Experimental investigation of partially premixed methane–air and methane–oxygen flames stabilized over a perforated-plate burner

Abstract

In this work, two sets of experiments were performed including air and oxy-combustion premixed flames stabilized over a perforated-plate burner. A set of experiments was performed considering air–fuel combustion in order to identify a range of equivalence ratio for stable flame operation at fixed oxidizer Reynolds number and over a range of premixing ratio (namely 7, 25, 45, 67 and 128). Also, effects of premixing ratio (L/D) on lower and upper flammability limits over a range of oxidizer Reynolds number were investigated. Another set of experiments was performed considering oxy-fuel combustion in order to identify a range of equivalence ratio for stable flame operation at fixed oxidizer Reynolds number. The study documents the visual flame length, appearance and color and identifies the extinction mechanism outside the flammability limits for both air and oxy-combustion flames over wide ranges of operating equivalence and premixing ratios. For air combustion flames, the results showed wide ranges of flammability limits at lower premixing ratios and tight ranges of flammability limits at higher premixing ratios. This can be attributed to the increase in the degree of flame diffusivity while reducing the premixing ratio and, as a result, more stable flame is obtained. Reductions in the emissions of NO and CO were observed while increasing the premixing ratio for the air combustion case. For the oxy-combustion flames, the results showed stable flame operation at oxygen fraction of 36%; however, the flammability limits were approximately 20% lower as compared to those of air combustion flames. Flash back was observed when operating oxygen fraction exceeded 40% and flame stability was affected badly with the decrease of oxygen fraction. The visual flame length was longer in cases of oxy-combustion flames as compared to those of air combustion.