## 4 Results and Discussions  
### Effect of Channel Outlet Configuration  
Figure 6 shows the streamlines and turbulence kinetic energy (TKE) distributions on the longitudinally middle sections for five outlet configurations at the \(Re\) of 30,000. Obviously, the OC1-3 cases produce similar flow structures in the main channel, that is, the coolant flows vertically into the channel from the inlet, and is laterally deflected by the extraction slots; then the fluid obliquely impinges on the top wall and flow separation occurs, producing a teardrop shaped closed vortex in the up-right area. However, differently, the vortex area of OC2 is significantly larger than that of OC1 and OC3 cases, which is because the TKE of the second passage outlet region (that is, the inlet region of the main channel) is at a high level, thus whose pressure is lower, resulting in a stronger extraction effect and more evident flow deflection. Therefore, for the OC2 case, the impingement point on the top wall is closer to the slot side, leading to the appearance of a larger-scale vortex. While for the OC1 case, the lateral extraction effect is uniformly distributed because the pressure at each slot outlet is equal; for the OC3 case, the TKE and pressure distributions of second passage are reverse to that of the OC2 case, thus the vortex area is even slightly smaller than the OC1 case. For the OC4 case, due to the top region outflow, the upper flow was substantially parallel to the side wall, and the lower flow was deflected by the side extraction, thus no flow separation occured and no vortex was generated in the main channel. For the OC5 case, the coolant was deflected to both sides because of the two-way lateral slots, then the fluid impingement and flow separation occurred on the central portion of the top wall, thus producing a basically symmetrical flow structure, for which no obvious vortex was generated. Furthermore, it can be found that large vortices were produced in the upstream slots; along the flow direction, the fluid deflection effect was continuously accumulated and the vortices were compressed. Figure 7 shows the Nusselt number distributions on the endwalls for five outlet configurations at the \(Re\) of 30,000. It can be seen that, the OC1-3 cases present similar heat transfer distributions, that is, the \(Nu\) is high at the entrance area and then rapidly decreases as it flows downstream, simultaneously, the heat transfer isopleth is gradually inclined, which is consistent with the flow deflection driven by the side extraction. The semi-elliptical high-\(Nu\) regions appears between the lateral slots. Moreover, the \(Nu\) near the top wall is improved by the impingement, while the heat transfer performance in the up-right area is deteriorated due to the large closed vortex. Among the three cases, the OC2 had  
the largest low-\(Nu\) area, while the OC3 produced the smallest low-\(Nu\) area. For the OC4 and OC5 cases, there was no obvious heat transfer deterioration. Specifically, the top region outflow weakened the lateral extraction effect in the OC4 case, and whose \(Nu\) distribution was basically stratified along the flow direction; the local heat transfer near the top wall was enhanced in the OC5 case, while several small area heat transfer deteriorations appeared round the upper sidewall. Generally, the \(Nu\) distribution is consistent with the flow structure for all cases. The OC4 produced a higher average \(Nu\) and a smaller low-\(Nu\) area, thus has the best heat transfer performance. Figure 8 presents the variations of Nusselt number and friction with \(Re\) for different channel outlet configurations. It can be seen from Figure 8a that, at the \(Re\) of 10,000, the \(Nu\) is close for all cases; while the \(Nu\) difference becomes obvious as the \(Re\) increases. Generally, the \(Nu\) of OC1, OC4, and OC5 increased linearly with \(Re\), and OC4 produced the largest heat transfer augmentation within the \(Re\) range of 10,000-100,000, with the maximum \(Nu\) of 253.2.