Characteristics of opposed flow partially premixed flames in mesoscale channels at low strain rates

Abstract

This study investigates opposed flow partially premixed flames (OFPPFs) in mesoscale horizontal channels. Flame stabilization conditions and behaviors were experimentally investigated for methane and propane, based on experimental parameters including flow velocity, nozzle distance, nozzle width, channel gap, and the equivalence ratios of the mixtures. Various flame structures were formulated, as partially premixed flames (PPFs) can intrinsically formulate three flame branches: lean premixed flame (LPF), non-premixed flame (NPF), and rich premixed flame (RPF), depending on the flow conditions. Three extinction limits were obtained: higher strain rate (HSR), lower strain rate (LSR), and flashback. The HSR extinction limits were extended through premixing and the increase of channel gap scale. Some distinctive flame characteristics were found in LSR conditions, which were the main focus of investigation. More specifically, the trends of LSR extinction limits were opposite in terms of the lean and rich sides regarding to the premixing ratio, and the flame oscillation near LSR extinction limits was affected by the premixing ratio. A simple analysis was conducted to explain flame oscillation phenomena. It was found that the flame oscillation was induced by the self-propagation of a flame edge faster than flow velocity. Its periodic motion can be explained by the existence of a new flame stabilization condition and a transient variation of gas concentrations caused by the dilution of burned gas along the stream. The LSR extinction characteristics of PPFs can be explained by variation in thermal thickness or effective heat loss from the flame. These results not only help us understand flame characteristics in mesoscale combustion spaces, but also provide important clues regarding the structure of ordinary opposed flow flames.