*2.1. Physical Model*

As shown in Figure 1a, a three-dimensional rectangular fluid channel is built on the basis of the rectangular coil. The length of the fluid channel is 888 mm, while the height and width of the inlet and outlet are 245 mm. Besides, the length, width, and height of the rectangular coil are 365 mm, 85 mm, and 20 mm respectively, and the distance from the rectangular coil to the inlet and outlet is 261.5 mm, and the distance to the upper and lower boundaries is 112.5 mm. Moreover, as illustrated in Figure 1b, the rectangular coil is composed of copper wire surrounded by a 0.5 mm thick insulating layer.

**Figure 1.** (**a**) Diagrams of a channel with coil and vortex generators, (**b**) specific graph of the coil, (**c**,**d**) specific graphs of vortex generator, (**e**) four different distribution types, (**f**,**g**) slice diagrams in *X* and *Z* directions.

Figure 1c shows the detailed data of the vortex generator. The length and width of vortex generators are 30 mm, 1 mm respectively, while the height, which is defined as *h*, of the vortex generator, is 7.5 mm. Furthermore, the attack angle of the vortex generator is defined as *α*, and the distance between each pair of wings is 15 mm. As presented in Figure 1d, when investigating the best arrangement of the vortex generator, the longitudinal distance between two adjacent vortex generators is *x*, while the transverse distance is defined as *y*. Additionally, besides the variety of the longitudinal and transverse distance, four different vortex generator distribution modes are shown in Figure 1e, which

are specified as *Flow-Up (FU), Flow-Down (FD), Flow-Up-Down (FUD), Flow-Down-UP (FDU)*. It is worth noting that when investigating the influence of vortex generator arrangement on heat transfer performance, this paper first explores *x* = 2*h*, 4*h*, 6*h* and four different distribution modes to obtain a better longitudinal distance and distribution mode. On this basis, *y* = 0.25*h*, 0.5*h*, 0.75*h* are discussed to explore the influence of the transverse distance of vortex generator on heat transfer. Based on the above results, a relatively better heat transfer arrangement can be obtained.

When discussing and analyzing the effect of vortex generator arrangement on heat transfer, there are six different slices in *X* and *Z* directions, specified as *Slice X*1, *Slice X*2, *Slice X*3, *Slice Z*1, *Slice Z*2, *Slice Z*3. The *X* coordinates of *Slice X*1, *X*2, and *X*<sup>3</sup> were −40 mm, −10 mm, and 20 mm separately, while the *Z* coordinates of *Slice Z*1, *Z*2, and *Z*<sup>3</sup> were 10 mm, 25 mm, and 42.5 mm respectively. On this foundation, the straight line 3.75 mm above the coil is selected on the slice to obtain the temperature data to explore the uniformity of temperature distribution. Since only one pair of vortex generators are used to discover the change of attack angle, while two pairs of vortex generators are used to explore the distance and arrangement, the position of the slice relative to the vortex generator changes slightly, which can be observed in Figure 1f,g.
