Numerical Simulations of Two-Phase Turbulent Combustion in Spray Burners

Investigation of subgrid-scale mixing of mixture fraction and temperature in turbulent partially premixed flames

<div class="section abstract"><div class="htmlview paragraph">Downsizing or higher compression ratio of SI engines is an appropriate way to achieve considerable improvements of part load fuel efficiency. As the compression ratio directly impacts the engine cycle thermal efficiency, it is important to increase the compression ratio in order to reduce the specific fuel consumption. However, when operating a highly boosted / downsized SI engine at full load, the actual combustion process deviates strongly from the ideal Otto cycle due to the increased effective loads requiring ignition timing delay to suppress abnormal combustion phenomena such as engine knocking. This means that for an optimal design of an SI engine between balances must be found between part load and full load operation. If the knocking characteristic can be accurately predicted beforehand when designing the combustion chamber, a reduction of design time and /or an increase in development efficiency would be possible. A turbulent combustion simulation is required to estimate the pressure and temperature trace in the cylinder for knocking analysis.</div><div class="htmlview paragraph">In this research, the verification for the turbulent combustion and knocking were done by using the detailed chemistry solver with multi-zone modeling integrated into CONVERGE CFD code. Thereby, it was found that a reduced reaction model can reproduce turbulent combustion and knocking phenomenon under some operation conditions. The verification results show that the pressure trace in the cylinder, Knocking timing and position can be validated with experiment results accurately.</div></div>

This paper briefly describes our recent efforts on the modeling and numerical simulations of two-phase turbulent reacting flows in realistic combustion systems with a new large-eddy simulation (LES) model. The model is constructed based on the two-phase extension of scalar filtered mass density function (FMDF) and a Lagrangian-EulerianLagrangian mathematical/numerical methodology. In this methodology, the “resolved” fluid velocity field is obtained by solving the filtered form of the compressible Navier-Stokes equations with a high-order finite difference scheme. The liquid (droplet) phase and scalar (temperature and species mass fractions) fields are both obtained by stochastic Lagrangian models. There are two-way interactions between the phases and all the Eulerian and Lagrangian fields. The LES/FMDF is used for systematic analysis of turbulent combustion in the spray-controlled dump combustor and double-swirl spray burner for various flow and spray parameters. The effects of fuel type, spray angle, mass loading ratio, droplet size distribution, fuel/air composition, wall, and inflow/outflow conditions on the combustion are investigated. It has been found that the main features of the turbulence and combustion are modified by changing the inflow/outflow conditions. The LES/FMDF results also confirm the significance of the spray parameters.

Large-eddy simulations of turbulent flows in internal combustion engines

This paper provides a brief review of the scalar filtered mass density function (FMDF) model. The FMDF is a subgrid-scale probability density function (PDF) model for large eddy simulation (LES) of turbulent combustion and is obtained by the solution of a set of stochastic differential equations by a Lagrangian Monte Carlo method. The applicability and the validity of the LES/FMDF are established by simulating various low and high speed, single- and two-phase turbulent reacting flows. The LES/FMDF results are found to be consistent and comparable to DNS and experimental results for different flows.

Advances in high-performance computational capabilities enable scientific simulations with increasingly realistic physical representations. This situation is especially true of turbulent combustion involving multiscale interactions between turbulent flow, complex chemical reaction, and scalar transport. A fundamental understanding of combustion processes is crucial to the development and optimization of next-generation combustion technologies operating with alternative fuels, at higher pressures, and under less stable operating conditions, such as highly dilute, stratified mixtures. Direct numerical simulations (DNS) of turbulent combustion resolving all flow and chemical features in canonical configurations are used to improve fundamental understanding of complex flow processes and to provide a database for the development and validation of combustion models. A description of the DNS solver and its optimization for use in massively parallel simulations is presented. Recent DNS results from a series of three combustion configurations are presented: soot formation and transport in a nonpremixed ethylene jet flame, the effect of fuel stratification in methane Bunsen flames, and extinction and reignition processes in nonpremixed ethylene jet flames.

A combustion model based on the flame surface density (FSD) concept is developed and implemented for large eddy simulation of turbulent nonpremixed combustion of wood pyrolysis gas and air. In this model, the filtered reaction rate ω̇̄α of species α is estimated as the product of the consumption rate per unit surface area ṁα and the filtered FSD Σ̄. This approach is attractive since it decouples the chemical problem from the description of the turbulence combustion interaction. The filtered FSD is modeled as the product of the conditional filtered gradient of mixture fraction and the filtered probability density function. This approach is validated from direct numerical simulation (DNS) using spatial filtering operation. Results show that the proposed FSD model provides a good description for the filtered reaction rate of wood pyrolysis gas. Temperature predicted by large eddy simulation agrees well with that filtered from DNS data.

Experimental study of scalar filtered mass density function in turbulent partially premixed flames

For Large Eddy Simulation (LES) of turbulent combustion, as the turbulent flow as well as the thin flame front are directly filtered by the cut-off scale, resolution of the computational grid plays a very important role in this case and represents always a quality determining factor. In addition, the fluctuation of heat release is found to be the main source for noise generation from turbulent combustion, which is attributed to the interaction of the turbulent flow and the combustion reaction. As the flame is thickened or filtered respectively by the computational grid, it becomes less sensitive to fluctuations of the flow as well, so that the emitted noise from the flame due to unsteady heat release is affected by the grid resolution in LES combustion modeling as well. The current work represents an investigation with respect to these aspects. For this purpose, LES and DNS simulations for a realistic jet flame at moderately turbulent condition have been carried out. The LES calculations have been performed on computational grids with different resolutions (0.4/3.2/10.7 million cells) on the HP XC4000 cluster of the Steinbuch Centre for Computing (SCC) at the KIT. In order to assess predictability of the LES methodology, a three-dimensional DNS simulation on a grid with 54 million cells has been carried out on the Cray XE6 (HERMIT) of the High Performance Computing Center Stuttgart (HLRS). The comparison of the LES solution with experimental and DNS data allows an evaluation of the influence of the grid refinement to a great extent.

An algebraic concentration moment (ACM)-PDF turbulent combustion model is proposed. It is formulated by jointly utilizing the explicit algebraic expressions for the second-order-moment of concentration fluctuation and the presumed probability density function (PDF) of gas instantaneous temperature. A set of analytical expressions for the time-averaged temperature relevant quantity is obtained for the closure of the time-averaged reaction rate. The model is applied to the simulation of turbulent flow and combustion in a swirl combustor. The calculated gas velocity, temperature, species concentrations, and turbulent fluctuating velocity are in agreement with the measured data. They are much improved over those obtained by the EBU-Arrhenius model.

This paper presents an analysis of the discretization errors in the non-equilibrium models for the subfilter variance of the mixture fraction, a key quantity to model in large eddy simulation (LES) of turbulent mixing and combustion. Two discretely distinct formulations of the non-equilibrium models that solve the transport equations to obtain the subfilter variance, i.e., the second moment transport equation (STE) and the variance transport equation (VTE), are analyzed. By deriving discrete equations for the evolution of subfilter variance by the two formulations, it is seen that the difference originates primarily from the product rule of differentiation applied to the scalar convection term, which does not hold discretely. LES of scalar mixing in a planar jet is performed to illustrate the outcome of the analysis. Results show that the discrete product rule error is significant and of the same order as the production and dissipation terms on average. A priori analysis using direct numerical simulation (DNS) data for scalar mixing in homogeneous isotropic turbulence is also performed. From the analysis, it is seen that the VTE model under-predicts the subfilter variance, whereas the STE model over-predicts it substantially with sharp oscillations.

The conditional filtering method is proposed as a subfilter combustion model for large-eddy simulation (LES) of turbulent nonpremixed combustion. The novel method is based on conditional filtering of a reactive scalar field and an extension of conditional moment closure (CMC) for LES. Filtering conditioned on isosurfaces of the mixture fraction is adopted to resolve small-scale mixing and chemical reactions in nonpremixed combustion. The conditionally filtered equations are derived and the closure assumptions are discussed. A priori tests are performed using direct numerical simulation data for reacting mixing layers. The primary closure assumption on the subfilter flux in mixture fraction space is shown to work much better than the corresponding closure for the Reynolds averaged CMC due to resolved large-scale fluctuations of the scalar dissipation rate and of reactive scalars. Results show that first-order closure of the reaction rate performs well except for the boundaries of flame holes. In the boundaries of flame holes, fluctuations of reactive scalars around the conditionally filtered values are large enough for the effects of higher-order correlations to be significant. The accuracy of the first-order closure is less sensitive to the level of local extinction than that of first-order CMC, since large-scale fluctuations of reactive scalars on isosurfaces of the mixture fraction are resolved. This shows that extinction processes occur primarily over length scales comparable to the large scales of the turbulence. The integrated conditional filtering approach is introduced to reduce the computational cost and to resolve the low probability problem in the conditional filtering method. While the assumption of homogeneity in the integration direction is not as good as in the conditional average, the integrated formulation is shown to represent the extinction process caused by large-scale fluctuations of the scalar dissipation rate quite well.

Large eddy simulation (LES)/ Probability Density Function (PDF) approaches are now well established and can be used for simulating challenging turbulent combustion configurations with strong turbulence chemistry interactions. Transported PDF methods are known to be computationally expensive compared to flamelet-like turbulent combustion models. The pre-partitioned adaptive chemistry (PPAC) methodology was developed to address this cost differential. PPAC entails an offline preprocessing stage, where a set of reduced models are generated starting from an initial database of representative compositions. At runtime, this set of reduced models are dynamically utilised during the reaction fractional step leading to computational savings. We have recently combined PPAC with in-situ adaptive tabulation (ISAT) to further reduce the computational cost. We have shown that the combined method reduced the average wall-clock time per time step of large-scale LES/particle PDF simulations of turbulent combustion by 39%. A key assumption in PPAC is that the initial database used in the offline stage is representative of the compositions encountered at runtime. In our previous study this assumption was trivially satisfied as the initial database consisted of compositions extracted from the turbulent combustion simulation itself. Consequently, a key open question remains as to whether such databases can be generated without having access to the turbulent combustion simulation. Towards answering this question, in the current work, we explore whether the compositions for forming such a database can be extracted from computationally-efficient low-dimensional simulations such as 1D counterflow flames and partially stirred reactors. We show that a database generated using compositions extracted from a partially stirred reactor configuration leads to performance comparable to the optimal case, wherein the database is comprised of compositions extracted directly from the LES/PDF simulation itself.

A laboratory-scale chamber is convenient for combustion scenarios in the practical analysis of industrial explosions and devices such as internal combustion engines. The safety risks in hazardous areas can be assessed and managed during accidents. Increased hydrogen usage in renewable energy production requires increased attention to the safety issues since hydrogen produces higher explosion overpressures and flame speed and can cause more damage than methane or propane. This paper reports numerical simulation of turbulent hydrogen combustion and flame propagation in the University of Sydney's small-scale combustion chamber. It is used for the investigation of turbulent premixed propagating flame interaction with several solid obstacles. Obstructions in the direction of flow cause a complex flame front interaction with the turbulence generated ahead of it. For numerical analysis, OpenFOAM CFD software was chosen, and a custom-built turbulent combustion solver based on the progress variable model—flameFoam—was used. Numerical results for validation purposes show that the pressure behaviour and flame propagation obtained using RANS and TFC models were well reproduced. The interaction between larger-scale flow features and flame dynamics was obtained corresponding to the experimental or mode detailed LES modelling results from the literature. The analysis revealed that as the propagating flame reached and interacted with obstacles and the recirculation wake was created behind solid obstacles, leaving traces of an unburned mixture. The expansion of flames due to narrow vents generates turbulent eddies, which cause wrinkling of the flame front.

Machine learning tabulation of thermochemistry in turbulent combustion: An approach based on hybrid flamelet/random data and multiple multilayer perceptrons