**3. Grid Independence Test**

As illustrated in Figure 2a, all models calculated in this paper use hexahedral mesh. When figuring out the grid near the wall, it is necessary to take the thickness of the boundary layer into account, thus the use of the dimensionless number Y plus is required for characterization [33]. When calculating the grid spacing of the first layer, Y plus = 1 is taken to obtain the grid spacing of the first layer. After the grid spacing of the first layer is determined as 0.02 mm, it is extended outward in the proportion of 1.1 ratio. Before the numerical simulation, the grid independence is analyzed with five grid numbers, i.e., <sup>262</sup> × 104, 421 × 104, 596 × <sup>10</sup>4, 685 × <sup>10</sup>4, and 799 × 104. From Figure 2b, it can be observed that with the increase of the number of grids, the growth of *Nu* and *f* gradually decreases. Besides, it can be seen from Figure 2c that the maximum temperature of copper wire decreases and the Δ*P* increases with the increase in grid number. When the number of grids is greater than 685 × 104, the *Nu*, *<sup>f</sup>* , maximum temperature, <sup>Δ</sup>*<sup>P</sup>* change little, and when the number of grids changes from 685 × <sup>10</sup><sup>4</sup> to 799 × <sup>10</sup>4, the relative change rates are less than 0.3%. Therefore, considering the accuracy and calculation time, a grid number of 685 × <sup>10</sup><sup>4</sup> is adopted in the present study.

**Figure 2.** (**a**) Representative grids in numerical simulation, (**b**,**c**) grid independence test results.
