Numerical and Experimental Calculation of CHTC in an Oil-Based Shaft Cooling System for a High Speed High Power PMSM

The convective heat transfer coefficient (CHTC) is a critical parameter that is required for developing an accurate and efficient thermal design of electrical machines. However, the existing empirical CHTC correlations are invalid for an oil-cooled hollow-shaft rotor. On this basis, a simplified numerical model based on computational fluid dynamics methods is developed in this paper to provide a qualitative understanding of the rotational effects on the convective heat transfer across a range of operation speeds. Then experiments are undertaken to validate the data obtained from numerical models and to estimate the impact parameters on the CHTC, such as the rotational velocity, coolant flow rate, and coolant temperature. On the basis of the numerical and the experimental results, it is concluded that the rotation can significantly increase the CHTC of the shaft inner wall surface above the level of the stationary condition. However, the axial flow rate and the viscosity of the coolant have less influence on convective heat transfer for the high rotational speeds. As a result of such analysis, a general correlation is defined by using Nusselt numbers as a function of rotational Reynolds numbers and Prandtl numbers.