The capillary length scale determines the influence of bubble-fin interactions and prediction of pool boiling from heat sinks

Abstract

Heat sinks have the capability of increasing the operating heat flux limits of two-phase immersion cooling for passive electronics thermal management. For arrays of fins, investigation of the heat-flux-dependent variation of boiling regimes that can manifest along the fin height is required to predict performance and facilitate heat sink design. Existing methods for the prediction of fin boiling heat transfer which account for a variable heat transfer coefficient along the fin height assume single, isolated fins that behave like a boiling flat surface. When applied to fin arrays, thresholds for fin height and array spacing where this assumption holds true are not known. To establish when fins in an array can be described as isolated and follow flat surface boiling behavior, pool boiling experiments are performed using copper heat sinks in two fluids with vastly different properties: HFE-7100 and water. The spacing and height of the longitudinal fins are varied with respect to the capillary length scale, Lb, of both fluids, and high-speed visualizations enable the identification of different boiling regimes and bubble confinement between fins. Predictions based on the single, isolated fin assumption are compared to the experimental boiling curves. Heat transfer from heat sinks with both height and spacing above the capillary length scale is accurately predicted in both fluids. However, spacings smaller than Lb lead to bubble confinement, which causes the superheat at each heat flux to be lower than the predictions using the flat surface, particularly at low heat fluxes. Further, heights shorter than Lb are unable to support boiling along the fin sidewall once film boiling initiates at the base. This work firmly establishes the fluid capillary length as the key length scale at which these confinement and height effects need to be considered for accurate prediction and design of heat sinks for two-phase immersion cooling applications.