Thermal management of hairpin winding traction motors in electric vehicles: Parametric evaluation of impinging oil jet cooling using CFD simulations

Abstract

Efficient thermal management is critical for ensuring the performance and longevity of high-power-density electric traction motors, particularly those using hairpin windings. This study aims to address the challenges of localised cooling in such systems by characterising oil jet impingement cooling using computational fluid dynamics (CFD) simulations. A detailed geometric model of hairpin windings was implemented to capture fluid-winding interactions within a 45-degree sector of the motor end winding region. The study examines key design parameters affecting cooling performance through a parametric analysis, including nozzle geometry (diameter, orientation, and distance), mounting configuration, and operational conditions (flow rate and inlet velocity). The numerical model demonstrated good agreement with experimental data, particularly in predicting heat transfer coefficient trends across different flow conditions. Results revealed that axial configurations with 2 mm nozzle diameters achieved higher heat transfer performance (940 W/m²K at 0.75 kg/min) compared to radial configurations, though this performance showed sensitivity to operational parameters. Additionally, critical velocity thresholds were identified, marking transitions between gravity-dominated and momentum-driven heat transfer mechanisms, with optimal nozzle orientations varying accordingly. Both top-mounted and radially mounted nozzle configurations were studied, with top-mounted configurations demonstrating superior spatial cooling distribution, achieving up to 370$\text{\%}$ enhancement in adjacent winding cooling at higher angular positions. These findings provide practical design guidelines for implementing efficient jet cooling systems in high-performance electric traction motors, particularly for optimising multi-nozzle array configurations to achieve effective thermal management.