A Comparison of the Thermohydraulic Performance of Oil-Cooled Heat Sink Geometries for Power Electronics

Using oils as a coolant for electrical drivetrains has several advantages. Firstly, the lubricant and coolant circuits can be combined, thereby eliminating the (water-glycol) coolant circuit and reducing the component volume and cost by using a single oil circuit. Secondly, using oils can enable direct contact cooling technologies, for example direct winding cooling for electric motors, which can significantly improve the thermal performance. The drawback of using oil as a coolant are its inferior heat transfer characteristics compared to water-glycol mixtures (mainly higher viscosity and lower thermal conductivity). This increases the complexity for cooling the power electronics of the drivetrain. This paper presents a comparison of different geometries for the heat sink used to cool the inverter of an electrical drivetrain. For all assessed geometries, the oil is in direct contact with the base plate of the inverter, while the heat sink acts both as a flow disturbance and as a fin. The heat sink geometries under consideration are: multiple parallel rectangular channels, offset strip fin inserts, impinging flows, pin fins and metal foam fins. Analytical models are constructed based on correlations from literature, determining the heat transfer coefficient and pressure drop. The effect of the thermal interface resistance from base plate to heat sink on the fin efficiency is analyzed by considering different bonding techniques. The inlet oil mass flow rate is varied and the results of the thermohydraulic models are analyzed by the Pareto front of the thermal resistance as a function of the required pumping power.