Effects of equivalence ratio on the extinction stretch rate of cylindrical premixed flames

The actual flame stretch rates of the stretched cylindrical premixed flame and counterflow twin flames were measured with LDV for the methane-air and propane-air mixtures over a wide range of equivalence ratios. The stretch rate at the flame center at the point of flame extinction was considered to be the maximum stretch rate for the existence of cylindrical and plane flamelets. It was shown that the actual flame stretch rate at the point of flame extinction for the stretched cylindrical flame was smaller than that for the counterflow twin flames except for the case of very rich propane-air mixtures. The conditions for the existence of the cylindrical flamelet in turbulent premixed flames, which has a similarity to the fine structure of turbulent premixed flames proposed by Chomiak (1970, 1972, 1977), were discussed using the stretch rate data at the extinction point and the turbulence parameters. It was found that the cylindrical flamelet could not exist in highly turbulent flames where the distributed reaction zones were realized because of the limitation of resistibility to the extinction due to local strain. Furthermore, the cylindrical flamelet should be limited to mixtures such as rich propane-air and lean methane-air in which the minimum flame diameter at the extinction point is comparable to the Kolmogorov microscale.