A comparative study of flow boiling heat transfer and pressure drop characteristics in microgap and microchannel heat sink and an evaluation of microgap heat sink for hotspot mitigation

Two phase microgap heat sink has a large potential to minimize the drawbacks associated with two phase microchannel heat sink, especially flow instabilities, flow reversal and lateral variation of flow and wall temperature between channels. This new concept of the two-phase microgap heat sink is very promising due to its high heat transfer rate and ease of fabrication. However, comparison of the performance of microgap heat sink (heat transfer, pressure drop and instability characteristics) with some conventional heat sink has not been investigated extensively. In this study, experiments have been conducted to investigate the heat transfer and pressure drop characteristics of deionized water (DI) in microgap heat sink and compare these experimental results with similar data obtained for microchannel heat sink. These studies are carried out with the inlet DI water temperatures 86 °C at different mass fluxes ranging from 400 to 1000 kg/m2 s, for effective heat flux 0–85 W/cm2. High speed flow visualizations are conducted simultaneously along with experiments to illustrate the bubble characteristics in the microchannel and microgap heat sink. Experimental result shows that microgap heat sink performs better at high heat flux and low mass flux due to confined slug and annular boiling dominance and consequent delay of dryout phase. So, this microgap technology is promising and an effective method to dissipate very high heat fluxes in compact space with a smaller rate of coolant flow. Moreover, pressure drop is higher in microchannel than microgap heat sink at all the heat fluxes. In addition, encouraging results have been obtained using microgap as it can potentially mitigate local hotspot, reduce flow instabilities, flow reversal and maintain uniform wall temperatures over the heated surface.