The heat transfer coefficient (HTC) is a criticalparameter that is required for accurate thermal modelling ofelectrical machines. This is often achieved from empiricalcorrelations of ideal geometries or computational fluid dynamics(CFD) simulations. This paper presents a novel technique usingdouble-sided thin film heat flux gauges for measuring the HTCfrom a direct oil-cooled electrical machine with segmented stator.While thin film gauges are often used in transient measurementsof the HTC on gas turbine components, their application toelectrical machines has been largely unexplored. This is the topicof this paper. Due to the large viscosity of the coolant, the transienttechnique was found to be inadequate and a steady stateadaptation for oil-cooled machines was developed. This paperexplores the challenges linked with this measurement techniquewhen applied to oil-cooled machines, and develops new nondimensionalcorrelations of the Nusselt number with Reynoldsnumber. These correlations are applicable to machines withdifferent geometries, flow and coolant properties. Theexperimental results were compared to CFD simulations andexisting pipe flow correlations. It is shown that these underpredictthe HTC by approximately 60% at Re = 20. The discrepancygradually decreases to around 10% at Re = 200.
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