Application of optical diagnostics to the quantification of soot in n-alkane flames under diesel conditions

In the present paper, three different soot-measuring techniques, namely laser extinction method (LEM), 2-color pyrometry (2C) and laser-induced incandescence (LII) have been simultaneously employed to characterize soot distribution inside a diesel flame. Two single-component fuels (n-Decane and n-Hexadecane) and two derived blends (50%Dec/50%Hex and 30%Dec/70%Hex) have been used. Tests have been performed at an optical diesel engine, under different in-cylinder conditions. The study has been complemented with the measurement of ignition delay and Lift-off length. The present work pursues a twofold objective. On the one hand, the effect of fuel properties on soot formation has been analysed, under different engine operating conditions. On the other hand, sensitivity and performance of the three optical techniques has been evaluated, identifying their main advantages and drawbacks in the framework of the current study. LEM has been considered as the reference technique, as the measurement principle can be implemented without important limitations associated to the other two. Results highlight that larger molecules produce more soot than the smaller ones, with both reactivity and soot formation changing with the proportion of the heavier fraction. Despite describing similar trends, LEM and 2C do not provide the same KL values, with the pyrometry reaching some sort of saturation when increasing flame soot. A detailed analysis confirms that 2-Color measurements are strongly biased by soot and temperature distribution inside the flame. Nevertheless, it could still be a good option for low sooting conditions. On the other hand, an attempt to calibrate LII signal by means of LEM measurements has been reported. This approach should make it possible to obtain additional information on the soot spatial distribution. However, inconsistencies have been identified which stem from the inherent limitations of LII technique in highly sooting conditions.