Ignition, lift-off, and soot formation studies in n-dodecane split injection spray-flames

Abstract

A close-coupled double injection strategy with two 0.5-ms injections separated by a 0.5-ms dwell is implemented. Studies are performed in a constant volume pre-burn type combustion vessel over two ambient temperatures (900 and 800 K) at constant density (22.8 kg/m3) with 15% O2 by volume in the ambient. The aim of this work is to investigate the establishment and dependence of ignition delay and flame stabilization on the ambient temperature conditions especially for the main injection, and thereby investigating eventual soot production. Simultaneous schlieren and planar laser -induced fluorescence experiments as well as three-dimensional Reynolds-averaged numerical simulation computational fluid dynamic modeling with chemical kinetics in every computational fluid dynamic cell were performed. It was observed experimentally that at 900 K, the second injection is injected in a high-temperature combustion recessed ambient of the first injection whereas at 800 K it is injected in a low temperature, possibly reactive species environment. It was found from Reynolds-averaged numerical simulation modeling that combustion recession at 900 K in the present case entails rich presence of hydroxyl radical species and also the ambient of 800 K is source of reactive radicals like peroxides, leading to acceleration of main ignition. Flame stabilization of the second injection occurs closer to the injector due to short ignition delays with flame being sustained in the fuel–air premixing zone. Flame stabilization of the second injection was found to follow a premixed flame propagation mechanism. Investigation in mixture fraction and temperature space of pilot-main spray combustion revealed that the lower lift-off of main results in lower air-entrainment which causes richer ignition of main resulting in quicker and higher soot formation. The effect of the second injection in enhancing the oxidation of soot from the first injection by inducing enhanced mixing was also revealed.