Experimental Investigation of Velocity Gust in High Pressure Turbines

Sammanfattning: This thesis investigates the unsteady three-dimensional gustfield in high subsonic and transonic turbine stages based onexperimental velocity data. The generation, propagation andinteraction mechanisms of the velocity gust in high pressureturbines are studied. Two approaches are followed: In aparametric study the tendencies of the velocity gust dependenton a number of stage parameters are examined. Secondly, theunsteady velocity flow field of one stage operating point iscarefully studied. This aims to receive a detailed picture ofvortical and potential velocity gust components, with andwithout shock contributions, and their interaction with therotor flow field.A three-dimensional Laser-two-Focus anemometer has been usedto measure the periodic three-dimensional velocity andturbulence intensity in subsonic and transonic turbine flow.The application of the measurement technique to the wind tunnelfor rotating annular cascades and the data evaluation aredescribed.For the parametric study the stator vane count, the axialgap between stator and rotor, the rotor turning frequency andthe flow regime subsonic vs. transonic have been changed. Thestator induced wake and shock propagate in different downstreamdirections. Dependent on the axial gap between stator and rotorthe rotor blades experience either two separated velocitydistortions or one superposed and re-enforced velocitydistortion. Accordingly, the computed blade surface pressureperturbations (not part of this thesis) show a differentweighting of the first andhigher harmonic amplitudes. At therotor exit the gust and the non time-resolved unsteadiness areclosely related to the stage loss. Highest unsteadiness andloss occur for strong negative off-design incidence, possiblydue to boundary layer separation on the rear blade suctionside. Lowest unsteadiness and loss occur not at design inletangle, but at strong positive off design. Data from stator onlyconfigurations represent the time-averaged rotor inlet flowfairly well. It might be used for simple gust computations tocalculate the unsteady rotor aerodynamics.The detailed flow field study shows the distribution of thegust components and the Mach number perturbation due to thestator induced wake and trailing edge shock. The computedunsteady pressure on the front blade suction side, the forcingfunction, is mainly affected by the pressure perturbations fromthe stator trailing shock, i.e. potential gust. This shock hasonly a minor influence on the velocity field. Furtherdownstream in the rotor passage the vortical gust near and theforcing function on the suction surface can be related to eachother. Closer to the casing and at lower Mach numbers increasedeffects by the stator wake and secondary vortices, resp.decreased stator shock effects are observed. The gust is alsosplit up into potential and vortical components using atwo-dimensional incompressible analysis using harmonicallydecomposed gust data. No reasonable results are obtained forthe potential gust component. This is attributed to the missingconsideration of compressibility effects and the pressure gust,especially at stator exit Mach numbers close to one.Qualitative agreement to the time-resolved analysis is foundfor the vortical gust distribution in the rear rotorpassage.A three-dimensional gust structure was examined, theinteraction of the stator wake with the relative eddy. Thegeneration of turbulence intensity due to the chopping ofstator wake and secondary vortices by the blades is described.Finally, stator trailing edge shocks and their time-dependentrelative strength to each other can be identified by the spaceand time-resolved turbulence intensity.Recommendations for future work are given.Keywords:turbomachinery, unsteady aerodynamics, gust,forcing function, forced response, transonic flow, annularcascade, Laser-Two-Focus Anemometer, experiments