Experimental investigation of propane-air flame propagation in narrow circular ducts

An experimental investigation was performed of propane-air flames propagating in narrow, horizontal, circular ducts with the purpose of establishing flame propagation phenomenology and comparing with recent theoretical findings that correlate propagation speed with heat release. Videos were taken at various axial locations to survey the entire development of the flame along the tube length. The phenomenology of the flame under various mixture compositions is discussed. Some of the features observed include stretching and undulation of the flame front, including inversion of the flame front at the leading edge-a phenomenon sometimes referred to as" tulip-shaped" flames. In addition, a sizeable increase in propagation speed and flame instability of fuel-rich and stoichiometric cases over lean cases was observed. These observations seem to undercut the traditional model of a stable flame shape with a uniform propagation speed. In addition, information regarding the propagation of the flame front over time was extracted via image processing. It was observed that the flame front accelerated as it traveled down the tube. The effects of factors such as stoichiometry, heat release, and tube length on the flame speed, travel time, and flame phenomenology were examined and compared with theoretical predictions.