Local mechanisms prior to flow boiling of a subcooled liquid in rectangular micro-channel heat sinks

This study explores the corresponding local mechanisms prior to the flow boiling of a subcooled liquid in rectangular micro-channel heat sinks. The study implemented a numerical model based on the Volume of Fluid (VOF) in which corresponding governing equations were solved using computational fluid dynamics (CFD). Although several works have been done in flow boiling, none are in pre-flow boiling conditions. In this model, a computational domain of micro-channel heat sink in three dimensions, including the sub-domains of solids and fluid, was created to consider the conjugate heat transfer for a better estimate of heat transfer. The mass fluxes employed in this study were between 115 and 389 kg/m2 s, and in all conditions, the heat flux and inlet temperature were set as 1561 kW/m2 and 23°C respectively. The micro-channel heat sink was operated at the atmospheric pressure, and the corresponding substrate material and working fluid chosen were silicon and water, respectively. The numerical results were validated against previous experimental work and agreed well with the experimental data. Although bubble nucleation was absent, heat transfer in micro-channels behaved as the characteristic of nucleate boiling with incomplete evaporation inducing the liquid-vapour mixture on the heating surfaces. When mass fluxes increase, the durations between the onset of phase change and the onset of flow boiling are higher, and the heat fluxes at the onset of phase change are higher. For all tested conditions, the peaks of non-dimensional local heat flux of phase change are evident. Also, the peaks occur only when the liquid phase is in direct contact with the solid surface and become significantly low at the mixture locations. This study aims to understand the mechanisms better immediately before the flow boiling of subcooled liquids in the micro-channel heat sinks to develop more reliable approaches to predict heat transfer. © 2024 Elsevier Ltd