Abstract: The effect of a surface roughness on heat transfer is studied in two electric-motor-related cases. A flow in a clearance and between the cooling fins of the ABB HXR 400L electric motor is simulated numerically. The clearance is simplified to a two dimensional slot and, therefore, the calculation is not valid in the ends of the rotor. Furthermore, the clearance is modelled utilizing symmetry and only a sector of 30◦ is modelled. The air flow around the cooling fins is modelled by calculating one fin space and its surroundings. FINFLO flow solver, developed at Helsinki University of Technology, is used in the simulations. For the turbulence modelling k-ω model is utilized.

Main result: A surface sand-roughness of 46 µm is used in a case of the clearance of an electric motor. The improvement of the equivalent conductivity is 0.4 % with open key slots and 4 % with filled key slots. A surface roughness of 1.5 µm has no effect on the heat transfer coefficient in a flow around the cooling fins while a roughness of 260 µm improves heat transfer coefficient by 1.8 %.