Numerical study on flame propagation of a fuel droplet array in a high-temperature environment under microgravity

A series of numerical simulations was conducted to observe the flame propagation of a fuel droplet array in a high-temperature environment and revealed details of the propagation process. Based on theoretical considerations, various modes of flame propagation were predicted for characteristic parameters that represented the kind of fuel, droplet spacing S, droplet diameter d, and ambient temperature. In the present study, droplet spacing was employed as a control parameter to investigate the effect of time scales among the heat transfer to an unburned droplet, formation of flammable premixed gas, and ignition of premixed gas. A fuel droplet was assumed to be decane with 1 mm initial diameter. The initial ambient temperature was set at 573 K, which was lower than the autoignited temperature of decane. When the spacing was small, the flame front went straight in the premixed gas layer formed at the outer region of a droplet array. When the spacing was relatively large, the flame front traveled through the premixed gas layer between droplets. When spacing was large, the unburned droplet ahead of the flame front was ignited before the front expanded to the premixed layer of the unburned droplet. The ignited region was immediately combined with the propagating flame. Hence, this process was considered as intermediate between premixed propagation and propagation with discontinuous ignition. Also, the trend of flame propagation rate versus S/d was in agreement with the trend of experimental results obtained at room temperature.