## 4 Results and Analysis
### Heat Transfer Characteristics of Target Surface  
Figure 15 shows the distribution of the \(Nu\) number ratio (\(Nu/Nu_{st}\)) on the target surface of the two flow guiding elements. Here, \(Nu\) is local Nusselt number, and \(Nu_{st}\) is local Nusselt number at the stagnation point. The distribution of \(Nu\) ratio on the target surface with \(0^{\circ}\) and \(45^{\circ}\) flow guiding elements shows a random distribution due to the randomness of the jet impact, and the effective area of heat transfer is roughly equivalent. Heat transfer enhances at the four corners of the jet (dashed box in the figure) with the \(0^{\circ}\) straight hole. The \(0^{\circ}\) straight hole makes the velocity at the boundary of the channel smaller than that at the boundary of the central circular hole, and the velocity in the radial direction of the groove decays faster, resulting in a difference in heat transfer at the four corners of the target surface (red dotted frame dashed box in the figure). In Figure 15, the swirling action of the spiral groove of the \(45^{\circ}\) spiral hole increases the dispersion of the jet medium at the boundary of the spiral groove and eliminates the  
Figure 14: Distribution of root mean square of the axial and radial fluctuating velocities. Figure 15: Cloud plot of the distribution of instantaneous \(Nu\) number ratio on the target surface. difference of heat transfer caused by the speed decay of the boundary between the spiral groove and the central circular hole. To further study the difference in heat transfer characteristics between two holes, Figure 16 shows the distribution of \(Nu/Nu_{st}\) in the \(x\) direction. It can be seen that \(Nu\) maintains a high value with two flow guiding elements in the range of \(x/D\) = 0-0.5, and decays rapidly in the range of \(x/D\) = 0.5-0.75. In the range of \(x/D\) = 0.75-1.75, the decrease rate of \(Nu\) slows down and fluctuates within a certain range. At \(x/D\) = 1.75, the instantaneous \(Nu\) decreases again greatly, and then the value decreases linearly along the direction of increasing the radius in the range of \(x/D\) = 1.75-3.5. The changing trend of the instantaneous \(Nu\) is in accordance with the variation law of the \(Nu\) of the target surface with the general impinging jet. It should be pointed out that the fluctuation of instantaneous \(Nu\) in different ranges is not the same. It can be seen that the fluctuation peaks appear in the A and B regions with the 45\({}^{\circ}\) spiral hole in the range of \(x/D\) = 0-0.5, and the peak appears in the B region with the 0\({}^{\circ}\) straight hole near \(x/D\) = 0.5. In the range of \(x/D\) = 0.75-1.75, there are two peaks with large spacing in the C region with the 45\({}^{\circ}\) spiral hole, while the instantaneous \(Nu\) of the 0\({}^{\circ}\) straight hole has four peaks with different fluctuation amplitudes in the C region. In the range of \(x/D\) = 1.75-2.6 of the D region, there are two peaks of approximately \"sinusoidal\" change with the 0\({}^{\circ}\) straight hole. Although there are two peaks with the 45\({}^{\circ}\) spiral hole, the corresponding \(Nu\) peak is greater than that of the 0\({}^{\circ}\) straight hole. In general, the \(Nu\) number of the jet impacting target surface with the 45\({}^{\circ}\) spiral hole is about 5-10% higher than that of the 0\({}^{\circ}\) straight hole, and the fluctuation is more severe in each sub-area. Figure 17 shows the temperature distribution of near wall in the jet core and wall jet region. In Figure 17a, the temperature boundary layer of the 0\({}^{\circ}\) straight hole is suddenly thickened at \(x/D\) = 0.1, so that the effective erosion of the jet core region is weakened, resulting in a sharp decrease in \(Nu\) in the corresponding position. In the range of \(x/D\) = 0.1-0.5, as the jet core region impacts the target surface and deflects into the adherent flow, the temperature boundary layer is slowly thickened, so that the \(Nu\) of the target surface decreases.