Investigation of the evaporation heat transfer mechanism of a non-axisymmetric droplet confined on a heated micropillar structure

Abstract

In scaling computer performance, closely integrating heterogeneous components in a single system-in-package assembly provides faster signal communication and a tighter footprint – but also generates more heat: state-of-the-art microelectronic devices can produce more than 1 kW cm−2. The next generation high-powered electronics will need two-phase liquid cooling, such as droplet evaporation, which utilizes the latent heat of vaporization to remove excessive heat. Compared to traditional single-phase cooling techniques, two-phase cooling offers both high efficiency and an exceptionally high heat dissipation rate. Although droplet evaporation has been explored for over a hundred years, many fundamental transport mechanisms are still not well understood. For example, most droplet evaporation studies focused only on spherical droplets, which possess a uniform curvature, κ (for a low Bond Number), along the meniscus interface. Evaporation from a non-spherical droplet, due to the change in perimeter-to-area ratio and the meniscus curvature, exhibits very different interfacial mass transport features from a spherical droplet. In particular, a higher perimeter-to-solid-liquid-area ratio will yield a larger fraction of thin film region and therefore a smaller thermal resistance in an evaporating droplet, while a high local curvature will facilitate a stronger local vapor diffusion rate. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on heated porous micropillar structures. We explore the equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes, using the Volume of Fluid (VOF) method, and we use a simplified Schrage model [1] to study the evaporative mass transport at the liquid-vapor interface. The results show that microdroplets evaporating on a triangular substrate possess 12.8% smaller effective film thickness compared to that on a circular substrate, due to a longer length of the contact line. During the evaporative heat transfer process, the triangular-based droplets also exhibit the smallest temperature difference between the droplets’ solid-liquid interface and ambient temperature, which leads to a higher heat transfer coefficient (21% larger than a spherical droplet at a supplied heat flux of 500 W/cm2). When the supplied heat increases to a higher value (e.g., 1000 W/cm2), the shape effect becomes less significant where the diffusion resistance is dominated by the liquid-vapor interfacial temperature instead of the meniscus curvature.