Automotive electric drive unit designs are often limited by installation space and the related environmental conditions. Electrical losses in various components of the motor such as stator, rotor and coils can be significant and as a result, the thermal design can become a bottle neck to improve power and torque density. In order to mitigate the thermal issue, an effective liquid cooling system is often employed that ensures sufficient heat dissipation from the motor and helps to reduce packaging size.
Although both stator and rotor are cooled in a typical motor, this paper discusses a multiphase oil-air mixture analysis on a spinning hollow rotor and rotor shaft subjected to forced oil cooling. Three-dimensional computational fluid dynamics (CFD) conjugate heat transfer (CHT) simulations were carried out to investigate flow and heat transfer. The effect of centrifugal force, shaft RPM, density gradients and secondary flows were investigated.
Initially, the computational model was validated with bench test data in terms of pressure loss and temperature data for a specific flowrate of oil. Later, the model was simulated using a range of shaft RPM, oil flow rates and rotor heat loss maps. Overall, the centrifugal force in the spinning shaft did not influence density gradients and secondary flow, hence had minimal effect on heat transfer. However, it has significant effect on hydraulic loss and phase distribution. Phase distribution of oil and air in certain regions within the shaft does affect overall heat transfer.