Increasing the annulus area of the last stage in steam turbines provides an effective way of increasing power output and improving the efficiency by reducing the amount of lost kinetic energy. The 50-inch and 60-inch last stage blades, which provide some of the world’s largest exhaust annulus areas, have been developed for 3600 rpm and 3000 rpm units respectively. The blades are designed to suppress the increase in losses caused by the supersonic inflow, and to ensure reliability. This paper describes the verification of the aerodynamic performance through four-stage model steam turbine tests. In addition, the unsteady flow phenomena caused by shock wave interactions near the tip between the last stage stator and rotor are clarified by unsteady flow measurements and calculations.
The characteristic of total-to-static efficiencies and flow distributions measured with pneumatic five-hole probes are obtained as designed. Static pressure recovery is also confirmed by static pressure measurement on the exhaust hood walls and three-dimensional turbulent flow analysis.
Regarding the unsteady stator-rotor interactions, the flow fluctuations such as static pressures and flow angles become relatively larger, when the travelling bow shock waves emanating from upstream of the rotor leading edge impinge near the stator trailing edge, because the axial distance between the emanating and impinging position of the shock wave is smaller. The static pressure on the suction surface of the rotor blade becomes locally large when it comes to the same circumferential portion as the stator trailing edge. However, the mass flow rate and loss variations caused by the unsteadiness are small, because the inlet relative Mach number is as small as 1.25, so that the strength of the shock wave is small.