Shigeki Senoo

Numerical Simulations of Unsteady Wet Steam Flows With Non-Equilibrium Condensation in the Nozzle and the Steam Turbine

Numerical techniques for non-equilibrium condensing flows are presented. Conservation equations for homogeneous gas-liquid two-phase compressible flows are solved by using a finite volume method based on an approximate Riemann solver. The phase change consists of the homogeneous nucleation and growth of existing droplets. Nucleation is computed with the classical Volmer-Frenkel model, corrected for the influence of the droplet temperature being higher than the steam temperature due to latent heat release. For droplet growth, two types of heat transfer model between droplets and the surrounding steam are used: a free molecular flow model and a semi-empirical two-layer model which is deemed to be valid over a wide range of Knudsen number. The computed pressure distribution and Sauter mean droplet diameters in a convergent-divergent (Laval) nozzle are compared with experimental data. Both droplet growth models capture qualitatively the pressure increases due to sudden heat release by the non-equilibrium condensation. However the agreement between computed and experimental pressure distributions is better for the two-layer model. The droplet diameter calculated by this model also agrees well with the experimental value, whereas that predicted by the free molecular model is too small. Condensing flows in a steam turbine cascade are calculated at different Mach numbers and inlet superheat conditions and are compared with experiments. Static pressure traverses downstream from the blade and pressure distributions on the blade surface agree well with experimental results in all cases. Once again, droplet diameters computed with the two-layer model give best agreement with the experiments. Droplet sizes are found to vary across the blade pitch due to the significant variation in expansion rate. Flow patterns including oblique shock waves and condensation-induced pressure increases are also presented and are similar to those shown in the experimental Schlieren photographs. Finally, calculations are presented for periodically unsteady condensing flows in a low expansion rate, convergent-divergent (Laval) nozzle. Depending on the inlet stagnation subcooling, two types of self-excited oscillations appear: a symmetric mode at lower inlet subcooling and an asymmetric mode at higher subcooling. Plots of oscillation frequency versus inlet sub-cooling exhibit a hysteresis loop, in accord with observations made by other researchers for moist air flow. Copyright

Development of Design Method for Supersonic Turbine Aerofoils Near the Tip of Long Blades in Steam Turbines: Part 2 — Configuration Details and Validation

Both inflow and outflow velocities near the blade tip become supersonic when the blade length exceeds a threshold limit. The aerofoil near the tip of such a long blade has four features that demand an original supersonic turbine aerofoil design: supersonic flow in the entire field, high reaction, large stagger angle, and large pitch-to-chord ratio. This paper describes design method development for the supersonic turbine aerofoil.First, the aerofoil shape is defined using a curve with continuity in the gradient of the curvature. Second, six loss generation mechanisms are clarified by turbulent flow analysis. Third, an allowable design space between the pitch-to-chord ratio, the stagger angle and the axial-chord-to-pitch ratio is clarified by formulating three geometrical constraints to accelerate supersonic flow smoothly. When there is no solution in the theoretically allowable design space because of the large pitch-to-chord ratio, methods to reduce shock wave losses are proposed. Increasing the outlet metal angle of the pressure surface by around 10 deg from the theoretical outlet flow angle reduces the loss caused by the trailing shock wave. The physical mechanism for this is as follows: the increased outlet metal angle increases the outlet flow passage area so that the overexpansion is suppressed downstream from the flow passage. Fourth, both a cusped leading edge and an upstream pressure surface which has both an angle corresponding to the inflow angle and near-zero curvature can reduce the loss caused by the leading shock wave and satisfy the unique incidence relation. Finally, the aerodynamic performance of the supersonic turbine cascade and the design method are validated by supersonic cascade wind tunnel tests.Copyright

Unsteady Wet Steam Flow Measurements in a Low-Pressure Test Steam Turbine

Turbo Machinery Research Department, Research & Development Center, Mitsubishi Hitachi Power System, Ltd. 1-1, Saiwai-cho, 3-chome, Hitachi-city, Ibaraki, 317-0073 Japan, chongfei_duan@mhps.com, koji_ishibashi@mhps.com, shigeki1_senoo@mhps.com Laboratory for Energy Conversion, Department of Mechanical and Process Engineering, ETH Zurich IET, ML J 33, Sonneggstr, 3, CH-8092, Zurich, Switzerland bosdas@lec.mavt.ethz.ch, michel.mansour@lec.mavt.ethz.ch, rabhari@lec.mavt.ethz.ch Department of Mechanical Engineering, Aristotle University of Thessaloniki 54124, Thessaloniki, Greece anestis.kalfas@lec.mavt.ethz.ch

Nuclear Steam Turbine With 60 inch Last Stage Blade

Nuclear steam turbines can be classified into two categories, one for BWR reactors where some countermeasures are taken for radiated steam and water, the other is for PWR reactors and PHWR (CANDU) reactors where steam and water are not radiated. As for Low Pressure section, there is some difference in LP rotor end structure, and LP last three stage blade components can be applied to all reactor types. The trend in nuclear power equipment is in a direction of larger capacity. In response to this trend, longer last stage blade is required if same number of casing is kept to make nuclear turbines reasonably compact. This paper addresses some of the key developments and new technologies to be employed focusing on longer Last Stage Blade (LSB) development with Continuous Cover Blades (CCB), and other enhancements in product reliability and performance.Copyright

Unsteady Wet Steam Flow Field Measurements in the Last Stage of Low Pressure Steam Turbine

Modern steam turbines need to operate efficiently and safely over a wide range of operating conditions. This paper presents a unique unprecedented set of time-resolved steam flowfield measurements from the exit of the last two stages of a low pressure (LP) steam turbine under various volumetric massflow conditions.The measurements were performed in the steam turbine test facility in Hitachi city in Japan. A newly developed fast response probe equipped with a heated tip to operate in wet steam flows was used. The probe tip is heated through an active control system using a miniature high-power cartridge heater developed in-house.Three different operating points, including two reduced massflow conditions, are compared and a detailed analysis of the unsteady flow structures under various blade loads and wetness mass fractions is presented. The measurements show that at the exit of the second to last stage the flow field is highly three dimensional. The measurements also show that the secondary flow structures at the tip region (shroud leakage and tip passage vortices) are the predominant sources of unsteadiness at 85% span. The high massflow operating condition exhibits the highest level of periodical total pressure fluctuation compared to the reduced massflow conditions at the inlet of the last stage. In contrast at the exit of the last stage, the reduced massflow operating condition exhibits the largest aerodynamic losses near the tip. This is due to the onset of the ventilation process at the exit of the LP steam turbine. This phenomenon results in 3 times larger levels of relative total pressure unsteadiness at 93% span, compared to the high massflow condition. This implies that at low volumetric flow conditions the blades will be subjected to higher dynamic load fluctuations at the tip region.Copyright

Development of Design Method for Supersonic Turbine Aerofoils Near the Tip of Long Blades in Steam Turbines: Part 1—Overall Configuration

The purpose of this paper is development of the design method for supersonic turbine aerofoils. In particular, a design method is established for four fundamental parameters which determine the overall configuration of the aerofoils: inlet angle, outlet angle, pitch-to-chord ratio, and stagger angle.The developed design method is constructed as follows. Three parameters of a velocity triangle, the inlet flow angle, inflow Mach number and pressure ratio, are selected as predetermined design parameters. The inlet angle is coincident with the inlet flow angle. The outlet angle is formulated as a function of the three design parameters using aerodynamic theory. An allowable design space between the pitch-to-chord ratio and the stagger angle is clarified by formulating three geometrical constraints to accelerate supersonic flow smoothly. The three geometrical constraints are the inlet and outlet flow passage areas derived from the design parameters and the no-inflection-point condition on the aerofoil surface. Good performance of supersonic turbine aerofoils designed by the developed method is confirmed using computational fluid dynamics. There is no strong shock wave.When there is no solution in the theoretical allowable design space because of the large pitch-to-chord ratio required for low centrifugal stress, the following two methods enable the feasible design space to be enlarged without a large increase in the energy loss. One is to ease the restriction of the outlet flow passage area. The other is to increase the outlet flow angle of the pressure surface by about 10 deg in the axial direction from the theoretical angle. Their effectiveness is also validated by computational fluid dynamics.Copyright

Non-Equilibrium Homogeneously Condensing Flow Analyses as Design Tools for Steam Turbines

In order to get the details of flow fields in steam turbines, three-dimensional turbulent flow calculations are useful. However in a design procedure, three-dimensional flow calculations are only possible in the last design stage, because they need in-depth boundary conditions of both geometries and flows. At such a late time in the procedure, it is difficult to go back and change main design parameters, such as flow area and stage load. Both three-dimensional flow patterns and non-equilibrium condensation caused by rapid expansions of steam have important roles with respect to steam turbine performance particularly in low-pressure sections. The steam turbine internal efficiency can be improved by taking account of these effects in the early design stage, especially in flow pattern design. This paper describes a multi-stage through-flow calculation technique including both three-dimensional flow efffects and phase changes from vapour to small droplets. To compute the high-speed two phase steam flow, a flux-splitting procedure including non-equilibrium homogeneously condensation is introduced. Three-dimensional blade forces are calculated by using angles of both blade camber and radial lean. The blade camber lines can be decided without in-depth blade geometries. Therefore this computational technique is applicable in the flow pattern design. The calculation results agree well with fully three-dimensional flow calculation and the calculation can predict supersaturating states and Wilson lines which are defined as the maximum supercooling.© 2002 ASME

A Numerical Method for Turbulent Flows in Highly Staggered and Low Solidity Supersonic Turbine Cascades

Two main problems are associated with conventional numerical methods for simulating turbulent flows in high-reaction-type supersonic turbine cascades near the tip of the last stage blade in a steam turbine: the large skewness of computational grids and treatments of boundary conditions when the shock waves hit boundaries. This paper presents a numerical method to deal with these issues. A grid generation technique which uses five-block structured grids has been developed. The orthogonality of the grid is good even for highly staggered and low solidity cascades. In addition, the grids are completely continuous at the boundary between the blocks and at the periodic boundaries. Both the gradient of the grid lines and the change rate of the grid widths connected smoothly. As a result, shock waves can be captured accurately and stably. The inflow and outflow boundary conditions based on the two-dimensional characteristic theory have been applied and diminished the spurious reflections and fluctuations of shock waves at both the inlet and outlet boundaries. Therefore the non-physical reflection does not affect the flow in the cascades. A low Reynolds number k-e turbulent model has been proposed. Distance from a wall is not used as the characteristic length of turbulent flows so that the turbulent model can be applied to a wake and a separation flow. The validity of the numerical method was verified by comparisons of the pressure distributions on the blade, the loss coefficients, and flow angles with linear cascade experiments of transonic compressor cascades.Copyright

Unsteady Flow Field and Coarse Droplet Measurements in the Last Stage of a Low Pressure Steam Turbine With Supersonic Airfoils Near the Blade Tip

The largest share of electricity production worldwide belongs to steam turbines. However, the increase of renewable energy production has led steam turbines to operate under part load conditions and increase in size. As a consequence long rotor blades will generate a relative supersonic flow field at the inlet of the last rotor. This paper presents a unique experiment work that focuses at the top 30% of stator exit in the last stage of an LP steam turbine test facility with coarse droplets and high wetness mass fraction under different operating conditions. The measurements were performed with two novel fast response probes. A fast response probe for three dimensional flow field wet steam measurements and an optical backscatter probe for coarse water droplet measurements ranging from 30 up to 110μm in diameter. This study has shown that the attached bow shock at the rotor leading edge is the main source of inter blade row interactions between the stator and rotor of the last stage. In addition, the measurements showed that coarse droplets are present in the entire stator pitch with larger droplets located at the vicinity of the stator’s suction side. Unsteady droplet measurements showed that the coarse water droplets are modulated with the downstream rotor blade-passing period. This set of time-resolved data will be used for in-house CFD code development and validation.© 2016 ASME

The Effects of the Tangential Leans for the Last Stage Nozzles of Steam Turbine

In general, since the aspect ratio defined by the ratio of blade height and blade cord length of the last stage blades (LSBs) of a low pressure steam turbine is considerably larger compared to those of other stages, there is anxiety concern about the possibility of flow separation in the vicinity of the root section of the blades. In addition to the above fact, possible efficiency deterioration due to supersonic inflow resulting from the usage of longer blades has to be avoided to obtain an efficient steam turbine. In this paper, the effects of various types of leans for the last stage nozzles have been studied with the help of 3D-CFD. Then, several characteristics charts related to the stage performance for each type of lean including the s-shaped lean were created. By applying well-designed s-shaped lean nozzles, the undesirable effect of supersonic inflow could be significantly mitigated, giving a fair improvement in stage efficiency.Copyright