Effect of inflow conditions on transonic turbine airfoil limit loading

Abstract

To better understand airfoil limit loading and the effect inflow conditions have on local efficiency, a computational fluid dynamic investigation was performed for four different transonic turbine airfoils under sub-critical, critical, and supercritical conditions. A computational baseline was established using data previously collected at the Pratt and Whitney Canada High-Speed Wind Tunnel at Carleton University near design conditions using the Reynolds-Averaged Naiver-Stokes shear stress transport k − ω turbulence model with γ transition. The effects of inflow conditions on aerodynamic performance were examined by varying incidence by ± 20°, mainstream turbulence intensity from 5 to 20% and mainstream turbulent length scale from 1 to 100% of the airfoil pitch. Quantitative data of mass-flow averaged Mach numbers, mass-flow averaged flow angles, surface isentropic Mach number distributions and mass-flow averaged total pressure loss coefficients were collected and are presented alongside flow visualizations of numerical Schlieren images to allow for a detailed description of the entire flow domain. Similar to previous experimental work the limit loading pressure ratio and the mass-flow averaged outlet flow angle were strongly correlated with the airfoil outlet metal angle. The influence of inflow conditions was minimal on the exit flow profile with the exception of the mass-flow averaged total pressure loss coefficients. Results show incidence variation to change the total pressure loss coefficient depending on the airfoil, whereas, turbulence intensity and turbulent length scale predicted a drastic increase in loss with increased turbulence level for all airfoils considered.

A characterization of unsteady effects for transonic turbine airfoil limit loading

“…Expanding on this work, Owen investigated the effects of off-design inflow conditions for four different transonic turbine airfoils over a similar range of pressure ratios by varying inflow boundary conditions: incidence from ±20°, turbulence intensity from 5 to 20% and turbulent length scale from 1 to 100% of the airfoil pitch. 6 Similar to previous experimental work, it was shown that the limit loading pressure ratio and the mass-flow averaged outlet flow angle were strongly correlated with the airfoil outlet metal angle. The influence of inflow conditions on the exit flow profile was found to be minimal with the exception of the mass-flow averaged total pressure loss coefficient.…”

“…Furthermore, the large change in outlet flow angle can drastically decrease the efficiency of downstream blade rows through a change in incidence angle. A quick interpolation of results from Owen, 6 shows that for a change of incidence by less than 2°, such as the change from critical to limit loading, total pressure loss is expected to rise by roughly 1%. However, as the incidence angle is significantly changed (>9°), such as is the case for supercritical loading, the estimated loss increase is roughly 25%.…”

“…The size of this pressure ratio range has been discussed by Owen and shown to be dependent on the airfoil geometry, specifically the airfoil stagger angle and trailing edge blockage ratio. 6 Further increase of the pressure ratio causes the TEPSS to obtain sufficient flow turning to completely miss the surface of the adjacent airfoil, moving into the base pressure region downstream of the trailing edge. This is the point of maximum loading and is defined as the limit loading condition.…”