Local Heat Transfer Coefficient Measurements on Shaft Spray Cooled End Windings

High power density electrical machines are mainly limited due to their cooling system. Oil spray cooled end windings are one possibility to significantly increase the power density. However, foundations for the estimation of the cooling capability are rather scarce in the design process. Different approaches to estimate the global heat transfer coefficients for spray cooled end windings exist in literature. However, local heat transfer coefficients, which are essential for the prediction of hot spots, can not be estimated. In this contribution, we present local heat transfer coefficient measurements for shaft spray cooled end windings. It is shown that the existence of a sump remarkably lowers the heat transfer coefficient, whereas it is almost constant over the remaining circumference. The rotational speed has a minor influence on the heat transfer. The latter is only increasing for very high speeds. A modeling approach for the Nusselt number is developed to apply it to different coolants. The achieved mean absolute percentage error between experimental and modeled Nusselt is 10.8%.