## 5 Conclusions  
Based on the typical internal cooling structure of a turbine blade trailing edge, trapezoidal cooling channels with lateral extraction slots were modeled in this paper. Five channel outlet configurations, i.e., no second passage (OC1), radially inward flow second passage (OC2), radially outward flow second passage (OC3), top region outflow (OC4), both sides extractions (OC5), and three dimple/protrusion arrangements (all dimple, all protrusion, dimple-protrusion staggered arrangement), were considered. Numerical investigations were carried out, within the \(Re\) range of 10,000-100,000, to analyze in detail the flow structures, heat transfer distributions, average heat transfer and friction characteristics and overall thermal performances. Specific conclusions were as follows:  
1. For five channel outlet configurations, the heat transfer level is high at the entrance area and then rapidly decreases as it flows downstream. Under the effect of lateral extraction, the coolant is deflected toward the slots, and the semi-elliptical high-\(Nu\) regions appear between the lateral slots. For the OC1, OC2 and OC3 cases, the heat transfer deterioration occurred in the up-right area due to the formation of a teardrop shaped closed vortex, which was more obvious in the OC2 case, while the heat transfer performances of OC4 and OC5 were at high levels, and OC4 produced the best heat transfer distribution;
2. Among five channel outlet configurations, OC4 produced the largest heat transfer augmentation within the \(Re\) range of 10,000-100,000, with the maximum \(Nu\) of 253.2; while the OC2 presented the worst heat transfer performance, and maximum difference of \(Nu\) between OC2 and OC4 was up to 37.4%. The \(Re\) had little effect on the flow resistance of the trailing edge channels studied in the paper. Taking the OC1 case as a baseline, when introducing the second passage, the friction was significantly increased by 27.6-40.6%; when the top region outflow or both sides extractions were applied, the friction was reduced by 69.9-70.2%, or even more;  
Figure 13: Variations of overall thermal performance with Re for different dimple/protrusion arrangements. In the dimple area, 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. In the protrusion area, the coolant impinged on the protrusion leading edge and flowed downstream along the spherical protrusion surface as well as the side edges, then separated at the rear area and reattached at the downstream region, generating a large-scale separation vortex. Therefore, the heat transfer performance in the dimple trailing edge and protrusion leading edge was greatly improved;
4. For different dimple/protrusion arrangements, the protrusion case produced the largest \(Nu\), with a 16.3% improvement at maximum compared with the smooth case, simultaneously, producing an additional friction of 8.5-10.8%. The dimple case had the lowest \(Nu\), with an only 0.5% additional friction at maximum relative to the smooth case. The heat transfer and friction characteristics of the dimple-protrusion case were between those of dimple case and protrusion case;
5. Three dimple/protrusion cases show similar \(TP\) variation law, that is, increases first and then decreases with larger \(Re\), and the maximum \(TP\) is obtained at the \(Re\) of 30,000. The \(TP\) of the dimple case and the dimple-protrusion case was relatively close, while the protrusion case produced a significantly higher \(TP\), whose maximum value reached 1.13, indicating a 13% improvement in the overall thermal performance compared with the smooth case. Taken together, in the cooling design of a blade trailing edge with lateral slots, channel outlet configurations with top region outflow or both sides extractions, and the arrangement of protrusions should be given priority in practical applications.