Characteristics of combustion and pollutant formation in swirling flames

Abstract

Detailed measurements are presented of gas species concentrations and temperature in the flame formed around a fuel (propane) jet surrounded by a swirling air flow. Attention is given to the interrelation between the flame structure and the formation and emission characteristics of air pollutants such as NO x , CO, and hydrocarbons. It is noted that the mixing and combustion of the fuel layer flowing around the recirculation zone in the swirling flame structure control the formation and emission of NO x , CO, and hydrocarbons. Two typical flame configurations are found to arise according to the primary air ratio: Low NO x formation occurs in a flame (type B) having a high primary air ratio. This results from the fact that the fuel layer around the recirculation zone burns at relatively low temperature because of dilution by mixing with the surrounding excess air prior to the combustion, and the temperature of the burnt hydrocarbons are liable to be exhausted because the hydrocarbons in the fuel layer are excessively diluted and spill away through the circumferential part of the bulk flow. For another flame (type A) having a low primary air ratio, increased NO x is formed in the flame and appears in the exhaust because the fuel layer burns at relatively high temperature and the slow mixing retards the temperature decay. Effects of a wide range of combustion parameters on NO x , CO, and hydrocarbon emissions are examined in connection with the flame configuration.