An experimental study to compare the smoking characteristics of diffusion flames of propylene diluted nitrogen, argon, carbon dioxide and helium was performed. The mass flow rate of propylene at smoke point condition, which was defined as the critical fuel mass flow rate (CFMFR), was first determined. Then, CFMFR was divided into ten different fractions for the study of the mechanism of inert gas dilution on smoke point. The mass flow rate of each different inert gas to achieve the smoke point condition was then determined in the same manner. Flame radiation and the visible flame height for all the diluted fuel flames were measured. The axial soot concentration profiles of nitrogen-diluted smoke point flames were also measured using the laser induced incandescence (LII) method for selective conditions. The inert gas dilution study showed two distinct regions (chemical and momentum controlled regions). The study shows the amount diluent needed to achieve smoke point was in the decreasing order of Ar, CO2, N2 and He on mass basis. The analysis of the results showed that the main reason for this phenomenon was the heat sink capability of the gas. Hence, the specific heat of the gas was an important parameter. In general, nitrogen-diluted flames had higher flame length than other inert gas diluted flames. At higher CFMFR, in helium-diluted flames radiation was higher than in other flames.

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