Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.
Accurate heat transfer analysis of the rotor-stator system in disk-type electrical machines is crucial for design purpose, especially to prevent overheating. If the internal temperature in the machine exceeds the critical value (Tc=150°C), it results in demagnetization in the rotor. A high temperature increases the resistivity of the copper windings, which negatively affects the efficiency of the machine. The study undertaken here aims to provide analytical modeling of the discoidal system which is applicable to various geometries and boundary conditions. The rotor-stator configuration is enclosed in a cylindrical chamber, and all the surfaces within the machine are considered isothermal, each on their own temperature. The average convective heat transfer coefficients are defined, taking into account the bulk flow temperature as the reference temperature instead of the ambient temperature. The latter is typically used in literature but has almost no influence on the heat transfer. Thereafter, in order to estimate the bulk fluid temperature, a linear correlation between the surface temperature of the rotor, the stator and the cover is made, based on CFD simulations. These are performed for different rotational Reynolds numbers and gap size ratios at different combinations of surface temperature of the rotor, the stator and the cover. Results have been compared with the available data in literature and good agreement has been found. It is showed that there is a gap size ratio for which the average convective heat transfer for the stator surface reaches a minimum. Additionally, it is found that the proposed correlations for the convective heat transfer in the rotor and the stator surface in the gap are totally applicable to the disk type electrical machine with different working conditions.