The purpose of the ''International Wet Steam Modeling Project'' is to review the ability of computational methods to predict condensing steam flows. The results of numerous wet-steam methods are compared with each other and with experimental data for several nozzle test cases. The spread of computed results is quite noticeable and the present paper endeavours to explain some of the reasons for this. Generally, however, the results confirm that reasonable agreement with experiment is obtained by using classical homogeneous nucleation theory corrected for non-isothermal effects, combined with Young's droplet growth model. Some calibration of the latter is however required. The equation of state is also shown to have a significant impact on the location of the Wilson point, thus adding to the uncertainty surrounding the condensation theory. With respect to the validation of wet-steam models it is shown that some of the commonly used nozzle test cases have design deficiencies which are particularly apparent in the context of two-and three-dimensional computations. In particular, it is difficult to separate out condensation phenomena from boundary layer effects unless the nozzle geometry is carefully designed to provide near-one-dimensional flow.
The wide use of fir-tree root and groove in turbine structures prompts the expectation to find optimum configurations, which ensure that stresses are in the safe limits to avoid mechanical failure. To perform the optimization, the reasonable characterization of root configuration is required. The existing researches characterized the fir-tree root with straight line, arc or even elliptic fillet, then the parameters of these features were defined as design variables to perform root profile optimization. However, this feature-based optimization technique yields configuration which is only optimal under the feature assumption, the question why choose these feature and whether there is a better feature modeling technique is difficult to answer. In this work, instead of the feature-based method, spline curves technique is involved to characterize the root and groove configuration, and the horizontal coordinates of the control points are selected as design variables, which are modified in the vicinity of their initial values during optimization process. The objective function is to minimize the peak stress in the root and groove regions. With the Multi-island genetic algorithm, the optimal fir-tree root configuration can be obtained with better stress distributions and low stress concentrations. The proposed spline-based optimization approach may shed lights on the conceptual design of blade root and can be easily extended to other industrial equipment design.
Experimental tests were implemented on a wet steam test rig to investigate the effects of location, shape and width of a suction slot on the water removal performance of a hollow stator blade. A straight cascade with varying outlet Mach numbers and suction pressure differences was used for the tests. The inlet flow conditions were consistent with the real running condition before the last stage stator of a 1000-MW nuclear steam turbine. Results show that the flow Mach number and suction pressure difference affect the amount of water removed. A moderate increase in the suction pressure difference triggers water film vaporisation, which decreases water removal performance. The amount of water removed continuously increases, as the slot location moves from 0.24 to 0.42 times the axial chord in the suction surface. Compared with the straight slot, the step-shaped slot cannot improve the water removal performance. On the contrary, the result is poor when the Mach number is above 0.7 because additional sharp corner leads to more serious water vaporisation. A suction slot with an arc-shaped inlet significantly improves the water removal performance by eliminating water film vaporisation under the test conditions. A 0.35-mm-width suction slot is apt to allow water film across, and a 2-mm-width suction slot cannot form an effective suction pressure difference along the slot height, both leading to poor water removal performance. Meanwhile, 0.7-and 1-mm-width suction slots promote good water removal performance, but the latter is less affected by water vaporisation.
In this study, a turbine stage together with diaphragm seals and shroud seals were chosen for numerical investigation. As the baseline design, labyrinth seals’ leakage and blade stage efficiency were analyzed firstly. The results illustrates that as a custom seal, although simplicity, reliableness, and easy replacement make labyrinth seals widely used in steam and gas turbine, but additional power loss caused by excessive leakage flow affects the efficiency of turbine. In order to enhance the efficiency of the turbine stage by leakage reduction, the labyrinth seals at diaphragm and shroud were replaced by brush seals and honeycomb seals respectively. Porous medium method was used to simulate the flow in bristle pack of brush seals, and the pressure drop in bristle was explored by Darcy law. Pressure distribution and flow field details of honeycomb seals were also researched by CFD method. Radial clearance has a direct influence on leakage, so the clearance effect was analyzed in this paper. Lastly, for the stage together with brush seals and honeycomb seals researched, results show that comparing to custom labyrinth seals, the reduction leakage was approximate 30% and the improvement of stage efficiency was 0.6%.
In addition to mechanical losses, such as interphase drag loss, braking loss and pumping loss 1 , the presence of wetness in steam turbine can also lead to many additional losses in thermodynamics, profile, shock wave and blade end, etc. The additional thermodynamics loss is caused by nonequilibrium condensing flow where the rapid-expansion pure steam will be in supercooled state, and the heat transfer process is anisothermal 2 . Many experimental and numerical investigations on nonequilibrium condensing flow have been conducted for years, where the classical condensation theory and the growth rate of water droplets have been validated and studied in Laval nozzles 3-5 and turbine cascades [6][7][8] . Moreover, experiments have been carried on in model or full-scale LP steam turbines
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