A Direct Moment Closure Model For Large Eddy Simulation Of Turbulent Combustion

In the context of global warming,developing more advanced and environmentally friendly energy utilization technologies is crucial for achieving energy conservation and emission reduction.Deeply understanding and researching the complex turbulent combustion phenomena in fossil fuel combustion processes is essential for controlling and reducing carbon emissions in energy utilization processes.With the continuous development of computer technology and numerical computing theory,computational fluid dynamics(CFD)methods have gradually become important tools for studying turbulent combustion.Large-eddy simulation(LES)method,as a numerical simulation method that balances accuracy and efficiency,has broad prospects in the study of turbulent combustion mechanisms and industrial application simulations.However,the governing equations of LES contain unresolved terms,especially the multiscale and strongly non-linear chemical reaction rate terms,which are the major obstacles limiting the application of LES methods in turbulent combustion research.To address this issue,researchers have proposed various turbulent combustion models.Currently,turbulent combustion models can be roughly divided into two categories.One category is based on specific physical assumptions,which decouples the turbulent combustion problem,reducing computational costs but sacrificing the accuracy and generality of the model.The other category of models is derived based on theoretical principles,usually having higher accuracy and generality,but they require solving a large number of transport equations or complex chemical reaction mechanisms,leading to higher computational costs.Therefore,this study aims to develop a turbulent combustion model and LES numerical method that are highly accurate,general,and efficient.Firstly,in this study,a new turbulent combustion model,namely the Direct Moments Closure(DMC)model,is proposed based on the dynamic second-order moment closure(DSMC)model.This model decomposes the filtered progress reaction rate into mean and fluctuating terms using mathematical transformations,avoiding the introduction of third-order moments and eliminating the need for the assumption of neglecting third-order moments.The model achieves theoretical completeness.Additionally,the accurate closure of filtered progress reaction rates is achieved by incorporating temperature gradient model,dynamic K model,algebraic moment model,and dynamic subgrid-scale method.Efficient and accurate closure method for multi-component reaction steps is proposed to overcome the limitations of the dynamic second-order moment closure model in handling multi-component reaction steps.To verify the generality of the DMC model in flame with different modes,a-priori study and a-posteriori validation of the DMC model have been conducted based on direct numerical simulation(DNS)databases of premixed and non-premixed flames.The accuracy of the model for subgrid-scale temperature fluctuations,subgrid-scale temperature-component correlation moments,filtered progress reaction rates,and filtered species production rates is explored.The results show that the DMC Model,which incorporates subgrid-scale fluctuation information,can accurately calculates filtered progress reaction rates and filtered species production rates.Its accuracy is significantly higher than that of Finite Rate Chemistry(FRC)model,reducing prediction errors by over 70%.Compared with the DSMC model,the DMC model,which does not neglect third-order moments,achieves higher accuracy,further reducing prediction errors by20%.To investigate the accuracy of the DMC model in simulating experiment flames,a partially premixed dimethyl ether jet flame experiment is studied using the large eddy simulation method with the DMC model.By comparing with experimental measurements,it is found that the DMC model accurately predicts the distributions of scalar quantities such as velocity,temperature,mixture fraction,and mass fraction of species in the flame.In certain regions,the performance of the DMC model is superior to that of Conditional Moment Closure(CMC)model and Probability Density Function(PDF)models.Moreover,based on the numerical simulation results,the flow development and flame characteristics of the partially premixed dimethyl ether flame are investigated.To enhance computational accuracy and reduce computational costs,a coupled adaptive algorithm for the large eddy simulation with DMC model is developed.This method integrates In-Situ Adaptive Tabulation(ISAT),Dynamic Adaptive Chemistry(DAC),Tabulation of Dynamic Adaptive Chemistry(TDAC),Adaptive Mesh Refinement(AMR),and Dynamic Load Balancing method(DLB).A comprehensive analysis and evaluation of the coupled methods are conducted from the perspectives of simulation accuracy and computational efficiency.It is found that the TDAC method can significantly reduce computational costs while ensuring accuracy.With appropriate tolerance,the TDAC method reduces computational costs by 58%,achieving a chemical computation speedup of 2.8 times.Additionally,the AMR-DLB method,dynamically refines the turbulent combustion regions in real-time,further improving the prediction accuracy of large eddy simulation.To expand the industrial application prospects of the DMC model,a numerical method for simulating gas-liquid two-phase combustion based on the Euler-Lagrange framework is developed.Using this method,a series of ethanol spray jet lift flame experiments is simulated.By comparing with experimental data,it is found that the temperature distribution predicted by the DMC model with large eddy simulation is in good agreement with experimental measurements.Moreover,it outperforms the predictions of the Dynamic Thickened Flame(DTF)model in the upstream and midstream regions.The DMC model demonstrates excellent prediction capabilities in flame lift-off height and average droplet size.Based on the numerical simulation results,the flame characteristics and lift-off mechanism of the spray jet flame under different equivalence ratios are explored.