Experimentally Measured Effects of Incidence Angle on the Adiabatic and Overall Effectiveness of a Fully Cooled Turbine Airfoil With Showerhead Shaped Holes

Abstract

Manufacturing and assembly variation can lead to shifts in the inlet flow incidence angles of a rotating turbine airfoil row. Understanding the sensitivity of the adiabatic film cooling effectiveness to a range of inlet conditions is necessary to verifying the robustness of a cooling design. In order to investigate the effects of inlet flow incidence angles, adiabatic and overall effectiveness data were measured in a low speed linear cascade at 0° and 10° of the designed operating condition. Tests were completed at an inlet Reynolds number of Re=120000 and a turbulence intensity of Tu = 5% at the leading edge of the test article. Particle Image Velocimetry was used to verify the incident flow angle for each angle studied. The test section was first adjusted so that the pressure distribution and stagnation line of the airfoil matched those predicted by an aerodynamic CFD model. IR thermography was then used to measure the adiabatic effectiveness levels of the fully-cooled airfoil model with nine rows of shaped holes of varying construction and feed delivery. Measurements were taken over a range of blowing ratios and at a density ratio of DR=1.23. This process was repeated for the two incidence angles measured, while the inlet pressure to the airfoil model was held constant for these incidence angle changes. Differences in laterally adiabatic effectiveness across the airfoil model were most evident in the showerhead, with changes as large as 0.2. The effect persisted most strongly at s/D=±35 downstream of the stagnation row of holes, but was visible over the whole viewable area of 160 s/D. The effect was due to the stagnation line affecting the film at the showerhead row. Due to this effect, the showerhead was investigated in detail, including effects of the stagnation line shift as well as the influence of the incidence angle on the overall effectiveness of the showerhead region. It was found that the stagnation line has the tendency to dramatically increase the near-hole adiabatic effectiveness levels when positioned within the breakout footprint of the hole. The effect persisted for the overall effectiveness study, since the hole spacing for this particular configuration was wide enough that the through hole convection was not completely dominant. This is the first study to present measured effectiveness values over both the pressure- and suction-side surfaces of a fully-cooled airfoil for appreciably off-nominal incidence angles as well as examine adiabatic and overall effectiveness levels for a conical stagnation row of holes.

Rotor Cascade Assessment at Off-Design Condition: An Aerodynamic Investigation on Platform Cooling

“…The flow angle measured at the theoretical 0 • incidence case is considered as a reference. The slightly negative incidence measured at the theoretical 0 • case is probably due to the turbulence generator, which induces a flow deviation upstream of the cascade, similar to [21,28].…”