Effects of Rotating Blade Wakes on Separation and Pressure Recovery in Turbine Exhaust Diffusers

Abstract

Highly efficient turbine exhaust diffusers can only be designed by taking into account the unsteady interactions with the last rotating row of the turbine. Therefore, a scale model of a typical gas turbine exhaust diffuser consisting of an annular and a conical diffuser is investigated experimentally. To investigate the influence of rotating wakes, a variable-speed rotating spoke wheel with cylindrical spokes as well as with NACA bladed spokes generates high-energy turbulent wakes simulating turbine rotor wakes. For the rotor with the NACA blades, the drive of the wheel is run in motor as well as in generator mode. Additional measurements in a reference configuration without a spoke wheel allow the detailed analysis of changes in the flow pattern. 3-hole pneumatic probes, static pressure taps, as well as a 2D-Laser-Doppler-Velocimeter (LDV) are used to investigate velocity profiles and turbulent characteristics. Without the wakes generated by a spoke wheel, the annular diffuser (with a 20° half cone opening angle) separates at the shroud for all swirl configurations. Increasing the swirl results in increasing pressure recovery at the shroud whereas the hub boundary is destabilized. For a non-rotating spoke rotor and low swirl numbers, the 20° annular diffuser separates at the shroud. Increasing the swirl number, a strong deceleration of the axial velocity at the shroud is generated without separation and a higher pressure recovery is achieved. The boundary layer at the shroud of the 20° annular diffuser separates for all operating points with the bladed rotor. A partly stabilized 20° annular diffuser can only be achieved for much higher values of the flow coefficient than that for the design point. At this high mass flow, the NACA-bladed rotor operates as a turbine, resulting in the generator mode of the electric drive. Contrary to the numerical design calculations, the flow at the shroud of a 15° annular diffuser does not separate for all swirl configurations in the experiment. Pressure recovery of the 15° annular diffuser can be increased by increasing the inlet swirl whereas the hub boundary layer is destabilized. For the NACA bladed rotor, the flow in the 15° annular diffuser as well as the pressure recovery strongly depend on the flow coefficient. For flow coefficients lower than the design value, the flow partly separates at the shroud whereas large flow coefficients result in increased pressure recovery. The pressure recovery also depends on the direction of swirl and thus the swirl number.