In an effort to reduce the charging time of electric vehicles (EVs), the thermal management of the charging system has emerged as one of the most urgent issues. The charging rate has been restricted to avoid thermal failure especially in charging cable. Recently, a direct contact cooling technique that utilizes subcooled flow boiling for charging cables has been proposed and has shown promising thermal management performance. However, there has been a lack of clear explanation regarding its thermal characteristics due to its intrinsically complicated flow features. This study investigates the thermal characteristics of subcooled flow boiling in a horizontally aligned concentric annular tube intended to simulate the direct contact cooling technique employed for charging cables. For the experimental investigation, the wall temperature measurements and high-speed flow visualization images acquired from the transparent test module annulus, with inner and outer diameters of 6.35 mm and 22.0 mm, were utilized. Three key axial thermal characteristics of subcooled flow boiling in annulus have been analyzed, and they are flow regime transition, variation of heat transfer mechanisms, and occurrence of critical heat flux (CHF). The transition of flow regimes and the variation of dominant heat transfer mechanisms in the axial direction are mutually influencing due to the coupled effects between the hydrodynamic and thermal characteristics of simultaneously developing flows. The flow regime changes from partially developed boiling (PDB) to fully developed boiling (FDB) as the intensities of nucleate boiling and bubble recondensation vary in the axial direction, while the dominant heat transfer mechanism shifts from single-phase convection to nucleate boiling, corresponding to the flow regime transition from PDB to FDB. The departure from nucleate boiling (DNB) type CHF is observed, manifested by the wavy interface propagating from the far downstream to the upstream region, and the intensities of nucleate boiling and bubble recondensation are also identified as the dominant parameters in determining the occurrence of CHF. Charging time estimation reveals that the proposed method can achieve an 80 % charge for 100-kWh EV batteries in 98 s, while maintaining the cable wire temperature below the safety limit of 80 °C. This strongly supports its potential as a promising thermal management technique for future ultra-fast charging systems.

中文翻译：

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超快电动汽车充电电缆水平同心环内过冷流沸腾热特性实验研究


为了减少电动汽车 (EV) 的充电时间，充电系统的热管理已成为最紧迫的问题之一。充电速率受到限制以避免热故障，尤其是充电电缆。最近，人们提出了一种利用过冷流动沸腾对充电电缆进行直接接触冷却的技术，并显示出有希望的热管理性能。然而，由于其本质上复杂的流动特征，对其热特性缺乏明确的解释。本研究研究了水平排列的同心环形管中过冷流沸腾的热特性，旨在模拟充电电缆所采用的直接接触冷却技术。在实验研究中，利用了从内径和外径分别为 6.35 毫米和 22.0 毫米的透明测试模块环带获取的壁温测量值和高速流动可视化图像。分析了环空过冷流动沸腾的三个关键轴向热特性，即流型转变、传热机制的变化和临界热流密度（CHF）的出现。由于同时发展的流动的流体动力和热特性之间的耦合效应，流态的转变和轴向方向上主要传热机制的变化是相互影响的。 随着核态沸腾和气泡再凝结的强度沿轴向变化，流态从部分发展沸腾（PDB）变为完全发展沸腾（FDB），而主要传热机制从单相对流转变为核态沸腾，相应从 PDB 到 FDB 的流态转变。观察到与核态沸腾 (DNB) 型 CHF 的偏离，表现为从远下游到上游区域传播的波状界面，并且核态沸腾和气泡再凝结的强度也被确定为确定 CHF 发生的主要参数。充电时间估计表明，该方法可以在 98 秒内为 100 kWh 的电动汽车电池充电 80%，同时保持电缆线温度低于 80 °C 的安全极限。这有力地支持了其作为未来超快速充电系统的一种有前途的热管理技术的潜力。