A Dynamic Second-Order Moment Closure(DSMC) Model For Large Eddy Simulation Of Turbulent Combustion

More and more combustion systems have been applied to transportation and power generation.The pollution of those induces is becoming a critical problem in our environment.It is important to control the combustion.There are three numerical method,namely direct numerical simulation(DNS),Large-eddy simulation(LES)and Reynolds-averaged Navier-Stokes(RANS)for the study of turbulent flow with chemical reaction.The DNS is a significant tool for fundamentals of turbulent combustion and combustion model confirmation.Although DNS of laboratory-scale turbulent combustion has been successfully performed in the recent years,it is still not practical to be applied to realistic industrial combustion because of prohibitive computational demand.The major limitation of DNS is its huge requirement of computational resources.Both RANS and LES are less expensive in computational cost since closure models on turbulence and combustion are used to reduce the calculation.Compared to RANS that has been widely used for industrial applications LES has been demonstrated to be able to provide more accurate information and useful qualitative understanding of turbulent combustion phenomena.In the previous DNS studies,it is noted that both premixed flames and non-premixed flames coexist in either traditionally defined premixed turbulent combustion or non-premixed combustion because of turbulence-chemistry interactions.So,it is important puzzle for a combustion model to deal with the multi-regime problem.To overcome the dependency of combustion model on combustion regime,a new dynamic second-order moment closure model is developed for large eddy simulation of turbulent combustion.The averaged reaction rate is directly closed in the form of Arrhenius law.The whole temperature exponential function is treated as a single variable to avoid the series expansion.In the sub-grid source term,the unresolved terms are closed using the algebraic form which can be derived from the unresolved terms' transport equations.All the coefficients in the model are evaluated dynamically.A-priori validation using a DNS database,The results show that dynamic second order matrix combustion model is credible.Secondly,a-posteriori validation by LES of Sandia pilot jet Flame series(Flame C,D,E and F)are performed with one step reaction chemical mechanism.The results demonstrate the the efficiency to well predict turbulent combustion.and then,the results from the combustion model and the original second-order moment(original-LES-SOM)have been compared with the data from Flame C.It is found that the original-LES-SOM could give good results.And the performance of the new combustion model is better than it.The sub-grid effect in viscosity in LES of flame D has been studied.The counter effects zone of turbulent sub-gird viscosity has been observed.The sub-grid effects in combustion model in Flame E and F have been carefully studied.Results show that the sub-gird plays a reverse role in reaction.those means that omit the second-order term will make the reaction artificial faster.Then,the combustion model has been extended to muti-step chemistry in a Sydney swirl burner(SM1).And the results has been compared to the results of one-step chemistry.results show that the chemistry plays a little influence in flow development of the flame SM1.Both chemisty could give reasonable results,but the muti-step chemistry play better.Finally,the dynamic grid combustion model extends to liquid phase combustion.In order to testing the accuracy of the combustion model,three dimensional swirling spray combustion were studied by DNS and LES.The results show that the combustion model could capture the complicated flame structure,premixed flame and non-premixed entanglemently concurrence.And also colud give good predicts in species.