Axial Oil-Jet Impingement Cooling of Hairpin End-Windings in EV Traction Motors for Improving Thermal Management: A Computational Fluid Dynamics (CFD) Study

Abstract

The increasing desire to produce compact and efficient EV traction motors while achieving high power density has created thermal management issues, particularly in the hotspot regions of motors, including the stator-end-winding region. This numerical study aims to investigate the performance of axial oil impinging jet cooling systems on the hairpin end-winding of traction motors by means of computational fluid dynamics (CFD) modeling. The model includes a geometry of a 30-degree section of end winding with a 2 mm jet nozzle diameter. It is solved using a multiphase flow utilizing the Volume of Fluid (VOF) method between oil and air. This numerical study examined two parameters that affect heat dissipation performance through a parametric sensitivity analysis under fixed operating conditions (oil inlet temperature = 42.5 ℃), including the effect of various oil types (silicone, ATF, and synthetic) and the effect of jet orientations (axial vs. radial). The study conducted mesh sensitivity analysis over three mesh sizes, 0.5 mm, 0.2 mm, and 0.15 mm, in the oil-air interface region. The fine mesh findings were validated using published numerical data at 0.5 m/s, demonstrating good agreement with 7.8% deviation in heat transfer coefficient (HTC) at the central winding (0°), which is the primary focus region of this investigation. The parametric study’s results revealed that ATF oil surpassed the silicone and synthetic oils in both average HTC and oil wetting at the center winding, resulting in an average HTC of 190 W/m²k, an increase of 31.1% to synthetic oil, and 15.15% to silicone. In terms of oil wetting, ATF displays 13.1%, and the synthetic oil shows higher results than silicone with 11.6% compared to 9.72%, respectively. Additionally, changing the jet from axial (90°) to radial (0°) improved the cooling efficiency; the radial jet outperformed the axial jet in both average HTC and oil wetting at the center segment, obtaining 284.1 W/m²k and 24.1%, achieving an increase of 72.1% and 147.9% relative to the axial jet, respectively. Findings support radial configuration and ATF selection at low flow rates. Future work should examine the effect of higher flow rates, multi-nozzle array design, and larger nozzle diameter to achieve effective thermal management.