Flame propagation: the measurement of burning velocities of slow flames and the determination of limits of combustion

Abstract

A method of measuring burning velocities of combustible gas/air mixtures near the limits of combustion is developed, using a burner providing a flat disk-shaped flame. The method is applied to methane, propane, n -pentane, n -heptane, ethylene, acetylene and benzene flames of velocities 5 to 10 cm/s. The characteristics of the burner and the flames are discussed; the time during which unburnt gas can be heated by conduction from the flame appears to be insufficient for purely thermal initiation, and the reactions are therefore probably started by diffusion of radicals from the flame front. The burning velocities of these slow flames are mainly dependent on reaction heat which determines the temperature of the flame and the rate of the reactions. The effect of some additives (1/2 % by volume) on the burning velocities in the range investigated and on the lower limits is thermal; no catalytic influence wets found. The flat flame is only stable within a fairly narrow range of velocity; cusped flames are readily formed, particularly with mixtures rather richer in oxygen than upper-limit mixtures. The flat-flame burner is also applied to the determination of limits of combustion. The lower limits are lower when determined in this manner compared with the standard tube method. The limit determinations can be carried out easily over a wide range of mixtures. The effect of argon on the limits is consistent with the change in specific heat of the mixture, but carbon dioxide appears to have a further effect.