Thermal characteristics of refrigerant flow boiling in two mini-channel heat sinks of different aspect ratios for battery thermal management

•Flow pattern and heat transfer performance of refrigerant flow boiling in MiCHSs investigated.•Effects of OEV, heating power and aspect ratio analyzed and discussed.•MiCHS2 with square channels better than MiCHS1 with narrow channels due to thinner base liquid film.•Heat transfer rate increases but battery temperature first decreases and then increases with OEV.•Higher heating power increases local heat transfer coefficient. The two-phase refrigerant cooling system has excellent thermal management performance, which can provide good temperature control of lithium-ion batteries. In this paper, the thermal performance of mini-channel heat sinks with different aspect ratios was investigated for a battery module with a small-scale vapor compression refrigeration system. Two mini-channel heat sinks (MiCHSs), one with a narrow channel cross section of 2 mm × 4.5 mm (MiCHS1) and the other one with square channel cross section of 3 mm × 3 mm (MiCHS2), were fabricated with a visualization window and connected in parallel in the system. The experiments were run at different openings of expansion valve (OEV) and heating power. With the increase of OEV, the refrigerant subjected to fixed heating power in the mini-channels experiences different flow patterns of annular flow, slug flow and small-bubble flow successively. The best cooling performance can be achieved in slug flow, corresponding to an optimal OEV. The optimal OEV value increased from 32 % to 55 % with the increase of heating power ranging from 100 W to 200 W. Under the same optimal OEV condition, the heat transfer coefficient and average temperature of MiCHSs increase with the increase of heating power for both MiCHSs. The channel aspect ratio has an effect on the heat transfer performance of MiCHS. Compared with the narrow channels of MiCHS 1 with a large aspect ratio, the flow boiling in square channel of MiCHS2 can achieve a better cooling performance due to the well-confined bubbly flow and thinner liquid film on the base of the mini-channels.