Effect of wall surface roughness on droplet cooling over a rectangular heated plate in an enclosure

Study of evaporation of single droplet plays a very important role in analyzing the process operations of fuel cooling and spraying systems. This has been a pioneering area of research among chemical and heat transfer engineers. In the present work, an experimental and numerical study of the dynamics of droplet evaporation inside a rectangular enclosure is carried out. The width of the enclosure is less than the spreading radius of the droplet which generates two contact angles, one with the heated surface and the other with the enclosure and this makes the study important. The droplet absorbs a considerable amount of heat as it evaporates on the plate thereby reducing its temperature. The introduction of enclosure enhances the cooling effect when compared to droplet cooling in the plate without enclosure. The effect of heat transfer with the introduction of rectangular enclosure is studied based on the numerical analysis. Turbulence modelling is used with standard wall treatment to study the effect of surface roughness on droplet evaporation in the presence of a rectangular enclosure. A change in the dynamics of the droplet with change in surface roughness of the heated plate and wall enclosure is reported in terms of variation in contact angle and heat transfer characteristics. INTRODUCTION Analysis of evaporation of water droplets over heated surfaces is of great interest to engineers because of its diverse applications in the field of heat transfer. The characterization of the dynamics of water droplets is pivotal in studying the cooling effect produced on the heated plate. The cooling of a hot surface with water droplets is one of the effective ways used in various cooling systems. It has wide applications in nuclear reactors, turbines etc. In droplet cooling technique, the major factors that affect the cooling process are droplet size, nature of the heated plate and wall surface roughness. Initial studies on water droplets levitating on a hot surface were carried out by Tamura and Tanaswa [1]. They classified four regimes during droplet evaporation as: film evaporation, nucleate boiling, transition and spheroidal vaporization. Analysis of droplet mechanics at various initial stages of droplet evaporation was presented by Hubbard et al [2]. Droplet evaporation at initial stages requires detailed analysis as it differs considerably from quasi steady state. An important parameter to characterize the droplet dynamic characteristics is the contact angle. Bernardin et al. [3] analysed the contact angle dependence for water droplets on Aluminium surfaces. It was seen from the analysis that the contact angle is an important parameter to be studied to understand the dynamics of water droplets. They also developed an empirical relation between the contact angle and the temperature of the heated surface. Surface characteristics play an important role in changing the dynamics of water droplet during evaporation. Surface roughness and its effects on droplet evaporation was systematically analysed by Kamnis and Gu [4]. Wachters and Westerling [5] described experimental observations involving the dynamics of droplet of size 2mm in a gold surface. Yet another interesting study of single droplet on a glass surface was reported by Kurokawa and Tada [6]. Xiong and Yuen [7] studied the impact of single droplet of water and hydrocarbon fuels. Di Marzo and Evans [8] performed experiments to analyze variation in temperature with droplet impingement on an aluminium surface. Though considerable attention has been paid to experimental analysis, numerical studies on droplet dynamics have gained acceptance widely. This is because, numerical analysis provides an easier means to analyse transient characteristics such as velocity and temperature distribution. Out of several numerical approaches available for multiphase flows, Volume Of Fluids is a robust modelling tool used to study the phase fraction distributions during droplet evaporation as presented by Harshavardhan et al [9]. In the present work, this tool has been effectively used along with experimental support to understand the droplet dynamics and heat transfer characteristics on the evaporation of droplet on heated plate surrounded by an enclosure. The presence of enclosure is found to play a major role on the cooling of the heated plate. The effect of wall surface roughness on the droplet dynamics is also studied