Dilute spray flames of ethanol and n-heptane in the transition to mild combustion

Abstract

The structure and stabilisation mechanisms of ethanol and n-heptane spray flames are investigated in this study. The burner configuration involves a dilute spray of dispersed droplets which is produced and transported via a carrier gas stream of air to the reaction zone, where the flames are stabilised by a hot coflow of combustion products. A range of coflow conditions were implemented for the different flame cases, allowing the effects of the coflow oxygen (O2) concentration and temperature to be examined independently. The resulting flames were analysed using three simultaneous laser diagnostic techniques, enabling the combined planar imaging of the hydroxyl (OH) and formaldehyde (CH2O) radicals, along with the location of droplets. For both fuel types, a noticeable shift in stabilisation behaviour was observed with a variation in the coflow O2 concentration from 11% to 3%, while the coflow temperature was not seen to have a significant impact. These flames also show an interesting departure from the typical behaviour observed for gaseous and prevaporised flames in a similar configuration, particularly for coflow conditions that are typically associated with the transition to the mild combustion regime.