The Role of Rotor Tip Clearance on the Aerodynamic Interaction of a Last Gas Turbine Stage and an Exhaust Diffuser

Abstract

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 on this interaction. Flow measurements downstream of a linear cascade of turbine blades with tip gap have been performed in a low speed cascade wind tunnel. The geometry of the cascade corresponds to the tip section of a gas turbine of fairly recent design. Five tip gaps lying in the typical range were investigated. The two essential results are the leakage loss and the underturning in the end wall region. The flow field of the turbine cascade supplied the inlet boundary condition for the subsequent numerical investigation of the flow field in an annular diffuser. The geometry of the annular diffuser is based on dimensions of exhaust diffusers of some heavy duty and aeroderivative gas turbines. The result of the investigation is the diffuser pressure recovery factor as a function of the gap of the upstream cascade. The results from the cascade measurements and the diffuser computations have then been coupled by means of an interaction model. For gaps of practical interest, specific work and efficiency of the last gas turbine stage followed by an exhaust diffuser are independent of the rotor gap.