Abstract—This paper copes with the numerical simulation for convective heat transfer in the stator disk of an axial flux permanent magnet (AFPM) electrical machine. Overheating is one of the main issues in the design of AFMPs, which mainly occurs in the stator disk, so that it needs to be prevented. A rotor-stator configuration with 16 magnets at the periphery of the rotor is considered. Air is allowed to flow through openings in the rotor disk and channels being formed between the magnets and in the gap region between the magnets and the stator surface. The rotating channels between the magnets act as a driving force for the air flow. The significant non- dimensional parameters are the rotational Reynolds number, the gap size ratio, the magnet thickness ratio, and the magnet angle ratio. The goal is to find correlations for the Nusselt number on the stator disk according to these non-dimensional numbers. Therefore, CFD simulations have been performed with the multiple reference frame (MRF) technique to model the rotary motion of the rotor and the flow around and inside the machine. A minimization method is introduced by a pattern-search algorithm to find the appropriate values of the reference temperature. It is found that the correlations are fast, robust and is capable of predicting the stator heat transfer with a good accuracy. The results reveal that the magnet angle ratio diminishes the stator heat transfer, whereas the rotational Reynolds number and the magnet thickness ratio improve the convective heat transfer. On the other hand, there a certain gap size ratio at which the stator heat transfer reaches a maximum. Keywords—Axial flux permanent magnet, CFD, magnet parameters, stator heat transfer. I. INTRODUCTION HE AFPM machines are highly efficient devices that have been widely used in the applications where the axial length of the machine is a limiting factor [1]. Due to their compact construction, overheating, which is one of the main issue in the design of electrical machines, becomes much more severe and this needs to be prevented. In fact, demagnetization occurs in the permanent magnet materials once the temperature exceeds 150 ºC (for SH type NdFeB magnets) [2]. The resistivity of the copper winding in the stator disk increases with temperature and this deteriorates the efficiency of the machine. Moreover, the lack of insight about the thermal performance of the machine entails the designer to Alireza Rasekh is with the Department of Flow, Heat and Combustion Mechanics, Faculty of Engineering and Architecture, Ghent University, Ghent, Belgium (phone: +32-489-305832, fax: +32-926-43590; e-mail: alireza.rasekh@ugent.be,). Peter Sergeant is with the Department of Electrical Energy, Systems and Automation, Faculty of Engineering and Architecture, Ghent University, Ghent, Belgium (e-mail: peter.sergeant@ugent.be). Jan Vierendeels is with the Department of Flow, Heat and Combustion Mechanics, Faculty of Engineering and Architecture, Ghent University, Ghent, Belgium (e-mail: jan.vierendeels@ugent.be). provide excessive cooling by external pump or ventilator to avoid overheating. For these reasons, it is vital to have a deep knowledge of the cooling in the AFPM machines. Thermal study of the AFPM machines has gained much attention over the past few years [3], [4]. Moradnia et al. [5] implemented the CFD modeling of an electrical machine by MRF method. It was reported that this approach is roughly useful as it makes very accurate prediction of the airflow characteristics in the machine. Pyrhönen et al. [6] improved the cooling of the AFPM machines by using copper bars as extra heat carriers in the construction. Chong et al. [7] performed the numerical modeling of the direct-drive AFPM generator operating at 100 rpm for power level of 25 kW and built the experimental set up to validate their results. They showed that some ventilation openings at the side of the machine could enhance the convection heat transfer in the machine. Lim et al. [8] conducted an experiment to assess the convective heat transfer coefficient in an AFPM machine. They indicated that the effects of natural convection are significant in thermal evaluation of the machine. In an AFPM machine, most heat losses occur in the stator winding and the main heat transfer mechanism to expel the heat from this region is by convection. Therefore, the convective heat transfer in the stator disk of the AFPM machines has been investigated in the literature. In this regard, Yuan et al. [9] investigated the turbulent heat transfer on the stator surface and the flow characteristics in the gap between the disks. They showed that there is an optimum rotor-stator distance where the heat transfer on the stator side reaches a maximum. Howey et al. [10] measured the stator heat transfer in a discoidal configuration using an electrical heater array. They found that the local Nusselt number increases at the periphery due to ingress of air toward the stator. Rasekh et al. [11] studied the effects of holes in the rotor side on convective heat transfer on the stator facing rotor in the gap. It was concluded that the presence of holes is beneficiary for the stator heat transfer as air is allowed to enter into the air-gap through the holes, resulting in a net radial flow in the gap region between the rotor and stator. Recently, Rasekh et al. [12] presented correlations for the convective heat transfer in the simplified discoidal system of an AFPM machine. The model is able to predict the convective heat transfer for different values of the rotational Reynolds number and the gap size ratio. The average bulk fluid temperature adjacent of each surface was utilized as the reference temperature instead of the ambient temperature to calculate the convective heat transfer coefficient. By doing so, the correlations for the surfaces in the gap became CFD-Parametric Study in Stator Heat Transfer of an Axial Flux Permanent Magnet Machine Alireza Rasekh, Peter Sergeant, Jan Vierendeels T World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:11, No:1, 2017 61 International Scholarly and Scientific Research & Innovation 11(1) 2017 scholar.waset.org/1307-6892/10006001 International Science Index, Mechanical and Mechatronics Engineering Vol:11, No:1, 2017 waset.org/Publication/10006001