Kiyoshi Segawa

A High Performance Optimized Reaction Blade for High Pressure Steam Turbines

A higher efficiency gain is necessary for steam turbine plants to reduce their fuel consumption rate and lessen their environmental disruption factor. Power plant manufacturers have continued to make an effort to raise steam turbine internal efficiency by developing new technologies. High pressure (HP) steam turbines should have increased efficiency owing to relatively shorter blade height compared with other turbine sections (intermediate and low pressure turbines). In order to increase efficiency, it is important to improve the steam path determined by design parameters such as degree of reaction, number of stages and rotor diameter and to develop a high performance blade applied to it. The advanced computational fluid dynamics (CFD) technique is a useful design tool, and has come to be applied generally to evaluate energy loss. A new rotating blade has been developed for small and mid-class steam turbines with a shorter blade height. The robust design method, based on the statistical theory for design of experiments, is used for the blade root profile design. It is combined with the inverse method and 2-D turbulent blade-to-blade flow analysis to evaluate the aerodynamic performance. The blade configuration is expressed by four control factors, which are turning angle, leading edge radius, pitch-chord ratio and maximum blade loading location. Linear cascade experiments are also carried out due to verify the blade performance under the optimized conditions obtained by the robust design. Consequently, the blade section has a blunt-nose, flat incidence characteristics and low energy loss, compared with the conventional one and the optimized conditions given by the robust design are aerodynamically reasonable. Finally, air turbine model tests and 3-D Reynolds-averaged Navier-Stokes analyses are performed to investigate the detailed flow pattern and stage performance of the new optimized reaction blade. An experimental investigation is still important to evaluate the performance in the real turbine stage structure, while the numerical analysis method is used based on the implicit TVD scheme with the modified k-e turbulence model. It is found that the new optimized reaction blade has greatly improved stage efficiency of about 1.5% at the design point including the effect of leakage flow (3% improvement in stage efficiency excluding leakage flow) and realized an increase of pitch-chord ratio by about 35%. Consequently, the new optimized reaction blade is considered effective to raise the internal efficiency of the high-pressure steam turbine with improved steam path.Copyright

Computations for Unsteady Compressible Flows in a Multi-Stage Steam Turbine With Steam Properties at Low Load Operations

A computational technique for compressive fluid in multistage steam turbines which can allow for thermodynamic properties of steam is presented. The understanding and prediction of flow field not only at design conditions but also at off-design conditions are important for realizing high-performance and high-reliability steam turbines. Computational fluid dynamics is useful for estimations of flows. However, current three-dimensional multi-stage calculations for unsteady flows have two main problems. One is the long computation time and the other is how to include the thermodynamic properties of steam. Properties of the ideal gas, such as equations of state and enthalpy formula, are assumed in most computational techniques for compressible flows. In order to shorten the computation time, a quasi-three-dimensional flow calculation technique is developed. In the analysis, system equations of conservation laws for compressible fluid in axisymmetric cylindrical coordinates are solved by using a finite volume method based on an approximate Riemann solver. Blade forces are calculated from the camber and lean angles of blades using momentum equations. The axisymmetric assumption and the blade force model enable the effective calculation for multi-stage flows, even when the flow is strongly unsteady under off-design conditions. In order to take into account steam properties including effects of the gas-liquid phase change and two-phase flow, a flux-splitting procedure of compressible flow is generalized for real fluid. Density and internal energy per unit volume are selected as independent thermodynamic variables. Pressure and temperature in a superheated region or wetness mass fraction in a wet region are calculated by using a steam table. To improve computational efficiency, a discretized steam table matrix is made in which the density and specific internal energy are independent variables. For accuracy and continuity of steam properties, the second order Taylor expansion and linear interpolation are introduced. The computed results of last four-stage low-pressure steam turbine at low load conditions show that there is a reverse flow near the hub region of the last (fourth stage bucket and the flow concentrates in the tip region due to the centrifugal force. At a very low load condition, the reverse flow region extends to the former (i.e. the first to third) stages and the unsteadiness of flow gets larger due to many vortices. Four-stage low pressure steam turbine tests are also carried out at low load or even zero load. The radial distributions of flow direction downstream from each stage are measured by traversing pneumatic probes. Additionally pressure transducers are installed in the side wall to measure the unsteady pressure. The regions of reverse flow are compared between computations and experiments at different load conditions, and their agreement is good. Further, the computation can follow the trends of standard deviation of unsteady pressure on the wall to volumetric flow rate of experiments. The validity of the analysis method is verified.Copyright