Toward the improvement of performance of the electric vehicle (EV), the design of the motor shape appropriate to a good heat removal is important. A typical EV motor is composed of a pair of coaxial cylinders with the fixed outer cylinder (stator) and the rotating inner cylinder (rotor). The gap of the coaxial cylinders is extremely narrow, i.e. 1 mm. In addition, the EV motor may have axial slits on the stator wall is considered. Thus, thermo-fluid characteristics of rotating coaxial cylinders with these features are important for the viewpoint of optimal design of the EV motor.

The aim of the present study is to clarify thermo-fluid characteristics in the narrow gap between a pair of coaxial cylinders simulating an EV motor at a high Reynolds number. The flow behavior in slits between the rotor and stator was measured by PIV with fluorescent particles distributed in the water, and was numerically examined.

Experimental apparatus simulating the EV motor was made of acrylic, and flow inside the EV motor was visualized by tracer particles. Using a high-speed video camera and Nd:YAG laser, flow in slits was observed. Visualization results show Taylor-Couette flow in the gap between the rotor and stator. Moreover, pathlines in the slit was observed, where a vortex was observed in the slit.

In the numerical calculation, two cases were executed to elucidate the slit effect: Case A is the stator without slits and Case B is the stator with slits. As a result, heat flux of Case B in the high rotational speed was increased compared with that of Case A. Hence, it was implied that the heat flux was increased by the presence of vortex of the internal slits.

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