Flow boiling in micro-pin fin heat exchangers and comparison with correlations

• Flow boiling in micro-pin fin heat exchangers. • Flow boiling patterns identified as bubbly flow, mixed and vapour layer flow. • Different nucleation bubble dynamics observed at the pin upstream and downstream. • Heat transfer rate depends on heat flux and pressure, but negligibly on mass flux. • Existing design heat transfer and pressure drop correlations were evaluated. The thermo-fluid performance of micro-pin fin heat exchangers has recently received extensive attention from the research community engaged in developing thermal management systems for high heat flux devices. Two-phase flow in these geometries could provide better thermal performance compared to other designs. However, more studies are still required to understand the effect of the control parameters on the fundamental flow boiling characteristics. Therefore, the present study aimed to examine experimentally the performance of micro-pin fin heat exchangers at different operating conditions. Staggered diamond micro-pin fins having a pin height of 1 mm and pin width of 0.6 mm were manufactured on a total base area of 20 mm × 25 mm. HFE-7100 was tested at a system pressure (inlet pressure) of 1, 1.5 and 2 bar, mass flux from 100 to 250 kg/m 2 s and 5 K inlet sub-cooling, while the wall heat flux was varied up to 324 kW/m 2 . The heat flux was increased gradually until the maximum thermal limit was achieved. Flow pattern features and bubble nucleation around the pins were visualised using a high-speed, high-resolution camera. A base heat flux up to 0.63 MW/m 2 was recorded without reaching the dryout region or the critical heat flux. Low substrate surface temperature, i.e. less than 85 °C, and stable flow without flow reversal and hysteresis were achieved in this geometry, making flow boiling in micro-pin fin heat sinks suitable for cooling electronics. Nucleate boiling was found to be present for the entire range studied. The effect of heat flux and pressure on the heat transfer rates was significant, while the mass flux effect was marginal for the range studied. Ten existing heat transfer and pressure drop correlations were evaluated, and a good prediction was found by some of them. The prediction of the pressure drop by existing correlations improved when the pin dimensions and the space between them was introduced in the two-phase friction multiplier.