Numerical Investigation Of Wet Steam Flow In The Low Pressure Stage Of Steam Turbine

Steam turbine is a key component in thermal and nuclear power plants and its performance has great effect on the economy and safety of electrical industry. The steam in low-pressure stages of modern larger power steam turbine-generator units is generally expanded across saturation line and rear stages operate in wet-steam conditions. The mechanism of droplet formation in steam turbine and the effect of droplets on the efficiency and reliability of turbine components are important tasks in the energy power engineering field. The wet steam flows in low pressure steam turbines are generally complex and highly three-dimensional. Therefore, it is very difficult to measure the flow fields accurately especially parameters related to water droplets. Moreover, the experimental devices are usually very expensive. Compared to the experimental method, the numerical investigations are easier to realize. Especially in recent years, with the advent of high performance computing machines, it is possible to investigate the highly three-dimensional condensing flow in steam turbine utilizing the numerical method.In this work, an Eulerian-Eulerian multiphase model already implemented in the commercial software ANSYS CFX is employed to simulate the non-equilibrium cond-ensing flow in a low pressure steam turbine. The equations of the model are formulated utilizing the corrected classical nucleation theory and the droplet growth model proposed by Gyarmathy. The multiphase model is used to simulate the spontaneous condensing flow in Laval nozzles and two-dimensional cascades. The predicted results are in good agreement with the measurement data, demonstrating the accuracy and reliability of the model utilized. In addition, the reasonable range of NBTF (nucleation bulk tension factor) for simulating the non-equilibrium condensing flow of low pressure steam is obtained. The non-equilibrium condensation model with different values of NBTF is applied to a 1000MW fossil-fired low pressure steam turbine to study the effect of droplet surface tension on the condensing flow in steam turbine. The optimal value of NBTF is determined. The main work in this paper can be concluded as followings:(1) The condensing flow in the last stage of this unit is simulated by the inhomo-geneous multi-phase model. Methods to predict the turbulent and inertial deposition of fog droplets and coarse droplets are proposed and employed to calculate the droplet deposition rate on the stationary blade and moving blade of the last stage. The calcul- ated results for different droplet diameters at the inlet of the last stage are in good agreement with the results of Yau and Starzmann, demonstrating the accuracy of the methods proposed. In the context of 3D multi-phase flow solutions and the droplets deposition predictions on the blades, a physically consistent 3D method to evaluate the moisture loss is presented. Within the method, moisture losses are divided into six different categories namely thermodynamic loss, drag loss of fog droplets, drag loss of coarse droplets, impact loss, capturing loss and centrifuging loss. All six categories as well as the total moisture loss in the last stage are determined for different diameters of fog droplets at the inlet of the last stage. Results indicate that inlet droplet diameter has significant effect on the magnitudes of moisture losses and relative fractions of each category. Applying the modified Baumann rule proposed by Rreimeier et al to the studied last stage, the predicted results is quite consistent with the value of the moisture loss coefficient of the case Dxl predicted by the 3D method proposed in this work. However, the modified Baumann rule cannot consider the effect of droplet size and the span-wise variations of steam parameters on the moist-ure loss.(2) In the open literatures, there is a lack of studies dealing with the effect of blade roughness on the flow in the wet steam stage of steam turbine. In this work, single-stage and multi-stage models are employed to investigate the effect of blade roughness on the stage performance and spontaneous nucleating processes in the nucl-eating stage of large power fossil-fired steam turbine. During the computing processes, the blade roughness is only imposed on the stationary blade and moving blade of the penultimate stage. The predicted stage efficiency, span-wise distributions of aero-dynamic loss and wetness fraction and thermodynamic loss for different magnitudes and locations of blade roughness are compared. The results indicate that the blade roughness degrades the stage efficiency and weakens spontaneous nucleation in blade passages.(3) Unsteady simulations of the flow in the penultimate stage considering the effect of non-equilibrium condensation are carried out. The results indicate that the unsteady flow in the penultimate stage is mainly caused by the trailing edge shock waves from the stationary blade and the strength of the unsteadiness is decreased from hub to shroud. The potential field caused by the relative motion between stator and rotor also contributes to the unsteadiness. But it is much weaker than the unsteadiness caused by trailing edge shock waves. A comparison of the calculated results by steady and unsteady models shows that the inherent unsteadiness changes the magnitudes and distributions of wetness faction and droplet size at the outlet of rotor. Moreover, the inherent unsteadiness also degrades the stage efficiency. The effect of axial spac-ing between the stationary and moving blade on the unsteady and steady condensing flow in the penultimate stage is studied by simulations for another two cases with larger and smaller axial spacing.