Numerical study of high temperature diluted air combustion in turbulent nonpremixed flames using an unsteady flamelet model

Abstract

The high temperature diluted air combustion, which improves the flame stability while lowers the $NO_x$ emission level, has been numerically investigated. The Favre-averaged Navier-Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the laminar flamelet concept with the standard $\kappa - \varepsilon$ turbulence model. The $NO_x$ formation is estimated by solving the Eulerian particle transport equations in a postprocessing mode. The numerical model was extensively validated against two test problems, i.e., the $CH_4/H_2/N_2$ jet flame and the $CH_4/H_2$ stabilized bluff body flame, because of ample experimental data base. The flow, temperature and species except NO are well captured by the steady flamelet model. The NO concentration is overpredicted by the steady flamelet model but the unsteady calculation gives much better agreement with the experimental data. In particular, the NO concentration is sensitive to the number of flamelet particles. The calculations with different number of flamelet particles show that the results for NO improve with increasing number of flamelet particles. However, the results do not much change when the particle number exceeds six. The NO concentration with six flamelet particles may be considered to be independent of the number of flamelet particle. In order to examine the characteristics on high temperature diluted air combustion, calculations are performed for a coaxial jet flame and a turbulent jet flame in a crossflow for various conditions of inlet air temperature and oxygen concentration. The results for the temperature distribution and the NO formation are presented and discussed. The carbon dioxide is a more effective diluent than the nitrogen in reducing the maximum flame temperature and NO formation. The relative importance of the thermal, prompt, $N_2O$ intermediate mechanisms for NO formation is discussed using detailed chemical kinetics. Also, a production rate analysis of ...