Numerical Prediction of the Spontaneous Ignition of Cool Flame for the Microgravity Experiment by Using Sounding Rocket

Abstract

A microgravity experiment is going to be held in 2022 by using sounding rocket to clarify a cool flame dynamics of multiple n-decane droplets such as ignition delay time, ignition location, cool flame propagation, and so on. To predict the cool flame dynamics near ignition limit, 2D axisymmetric unsteady numerical calculation was carried out. A single droplet, droplet pairs whose inter-droplet distances are 8 mm and 16 mm, and droplet array with inter-droplet distance of 8 mm are simulated. Ambient temperature, pressure, and initial droplet diameter are 550 K, 1.0 atm, and 1.0 mm respectively. To acquire the mixture reactivity for the identification of the cool flame ignition start point, 0D reaction calculation is also carried out. It is found that the highly reactive mixture distributes in the outside of the droplets where temperature is high and mixture fraction is low. Temperature rise caused by the cool flame generation starts from the outside of the droplet pair, and the cool flame surrounds the droplet pair with small inter-droplet distance. In the case of the droplet array, the cool flame is generated at the outside of the array. As tracing the highly reactive mixture region, it propagates inward with average propagation velocity of 7.3 cm/s, and the cool flame propagation velocity accelerated just before burned out.