Autoignition characteristics of laminar lifted flames in coflow jets

Autoignition characteristics are an important parameter in designing diesel or PCCI engines. Especially diesel spray flames are lifted from the nozzle and initial flame is formed by an autoignition phenomena. The lifted nature of diesel spray flames influences soot formation since air will be entrained into the spray core by the entrainment of air between the region of nozzle and lifted flame base. The objective of the present study is to identify the autoignition characteristics by adopting a coflow jet as a model problem. The propane jets have been injected into high temperature air and autoignition and lifted flame characteristics have been investigated experimentally. The result showed that the fuel jet in the high temperature coflow air with the initial temperature larger than 860 K was autoignited by itself without having any external ignition source. The liftoff height of the autoignited lifted flame increased nonlinearly with the fuel jet velocity, in much the similar behavior as to non-autoignited lifted flames at relatively low temperature. However, the data reduction demonstrated that the liftoff height of the autoignited flame exhibited marked difference as compared to non-autoignited cases, where the liftoff height was controlled by the balance of the propagation speed of lifted edge flame and local flow velocity. At a certain excessive jet velocity, the autoignited lifted flame had a repetitive behavior of extinction and reignition at a critical liftoff height. An analysis for this critical autoignition showed that the ignition delay time for the adiabatic case should be modified by considering heat loss to ambience. A liftoff height correlation for autoignited lifted flames has also been determined.