Heat-transfer mechanisms of lean premixed CH4/air flame in a ceramic granular bed burner

Abstract

This study investigates the heat-transfer mechanism of premixed flames in a granular bed by conducting experiments and numerical simulations. The goal is to understand the heat-transfer phenomenon between the flames and the ceramic granules and among the ceramic granules themselves. This heat-transfer phenomenon involves equivalence ratios and firing rates of premixed flames because these two parameters affect the structural characteristics of flames and alter the heat transfer mechanism. These experimental results indicate that the temperature of the premixed flame increased as the firing rate increased. Additionally, the absolute propagation speed of the flame was observed to increase as the firing rate increased at a high equivalence ratio; however, the absolute propagation speed was observed to decrease as the firing rate decreased at a low equivalence ratio. At low equivalence ratios, the firing rate of the flame was relatively low and partial heat loss resulted in flame instability. The numerical simulation results indicate that changing equivalence ratios and firing rates alters the structure of the premixed flames, and the preheat characteristics of premixed flames result in the flame approaching the flameless characteristic most often seen in high-temperature air combustion. The premixed flame heat-transfer mechanism is further identified to be related to the equivalence ratio and firing rate related to the equivalence ratio and thermal radiation effects. At high equivalence ratios, thermal radiation effects have a greater influence on premixed flames. At medium equivalence ratios, preheat conductions have a greater influence on premixed flames at low firing rates; however, preheat radiation has a greater influence on premixed flames as the firing rate increases.