Pressure extinction limits of non-premixed flames

Pressure extinction limits of non-premixed (diffusion) flames are studied theoretically by examining the two key non-dimensional parameters: Damkohler number and radiation-heat-loss parameter. It is found that there is a critical value of the reaction order, ncr, for a given flame and flow configuration. If the overall combustion reaction order n is less than ncr, a high-pressure flame extinction limit exists as a result of a small limiting Damkohler number. If the overall combustion reaction order n is greater than ncr, both heat loss and Damkohler number contribute to the existence of the low-pressure extinction limit. The contribution of the Damkohler number increases with flame intensity. Since the value of the critical reaction order ncr varies with flame configuration and flow type, three examples of diffusion flames adjacent to a condensed fuel are given in the paper: stagnation-point flame, side-stabilised flame and spherical flame. The reason for the different value of the critical ncr is due to the different pressure dependence of the flow residence time in the flame stabilisation zone of these flames.