## 3 Results and Discussion
### Comprehensive Performance under Stationary Condition
For case1, the maximum value is located in the inlet of pass2 (region 10). It is mainly due to the formation of an acceleration zone in region 10 after fluid impacts the wall. The fluid speeds up and takes away more heat. On the other side wall, the flow separation causes a stronger vortex disturbance, which is beneficial for heat exchange. Under the influence of the expansion effect of pass2, the velocity of the fluid drops to the minimum value when the fluid arrives at the second turning region. The poor heat exchange condition results in the minimum value of \(Nu\) in region 17. The characteristics of spiral flow destroy the boundary layer in the second turning region (region 18). For the inlet of pass3, there is a flow separation so that the \(Nu\) in region 19 is larger. Then, the disturbance disappears, the fluid accelerates outflow, and the wall \(Nu\) decreases first and then increases. After the structural arrangement of turning vane, on the whole, the wall \(Nu\)/\(Nu_{0}\) of each region increases with the arguments of the thickness of the vane. Specifically, the inlet region of the guide vane (region 8 and 17) is impinged by the cooling medium, which enhances the heat transfer at this region. Therefore, the wall \(Nu\)/\(Nu_{0}\) increases compared with case1. The impact and acceleration mode in the first turning region (region 9) significantly enhances the heat transfer level there. \(Nu\)/\(Nu_{0}\) increases obviously at the entrance of pass2 (region 10) due to the impingement of the inner and outer basin fluid. Affected by the turning vane, the disturbance in pass2 is reduced, and the development of the boundary layer makes the \(Nu\)/\(Nu_{0}\) of region 12-16 lower than that of case1. In the second turning region and its outlet (regions 18 and 19), the change of flow form and the collection of fluid also bring about the improvement of heat exchange capacity. The amelioration of flow uniformity is usually accompanied by the reduction of thermal exchange, so the \(Nu\)/\(Nu_{0}\) of pass3 is lower than that of case1. To compare the difference of the tip-wall cooling performance in each channel, the \(Nu_{\text{up}}\)/\(Nu_{0}\) is shown in Figure 10. The results shows that the \(Nu_{\text{up}}\)/\(Nu_{0}\) of each case decrease first and then increase with the enlargement of \(Re\). The \(Nu_{\text{up}}\)/\(Nu_{0}\) of case2-case6 is improved compared with case1. The thicker of the guide vane is, the greater the thermal exchange of the tip wall is. When \(Re\) = 50,000, the \(Nu_{\text{up}}\)/\(Nu_{0}\) of case6 is increased by 56.5%. It is mainly related to the fact that fluid acceleration effect in the out basin of the first turning region reinforces with the increase of the turning vane thickness. Figure 9: The wall \(Nu\)/\(Nu_{0}\) of SS along the flow direction, \(Re\) = 10,000. The friction characteristic of the channel is one of the momentous indexes to assess the comprehensive property of the channel. The variations of the _fff\({}_{0}\)_ with _Re_ for each case is shown in Figure 11. It can be obtained from the diagram that the thinner of the turning vane can play a better rectification effect, thus greatly reducing the resistance of the channel. When _Re_ = 10,000, case2 achieves a maximum drag reduction effect of 14.2%. At the same time, it should be noted that too thick turning vanes will increase the friction of the channel. This is mainly because as the thickness of the guide vanes increases, the flow area at the corner of the channel decreases, and a too-small cross area of the basin increases the resistance of the channel. The improvement of the thermal exchange of the channel is often accompanied by the reduction of the friction performance of the channel. The _TP_ comprehensively considers the heat exchange and friction characteristics, which can more intuitively reflect the comprehensive performance of the channel. The variation of _TP_ with _Re_ in different channels is shown in Figure 3:0 As the _Re_ increases, the _TP_ of each channel decreases first and then increases.