Recent studies have demonstrated that cylindrical hole with backward injection arrangement, of which the jets are injected reverse to the mainstream flow direction, outperforms its forward injection counterpart, of which the jets are injected along the flow direction, at high blowing ratio, since the jet starts to lift off typically for forward injection when blowing ratio is greater than 1.0. However, the backward injection configuration features a large separation and induces high heat transfer near the hole. Relative few studies have been conducted to mitigated the drawbacks of backward injection arrangements. The present study investigated several flat plate trenched hole arrangements with backward injection. Experiments were conducted in a low speed suction type wind tunnel. The trench width was varied from 2d to 4d for the backward arrangements with fixed trench depth of 0.75d. Besides, a simple backward and a trenched hole with forward injection, whose width is 2d and depth is 0.75d, were also studied as references. Transient thermal measurements were carried out for all the arrangements with IR camera. Detailed distributions of heat transfer coefficient were obtained. For each case, blowing ratio was varied from 0.25 to 4.0, while the density ratio was almost unity. Effects of injection angle, trench width and blowing ratio on the surface heat transfer distributions were obtained, and the results are presented and explained in this investigation.

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