| Summary: | Cellular formation in non-premixed flames is experimentally studied in an opposed-flow tubular burner that allows independent variation of the global stretch rate and overall flame curvature. All experiments were 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 formation from the onset of cells through extinction was analyzed in the tubular burner by flame imaging using an intensified CCD camera. Cells formed near extinction at low fuel Lewis numbers and low initial mixture strengths. The experimental onset of cellular instability is found to be at or slightly above the numerically determined 1D tubular flame extinction limit.
For fuel Lewis numbers less than unity, concave curvature towards the fuel retards combustion and weakens the flame and convex curvature towards the fuel promotes combustion and strengthens the flame. In the cell formation process, the concave flame cell midsection is weakened and the convex flame cell ends are strengthened. With increasing stretch rate, the flame thickness at the cell midsection decreases while the flame thickness of the cell edges is unchanged, with further increase in stretch rate, the flame breaks into cells and the cell formation process continues until near-circular cells are formed with no concave midsection. Further increase in the stretch rate leads to cell extinction. The results show the importance of local flame curvature in the formation of flame cells.
To assess hysteresis effects, three different procedures of decreasing the Damköhler number (positive process), as well as using those same procedures in the opposite progression of increasing the Damköhler number (negative process) were completed. Significant flame hysteresis was seen and the positive transition occurred at a lower Damköhler number than the negative transition.
Mechanical perturbations were conducted to show that the onset of cellularity could be realized at a higher Damköhler number. 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 Damköhler 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 are given.
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