Computational study on the flame and extinction behavior of a high enthalpy air slab

Abstract

A solid fuel ramjet has been recognized as a viable contender for atmospheric supersonic flight due to its performance and simple construction. However, the static temperature increases due to shock compression at the inlet that results in high temperature air entering the combustor. As such, the goal of this study is to analyze how the kinetics and flame sustainment properties are altered with this higher enthalpy air exposure. To examine these effects, a slab burner is modeled with gaseous using OpenFOAM© software. The reaction processes are modeled using complete University of California San Diego chemical-kinetic mechanism for burning in a hydrogen–air environment. Temperatures in the air intake range from 300 to 1000 K in 100 K increments, with the hydrogen fuel temperature kept at 300 K. In examining the conductive and convective heat flux data at the fuel surface, two distinct temperature regimes appeared for each respective heat flux mechanism. The results showed a large drop in conductive heat flux seen only between inlet temperatures of 300 and 500 K. Conversely, convective heat flux saw an increase in heat flux as a fully turbulent flow was approached for temperatures between 600 and 1000 K. In analyzing the Damköhler numbers of involved reactions, it was found that higher temperatures activated an endothermic chain branching reaction, which contributed significantly to the OH radical pool. This activation prevented a large drop in conductive heat flux for high inlet air temperatures. The results showed the impact of inlet temperatures on reaction pathways, which allows improved flame holding at higher air velocities.