Numerical Study on NO_x Production of Transitional Fuel Jet Diffusion Flame

Abstract

A direct numerical simulation of a two-dimensional jet diffusion flame developed in a co-flowing air stream was carried out using the GRI [Gas Research Institute] chemical reaction mechanism (NO_x included) in order to elucidate the mechanism of NO_x formation in turbulent flow fields. The governing equations were discretized and numerically integrated using the finite volume method. The temperature dependence of thermodynamical properties was taken into account ; transport properties were calculated according to the simplified transport model proposed by Smooke [Reduced Kinetic Mechanisms and Asymptotic Approximations for Methane-Air Flames, (1991), p. 1-28, Springer-Verlag]. Fuel jet velocity was found to have the same effects on the flame structure and the unsteady behavior as those concluded from the flame sheet approach ; by increasing the fuel jet velocity, the flow becomes turbulent with large fluctuations downstream, and the flame is disturbed by the enlarged fluctuations in the turbulent region downstream where local extinction takes place. From a comparison with the laminar counterflow diffusion flame, it was concluded that the hypothesis of the laminar flamelet model that "the instantaneous local structure of the turbulent flow field can be accurately simulated by the laminar diffusion flame", can be applied to the unsteady turbulent jet flame that includes the possibility of extinction and takes into account the NO_x formation process. As a result, it was verified that the formation of NO_x in the turbulent flow field has the same mechanism as that in the unsteady laminar diffusion flame even for the case of extinction.