*4.3. Effect of Attack Angle of the Vortex Generator*

Four different attack angles of vortex generators are used to explore the best choice, the four models differ only in the attack angle, and the other control conditions are consistent. By comparing the performance of the *R* of vortex generator under four different attack angles of 15◦, 30◦, 45◦, and 60◦, a better attack angle can be obtained. Figure 4a shows the change of the *R* of vortex generators with different attack angles in the process of Re increasing. It can be seen that the *R* is very close when the attack angles are 30◦ and 45◦, and both of them are better than 15◦ and 60◦ attack angles, which is similar to the law obtained by Lotfi et al. [35] and Gholami et al. [36] when studying the attack angles of vortex generators in the past. Considering that the maximum temperature has a greater impact on oil-cooled motors, when the *R* of different types have the same performance, the vortex generator with a lower maximum temperature is used for calculation and discussion. In order to more widely explore the effect of the vortex generator on the coil end heat transfer, we introduce the thermal enhancement factor <sup>Δ</sup>*<sup>T</sup> <sup>T</sup>*<sup>0</sup> , where <sup>Δ</sup>*<sup>T</sup>* represents

the difference between the maximum temperature of the channel with vortex generator and the maximum temperature of the channel without vortex generator, and *T*<sup>0</sup> represents the maximum temperature of the channel without vortex generator. As presented in Figure 4b, the thermal enhancement factor <sup>Δ</sup>*<sup>T</sup> <sup>T</sup>*<sup>0</sup> of the 45◦ attack angle is higher than that of the 30◦ attack angle. Therefore, the 45◦ attack angle vortex generator is selected as the basis for subsequent exploration.

**Figure 4.** Performance of (**a**) *R* and (**b**) thermal enhancement factor <sup>Δ</sup>*<sup>T</sup> <sup>T</sup>*<sup>0</sup> at different Re.

When the iterative error is less than 10−<sup>5</sup> and the maximum temperature does not change, the calculation is considered to be completed, and the data required for temperatures, velocities, and slices are extracted and analyzed. Under steady state conditions, Figure 5 shows pictures of a smooth channel without vortex generator at Re = 3993, Figure 5a shows the streamline diagram at 3.75 mm above the coil. Corresponding to this is the temperature contour diagram shown in Figure 5b. Moreover, Figure 5c,d shows the point line diagrams of the temperatures of different points extracted from the slices in the *X* and *Z* directions, respectively. Figure 6 shows pictures of the channel after installing a pair of vortex generators with an attack angle of 45 degrees. In Figure 6a,b, it can be seen that the region with low velocity in the streamline diagram shows higher temperature in the temperature contour diagram. Comparing with Figure 5a,b, due to the addition of vortex generator, two vortexes are formed behind the vortex generator, and the velocity at the two vortexes is higher than that in the adjacent region to a certain extent; therefore, this area shows lower temperature in the temperature contour diagram. However, by observing Figure 6c,d, since the thermal conductivity of the solid is significantly higher than that of the oil phase, the temperature of the vortex generator is higher than that of the oil phase on the same section, so the temperature of the vortex generator increases obviously at the place where the vortex generator is placed. In the middle of the two wings of the vortex generator and the area where the fluid flows through the vortex generator, the heat dissipation is accelerated due to the generation of the vortex, and the temperature is significantly lower than that in Figure 5c,d.

**Figure 5.** Diagrams of smooth channel under steady state conditions: (**a**) streamline diagram, (**b**) contour diagram of temperature, (**c**) slice temperature diagram in *X* direction, (**d**) slice temperature diagram in *Z* direction.

**Figure 6.** Diagrams of channel with 45◦ attack angle vortex generator under steady state conditions: (**a**) streamline diagram, (**b**) temperature contour diagram, (**c**) slice temperature diagram in *X* direction, (**d**) slice temperature diagram in *Z* direction.
