Influence of equivalence ratio and hydrogen addition on thermoacoustic instabilities in premixed methane and propane flames in vertical tubes

This research examines the behavior of premixed flames within a vertical tube, considering both an open and a closed end configuration. The study utilizes a pilot flame as the ignition source. The methane (CH4) and propane (C3H8) flames were tested at equivalence ratios (phi) ranging from 0.9 to 1.2 and constant hydrogen content at (R-H = 0.1). High-speed cameras captured the flame propagation, revealing that several mechanisms, including pressure and velocity, influenced the flame shape and propagation rate. Increasing equivalence ratios initially raised laminar burning velocity, peaking at phi = 1.1, before decreasing. For C3H8, flames with lower burning velocity exhibited higher underlying flame speeds and maximum pressures alongside larger flame surface areas. It suggests that velocity coupling is the primary mechanism causing acoustic instability in flames that propagate downward. CH4 flames achieve maximum burning speed at a mixture ratio of phi = 1.1. Notably, the peak pressure and underlying speed remain constant. A comparative analysis of CH4 and C3H8 flames utilizing the RH method showed a distinctive correlation in behavior. Moreover, a comprehensive summary of CH4 and C3H8 flame behaviors is included, with (R-H = 0-0.4) and (phi = 0.8-1.5).