Numerical Investigation on Flow Characteristics of Low Pressure Exhaust Hood Under Off-Design Conditions for Steam Turbines

Abstract

In this paper, numerical investigations on the aerodynamic characteristics of the last three stages and the exhaust hood for a large power steam turbine were conducted under a series of mass flow conditions (100% ∼ 10% of the design condition) using the commercial CFD software ANSYS-CFX. The single passages of the last three stages and the whole exhaust hood are combined together as the computational domain. The main objective of this present work is to analyze the aerodynamic performance and the flow behavior of the exhaust hood. The variations of the static pressure recovery coefficient and the total pressure loss coefficient while the mass flow rate decreasing were analyzed. The static pressure distributions along the diffuser surface under different flow conditions were illustrated. The development of the vortex near the outlet of the diffuser was demonstrated through the velocity vector distribution at the meridional plane of the exhaust hood. The windage conditions were analyzed under 20% and 10% mass flow rate of the design condition. In addition, the back flow phenomenon was observed when the mass flow rate was below 50% of the design condition, and it starts from the hub region of the last stage rotor and grows up along the radial direction. The back flow also induces a sharp turning on the span-wise distribution of the angle θ (defined in Fig. 9) at the outlet of the last stage rotor. The three-dimensional streamlines inside the exhaust hood under different mass flow conditions were also compared.