Investigation Of Unsteady Interaction Between Axial Turbine And Non-axisymmetric Exhaust System

Exhaust system in condensing steam turbines is used to recover leaving kinetic energy of the last stage turbine, while guiding the flow from turbine to condenser. During operation, strong flow interactions between the turbine and the exhaust system have a strong impact on the aerodynamic performance of turbine and exhaust, as well as on the blade safety. The present aerodynamic design and analysis system of the exhaust system and turbine stage should be improved to include such an impact.In this dissertation, the investigation on the mechanisms of the unsteady interactions between axial turbine and non-axisymmetric exhaust system was carried out. An experimental rig including a singe-stage turbine and an exhaust model was built up. By means of the data acquisition system based on the PXI technology and virtual instrument, the flow field in the coupled singe-stage turbine and exhaust model configuration was measured under three operational conditions. Numerical simulations for the coupled turbine and exhaust configuration were also performed. Based on the experimental and numerical results. the relationship between the non-axisymmetric unsteady flow in the exhaust system and the unsteady force on the turbine blade was explored. The influential mechanisms of inflow conditions and geometrical configuration of the exhaust on the flows and aerodynamic performance in the exhaust system were discussed. The performances of different exhaust configurations were compared and the methods to reduce the circumferential non-uniformity of the flow in the exhaust were brought forward. The main conclusions of the dissertation are as follows:The blade force varies circumferentially according to the non-axisymmetric distribution in static pressure at the turbine exit plane, which is set up by the exhaust system. The non-axisymmetric distribution in static pressure causes a low frequency fluctuation in the aerodynamic force of rotor blade. The results of experiment and simulation show that the influence of the non-axisymmetric flow field in the exhaust system on the unsteady blade force is much stronger than that of stator and rotor interaction. With the flow coefficient increasing, the amplitude of the high frequency fluctuation in the aerodynamic force decreases and the amplitude of the low frequency fluctuation increases for the stator blade, and the amplitude of the high frequency fluctuation increases and the amplitude of the low frequency fluctuation decreases for the rotor blade. The flow separation in the diffuser is the main cause for the flow unsteadiness in the exhaust. The frequency of the unsteady fluctuation in the exhaust is low and the corresponding amplitude is small. The unsteady flow in the exhaust also impacts on the flow in the turbine stage, e.g., the unsteady aerodynamic force on the blade has the same low frequence fluctuation as the flow in the exhaust.Meridional velocity at the hub of diffuser inlet reduces when the swirl angle increases or the total pressure decreases, which trends to result in flow separation in the diffuser. The vortices and flow mixing in the exhaust form the main part of the loss in the exhaust. The tip leakage flow of the rotor reduces the circumferential non-uniformity in the flow at the tip of diffuser inlet. The tip leakage flow turns stronger when the flow coefficient of turbine increases.Keeping the volute of exhaust system unchanged, reducing the axial length of the diffuser locating atθ=0°or adjusting the flow path in the hood by imposing an appropriate tangential flow angle distribution at diffuser inlet can improve the exhaust performance and reduce the flow non-uniformity.