Experimental investigation of hydrodynamics and heat transport during vertical coalescence of multiple successive drops impacting a hot wall under saturated vapor atmosphere

Abstract

In this study, hydrodynamics and heat transport during the vertical coalescence of multiple successive drops impacting a hot wall are analyzed experimentally. This study addresses the influence of wall superheat and the frequency of drop generation on the hydrodynamics and heat transport. The experiments are conducted under a pure vapor atmosphere with the refrigerant FC-72 at a saturation temperature of 54.5°C, corresponding to a system pressure of 0.94 bar. The drops are generated with a constant diameter of 1.14 mm and a constant impact velocity of 0.54 m s−1. An infrared camera with a high spatial and temporal resolution is used to capture the temperature field on the surface of the heater. The local heat flux distribution is derived from the temperature field by solving the transient three-dimensional heat conduction equation within the substrate. The total heat flow is evaluated by integrating the local heat flux over the footprint of the drop. The impact parameters (drop size and impact velocity) are evaluated through post-processing of the black/white images captured using a high-speed camera. The maximum spreading radius and maximum heat flow observed after the impact of each successive drop are higher than those observed after the impact of the previous drop. For instance, in the case of a wall superheat of 12.4 K and an impact frequency of 10 Hz, the maximum spreading radius and maximum heat flow observed after the impact of the fourth drop increased by approximately 35% compared with those observed after the impact of the first drop on a dry wall. The distribution of wall heat flux during the spreading phase of the second impacting drop is characterized by the appearance of two thin ring-shaped zones of high heat flux. The time evolutions of the drop shape and heat flow depend on the wall superheat and are independent of the frequency of drop generation within the studied range of parameters.