The study of the interactions between a low pressure steam turbine and axial-radial diffuser

Abstract

Specific power output from a Low Pressure (LP) steam turbine can be enhanced by increasing the stage efficiency or raising its pressure ratio; both methods are interlinked and must be dealt with together. The latter is achieved by connecting to the exhaust diffuser; space and cost constraints often insist the use of an axial-radial diffuser with high levels of diffusion. The present study aims to investigate the interaction between the last-stage blade and the axial-radial diffuser, which can influence the diffuser performance and thus the total work output from the stage. This work is carried out using CFD simulations of a generic last stage low pressure (LP) turbine and axial-radial exhaust diffuser attached to it. In order to determine the validity of the computational method, the CFD predictions are first compared with data obtained from an experimental test facility. A computational study is then performed for different design configurations of the diffuser and rotor casing shapes. The study focuses on typical flow features such as effects of rotor tip leakage flows and subsequent changes in the rotor-diffuser interactions. The results suggest that the rotor casing shape and diffuser configurations influences the rotor work extraction capability and yields significant improvements in the static pressure recovery