Experimental investigation into longitudinally fin-surface effect to subcooled flow boiling of HFE-7100 in horizontal annular tube for Ultra-fast EV charging cable TMS

Abstract

Among various current EV charger systems categorized depending on power rate, the one beyond 350kW is especially the promising candidate to cut charging time down significantly up to the level of oil vehicle. In terms of heat dissipation, a preceding paper confirmed the feasibility of realizing less than 5 minutes charging time at 3,893A by the application of subcooled flow boiling more effective compared to single-phase. As an advanced research, current study aims to explore an improvement of heat transfer coefficient and an investigation into two phase flow regimes by applying longitudinal fin configuration. The test module features 22mm outer diameter concentric annular tube aligned with center-located test modules having 10mm diameter baresurface to imitate real wires and cable of EV charger. Note that fin tube one has 2 by 5 mm sized rectangular shape 6-fins along flow direction throughout 300 mm test section. The each operating parameter ranged over 120~240 (kPa) of inlet pressure at 40 (kPa) interval, 10~30 (°C) of inlet temperature at 10 (°C) interval, 330.8~1002 (kg/m2s) for barewire and 434.6~1316.2 (kg/m2s) for fin-tube of mass velocity and 2.62~31.5 (W/cm2) of heat flux. It is verified that wire surface temperature significantly turned upon pressure under such highly subcooled flow boiling conditions. But, the mass velocity and temperature of incoming working fluid, a main parameter in case of single-phase cooling scheme, is investigated to show negligible effect into surface temperature over the range of two-phase regimes. From the visualization result, it is proven that buoyancy force gives rise to totally different flow boiling regimes between bare and fin-surface cases, due to the fin-blocking and downward facing effect. By virtue of fin configuration, a 33% improvement in heat transfer coefficient was achieved, and, leads to lower and uniform temperature distribution along flow direction, reduction in surface superheat. In addition, several subcooled flow boiling correlations were assessed and MAE were compared, especially by employing thermal circuit combined with precedent correlations for fin-tube.