NUMERICAL STUDY OF AN UNSTEADY NON-PREMIXED FLAME IN A POROUS MEDIUM BASED ON THE THERMAL EQUILIBRIUM MODEL

摘要

Non-premixed combustion in porous media, also known as diffusion combustion, is found in many burners and combustion systems for both household and industry. However, in terms of porous combustion, there have been far fewer studies of non-premixed as compared to premixed combustion. Moreover, most of the previous works regarding the non-premixed porous combustion were based on experiments. The present research numerically investigates the behavior of non-premixed combustion of CH4 in a porous medium. The mathematical model proposed consists of conservation of mass, momentum equation, energy equation and species equations. This set of equations is discretized based on a sixth-order accurate compact finite difference algorithm. The discretized equations are integrated according to the third-order Runge-Kutta method. The porous medium is defined as a pseudohomogeneous medium in which temperature gradients between two phases are assumed negligible. The proposed unsteady model is successfully validated with the published study. The model is able to correctly describe physical behavior of a non-premixed flame. The effects of porous materials on the combustion process are analyzed. It is found that porous combustion has a broader reaction zone than combustion in a free space. The location of the reaction zone is determined by diffusion velocity of the mixture gas. The porous structure made of SiC gives higher temperature than when Al2O3 is used since SiC has higher thermal conductivity and lower heat capacity. The developed model can be utilized as a tool for fast adaption of combustion systems and for optimizing the combustion efficiency.