Designing Low Pressure Turbines for Optimized Airfoil Lift

Abstract

In the past 10–15 years, substantial effort has been spent on increasing the airfoil lift especially in aero-engine low pressure turbines. This has been attractive, since increased airfoil lift can be used for airfoil count decrease leading to weight and hardware cost reduction. The challenge with this effort consequently has been to keep the efficiency at high levels. Depending on the baseline level of airfoil lift, an increase of 20–50% has been realized and at least partly incorporated in modern turbine designs. With respect to efficiency there is actually an optimum level of airfoil lift. Airfoil rows at a lift level below this optimum suffer from an excessive number of airfoils with too much wetted surface and especially increasing trailing edge losses. Airfoils at lift levels above this optimum suffer from growing losses due to high peak Mach numbers inside the airfoil row, higher rear diffusion on the airfoil suction sides, and increased secondary flow losses. Since fuel cost have been rising significantly, as has been the awareness of the environmental impact of CO2, it becomes more and more important to design low pressure turbines for an optimal trade between efficiency and weight to achieve the lowest engine fuel burn. This paper summarizes work done recently and in the past to address the main influences and mechanisms of the airfoil lift level, with respect to losses and efficiency as a basis for determination of optimal airfoil lift selection.