Reynolds Number Effects in a Low Pressure Turbine

This paper reports the results of an experimental and analytical study of Reynolds number (Re) effect on Low Pressure Turbine (LPT) performance. LPTs suffer both a loss in efficiency and flow capacity at altitude due to thicker boundary layers and higher viscous losses. Boundary layer separation can occur in highly loaded and/or high lift designs. The magnitude of the effect is stronger for smaller engines being designed for regional jets which may have cruising altitudes above 50K feet. There is a general lack of knowledge about performance degradation in commercial LPTs under these conditions. A test program was undertaken in a low pressure 3 stage axial turbine to quantify the effect of low Re on efficiency and flow lapse rate. Rig inlet pressures were varied from 0.27E+5 N/m2 (4 psia) to 4.40E+5 N/m2 (65 psia) to achieve over a 15 fold variation in Re. The chord based average Re varied from 30000 to 500000. Efficiency and flow function lapse of over 5% was measured. The fall off was non-linear with a rapid loss occurring at Re below 100000. 3D CFD analysis was conducted in parallel to predict overall performance but also understand loss details within the blade rows. Measured inlet profiles of total pressure, temperature and air angle, and exit static pressure were used as boundary conditions. Leakages and purge flows were modeled as source terms. A turbulence transition model and wall integration grids was used. The CFD results corroborate the test findings on the overall efficiency and flow capacity lapse rate. Analysis of blade row performance at high and low Re shows a sharp increase in profile loss at low Re.