Yoshiki Niizeki

Numerical Investigation of Steam Turbine Exhaust Diffuser Flows and Their Three Dimensional Interaction Effects on Last Stage Efficiencies

The purpose of this paper is to present the methodology for high accurate aerodynamic numerical analysis and its design application of steam turbine down-flow type exhaust diffusers including their three dimensional flow interaction effects on last stage efficiencies.Down-flow type exhaust diffusers are used in large scale steam turbines from 200MW to 1400MW class units for power generation plants mainly. The axial length of typical 1000MW class large scale steam turbines is about 30–40m and its four low pressure (LP) down-flow type exhaust diffusers occupy a large amount of space. The axial lengths and diameters of these exhaust diffusers contribute significantly to the size, weight, cost, and efficiency of the turbine system. The aerodynamic loss of exhaust hoods is nearly the same as that of stator and rotor blading in LP steam turbines, and there remains scope for further enhancement of steam turbine efficiency by improving the design of LP exhaust hoods.In the design process of last stages, the average static pressure in the last stage exit is introduced accurately using numerical analysis and experimental data of model steam turbines and model diffusers. However the radial and circumferential unsteady aerodynamic interaction effects between last stages and their exhaust diffusers are still need to be investigated to increase the accuracy of the interaction effect on the last stage efficiencies.This paper presents numerical investigation of three dimensional wet steam flows including three dimensional flow interaction effects on last stage efficiencies in a down-flow type exhaust diffuser with non-uniform inlet flow from a typical last stage with long transonic blades designed with recent aerodynamic and mechanical design technology.Copyright

Aerodynamic Interaction Effects From Upstream and Downstream on the Down-Flow Type Exhaust Diffuser Performance in a Low Pressure Steam Turbine

The purpose of this paper is to explain aerodynamic interaction effects from upstream and downstream on the down-flow type exhaust diffuser performance in a low pressure steam turbine. To increase exhaust diffuser performance, design data related to the aerodynamic interaction effects from upstream turbine stages and downstream exhaust hood geometry on the exhaust diffuser performance would be very useful. This paper presents numerical investigation of three dimensional wet steam flows in a down-flow type exhaust diffuser with non-uniform inlet flow from a typical last stage with long transonic blades designed with recent aerodynamic and mechanical design technology. Previous studies show that small scale model tests and CFD analyses of exhaust diffusers with uniform inlet flow conditions are not enough to investigate diffuser efficiency and detail diffuser flow mechanism because inlet flow structures including tip leakage flows and blade wakes superimposed from a last stage and several other upstream turbine stages in low pressure turbines affect flow separations that reduce the exhaust diffuser performance. Recent studies by the authors show that the introduction of radial distributions of velocities and flow angles at the inlet section of exhaust diffuser measured in a full scale development steam turbine increased the accuracy of numerical analysis of diffuser flow. In the present study, the computational domain was enhanced and the method of boundary condition definition was improved to increase the accuracy of boundary layer separation and separation vortex generation in wet steam flows. Using the improved method, the calculation results explained the aerodynamic interaction effects from upstream and downstream on the down-flow type exhaust diffuser performance.Copyright

A High-Order LES Turbulent Model to Study Unsteady Flow Characteristics in a High Pressure Turbine Cascade

In this work, unsteady viscous flow analysis around turbine blade cascade using a High-Order LES turbulent model is carried out to investigate basic physical process involved in the pressure loss mechanism. This numerical analysis is assessed to the wind tunnel cascade test. Basically, all the physical phenomena occurring in nature are the effect of some cause, and the effect can somehow be measured. However, to understand the cause, detail information regarding the visualization of the phenomena, which are difficult to measure, are necessary. Therefore, in our work, firstly the computed results are compared with the measured data, which are the final outcome of the cause (of the phenomena under investigation), to verify whether our physics-based model could qualitatively predict the measured facts or not. It was found that the present model could well predict measured data. Therefore, the rest of the computed information, which were difficult to measure, were used to visualize the overall flow behavior for acquiring some knowledge of the physical process associated with the pressure loss mechanism. Our study led to an understanding that the interaction of the vortex generated on the suction and pressure surface of the blade and the secondary vortex generated on the end-wall, downstream the trailing edge resulted in the formation of a large vortex structure in this region. This unsteady three-dimensional flow characteristic is expected to play an important role in the pressure loss mechanism.Copyright

Influence of Surface Roughness on Turbine Nozzle Profile Loss and Secondary Loss

The effects of surface roughness of both nozzle and end-wall on a turbine nozzle performance were investigated experimentally using liner cascade wind tunnel facility under the Reynolds number (Re) condition of Re = 0.3∼1.0 × 106 . With buffing, milling, sand blasting and shot blasting, the total of seven levels of the model surface roughness were realized. In order to clarify the effect of the nozzle surface roughness on the profile loss, total pressure losses were measured using three-hole probe for different levels of the surface roughness. It became clear the nozzle profile loss increases as Reynolds number increases for larger roughness group. In addition, it appeared the profile loss depends on not only maximum value of the surface roughness but also roughness conditions. In order to examine the effect of surface roughness on the secondary flow loss, spatial total pressure field of the secondary flow region was measured using three-hole probe for the cases of smooth or rough nozzle surface with smooth or rough end-wall. The secondary flow structures were recognized at the 5∼10% span-wise height region of the suction surface of the nozzle for all cases. With increasing the nozzle surface roughness, not only the profile loss but also net secondary flow loss increases, which is defined as the difference between the total pressure loss and the profile loss in the secondary flow region. However, increase of the end-wall roughness has higher effect on the net secondary flow loss increase. Difference of the effect between the nozzle surface roughness and the end-wall roughness on the nozzle secondary flow loss was discussed.Copyright

Development of a floating current turbine

Ocean current energy is one of promising power resource for Japan, since Kurosio Current, which pass through near the Japanese coast, is one of strongest ocean current in the world. We formed a consortium and started to study about a floating type current turbine system last year. A floating type concept with the twin-turbine is proposed as a minimum weight ocean current generator. The float is moored by a single mooring line and the tension force is balanced with the thrust force and the buoyancy of the turbine. This system is supposed to have a weathervane function and the depth is controllable by adjusting the buoyancy and the blade pitch. A conceptual design, turbine design, current measurement at off Izu Island and a computer code to simulate the state of whole system are shown in this paper.

Numerical Investigation of Three-Dimensional Wet Steam Flows in an Exhaust Diffuser With Non-Uniform Inlet Flows From the Turbine Stages in a Steam Turbine

The purpose of this paper is to present a numerical evaluation method for the aerodynamic design and development of high-efficiency exhaust diffusers in steam turbines, as well as to present the comparison between the numerical results and measured data in an actual real scale development steam turbine. This paper presents numerical investigation of three-dimensional wet steam flows in a down-flow-type exhaust diffuser that has non-uniform inlet flows from a typical last turbine stage. This stage has long transonic blades designed using recent aerodynamic and mechanical design technologies, including superimposed leakages and blade wakes from several upstream low pressure turbine stages. The present numerical flow analysis showed detail three-dimensional flow structures considering circumferential flow distributions caused by the down-flow exhaust hood geometry and the swirl velocity component from the last stage blades, including flow separations in the exhaust diffuser. The results were compared with experimental data measured in an actual development steam turbine. Consequently, the proposed aerodynamic evaluation method was proved to be sufficiently accurate for steam turbine exhaust diffuser aerodynamic designs.Copyright

An Upwind Eulerian-Eulerian Model for Non-Equilibrium Condensation in Steam Turbines

The present paper proposes an Eulerian-Eulerian two-phase model for non-equilibrium condensing flow in steam turbines. This model is especially suitable for upwind finite volume scheme. An approximate Roe type flux using real water/vapor property is constructed to calculate the upwind wet-steam flux. This flux fully couples the wetness fraction with other conservative variables in the Jacobian Matrix whose eigen-vector and eigen-value are analitically derived. A novel treatment of real wet-steam property is developed by constructing a 3-DOFs TTSE table according to IAPWS97 formulas. The table is actually a cubic and uses the mixture’s density, the mixture’s internal energy and wetness as independent variables. Besides homogeneous condensation, heterogeneous condensing is also integrated into the model, which facilitates simulating the effect of salt impurities. The above methods are validated through two nozzle and one turbine cascade calculations and finally applied to a model LP steam turbine stage. Results show that the current model is very robust and is able to correctly capture the non-equilibrium condensation phenomena.Copyright

Three Dimensional Optimum Design of a Steam Turbine Stage With NURBS Curves

Progress in the computer performance has enabled automatic optimization of the three dimensional shape of turbine blades with a large number of large-scale CFD (Computational Fluid Dynamics) calculations. This paper presents an advanced aerodynamic optimization system for turbine blades. The system can automatically find improved blade shapes that give better aerodynamic performance in a turbine stage and reduces human efforts to generate blade shape data, computational mesh, CFD input data etc. The system consists of three parts; parameter updating part, blade shape generation part, and aerodynamic performance evaluation part. In the parameter updating part, users can choose DOE (Design of Experiment) or evolutionary optimization method such as GA (Genetic Algorithm), ASA (Adaptive Simulated Annealing), etc. to define the parameters in each step. The shape generation part changes the blade shapes using NURBS curves whose control point parameters are defined in the parameter updating part. Three-dimensional CFD grid is automatically generated for the changed blade shapes and steady CFD calculation is used to evaluate the aerodynamic performance of the changed blades in a turbine stage. Stagger angle distribution in the radial direction was thought as one of the important design parameters of turbine blades because it determines the flow pattern in radial direction. Then it was chosen as an optimized parameter with NURBS curves in this system. First, DOE was used for the human optimization, in which the parameter range for the advanced optimization was estimated and the best shape obtained was used as the initial shape for the evolutionary optimization to explore better blade shape parameters. Stage loss of an exhaust stage of IP (Intermediate Pressure) turbine which contains relatively high aspect ratio was chosen as the objective. In spite that such kind of stage was considered not to be sensitive to three dimensional stacking, the results showed good performance enhancement.Copyright

Numerical Investigation of Exhaust Diffuser Performances in Low Pressure Turbine Casings

Low pressure (LP) exhaust hoods are an important component of steam turbines. The aerodynamic loss of LP exhaust hoods is almost the same as those of the stator and rotor blading in LP steam turbines. Designing high performance LP exhaust hoods should lead further enhancement of steam turbine efficiency. This paper presents the results of exhaust hood computational fluid dynamics (CFD) analyses using last stage exit velocity distributions measured in a full-scale development steam turbine as the inlet boundary condition to improve the accuracy of the CFD analysis. One of the main difficulties in predicting the aerodynamic performance of the exhaust hoods is the unsteady boundary layer separation of exhaust hood diffusers. A highly accurate unsteady numerical analysis is introduced in order to simulate the diffuser flows in LP exhaust hoods. Compressible Navier-Stokes equations and mathematical models for nonequilibrium condensation are solved using the high-order high-resolution finite-difference method based on the fourth-order compact MUSCL TVD scheme, Roe’s approximate Riemann solver, and the LU-SGS scheme. The SST turbulence model is also solved for evaluating the eddy viscosity. The computational results were validated using the measurement data, and the present CFD method was proven to be suitable as a useful tool for determining optimum three-dimensional designs of LP turbine exhaust diffusers.© 2011 ASME

Unsteady Viscous Flow Simulation around Turbine Blade

In this study, 3-D unsteady viscous flow analysis around turbine blade cascade is carried out to investigate basic physical process involved in the pressure loss mechanism. In this regard, the strategy of the present study is first to compare the predicted results with the experimentally measurable data to check the prediction ability of numerical method. As a consequence of that the computed results agree with the experimental results. Therefore computed results could be used for visualization of the overall flow behavior to gather knowledge about what physical phenomena are associated with the mechanism of pressure loss. Because all the experimentally results compared so far with the computed results are the final outcome of the cause. From computed results, it turned that structure of vorticity from suction side and pressure side of turbine blade is a factor of pressure loss mechanism.