Effects of the secondary air on the combustion characteristics of turbulent premixed CH4/NH3/air flames in a two-stage swirl combustor

Abstract

This study experimentally investigates the effects of secondary air injection on the flame structure and CO/NO emission characteristics of turbulent premixed CH4/NH3/air flames in a two-stage swirl combustor. The equivalence ratio (phi(p)) and velocity (U-p) of the primary fuel/air jet and the injection location (L-s) and velocity (U-s) of the secondary air jet are varied. The location of the secondary air injection of 30 and 250 mm are adopted to investigate two interaction cases in two-stage combustion (TSC): 1) a strong interaction where the secondary air is directly injected into the primary premixed flame and strongly affects the flame, and 2) a weak interaction where the secondary air is injected after the primary reaction is completed. A single-stage combustion (SSC) is also tested for comparison. It is found from averaged OH-PLIF images that the strong interaction cases of TSC with phi(p) > 1 can produce a significant amount of NO emissions due to the direct interaction between the primary flame and the secondary air, while the weak interaction cases can produce an appreciable amount of CO emissions due to the incomplete combustion of the secondary flames. This result is further confirmed by CO/NO measurements that the strong interaction cases produce higher NO emissions than the weak interaction cases, while the CO emissions of the weak interaction cases become significant and peak at phi(p) = 1.1 similar to 1.2. Based on the OH-PLIF images and CO/NO emission characteristics of TSC, the optimal conditions of the primary and secondary jets are proposed to ensure the reduction of NOx emissions with low level of CO emissions.