Exit-shape effect of film holes at pressure side on trailing edge cutback film cooling characteristics

Pressure side (PS) cutback structure has been widely employed in modern turbine blade trailing edge (TE) cooling design. However, hot gas flows around lip of TE slot to generate an impingement to cutback, resulting in a rapid decay of film cooling effectiveness and hence thermal damage of entire TE. Proper angled-ejections upstream of TE can suppress the downstream effectiveness-decay. Therefore, in present work, five types of film-holes were designed to discuss the effect of exit-shape of film-hole on TE film cooling characteristics, inlet boundary layer profile, fluid-interactions, discharge behaviors and total pressure losses. The studied models included cylindrical-hole, common and laidback fan-shaped film-holes, and converging slot-holes with two different exit-to-inlet area ratios ( ARs). The solo cutback cooling was chosen as a reference. Thermal tests were conducted in a hot wind tunnel featuring a simplified TE test section by infrared thermal technique. Engine-similar mainstream-to-coolant temperature ratio of 2.0 was controlled. Comparisons of experimental results revealed that the converging slot-holes can obtain the highest film cooling effectiveness at TE region and overall cooling effectiveness at PS film-plate, but the slightest increase of total pressure losses. Flow measurements using Particle Image Velocimetry technique indicated the converging slot-holes can modify the inlet boundary layer profile, suppress effectively the impingement of mainstream and change the flow separation features at cutback. In addition, applying shaped-holes can effectively suppress the flow ingestion under low coolant amounts. However, relative to the exit-shape effect, the coolant amount plays a more important role on the aerodynamic and thermal characteristics of TE cooling.

Effects of nozzle-exit boundary-layer profile on the initial shear-layer instability, flow field and noise of subsonic jets

The influence of the nozzle-exit boundary-layer profile on high-subsonic jets is investigated by performing compressible large-eddy simulations (LES) for three isothermal jets at a Mach number of 0.9 and a diameter-based Reynolds number of $5\times 10^{4}$, and by conducting linear stability analyses from the mean-flow fields. At the exit section of a pipe nozzle, the jets exhibit boundary layers of momentum thickness of approximately 2.8 % of the nozzle radius and a peak value of turbulence intensity of 6 %. The boundary-layer shape factors, however, vary and are equal to 2.29, 1.96 and 1.71. The LES flow and sound fields differ significantly between the first jet with a laminar mean exit velocity profile and the two others with transitional profiles. They are close to each other in these two cases, suggesting that similar results would also be obtained for a jet with a turbulent profile. For the two jets with non-laminar profiles, the instability waves in the near-nozzle region emerge at higher frequencies, the mixing layers spread more slowly and contain weaker low-frequency velocity fluctuations and the noise levels in the acoustic field are lower by 2–3 dB compared to the laminar case. These trends can be explained by the linear stability analyses. For the laminar boundary-layer profile, the initial shear-layer instability waves are most strongly amplified at a momentum-thickness-based Strouhal number $St_{\unicode[STIX]{x1D703}}=0.018$, which is very similar to the value obtained downstream in the mixing-layer velocity profiles. For the transitional profiles, on the contrary, they predominantly grow at higher Strouhal numbers, around $St_{\unicode[STIX]{x1D703}}=0.026$ and 0.032, respectively. As a consequence, the instability waves rapidly vanish during the boundary-layer/shear-layer transition in the latter cases, but continue to grow over a large distance from the nozzle in the former case, leading to persistent large-scale coherent structures in the mixing layers for the jet with a laminar exit velocity profile.