Numerical Investigations of Air Flow and Heat Transfer in Axial Flux Permanent Magnet Electrical Machines

Abstract

In this study an investigation of heat transfer in air cooled Axial Flux Permanent Magnet (AFPM) electrical machines is carried out. Efficiency of electrical machines is strongly influenced by an effective cooling which is provided by forced convection: air enters the generator through the opening in the machine enclosure and leaves it radially, as it is forced out by the rotating discs. The main goal is the enhancement of the heat transfer from the stator where heat is generated in the copper windings. On the other hand the heat transfer to the rotor needs to be minimised in order to keep the magnets' temperature as low as possible.
The cooling can be improved by acting on design parameters, such as the distance between the stator and the rotors (commonly named running clearance), the magnet depth, and by acting on operational parameters, such as the rotational speed.
The investigation is carried out by using Computational Fluid Dynamics (CFD) software to model air flow and heat transfer inside the AFPM machines. The experimental validation of the numerical models confirmed the capability of the CFD software in predicting the air mass flow rate and the heat transfer in the AFPM machines.
The thesis describes the effects of the above mentioned parameters on target quantities, such as the heat transfer coefficients on the generator surfaces, the air mass flow rate through the machine, and the resistive torque.
General correlations in non-dimensional form are obtained for the calculation of the heat transfer on the generator surfaces in the AFPM machines as a function of the above parameters. General correlations have also been obtained for the calculations of the non-dimensional air mass flow rate through the machine and for the non-dimensional resistive torque.
It was found that the corresponding relationships between the peripheral Reynolds number and local Nusselt numbers on the generator surfaces and the non-dimensional mass flow rate are linear. However, the dependency of Nusselt number on the non dimensional clearance and the magnet depth is non-linear.
For the investigated range of the parameters the following was established: an increase in the peripheral Reynolds number results in higher Nusselt number on both the rotating and stationary surfaces of machines; an increase in the running clearance results in the reduction of the Nusselt number on the machine surfaces; the magnetic segments installed on the surface of the flat rotor act as blades of a radial compressor increasing the air mass flow rate and the corresponding Nusselt number on the stator surface.
The combination of the non-dimensional running clearance equal to 9 10-3 and the non-dimensional magnet depth equal to 7.3 10-2 was found to be the one which provides the maximum heat transfer from the stator. This piece of information can be used in the design stage for improving the cooling of AFPM electrical machines without increasing the windage losses and contribute in this way to the enhancement of the overall efficiency of this type of machines.