The electric vehicle is a promising technology to reduce particulate matter and greenhouse gas emission in urban areas. The R&D sector of the automotive industry is developing advanced electric motors to meet today’s standards for traction. Motors with high power density, high torque density and reduced bulk are required, thus the need for advanced electro-mechanical design and materials. The thermal management of the motor is not of secondary importance, because intrinsic power losses lead to the rise of the motor temperature, causing a loss of performances and even electro-mechanical failure. The stator end-windings are a critical component because the electrical insulation degrades when exposed to high temperature.
Direct cooling with dielectric lubricant oil injections is an effective solution for mitigating the temperature of the end-windings and extend the operating range of the motor. Modelling the injections is difficult because of the complex thermal and mechanical interactions between the liquid, the air and the solid. More complexity is added by the end-windings intricated geometries and by the peculiar thermal properties of the oils, which are known as high-Prandtl fluids. More experimental research is needed to support the numerical analysis by providing estimates of the fundamental parameters of the heat transfer.
Free-surface jet impingement is considered. The known experimental literature proposes fundamental studies, which evaluate the convective coefficient and the correlations between the dimensionless numbers Nusselt, Reynolds and Prandtl. The literature is not abundant, not representative of the thermal range of the end-windings, not using fluid types of electric mobility and uses solid target shapes which allow simplifying assumptions on the heat transfer. The present research aims to extend this literature by fulfilling these knowledge gaps.
An experimental apparatus is built to replicate a simplified oil injection over a flat heated plate, within the temperature range of the end-windings. A method is developed to evaluate local convective coefficients, local Nusselt numbers and Nusselt correlations, relying on measurements and simulations. Various diagnostics are employed to characterise the solid plate and the liquid injection (before and after impingement), including an optical technique based on laser induced fluorescence.
The main findings of this research are organised in five chapters. In the first, the industrial framework, the state of art on jet impingement and the objectives of the research are explained. In the second, the technical details of the methods, materials, formulations, and assumptions, are given. In the third, the injection and the plate are characterised, by taking measurements of the plate temperatures with embedded thermocouples and by simulating the heat flux. In the fourth, the liquid is characterised after impingement, by taking measurements of thickness and temperature with diverse techniques, including thermocouples, contact needles, and imaging methods. In the fifth, the results from the previous chapters are combined to determine the convective coefficient, the Nusselt number and the Nusselt correlations, which are compared to the most similar literature. Lastly, the manuscript ends by reporting the conclusions and the perspectives for future work.
This research project contributes to better define the industrial, scientific and technical challenges related to the cooling of the end-windings with jet impingement and high-Prandtl fluids. Additional contributions are given by the development of new diagnostic methods and representative estimates of convective heat transfer.