Willinger, R. (1997). Beitrag zur aerodynamischen Wechselwirkung zwischen axialer Turbinenstufe und nachgeschaltetem Diffusor [Dissertation, Technische Universität Wien]. reposiTUm. https://resolver.obvsg.at/urn:nbn:at:at-ubtuw:1-12487
An investigation of the aerodynamic interaction between a last gas turbine stage and an exhaust diffuser is presented. Special attention is given to the influence of the rotor tip gap width on this interaction. A flow model accounts for the aerodynamic interaction of the two components. The model presented in this thesis is distinguished by a uniform set of parameters for the performance description of turbine stage and diffuser. Subsequently, flow measurements made downstream of a linear cascade of turbine blades with tip gap are presented. The geometry of the cascade corresponds to the tip section of a moving blade row of a gas turbine of fairly recent design. A five hole pressure probe was traversed immediately downstream of the trailing edge plane. The two primary results are the leakage loss and the underturning in the end wall region as a function of the gap width. The underturning of the gap flow increases with increasing gap width. This observation is in contrast to the common tip flow models which suppose an underturning independent of the gap width. The flow field downstream of the turbine cascade supplies the inlet boundary condition for the subsequent numerical investigation of the flow field in an annular diffuser. The Reynolds averaged Navier- Stokes equations were solved with a commercial finite-element based CFD- code. The recovery coefficient shows a slight increase with increasing gap width. The results from the cascade measurements and the diffuser computations were then coupled with the help of the interaction model presented. The investigation supplies the performance of the turbine stage-diffuser combination as a function of the rotor tip gap width. In the range of gap widths of practical interest, specific work and efficiency of the turbine stage-diffuser combination are independent of the rotor gap width. The investigation also supplies some conclusions concerning the preliminary design of annular diffusers behind axial turbine stages.