Stage Of Low-pressure Steam Turbine Research Articles

Evaluation of a New Droplet Growth Model for Small Droplets in Condensing Steam Flows

As the condensation phenomenon occurs in the low-pressure stages of steam turbines, an accurate modelling of the condensing flows is very crucial and has a significant impact on the development of highly efficient steam turbines. In order to accurately simulate condensing steam flows, it is essential to choose the right condensation model. Further research to enhance condensation models is of special importance because the outcomes of numerical studies of condensation models in recent years have not been entirely compatible with the experiments and there are still uncertainties in this area. Therefore, the main aim of this paper is to evaluate a proposed droplet growth model for modelling condensation phenomenon in condensing steam flows. The new model is derived to profit from the advantages of models based on the continuum approach for large droplets and those based on the kinetic theorem for small droplets, which results in the model being robust for a wide range of Knudsen numbers. The model is implemented into a commercial CFD tool, ANSYS Fluent 2022 R1, using UDFs. The results of the CFD simulations are validated against experimental data for linear cascades within the rotor and stator blade geometries of low-pressure steam turbine stages. The findings clearly demonstrate the superiority of the new model in capturing droplet growth, particularly for very small droplets immediately following nucleation. In contrast, widely used alternative droplet growth models tend to either underpredict or overpredict the droplet growth rate. This research significantly contributes to the ongoing efforts to enhance condensation modeling, providing a more accurate tool for optimizing the design and operation of low-pressure steam turbines, ultimately leading to a higher energy efficiency and a reduced environmental impact.

Second law evaluation and environmental analysis of biomass-fired power plant hybridized with geothermal energy

Two novel configurations for hybridization of geothermal and biomass energies are proposed and analyzed in this paper. In the first system (named Configuration 1) geothermal heat is employed for feed water heating, while in second one (named Configuration 2) the geothermal energy is employed for steam generation to be entered into the low pressure steam turbine stage. The method of hybridization of geothermal and biomass resources in this paper is quite different than the schemes proposed in previous researches. Another unique feature of the proposed hybrid configurations is the ability of utilizing geothermal resources with various temperature level. In order to assess the feasibility of proposed systems and to compare their performance, detailed simulation models are made based on the first and second laws. Also, the systems’ performances are evaluated from the environmental perspective using three exergo-environmental factors. In addition, the optimal operating conditions of the systems are determined with respect to the maximum exergy efficiency. For a wide range of practical operating conditions it is found that, the proposed Configuration 2 outperforms the Configuration 1 due to the direct steam injection into the law-pressure turbine in the former. Under the optimal operation, it yields 8.65 % higher exergy efficiency as well as 12.5% lower environmental damage effectiveness compared to Configuration 1. Also it is concluded that, the gasification and combustion processes own more than 55% of overall exergy destruction in both systems.

Investigation of Energy Loss on Fractional Deposition in Last Stages of Condensing Steam Turbine Due to Blade Shape and Moisture Droplet Size

The steam consumption in a turbine within an operating pressure range determines the effectiveness of thermal energy conversion to electric power generation in a turbo-alternator. The low pressure (LP) stage of the steam turbine produces largest amount of steam to shaft-power in comparison to other stages of turbine although susceptible to various additional losses due to condensation of wet steam near penultimate and ultimate stages. The surface deposition in blade is caused by inertial impaction and turbulent-diffusion. With increasing blade stagger angle along the larger diameter of blading, the fractional deposition of wet steam is largely influenced by blade shape. From this background, the aim of this work is to predict the effect of mathematical models through computational fluid dynamics analysis on the characterization of thermodynamic and mechanical loss components based on unsaturated vapor water droplet size and pressure zones in LP stages of steam turbine and to investigate the influence of droplet size and rotor blade profile on cumulative energy losses due to condensation and provide an indication about the possible conceptual optimization of blade profile design to minimize moisture-induced energy losses.

Numerical modeling and high-order scheme for wet steam flow

A mathematical model of three-dimensional nonequilibrium condensing wet-steam flow is established in Eulerian form, based on conservation laws for a mixture of steam and water droplets. The method of moments is introduced in modeling the droplet spectrum. To describe the nonequilibrium condensing process, models for classical nucleation and enhanced droplet growth are applied. A special high-order implicit scheme is constructed for this condensing flow model. Tables based on IAPWS-IF97 formulae are used in solving the thermal properties of wet steam. The numerical results for a two-dimensional supersonic nozzle and a low-pressure steam turbine stage are compared with experimental data. The good agreement indicates the effectiveness of the condensation model and numerical scheme.

Numerical investigation of condensing flow in the last stage of low-pressure nuclear steam turbine

In this work, the numerical simulation of condensing flow through the last stage of nuclear steam turbine with 1828mm long moving blades and wetness fraction in front of the last stage up to 13% is presented. Equilibrium condensation model and homogeneous non-equilibrium condensation model are employed to calculate the wet steam flow field. The non-equilibrium condensing flow is modeled on the basis of classical nucleation theory. The predicted Markov loss coefficients, stage enthalpy drop, wetness fraction, etc., of wet-steam flow obtained through equilibrium condensing model show discernable differences from those obtained through non-equilibrium condensing model, which results from the secondary nucleation. In addition, the upstream droplets diameter has, to some extent, impact on the non-equilibrium condensing flow with fixed upstream wetness fraction.

Sensors and Methods for Blade Damage Operational Assessment in Low-Pressure Steam Turbine Stages

The study deals with the opportunities for the assessment of damage to blades in low-pressure steam turbine stages under operation. So far existing methods are based on measurements and evaluation of blade vibrations. Calculations of fatigue cycles are used as a basis for an estimate of the residual life of the blades. A new approach using the analysis of impulse blade signals generated by non-contact stator sensors was applied. Basis for the assessment of blade damage are static characteristics and mutual position of blades. Geometrical and mechanical characteristics of blades change due to creation and progress of a crack. The presence of the crack leads to a change in position between adjacent blades. This method has been applied and verified by long-term measurements at the nuclear power plant Temelin. Other static methods based on blade untwisting and elongation are suggested for monitoring the state of turbine blades.

Characteristics of Moisture Losses in Low Pressure Steam Turbine Stages

Characteristics of Moisture Losses in Low Pressure Steam Turbine Stages

Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow through the last stage moving blades (LSMBs) of a low-pressure (LP) steam turbine falls below a certain limit, energy is returned to the working fluid rather than being extracted. This so-called “ventilation” phenomenon produces nonsynchronous aerodynamic excitation, which has the potential to lead to high dynamic blade loading. The aerodynamic excitation is often the result of a rotating phenomenon, with similarities to a rotating stall, which is well known in compressors. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. The analysis revealed that the rotating excitation mechanism observed in operating steam turbines is reproduced in the model turbine. A 3D computational fluid dynamics (CFD) method has been applied to simulate the unsteady flow in the air model turbine. The numerical model consists of the single stage modeled as a full annulus, along with the axial-radial diffuser. An unsteady CFD analysis has been performed with sufficient rotor revolutions to obtain globally periodic flow. The simulation reproduces the main characteristics of the phenomenon observed in the tests. The detailed insight into the dynamic flow field reveals information on the nature of the excitation mechanism. The calculations further indicate that the LSMB tip clearance flow has little or no effect on the characteristics of the mechanism for the case studied.

Application of quadrature method of moments to the polydispersed droplet spectrum in transonic steam flows with primary and secondary nucleation

This paper investigates the effectiveness of the Quadrature Method of Moments (QMOM) in representing droplet size distributions present in low-pressure steam turbine stages. In particular, distributions that result during transonic flow with sufficient supercooling and high expansion rate for primary and secondary nucleation to occur along the flow path. It is shown that the discretization of the droplet size distribution inherent in QMOM is robust for representing a polydispersed distribution with sizes several orders of magnitude apart, and originating from separate condensation phenomena. Inclusion of the QMOM method would thus provide a significant improvement in condensation models currently in use, which generally rely on a monodispersed representation of the droplet distribution.

Influence of upstream stator on rotor flutter stability in a low pressure steam turbine stage

Conventional blade flutter prediction is normally based on an isolated blade row model, however, little is known about the influence of adjacent blade rows. In this article, an investigation is presented into the influence of the upstream stator row on the aero-elastic stability of rotor blades in the last stage of a low pressure (LP) steam turbine. The influence of the upstream blade row is computed directly by a time-marching, unsteady, Navier-Stokes flow solver in a stator-rotor coupled computational domain. The three-dimensional flutter solution is obtained, with adequate mesh resolution, in a single passage domain through application of the Fourier-transform based Shape-Correction method. The capability of this single-passage method is examined through comparison with predictions obtained from a complete annulus model, and the results demonstrate a good level of accuracy, while achieving a speed up factor of 25. The present work shows that the upstream stator blade row can significantly change the aero-elastic behaviour of an LP steam turbine rotor. Caution is, therefore, advised when using an isolated blade row model for blade flutter prediction. The results presented also indicate that the intra-row interaction is of a strong three-dimensional nature.

Nucleating unsteady flows in low-pressure steam turbine stages

The present paper deals with the detailed numerical investigation of transonic, turbulent, unsteady two-phase steam flows with internal heat addition after partial condensation of the working fluid. The observed phenomena include homogeneously, homogeneously-heterogeneously and heterogeneously dominated condensation effects. The results concerning the different condensation mechanisms are initially discussed in terms of the turbulent, transonic flow through Laval nozzles. Further, an interesting phenomenon of nucleation and droplet growth inside turbulent boundary layers is presented. Application to transonic turbine flows in the rotor cascade VKI-1 shows the influence of homogeneous nucleation on the turbulent shock-boundary layer interaction. Finally, the different types of condensation models were compared to investigate the two-dimensional transonic two-phase flow in an entire turbine stage including high-frequency rotor-stator interactions.

Semiempirical Criterion of Erosion Threat in Modern Steam Turbines

A parametric relationship is developed for the degree of erosion to be expected in a low-pressure steam turbine stage operating with wet steam. This relationship involves geometrical parameters of blading, thermodynamic parameters of stage flow, properties of blade material, stable drop size, and impact velocity of attacking stream of droplets, the resulting impact pressure, and service time of the turbine. Although not all of the parameters are exactly known or calculable, most of the unknowns can be assumed to remain approximately equal for typical modern turbines. Therefore, a two-dimensional field can be specified with impact pressure as one coordinate and a simplified calculable parametric expression as the other coordinate. On such a field one should be able to plot lines of constant erosion severity, which has been done with the help of field service measurements from a large collection of steam turbines. The resulting chart provides a semi-empirical tool for the prediction of erosion in other turbines.