Transient ignition characteristics of swirling spray kerosene flames under high-altitude relight conditions

The effects of low temperature and pressure on the transient ignition process of swirling spray kerosene flames under high-altitude flight conditions were studied experimentally in an optical chamber. The transient flame and flow field variations during the ignition process are observed and analyzed through high-speed direct and schlieren images. Through image enhancement of color flame images, it is found that there are two different development modes in the ignition process. The first mode involves three stages: the formation stage of the fire nucleus, the initial flame kernel propagation stage, and the successful ignition stage. The flame is more likely to be distinguished in the second stage. In the second ignition mode, a flame kernel is formed first, which then propagates both upward and downward to achieve continuous combustion. With the reduction of ambient pressure, the ignition completion time is shortened while the flame height increases significantly. Under the same environmental pressure conditions, the ignition completion time increases slightly under low-temperature conditions. The ignition process and the flame morphology after ignition become more chaotic and irregular under low-temperature conditions, primarily because low temperatures worsen the fuel evaporation and atomization performance, resulting in an uneven distribution of droplets of different sizes. Besides phenomenological observation, quantitative velocity is estimated through schlieren images using an optical flow algorithm. The results indicate that reducing pressure causes the flame to spread faster in the flow field. Under the most extreme low pressure and temperature conditions, dramatic velocity gradients can be observed, which is the leading cause of the ignition failure.