Numerical and Experimental Investigation of Fluid Flow and Heat Transfer in Hairpin End Winding for an Oil-Injected Cooled Traction Motor

Abstract

A correct understanding of fluid flow and heat transfer characteristics in oil injection cooling for hairpin end winding is crucial for accurate thermal analysis. This article utilizes a meshless computational fluid dynamics (CFD) approach, based on moving particle simulation (MPS), to develop a simplified numerical model. The model provides qualitative insights into fluid dynamics and heat transfer mechanism in hairpin end winding. Experimental validation is conducted to compare the numerical data and identify key factors influencing oil coverage rate (OCR) and heat transfer coefficient (HTC), including the number and placement of inlets, coolant flow rate and temperature, and rotation speed. The results indicate that increasing the number of inlets and optimizing their placement enhances cooling uniformity. Furthermore, the HTC is strongly influenced by the coolant flow rate and temperature, with an average HTC of 130 W/m²/K achieved at an inlet temperature of $40~^{\circ }$ C and a flow rate of 8 L/min. The highest HTC values are found in the lower winding layers due to the downward flow of oil. Rotation also improves the OCR and HTC by approximately 20% and 37%, respectively, at 1000 r/min, as oil droplets are sprayed onto the winding surfaces, particularly near the rotor. However, at higher rotational speeds, the cooling efficiency stabilizes, and the effect of rotation becomes less significant.