*4.5. Effect of Transverse Distance of the Vortex Generator*

Through the above research results, eight transverse distributions are explored based on *FD-*4*h* and *FU-*4*h*. Figure 11a,b illustrates the change of the *R* of *FD-*4*h* and *FU-*4*h* with different transverse distances under different Re. According to Figure 11a, compared with other transverse distances, *FD-*4*h* without transverse distance still performs best, while in Figure 11b, *FU-*4*h*-0.5*h* has slight advantages. Therefore, as shown in Figure 11c, by comparing *FD-*4*h* and *FU-*4*h-*0.5*h*, the results can be conducted that when the Re is less than 4000, the comprehensive performance of *FD-*4*h* is better, while when the Re is greater than 4000, the effect of *FU-*4*h-*0.5*h* is better than other situations. Furthermore, from Figure 12, the conclusion that when the Re < 4000, the thermal enhancement factor <sup>Δ</sup>*<sup>T</sup> <sup>T</sup>*<sup>0</sup> of *FD-*4*h* still performs better, while when the Re > 4000, the *FU-*4*h*-0.5*h* tends to have some gradual preponderance.

**Figure 9.** Diagrams of *FD-*4*h* 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.

**Figure 10.** Diagrams of *FU-*4*h* 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.

**Figure 11.** Performance of *R* at different transverse distances (0.25 *h*, 0.5 *h*, and 0.75 *h*) and distribution modes, (**a**) *FD-4h*; (**b**) *FU-4h*; (**c**) relatively better distribution mode.

**Figure 12.** Performance of thermal enhancement factor <sup>Δ</sup>*<sup>T</sup> <sup>T</sup>*<sup>0</sup> at different Re.

Since the *FD*-4*h* situation has been analyzed above, there will be no more details here, and only the *FU*-4*h*-0.5*h* situation will be discussed. Figure 13a shows the flow line diagram

of *FU*-4*h*-0.5*h*, from which it can be found that the flow line in the center of the vortex generator is no longer a straight line, but a curve with secondary disturbance on the left side of the second pair of vortex generators. In addition, the vortexes generated on the right side of the two pairs of vortex generators have a superposition effect, which impacts the heat transfer effect. The temperature contour diagram of Figure 13b and the slice diagrams of Figure 13c,d verify the analysis of the streamline in Figure 13a, the lower temperatures at the left wing of the second pair of vortex generators and the rear area of the second pair of vortex generators can be obtained. In addition, compared with Figure 10, due to the influence of transverse distance, the maximum temperature in Slice *X*<sup>2</sup> in Figure 13c is no longer symmetrically distributed, but the temperature is higher on one side and lower on the other.

**Figure 13.** Diagrams of *FU*-4*h* − 0.5*h* 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.
