Characterisation of ethylene flames under a range of low-oxygen concentrations

Abstract

New understanding of turbulent ethylene (C2H4) jet flames, issuing into a range of preheated coflowing oxidisers with reduced oxygen (O2) concentrations are reported. These conditions emulate moderate or intense low-oxygen dilution (MILD) combustion. To resolve previously reported non-monotonic trends, comparisons are made for coflow O2 concentrations of 3%, 4%, 5%, 6%, 9% and 11% and coflow temperatures of 1250K, 1315K and 1385K. Instantaneous and simultaneously planar imaging measurements of temperature, hydroxyl radicals (OH) and formaldehyde (CH2O) were taken at eight downstream locations ranging from 9mm to 75mm. The new data reveal non-monotonic lift-off trends in the OH and CH2O formation heights at all three temperatures. Compared with extant measurements, the higher resolution and greater control of coflow composition provide a detailed exploration of the transitional behaviour that occurs when MILD combustion conditions are achieved. The results experimentally demonstrate non-monotonic variation in lift-off height, and show that the same behaviour is present in both the OH and CH2O formation heights, and occurs at all three temperatures, which is due to the movement of the location of the stoichiometric and most reactive mixture fractions to the coflow side of the jet shear layer. The results also show that increasing the oxidiser temperature does not have a significant effect on the OH number density for a given temperature but does result in a significant decrease in lift-off height and CH2O concentration.