Experimental investigation of hydrodynamics and heat transport during horizontal coalescence of two drops impinging a hot wall

Abstract

This paper presents an experimental study on hydrodynamics and heat transport during the horizontal coalescence of two drops impinging a hot wall. The study addresses the influence of distance between impact locations, the time interval between drop impact, and wall superheat on the transport processes. The experiments were conducted under a pure vapor atmosphere with the refrigerant FC-72 at a saturation temperature of 54°C, corresponding to a system pressure of 0.94 bar. The drops were generated with a constant diameter and a constant impact velocity. The temperature field at the surface of the heater was measured by an infrared camera with a high spatial and temporal resolution. The local heat flux distribution was derived from the temperature field by solving the transient three-dimensional heat conduction equation within the substrate. The total heat flow was evaluated by integrating the local heat fluxes over the footprint of the drop. The impact parameters (drop size and impact velocity), the time interval between drop impact, and the distance between impact locations were evaluated through post-processing of the black-and-white images captured employing a high-speed camera. The results for simultaneous drop impingement reveal that the heat transported from the wall to the fluid is not affected by the presence of neighboring drop as long as the drops do not contact each other. In the case of horizontal coalescence of drops, the heat flow is reduced both during the spreading and receding phases and during the sessile drop evaporation phase. This can be explained by the reduction of solid–liquid interface and of three-phase contact line length after the coalescence. An increase in wall superheat leads to reducing the footprint of the drop and increasing of heat flow. Asymmetric behavior of the drop hydrodynamics and heat flux is observed during the non-simultaneous drop impingement.