## 4 Results and Discussions
### Effect of Dimple/Protrusion Arrangement  
According to the results and analysis in Section 4.1, the outlet configuration OC4 is selected due to its excellent thermal performance, and dimples/protrusions are arranged on the endwalls to further improve the heat transfer characteristics. In this section, the effect of dimple/protrusion arrangement on the trailing edge internal flow, heat transfer, flow resistance, and overall thermal performance are investigated and discussed. Figure 9 shows the streamlines and velocity distributions on the cross sections for three dimple/protrusion arrangements at the _Re_ of 30,000. Three cross sections, namely \(x/D_{\text{h}}=0.8\), 4.6, and 8.5 were selected, corresponding to the entrance, middle and top regions, respectively. For the dimple case, in the cross section of \(x/D_{\text{h}}=0.8\), the coolant separates at the tail region of dimple away from the extraction slots, producing several small-scale vortices, while the mainstream is little affected. As the coolant flows to the lateral slots, the extraction effect is gradually enlarged and the secondary flow near the dimples is weakened, contributing to the disappearance of flow separation at the outermost dimple. Moreover, the low-velocity areas are produced near the endwalls, especially in the dimple cavities, while the velocity around the slots is higher, and the overall flow structure and velocity distribution are substantially symmetric about the centerline. As the coolant flows downstream, the extraction effect is continuously accumulated and the affected area expands inward. Therefore, in the cross sections of \(x/D_{\text{h}}=4.6\) and 8.5, the flow separation occurred only in the innermost dimple area. At the same time, the overall flow velocity decreased and low-velocity area gradually increased. Figure 9: Streamlines and velocity distributions on several cross sections for different dimple/protrusion arrangements, _Re_ = 30,000. For the protrusion case, due to the reduction of channel cross sections and the effect of impingement on the protrusion leading edge, a higher level of overall velocity, compared with the dimple case, was presented. There exist several small-scale vortices driven by the secondary flow in the leading edge of protrusions, especially for the inner protrusions. Local low-velocity area is produced on both sides of protrusions due to the weakened fluid disturbance. Similar to the dimple case, as the coolant flows downstream, the velocity of cross section gradually decreases and the secondary flow near the protrusion is weakened. For the dimple-protrusion case, the cross sections of \(\chi/D_{\text{h}}=0.8\) and \(4.6\) were occupied by the dimples and the cross section of \(\chi/D_{\text{h}}=8.5\) was occupied by the protrusions. It can be seen that, in the dimple cross sections, large-scale flow separation vortices appeared in the front half of inner dimple cavity. Although a similar overall flow structure as the dimple case was presented, the velocity level was higher than the that of dimple case, demonstrating that the interaction between the dimple and protrusion effectively enhances the flow mixing. The flow structure in the protrusion cross section was close to that of protrusion case. Figure 10 displays the Nusselt number distributions and local limiting streamlines on the endwalls for three dimple/protrusion arrangements at the _Re_ of 30,000. As can be seen from Figure 10a, the _Nu_ is high in the entrance area, and gradually decreases as the coolant flows downstream. Simultaneously, the high-_Nu_ area is deflected toward the lateral slots, which is especially noticeable near the slots. According to the local limiting streamlines, the coolant separates at the leading edge of the dimple and then reattaches at the tail region, forming a separation vortex in the front half of the dimple cavity.