Flame development characteristics inside a reverse-flow combustor at early stages of combustion

Characteristics of flame development and the influence of reverse-flow configuration on these characteristics few seconds after ignition are investigated experimentally. Simultaneous and high-speed OH* chemiluminescence and pressure measurements are performed. Methane is used as the fuel, and the fuel-air equivalence ratio is 0.7. Four experimental conditions pertaining to mean bulk flow velocities of 4.0 and 6.4 m/s along with two igniter rod positions (-5 and 1.5 mm with respect to a flame-holder) are tested. The results suggest the flame development can be categorized into ignition, stabilization, and transition phases. During the ignition phase, compared to closed ducts, the reverse-flow configuration allows for achieving relatively large values of the normalized flame edge velocity prior to reaching the stagnation wall. Once the flame reaches the wall, the pressure rate significantly increases. After this, the flame stabilizes on the flame-holder forming a Bunsen-type flame, which is followed by a long term oscillation in the heat release rate for the majority of the tested conditions. During this period, the pressure, the pressure rate, and the spatially-averaged heat release rate feature a peak at the acoustic frequency of the combustor. The normalized power spectrum densities of the pressure and spatially-averaged heat release rate collapse and follow power-law relations, agreeing with the results reported in the literature pertaining to unconfined flames. This suggests, despite significant influence of the stagnation wall during the ignition and stabilization phases, spectral characteristics of pressure and heat release rate are not influenced by the presence of the wall during the transition phase.