Effect of transverse trench on film cooling performances of typical fan-shaped film-holes at concave and convex walls

Abstract

By means of a time-resolved infrared thermal imaging system, the effect of transverse trench on the film cooling performances of a fan-shaped film-hole at typical concave and convex walls was discussed deeply, from a combined view of the time-averaged parameter evaluation and cooling unsteadiness level analysis. Harmful cooling unsteadiness with high level can be estimated by large standard deviation (SD) of transient film effectiveness, which is generated by the intensified temporal evolutions of vortex-footprints at wall. Comparisons of the time-averaged experimental results revealed the design of trench at curved walls can effectively improve the film effectiveness, completely different from the phenomenon of previous flat-plate studies. The increments in area-averaged effectiveness are nearly non-sensitive to cooling air flowrate, which can reach 27% and 16% at the concave and convex walls, respectively. Meanwhile, the trench can cause the relatively small aerodynamic loss at curved walls. Analysis of cooling unsteadiness can provide the new horizon to estimate the benefits of film cooling design. Thus, the transverse trench structure is strongly suggested at convex wall, due to the reduced level of cooling unsteadiness; however, the improved level of cooling unsteadiness can impede the direct application of transverse trench at concave wall. Steady numerical simulations were also conducted to provide the detailed knowledge of flow mechanisms. The variations of intensities of vortex-system result in the different jet reattachments downstream of trench caused by wall curvature. The difference of cooling unsteadiness levels is mainly determined by the turbulence quality of impingement to trench-edge. An additional information from the present study was introduction of trench can obviously change the trends in film effectiveness and SD-contour with wall curvature. The variations of turbulence dissipation cause a break of the wall curvature effect on film effectiveness at blowing ratio (BR) of 1.5, indicting the highest effectiveness is acquired by the convex model under small BRs whereas by the concave model under large BRs.