Cellular Instabilities from the Onset of Cells Until Extinction in a Non-Premixed Opposed-Flow Tubular Flame

Abstract

An experimental study of cellular instability of non-premixed opposed-flow tubular flames was conducted in flames burning H2 diluted with CO 2 flowing against air. The transitions to cellularity, cellular structures, and extinction conditions are 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 is analyzed by flame imaging using an intensified CCD camera. This new camera system is able to capture high resolution images of the three-dimensional cellular structures. Three different procedures of decreasing the Damkohler number (positive process), as well as using those same procedures in the opposite progression of increasing the Damkohler number (negative process) were completed. In doing so, it was possible to display significant flame hysteresis as well as show that the positive transition occurred at a lower initial mixture strength or higher stretch rate than the negative transition. Mechanical perturbations were conducted to show that the onset of cellularity was a fairly rigid flow condition; whereas 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 initial mixture strength or lower stretch rate than without perturbations. Images are shown of a new cellular instability regime at an initial mixture strength greater than unity and away from extinction conditions. Lastly, a qualitative explanation of flame rotation is given and a general categorizing of three distinct flame regimes is given.