Evaluation of Cool Flame Characteristics of Droplet Pairs with Two-dimensional Calculation

To investigate characteristics of cool and hot flame ignition of n-heptane droplet pairs with different sizes, 2-D numerical simulations were conducted. The initial droplet diameters were 0.4 mm and 1.0 mm, and spacing between the droplet centres (S_d) were 3 and 6 mm. The ambient temperature was 750 K, and the pressures were 0.1 and 0.3 MPa. Under each condition, cool flame ignition and two stage ignition were observed, respectively. When the pressure was 0.1 MPa and S_d was 3 mm, cool flame ignition occurred near the smaller droplet, resulting in cool flame apperance near the larger droplet. However, when S_d was 6 mm, each droplet independently showed cool flame ignition and subsequent appearance of cool flame around the larger droplet was not observed. At 0.3 MPa, because the diffusion of fuel vapor was mitigated, cool flame ignition independently occurred even if S_d was 3 mm. Regarding the hot flame, it occurred between the droplets regardless of S_d, which is probably due to the duplicated fuel source. The hot flame ignition occurred near the larger droplet, not in the middle point between the droplets. This is probably due to the larger Stefan flow from the smaller droplet.