Burning velocity and reaction rates in constant temperature non-adiabatic flames

Abstract

Flames with different fuel/oxygen ratios can be burnt at constant final temperatures on cooled porous burners, the constant temperature being maintained by changing burning velocity so that more heat is abstracted from the flames of greater calorific value. It is not obvious whether the required change of burning velocity is also associated with a change in the rate of fuel consumption, and this question has been investigated. In some lean hydrogen or hydrocarbon flames, the rate of fuel consumption has been found to be almost constant in constant temperature flames despite large changes in burning velocity with mixture ratio; and for such flames, the change in burning velocity is accounted for merely by the necessary change in heat abstraction. In fuel-rich hydrogen flames, however, the increased burning velocity of progressively richer mixtures is also associated with a faster reaction rate. In the course of the work, it was noted that the maximum rate of fuel consumption did not occur at a constant fraction of the final temperature or of the temperature rise, and this casts doubt on an assumption sometimes made to deduce ‘activation energies’ of flame reactions.