*4.4. Effect of Longitudinal Distance of the Vortex Generator*

After selecting 45◦ as the better attack angle of the vortex generator, the best longitudinal distance and distribution mode are explored. In this part, two winglets of the vortex generator are symmetrically distributed, with a total of 12 different cases. Figure 7a–d shows the variation of the *R* of vortex generators with *FU*, *FD*, *FUD,* and *FDU* distribution modes at different longitudinal distances at different Re. Through comparing the performance of the *R*, it can be concluded that in the three distributions of *FU*, *FD,* and *FUD*, the performance of longitudinal distance of 4*h* is better than that of 2*h* and 6*h*. Especially in the two cases of *FU* and *FD*, the performance of the longitudinal distance of 4*h* is significantly better than that of 2*h* and 6*h* at small Re. However, in the case of *FDU* distribution, the performance of longitudinal distance of 4*h* is similar to that of 6*h*. Therefore, as shown in Figure 7e, comparing the five cases of *FU-*4*h*, *FD-*4*h*, *FUD-*4*h*, *FDU*-4*h*, and *FDU-*6*h*, the conclusion that when the Re is less than 4000, the comprehensive performance of *FD-*4*h* is the best, while when the Re is greater than 4000, *FU-*4*h* has a slight advantage over other cases can be summarized. Besides, according to Figure 8, it can be concluded that the thermal enhancement factor <sup>Δ</sup>*<sup>T</sup> <sup>T</sup>*<sup>0</sup> of *FD-*4*h* performs more prominent when the Re < 4000, while the *FU-*4*h* tends to be better when the Re > 4000, and the overall thermal performance can be improved by up to 10.2%.

**Figure 7.** Performance of *R* at different longitudinal distances (2 *h*, 4 *h*, and 6 *h*) and distribution modes, (**a**) *FU*; (**b**)*FD*; (**c**) *FUD*; (**d**) *FDU* and (**e**) relatively better distribution mode.

**Figure 8.** Performance of thermal enhancement factor <sup>Δ</sup>*<sup>T</sup> <sup>T</sup>*<sup>0</sup> at different Re.

Figures 9 and 10 show pictures of *FD-*4*h* and *FU-*4*h*. By observing Figures 9a and 10a, four vortexes are formed behind the vortex generator due to the installation of two pairs of vortex generators at this time. In *FD-*4*h*, the vortexes are created at the contracted two wings of the vortex generator and develop towards the center of the vortex generator, while in *FU-*4*h*, the vortexes are formed at the expanded two wings of the vortex generator and develop towards both sides of the vortex generator. Through analyzing Figure 9c,d and Figure 10c,d, it can be found that, in *FD-*4*h*, the flow velocity at the center of the vortex generator is relatively slow and the temperature is high, while in the vicinity of the center, due to the formation of the vortex, more energy loss and temperature drop are generated, whereas in *FU-*4*h*, the flow velocity at both sides of the vortex generator is relatively slow and the temperature is high, and the vortexes generated near both sides speeds up the heat dissipation and reduces the temperature. Moreover, due to the change of flow mode, the position of the highest temperature point on the section also changes. There are two symmetrical peaks in *Slice Z*<sup>1</sup> in Figure 10d, while the two peaks in Figure 9d are asymmetrical.
