Effects of streamwise distance and density ratio on film-cooling effectiveness for double-jet film-cooling on a flat plate

Several double-jet film-cooling (DJFC) hole geometries on a flat plate are investigated in this paper. Pressure sensitive paint (PSP) is used to measure the film-cooling effectiveness on the flat plate. The streamwise distance between the DJFC holes varies as s/d=3.0, 4.0, 5.0, and 6.0, while three different spanwise distance conditions are considered (p/d=0, 0.5, and 1.0). The diameter (d) of the DJFC holes is 7 mm, and the pitch (P) between each set of the DJFC holes is 8d. The holes have an inclination angle (θ) of 35°, and a compound angle (β) of ± 45°. The density ratio (DR) varies as 1.0, 1.5, and 2.5, and the blowing ratio (M) is 0.5, 1.0, 1.5, and 2.0. Both effects of the streamwise distance between DJFC holes, and the density ratio on film-cooling effectiveness are focused. Results show, though the effect of the streamwise distance is not monotonic all the time, a general trend exists when comparing with the baseline case of s/d=3.0. For p/d=0, increasing the streamwise distance leads to wider lateral coverage and higher laterally averaged effectiveness; however, for p/d=0.5 and 1.0, an increased s/d may weaken the interaction between the two jets, thus the film coverage and effectiveness decrease. A higher density ratio reduces the jet momentum, therefore better attachment and higher effectiveness are obtained. Effects of the streamwise distance and the density ratio on double-jet film-cooling are more distinct when the blowing ratio rises. However, in most cases, both of them show minor influence on the range of lateral coverage, as it is mainly dominated by the spanwise distance. For each operation condition (DR and M), an optimal DJFC configuration (among the geometries in this study) is identified by comparing the area averaged effectiveness.