Non-premixed flame characteristics of opposed methane jets in coaxial narrow air stream tubes

Recently, there has been a mounting interest in adopting non-premixed flames in order to improve flame stabilization in small combustors. This study introduces a non-premixed flame that was formulated in a narrow tube by an opposed injection of methane jet into a coaxial air stream. This configuration enhanced both the mixing and heat exchange between the burned and unburned gases. Various flame behaviors were observed and their stabilization limits were investigated for various fuel–air-flow rates and tube diameters. The temperature profiles near the flame and flue gas concentrations were measured. It was found that the flame stabilization limits were determined by three mechanisms depending on the flow rates and tube diameters: non-premixed flame quenching, flat flame stretch, and curved flame stretch. These results extend our understanding of the characteristics of non-premixed flames in a narrow space.