Experimental study of cellular instability and extinction of non-premixed opposed-flow tubular flames

An experimental study of cellular instability of non-premixed opposed-flow tubular flames was conducted burning H2 diluted with CO2 flowing against air. The transitions to cellularity, cellular structures, and extinction conditions were determined as a function of the initial mixture strength, stretch rate, and curvature. The progression of cellular structures from the onset of cells through extinction was analyzed by flame imaging using an intensified CCD camera. Three different procedures of decreasing the Damkِhler number (forward process), as well as using those same procedures in the opposite progression of increasing the Damkِhler number (backward process) were completed. Significant flame hysteresis was seen and the forward transition occurred at a lower Damkِhler number than the backward transition. Mechanical perturbations were conducted to show that the onset of cellularity could be realized at a higher Damkِhler number than without perturbations. Once cellular instability was induced, it was possible to perturb the flame into multiple stable cellular states and extinguish the flame at a much higher Damkِhler number than without perturbations. Images are shown of rotating cellular flames and a cellular instability regime at an initial mixture strength greater than unity and away from extinction conditions. A qualitative explanation of flame rotation and a general categorizing of three distinct flame regimes is given.