Physically-Derived Figure of Merit (FOM) Quantifying the Cooling Performance of Fluids in Laminar Free-Surface Jet Impingement Cooling of Electrical Components

High heat fluxes occurring in modern electrical systems require direct cooling methods in which the electrical components come in direct contact with the cooling fluid. Hence electrically non-conductive fluids with thermal properties inferior to the ones of water have to be applied. In order to assess the cooling performance of such fluids a physically-derived figure of merit (FOM) is presented based on the ratio of the achieved heat flux and the required pumping power in an exemplary cooling application under laminar flow conditions. This cooling application is the stator cooling of an electrical engine through two free-surface impinging jets, each one aimed axially on each coil end of the stator teeth. Due to the high viscosities of most cooling fluids combined with small geometrical measures of modern electrical components, the assumption of a laminar flow is often justified. The FOM is derived analytically through generally accepted correlation functions and solely a function of the thermal fluid properties. Hence it is suitable for any other cooling configuration with laminar external flow conditions. The results of an experimental study of the considered cooling case are used to verify and further enhance the FOM.