Tomohiko Tsukuda

Aerodynamic and Structural Numerical Investigation of Unsteady Flow Effects on Last Stage Blades

The aim of this paper is to present some of research results of our current collaborative program to increase steam turbine efficiency with the development of high-performance blade and exhaust hood design methodology using large-scale aerodynamic and structural interaction analysis. Aerodynamic optimum designs of stator blades are already introduced in many designs of actual operating commercial steam turbine units. However, aerodynamic optimum designs of rotating blades are still difficult due to high centrifugal force and vibration stress on rotating blades. This paper focuses on rotating blades and exhaust diffusers that affect the flow field just downstream of last stage long blades. The large-scale high-accuracy CFD analysis of unsteady wet steam flows has been successfully introduced for simulations of low pressure exhaust diffuser using the Earth Simulator of Japan Agency for Marine-Earth Science and Technology. This result shows that the diffuser domain analysis can provide static pressure recovery coefficients and its circumferential deviations with enough accuracy for design use except correct location predictions of separations. The unsteady flow analyses of the typical designed last stage with the measured and calculated downstream static pressure distribution as the outlet boundary condition were conducted. The unsteady flow analyses of the typical designed low pressure exhaust diffuser with the measured and calculated upstream flow conditions as the inlet boundary condition were also conducted. Some of the calculated results were compared with measured data. The large-scale parallel computing Finite Element Analysis of turbine blades with inter-connection parts has been also successfully introduced on the Earth Simulator. The calculation result shows that the eigen frequencies of the present group of loosely-connected rotating blades correspond well to the existing measured data. For the next step, the unsteady structural analysis is being conducted with the calculated unsteady forces on the rotating blades as the FEA boundary conditions. Some of the FEA results are also presented in this paper.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

Influence of Wetness on Efficiency of the Full Scale Size Low Pressure Turbines

Efficiencies of 60Hz full size test turbines were measured in various wet steam conditions to reveal the wetness impact on the performance. We changed the wetness and stage load conditions independently under the condition of constant steam mass flow rate in the low pressure turbine. The test results told that the stage efficiency decreases with the increasing of wetness as many studies showed, furthermore, the stage efficiency decreases more in smaller load conditions than in the design point. In addition, blade length effects were examined by comparing two types of LP turbine to be found that the longer case got more deficits at the same wetness. Some theoretical evaluations were tried and a combination of some simple loss models explained the tendencies above, qualitatively. The evaluation showed that absolute value of mechanical wet loss such as braking loss remained unchanged regardless of load conditions, so in low load condition, ratio of mechanical loss to stage load increased, resulting decrease of stage efficiency. It also showed that increasing wet loss at the longer blade was mainly because higher circumferential velocity caused larger mechanical wet loss such as braking loss.Copyright

An Experimental Investigation of Thermal Wetness Loss in the Full Scale Size Low Pressure Turbine

Experimental investigation with a full scale low pressure steam turbine is carried out to reveal the thermal wetness loss. This paper focuses on thermal wetness loss of the last stage in a six-stage low pressure turbine. The temperature of superheating at the inlet of the last stage varies within a range of several tens of degrees under the condition that enthalpy at the stage outlet is below the saturation point. Radial distributions of pressure and temperature at the stage inlet and outlet are measured with rake probes and traverse probes, whereas the stage outlet enthalpy is identified by using the generator output and turbine mass flow rate. Higher stage efficiency is obtained the superheating inlet temperature becomes higher in this experimental condition. A three-dimensional CFD taking into consideration a non-equilibrium/equilibrium condensation model is carried out to understand the experimental results. A non-equilibrium condensation model can take into account the thermal wetness loss associated with supercooling and non-equilibrium condensation. The amount of thermal wetness loss is evaluated by comparing the results of non-equilibrium and equilibrium condensation models. The results show that the degree of superheating at the inlet of the stage affects the supercooling temperature distribution in the last stage flow path, resulting in lower thermal wetness loss at a higher inlet superheat condition.Copyright

An Experimental Flow Investigation of Low Pressure Turbine Stages With Various Wet Conditions

Detail flow characteristics of an actual size low pressure steam turbine stages under real operating conditions were examined in this paper. The main purpose of the experimental work was to obtain the radial distribution of the velocity triangles in the wet flow of the large size turbine, so that a series of tests were carried out with various wet conditions. Some particular changes of the flow pattern were observed at the exit of both the stator and the blade rows which would not predicted by steam turbine design tools.A kind of flow coefficient was defined and investigated as well as the steam velocities. With an assumption of the steam wetness distribution along the blade span, a tendency of the flow coefficient was appeared which was similar to previous work [7]. The wetness assumption was qualitatively verified by bore-scope observation of the water concentration on the stator surface and the fog condition of the steam path.Copyright