Two-phase heat transfer of multi-droplet impact on liquid film

•A 3D numerical model with implementing random disturbance subjected to Gaussian distribution is built up.•Peaks of heat transfer coefficient present at triple-phase contact lines in vicinity of vapor bubbles.•Reduced number of vapor bubbles by flow redirection results in lower heat transfer coefficient in crater.•Multi-droplet successive impact breaks uniformity of local flux in impact region. Multi-droplet impact on a liquid/vapor two-phase thin liquid film covering a heated wall is numerically studied using a three-dimensional model with an implement of a random disturbance subjected to Gaussian distribution. The temperature field reveals that liquid mass getting into crowns mainly originate from liquid film rather than droplets, while liquid mass of droplets constitutes the central liquid sheet. Despite minor effects of bubble configuration on interface evolution and overall trend of heat transfer coefficient in the impact region, it brings remarkable effect on the peaks of heat transfer coefficient at the triple-phase contact lines in the vicinity of vapor bubbles, values of which are higher than those in pure liquid phase, and vapor phase within bubbles. The heat transfer coefficient in the crater region is smaller than both the multi-droplet interaction region and the crown developing region, which can be attributed to the increased number of vapor bubbles in the latter two regions under the effect of flow redirection. Multi-droplet successive impact results in nonuniformity of local heat flux in the impact region, despite that this nonuniformity will be alleviated shortly because of continuous heating from the surface. Also discussed in this study include the effects of droplets vertical spacing and contact angle on the two-phase heat transfer performance in multi-droplet impingement.