## 3 Results and Discussion
### Comprehensive Performance under Rotating Condition
Due to the rotation effect, the velocity of the fluid in pass2 is reduced, and the flow impingement of the first protrusion structure is weakened. The heat transfer condition is deteriorated so that the \(Nu\) decreases. In the meantime, the low \(Nu\) region in the middle and lower reaches area expands. The rotation effect weakens the heat transmission ability in pass2. According to the division principles shown in Figure 7, the average wall \(Nu\) of regions 1-26 is calculated. The results are presented in Figures 16 and 17. Due to the inlet effect, the \(Nu\)/\(Nu_{0}\) is higher in region1. The effect gradually disappears near region3-4. Subsequently, due to the flow acceleration, the wall \(Nu\)/\(Nu_{0}\) of region 5-8 gradually elevates along the flow direction. For case1, the maximum value appears near the first turning zone. The flow direction of the cooling medium in pass2 is opposite to the direction of the centrifugal force. In addition, affected by the expansion of section, the flow speed of the working medium gradually decreases so that the \(Nu\)/\(Nu_{0}\) of the region 10-17 wall gradually decreases. The minimum value is taken in the second turning region entrance. The cooling medium at the outlet of the second turning region is biased to the side of RS\({}_{\text{TR}}\), mainly due to the channel profile and the Coriolis force. It leads to the improvement of the heat interchange and the appearance of 'RS\({}_{\text{TR}}\) > RS\({}_{\text{LE}}\)'. In region 19, owing to the spiral flow in the second turning zone, the wall \(Nu\)/\(Nu_{0}\) is slightly increased. Then, the spiral flow is consumed gradually. Finally, the outflow channel is accelerated under the joint action of centrifugal force and section contraction effect. The wall \(Nu\)/\(Nu_{0}\) of region 20-26 gradually increases. Figure 13: The streamlines of channels, \(Ro\) = 0.4. Figure 14: The vorticity based on Q criterion and streamline counters of the first turning region, \(Ro=0.4\). Figure 16: The wall \(Nu/Nu_{0}\) of RS\({}_{\rm TR}\), \(Ro=0.4\). Figure 15: The \(Nu\) counters, \(Ro=0.4\): (**a**) case\(1\); (**b**) case\(3\); (**c**) case\(5\). For case3 and case5, on the whole, the wall \(Nu\)/\(Nu_{0}\) in each region augments in varying degrees with the enlargement of the turning vane thickness. Specifically, the inlet zone of the turning vane (region 8 and region 17) is struck by the cooling medium, which strengthens the heat transfer there. The wall \(Nu\)/\(Nu_{0}\) of first turning region (region 9) rises in an impact and accelerated flow pattern. At the outlet of the first turning zone (region 10), the heat transfer capacity improved significantly due to the intersection and impingement operation of the fluid in the inner and outer basin. Since the rectification of the turning vane, the disturbance in pass2 decreases, and the development of the boundary layer makes the \(Nu\)/\(Nu_{0}\) in region 12-16 smaller than that in case1. In the second turning region and its outlet (region 18-19), there is also an enlargement in thermal exchange with the changes in flow state and the pooling of fluids. The improvement of flow uniformity is at the expense of heat transfer capacity, so the \(Nu\)/\(Nu_{0}\) of RS\({}_{\text{TR}}\) in pass3 is lower than that in case1. Figure 18 presents the variation of \(Nu_{\text{up}}\)/\(Nu_{0}\) of with \(Ro\) for different channels. It can be seen that with the enlargement of \(Ro\), the value increases first and then decreases. For case1, the maximum is around \(Ro\) = 0.3, while for case2-6, it is near \(Ro\) = 0.1. Within the range of \(Ro\) in this paper, the heat transmission capacity of the tip wall of case2-6 is greater than case1, and the thicker the guide vane, the greater of the improvement. When \(Ro\) = 0.1, the \(Nu_{\text{up}}\)/\(Nu_{0}\) of case6 is increased by 33.0% compared with case1. As \(Ro\) goes up, the additional amount of performance of case2-6 decreases. Figure 17: The wall \(Nu\)/\(Nu_{0}\) of RS\({}_{\text{LE}}\), \(Ro\) = 0.4.