## 4 Results and Discussions
### Effect of Dimple/Protrusion Arrangement
Consequently, the _Nu_ in the front half cavity is reduced, while the heat transfer around the dimple trailing edge as well as the downstream area is significantly improved, and this effect is gradually weakened as the coolant flows downstream. It can be seen from Figure 10b that the overall heat transfer level on the endwalls of protrusion case is obviously higher than that of dimple case. Combined with the local limiting streamlines, the coolant impinges on the protrusion leading edge and produces a small separation region; the fluid flows downstream along the spherical protrusion surface as well as the side edges, then separates at the rear area and reattaches at the downstream region, generating a large-scale separation vortex. Therefore, the heat transfer performance on the protrusion leading edge is evidently improved attributing to the impingement effect and flow mixing, while the _Nu_ on the tail area is reduced to a certain extent. Under the effect of lateral extraction, the abovementioned flow structure is deflected, which is particularly evident in the protrusion regions near the slots. Generally, most of protrusions are occupied by the high-_Nu_ area, while the local heat transfer deterioration appears at the junction area of top region outflow and lateral extraction, such as the third row of protrusions near the top region. According to Figure 10c, the local flow structures of dimple and protrusion are similar to that in Figure 10a,b. Differently, in the dimple-protrusion case, the composite flow structure was generated at the interaction of dimple and protrusion, and heat transfer enhancement area at the protrusion downstream was extended to the leading edge of dimple, but this effect was weakened along the flow direction. Moreover, it can be clearly seen that the enhanced heat transfer performance at the protrusion leading edge dropped more slowly compared with that at the dimple tail area. Consequently, the overall heat transfer performance of dimple-protrusion case is between the dimple case and protrusion case. Variations of Nusselt number and friction with _Re_ for different dimple/protrusion arrangements are presented in Figure 11. According to the Figure 11a, the _Nu_ of all cases increases linearly with larger _Re_, among which the protrusion produced the largest _Nu_ and the smooth case had the lowest _Nu_, while the heat transfer performance of dimple-protrusion case was between that of dimple and protrusion cases. Generally, at the same _Re_, there exists little difference in the heat transfer characteristics between different cases, and a 16.3% improvement at maximum can be achieved by the protrusion, compared with the smooth case. While in Figure 11b, significant differences in the friction for different cases can be found. As the _Re_ is increased from 10,000 to 50,000, the friction coefficient of all cases decreases and the declining trend gradually slows down; with the further increase of _Re_, the friction coefficient remains almost unchanged. Relatively speaking, the smooth case had the lowest friction, followed by the dimple case, while the protrusion produced the largest flow resistance, and the friction of dimple-protrusion case was between that of dimple and protrusion cases. Compared with the smooth case, an additional friction of 8.5-10.8% was generated by the protrusion; while for the dimple case, the value was only 0.5% at maximum. Figure 10: Nusselt number distributions and local limiting streamlines on the endwalls for different dimple/protrusion arrangements, \(Re=\) 30,000. Figure 12 shows the variations of relative Nusselt number and relative friction with \(Re\) for different dimple/protrusion arrangements. It can be seen from Figure 12a that three cases present similar \(Nu\) changing regularity, that is, increases first and then decreases with larger \(Re\), and the maximum \(Nu\)/\(Nu_{0}\) is obtained at the \(Re\) of 30,000.