A dual timescale model for micro-mixing and its application in LES-TPDF simulations of turbulent nonpremixed flames

The numerical simulation of modern aero-engine combustion chamber needs accurate description of the interaction between turbulence and chemical reaction mechanism. The Large Eddy Simulation(LES) method with the Transported Probability Density Function(TPDF) turbulence combustion model is promising in engineering applications. In flame region, the impact of chemical reaction should be considered in TPDF molecular mixing model. Based on pioneer research, three new TPDF turbulence-chemistry dual time scale molecular mixing models were proposed tentatively by adding the chemistry time scale in molecular mixing model for nonpremixed flame. The Aero-Engine Combustor Simulation Code(AECSC) which is based on LES-TPDF method was combined with the three new models. Then the Sandia laboratory's methane-air jet flames: Flame D and Flame E were simulated. Transient simulation results show that all the three new models can predict the instantaneous combustion flow pattern of the jet flames. Furthermore,the average scalar statistical results were compared with the experimental data. The simulation result of the new TPDF arithmetic mean modification model is the closest to the experimental data:the average error in Flame D is 7.6% and 6.6% in Flame E. The extinction and re-ignition phenomena of the jet flames especially Flame E were captured. The turbulence time scale and the chemistry time scale are in different order in the whole flow field. The dual time scale TPDF combustion model has ability to deal with both the turbulence effect and the chemistry reaction effect, as well as their interaction more accurately for nonpremixed flames.

Auto-ignition and stabilization mechanism of diluted H_2 jet flame

The controllable active thermo-atmosphere combustor(CATAC) has become a utilizable and effective facility because it benefits the optical diagnostics and modeling. This paper presents the modeling research of the auto-ignition and flames of the H2/N2(H2/CH4/N2,or H2/H2O2/N2) mixture on a CATAC,and shows curves varying with temperatures of auto-ignition delay,the height of the site of auto-ignition of lifted flames,and flame lift-off height. The results of auto-ignition delay and the lift-off height are compared the experimental results to validate the model. A turning point can be seen on each curve,identified with criterion temperature. It can be concluded that when the co-flow temperature is higher than the criterion temperature,the auto-ignition and lifted flame of the mixture are not stable. Conversely,below the criterion temperature,the mixture will auto-ignite in a stable fashion. Stabilization mechanisms of auto-ignition and lifted flames are analyzed in terms of the criterion temperature.