The effect of stretch on cellular formation in non-premixed opposed-flow tubular flames

Abstract

Cellular formation in non-premixed flames is experimentally studied in an opposed-flow tubular burner. This burner allows independent variation of the global stretch rate and overall flame curvature. In opposed-flow flames formed by 21.7% hydrogen diluted in carbon dioxide versus air, cells are formed near extinction with a low fuel Lewis number and a low initial mixture strength. Using an intensified CCD camera, the flame chemiluminescence is imaged to study cellular formation from the onset of cells to near extinction conditions. The experimental onset of cellular instability is found to be at or at a slightly lower Damköhler number than the numerically determined extinction limit based on a two-point boundary value solution of the tubular flame. 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 locally concave flame cell midsection is weakened and the locally convex flame cell ends are strengthened. With increasing 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. With increasing stretch rate, the flame thickness at the cell midsection decreases similar to a planar opposed-flow flame while the flame thickness at the cell edges is unchanged and can even increase due to the strengthening effect of convex curvature at the flame edges toward the low Lewis number fuel. The results show the existence of cellular flames well beyond the two-point boundary value extinction limit and the importance of local flame curvature in the formation of flame cells.