed flame front is found coupled with a trailing diffusion burning through the spray evaporation, and the spray effect on regime discrimination is manifested with transport budget analysis. A two-phase regime indicator is then proposed, by which the evaporation-dictated regime is properly described. Its intended use will rely on both gas and spray flamelet structures.     

Paradigms in turbulent combustion research

The development of the basic conceptual viewpoints, or paradigms, for turbulent combustion in gases over the last 50 years is reviewed. Significant progress has been made. Recent successes in the prediction of pollutant species and extinction/re-ignition phenomena in non-premixed flames are seen as the result of close interaction between experimentalists, theoreticians, and modellers. Premixed turbulent flames seem to be dependent on a much wider range of factors, and predictive capabilities are not so advanced. Implications for large eddy simulation (LES) and partially premixed combustion are outlined.     

A DNS evaluation of mixing and evaporation models for TPDF modelling of nonpremixed spray flames

In the present work, nonpremixed temporally evolving planar spray jet flames are simulated using both direct numerical simulation (DNS) and the composition transported probability density function (TPDF) method. The objective is to assess the performance of various mixing and evaporation source term distribution models which are required to close the PDF transport equation in spray flames. Quantities which would normally be provided to the TPDF solver by spray models and turbulence models are provided from the DNS: the mean flow velocity, turbulent diffusivity, mixing frequency, and cell-mean evaporation source term. Two cases with different Damköhler numbers (Da) are considered. The low Da case (Da-) features extinction followed by reignition while extinction in the high Da case (Da+) is insignificant. The TPDF modelling considers two mixing models: interaction by exchange with the mean (IEM) and Euclidean minimum spanning trees (EMST). Three models for distribution of the evaporation source terms are considered: EQUAL which distributes them in proportion to notional particles’ mass weight, NEW which creates new particles of pure fuel, and SAT which distributes the sources preferentially to notional particles close to saturation. It is found that the IEM model overpredicts the extinction when used with any evaporation model for both Da- and Da+ cases. The EMST model captures well the trend for extinction and reignition for the Da- case when it is coupled with the EQUAL evaporation model, but it overpredicts the extinction when coupled with the NEW or SAT evaporation model. For the Da+ case, all evaporation models reasonably capture the flame dynamics when coupled with EMST. The flame temperature in the mixture fraction space was examined to further assess the model performance. In general the EMST model results in narrow PDFs with little conditional fluctuation, while the IEM model produces bimodal PDFs with burning and partial extinction branches.     

Zone-adaptive modeling of turbulent flames with multiple chemical mechanisms

A novel zone-adaptive modeling method (AdaCM) with multiple chemical mechanisms is proposed for turbulent flames to achieve high-fidelity, yet computationally efficient, predictions. Specifically, a computational economical species transport model, e.g., the well-mixed model, via finite volume algorithm is employed with a simple mechanism as the base model for the whole computational domain, while the advanced transported probability density function (TPDF) method via Lagrangian particle tracking is employed with a detailed mechanism only for spatial regions with intense turbulence-chemistry interaction (TCI), denoted as the “PDF regions”. The PDF regions are dynamically identified based on local flow and flame characteristics and may evolve with time. A two-way particle/finite volume submodel coupling is formulated to ensure the composition consistency between submodels in the PDF regions and impose the correct interface conditions for composition and mass flow rate on the boundary of the PDF regions. With regard to transformation between different species representations in the mechanisms, a species reconstruction/reduction approach based on constrained chemical equilibrium is proposed to ensure element conservation and an adequate specification of unrepresented species at the model interface. The proposed adaptive modeling method has been applied to the well-known Sandia Flame D, in which the well-mixed combustion model with a 6-species, two-step global mechanism is employed as a base model and the high-fidelity TPDF with a 25-species skeletal mechanism is employed for regions with intense TCI. Results demonstrate the consistency in PDF regions between submodels with two distinct mechanisms. The predictions from the adaptive modeling are almost identical to those of TPDF and agree well with experimental measurements, illustrating the preservation of prediction accuracy in the adaptive method. In addition, the total number of computational particles in AdaCM for Flame D is only 18% of that for the stand-alone TPDF, and the recorded computational speedup is about 2.8.     

Simulation of spray combustion in a lean-direct injection combustor

Large-eddy simulation (LES) of a liquid-fueled lean-direct injection (LDI) combustor is carried out by resolving the entire inlet flow path through the swirl vanes and the combustor. A localized dynamic subgrid closure is combined with a subgrid mixing and combustion model so that no adjustable parameters are required for both non-reacting and reacting LES. Time-averaged velocity predictions compare well with the measured data. The unsteady flow features that play a major role in spray dispersion, fuel–air mixing and flame stabilization are identified from the simulation data. It is shown that the vortex breakdown bubble (VBB) is smaller with more intense reverse flow when there is heat release. The swirling shear layer plays a major role in spray dispersion and the VBB provides an efficient flameholding mechanism to stabilize the flame.     

A novel method to automate FGM progress variable with application to igniting combustion systems

This paper introduces a new method to formulate reaction progress variables for the application of FGM in combustion systems. The method involves a multiobjective optimisation to find a reaction progress variable that accurately reproduces complex reactive phenomenon of interest. In our current research, the method is applied to igniting non-premixed flames. The optimised progress variable combinations are evaluated for their accuracy in reproducing detailed chemistry results for the ignition of hydrocarbon fuels. Comparisons are made against conventional progress variable formulations used in the literature. The current approach takes into consideration the table resolution and error reduction for the application of FGM in combustion problems. Methods that rely on maximising the smoothness of the manifold or ensuring monotonic increase in progress variables alone are shown to be insufficient to capture ignition. The possibility of optimising the progress variable with emphasis on accurately resolving a particular zone or phase of combustion, such as ignition, while maintaining minimum data loss is demonstrated. Through its application in a number of igniting counterflow flames, the effectiveness of the current method is verified. Progress variables optimised using specific flame databases are shown to accurately reproduce the ignition delays even with moderate variations in boundary conditions of the respective flames.     

LES study of turbulent ethanol spray flames using CSE coupled with non-adiabatic chemistry tables

Conditional Source-term Estimation (CSE) is applied to three turbulent ethanol spray flames (EtF3, EtF6, and EtF8) in Large Eddy Simulation (LES). The objectives of this paper are to include the heat losses due to spray evaporation and gas radiation in the chemistry tabulation, assess the impact of these changes on the temperature and droplet statistics, and evaluate the performance of LES-CSE for the selected flames. The profiles of gas temperature, spray velocity, velocity root mean square (rms) and droplet size distribution are well reproduced in the simulations compared to available experimental data. Temperature underpredictions near the centreline are observed, in particular, at locations closer to the jet exit for flames with lower jet velocity. A wider flame is predicted in EtF8 compared to the experiment and regions of local extinction are visible. The use of non-adiabatic chemistry library results in a noticeable improvement in the temperature predictions near the peak locations, especially for flames with higher velocity and closer to the jet exit. The heat losses due to evaporation are larger than those from radiation, confirming the importance of including the evaporation effects in the chemistry tables. The droplet velocity is well predicted, except for EtF8 where an underprediction is observed far downstream. The velocity rms is slightly underpredicted at some locations, probably due to the simple stochastic model used. Overall, LES-CSE with non-adiabatic chemistry tables successfully captures the gas-spray quantities in the selected flames.     

Stratified turbulent Bunsen flames: flame surface analysis and flame surface density modelling

In this paper it is investigated whether the Flame Surface Density (FSD) model, developed for turbulent premixed combustion, is also applicable to stratified flames. Direct Numerical Simulations (DNS) of turbulent stratified Bunsen flames have been carried out, using the Flamelet Generated Manifold (FGM) reduction method for reaction kinetics. Before examining the suitability of the FSD model, flame surfaces are characterized in terms of thickness, curvature and stratification.

All flames are in the Thin Reaction Zones regime, and the maximum equivalence ratio range covers 0.1?φ?1.3. For all flames, local flame thicknesses correspond very well to those observed in stretchless, steady premixed flamelets. Extracted curvature radii and mixing length scales are significantly larger than the flame thickness, implying that the stratified flames all burn in a premixed mode. The remaining challenge is accounting for the large variation in (subfilter) mass burning rate.

In this contribution, the FSD model is proven to be applicable for Large Eddy Simulations (LES) of stratified flames for the equivalence ratio range 0.1?φ?1.3. Subfilter mass burning rate variations are taken into account by a subfilter Probability Density Function (PDF) for the mixture fraction, on which the mass burning rate directly depends. A priori analysis point out that for small stratifications (0.4?φ?1.0), the replacement of the subfilter PDF (obtained from DNS data) by the corresponding Dirac function is appropriate. Integration of the Dirac function with the mass burning rate m=m(φ), can then adequately model the filtered mass burning rate obtained from filtered DNS data. For a larger stratification (0.1?φ?1.3), and filter widths up to ten flame thicknesses, a β-function for the subfilter PDF yields substantially better predictions than a Dirac function. Finally, inclusion of a simple algebraic model for the FSD resulted only in small additional deviations from DNS data, thereby rendering this approach promising for application in LES.     

Subgrid combustion modeling of 3-D premixed flames in the thin-reaction-zone regime

Large eddy simulation (LES) is used to investigate three-dimensional (3D) lean premixed turbulent methane–air flames in the thin-reaction-zone regime. In this regime, the Kolmogorov scale is smaller than the preheat zone thickness, but larger than the reaction zone thickness. Past numerical studies of similar flames were primarily direct numerical simulation either in two-dimensions or using the artificially thickened flame approach in 3D. For an LES the effect of small (unresolved) scales on the scalar field must be, modeled accurately to capture the correct flame structure. A subgrid combustion model based on the linear-eddy-mixing (LEM) model is used within an LES framework (called LEM–LES hereafter) to capture the 3D flame-structure of the highly stretched premixed flames. A finite-rate, one-step methane–air chemistry with a non-unity Lewis number formulation is used in this study. The simulated flame structure resembles flames experimentally studied in the thin-reaction-zone regime. Even though the preheat zone is broadened by the penetration of small eddies, the chemical reaction zone remains thin and localized. This feature is captured properly in the current LEM–LES approach. The flame structure and other statistics such as the flame area evolution, curvature, and strain-rate statistics computed using the LEM–LES are also in good agreement with the past DNS studies.     

An assessment of large eddy simulations of premixed flames propagating past repeated obstacles

This paper presents an assessment of Large Eddy Simulations (LES) in calculating the structure of turbulent premixed flames propagating past solid obstacles. One objective of the present study is to evaluate the LES simulations and identify the drawbacks in accounting the chemical reaction rate. Another objective is to analyse the flame structure and to calculate flame speed, generated overpressure at different time intervals following ignition of a stoichiometric propane/air mixture. The combustion chamber has built-in repeated solid obstructions to enhance the turbulence level and hence increase the flame propagating speed. Various numerical tests have also been carried out to determine the regimes of combustion at different stages of the flame propagation. These have been identified from the calculated results for the flow and flame characteristic parameters. It is found that the flame lies within the ‘thin reaction zone’ regime which supports the use of the laminar flamelet approach for modelling turbulent premixed flames. A submodel to calculate the model coefficient in the algebraic flame surface density model is implemented and examined. It is found that the LES predictions are slightly improved owing to the calculation of model coefficient by using submodel. Results are presented and discussed in this paper are for the flame structure, position, speed, generated pressure and the regimes of combustion during all stages of flame propagation from ignition to venting. The calculated results are validated against available experimental data.     

LES/FGM investigation of ignition and flame structure in a gasoline partially premixed combustion engine

This paper presents a joint numerical and experimental study of the ignition process and flame structures in a gasoline partially premixed combustion (PPC) engine. The numerical simulation is based on a five-dimension Flamelet-Generated Manifold (5D-FGM) tabulation approach and large eddy simulation (LES). The spray and combustion process in an optical PPC engine fueled with a primary reference fuel (70% iso-octane, 30% n-heptane by volume) are investigated using the combustion model along with laser diagnostic experiments. Different combustion modes, as well as the dominant chemical species and elementary reactions involved in the PPC engines, are identified and visualized using Chemical Explosive Mode Analysis (CEMA). The results from the LES-FGM model agree well with the experiments regarding the onset of ignition, peak heat release rate and in-cylinder pressure. The LES-FGM model performs even better than a finite-rate chemistry model that integrates the full-set of chemical kinetic mechanism in the simulation, given that the FGM model is computationally more efficient. The results show that the ignition mode plays a dominant role in the entire combustion process. The diffusion flame mode is identified in a thin layer between the ultra fuel-lean unburned mixture and the hot burned gas region that contains combustion intermediates such as CO. The diffusion flame mode contributes to a maximum of 27% of the total heat release in the later stage of combustion, and it becomes vital for the oxidation of relatively fuel-lean mixtures.     

Simulation of diesel spray combustion using LES and a multicomponent vapourisation model

Diesel spray and combustion in a constant-volume engine cylinder was simulated by a large eddy simulation (LES) approach coupling with a multicomponent vapourisation (MCV) modelling. The simulation focused on the inclusion of the interaction between fuel spray and gas-phase turbulence flow at the sub-grid scale. The LES was based on the dynamic structure sub-grid model, and an additional source term was added to the filtered momentum equation to account for the effect of drop motion on the gas-phase turbulence. The multicomponent drop vapourisation modelling was based on the continuous thermodynamics approach using a gamma distribution to describe the complex diesel fuel composition and was capable of predicting a more complex drop vapourisation process. The effect of gas-phase turbulence flow on the fuel drop vapourisation process was evaluated through the solution of the gas-phase moments of the distribution in the present LES framework. A non-evaporative spray in a constant-volume engine cylinder was first simulated to examine the behaviours of LES, in comparison with a Reynolds-averaged Navier–Stokes (RANS) simulation based on the RNG k–? model. More realistic diesel spray structures and improved agreement on liquid penetration length with the corresponding experimental data were predicted by the LES, using a grid resolution close to that of RANS. A more comprehensive simulation of diesel spray and combustion in cylindrical combustor was also performed. Predicted distributions of soot particles were compared to the experimental image, and improved agreement with the experimental data was also observed by using the present LES and MCV models. Consequently, results of the present models proved that improved overall performance of the fuel spray simulation can be achieved by the LES without a significant increase in the computational load compared to the RANS.     

Large Eddy Simulation of a swirl stabilized spray flame

Large Eddy Simulations (LES) of kerosene spray combustion in an axial-swirl combustor have been carried out focusing on the effect of the evaporating droplets on the flame temperature and species concentrations. The LES-PDF methodology is used for both dispersed (liquid) and gas phases. The liquid phase is described using a Lagrangian formulation whilst an Eulerian approach is employed for the gas phase. The predictive capability of LES with sub-grid scale models for spray dispersion and evaporation is assessed placing emphasis on the effect of the unresolved velocity and temperature fields on the droplet evaporation rate. The results of the fully coupled LES formulation exhibit good agreement between the measured and simulated mean velocity fields. The global behaviour of the spray combustion, such as droplet dispersion and evaporation, are captured reasonably well in the simulations. It was found that the large velocity fluctuations observed in the shear layer strongly affect the evaporation rate and thus the temperature distributions. The present work also demonstrated the feasibility of LES to study complex flow features which are typical of gas-turbine combustion chambers.