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Battery thermal management (BTM) based on immersion liquid is a novel and promising technology due to its excellent thermal performance. However, the natural convection characteristics of immersion liquid are crucial yet often neglected for the design of immersion-based BTM. In this study, a typical BTM unit and module are designed utilizing a multi-component oil as the immersion liquid. To characterize the effect of natural convection, a rigorously validated model coupled with heat transfer and natural convection is developed. For the BTM unit, the impact of varying space occupied by immersion liquid is investigated. The increase in the thickness of the immersion liquid progressively enhances the natural convection effect. The Grashof number in the vertical interlayer of finite space helps assess the strength of natural convection of the BTM unit. Furthermore, a comparison of the BTM module with and without natural convection is performed. The results show that the natural convection effect enhances heat dissipation but reduces thermal uniformity in static mode. When the thickness of immersion liquid is 10 mm, the natural convection effect results in a decrease of 1.4 °C in the average battery temperature. Meanwhile, the battery temperature difference increases from 1.27 °C to 3.14 °C, and the average temperature of the terminals and busbars increases by >1 °C. This study can provide a foundation and reference for the BTM design based on immersion liquid.

Flow mal-distribution of refrigerant in small diameter tube heat exchangers is a great concern, which may lead to a 25% efficiency loss. Two-phase refrigerant distributors are set before evaporators to separate refrigerant into parallel paths uniformly. In this paper, a new type of distributor with two barrels is proposed. Experimental test and numerical simulation were both carried out to evaluate the performance and to understand internal hydrodynamic flow behavior. Compared with the previous distributor, it is found that the double-barrel distributor with proper parameters performs better. The relative error between experimental and simulation results is less than 15%, which proves the reliability of the established simulation model. Computation fluid dynamics (CFD) calculation indicates that the distribution performance is improved with properly larger bottom and top barrel diameters. With the increase of the bottom barrel diameter, beneficial reflux of refrigerant occurs in bottom barrel and when top barrel diameter is larger, little refrigerant flows directly into outlet capillary tubes without mixture or reflux. In addition, parameters such as top barrel diameter, top barrel height, bottom barrel diameter, bottom barrel height, mass flow rate and quality are studied by Taguchi Method to analyze the parameter sensitivity. The effect of the parameters listed below ranges from biggest to smallest: mass flow rate, bottom barrel height, quality, top barrel height, bottom barrel diameter and independent top barrel diameter. An optimized two-barrel distributor is achieved with proper top and bottom barrel diameters and larger bottom and top barrel heights.