Single-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.•Single-phase heat transfer coefficient in impinged region is remarkably enhanced due to droplets impingement.•A heat transfer blind spot situated under central liquid sheet in simultaneous impact is identified.•Cooling of heated wall in successive impact is achieved by contacting of trailing droplet with heated wall. Multi-droplet impact on a 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. Results show that successive impact results in higher splashing threshold for the trailing droplet than the leading one due to radial flow inside the residual film. Single-phase heat transfer coefficient in the impinged region is remarkably higher than the undisturbed film region, caused by enhanced convective heat transfer. A heat transfer blind spot situated under the central liquid sheet in simultaneous impact is identified, at which relatively high local temperature is observed and heat is more difficult to be rejected due to flow stagnation there. Cooling of the heated wall in successive impact is mainly achieved by contacting of the trailing droplet with the heated wall, rather than mixing of cold droplets and warm film.