Wet Steam Model Research Articles

CFD modelling of the condensation inside a Supersonic Nozzle: implementing customized wet-steam model in commercial codes

Non-equilibrium condensation of steam occurs in many jet and turbomachinery devices, such as supersonic nozzles, ejectors and across the last stages of steam turbines. Wet steam models are available in many commercial CFD codes and can represent the metastable behaviour of the flow with reasonable accuracy. Unfortunately, the use of built-in models does not allow freedom in the choice of model parameters and settings. In the present paper, a numerical model for the simulation of wet steam flow has been developed and implemented within a commercial CFD code (ANSYS Fluent) via user defined functions. The scheme is based on a single-fluid approach and solves the transport equation for a homogeneous mixture flow coupled with conservation equations for the number of droplets and liquid mass fraction. The model is compared against a well-known steam nozzle test-case.

CFD modelling of the condensation inside a cascade of steam turbine blades: comparison with an experimental test case

Non-equilibrium condensation of steam occurs in many jet and turbomachinery devices, such as supersonic nozzles, ejectors and across the last stages of steam turbines. Wet steam models are available in many commercial CFD codes and can represent the metastable behaviour of the flow with reasonable accuracy. Unfortunately, the use of built-in models does not allow freedom in the choice of parameters and settings. In the present paper, a numerical model for the simulation of wet steam flow has been developed and implemented within a commercial CFD code (ANSYS Fluent) via user-defined functions. The scheme is based on a Mixture-model approach. The model is compared with literature data for a 2D stationary cascade of a steam turbine blade.

Influences of area ratio and surface roughness on homogeneous condensation in ejector primary nozzle

Homogeneous condensation inside the ejector has been ignored by numerous researchers. In this paper, the wet steam model was utilized to study the influences of nozzle area ratio and surface roughness on homogeneous condensation in a primary nozzle. A substantial amount of latent heat is released during the homogeneous condensation process to increase static pressure and reduce Mach number. The increase of nozzle area ratio reduces condensation intensity and lowers nozzle pressure but increases liquid mass fraction. Furthermore, with the growth of surface roughness, the slightly increased dryness fraction is almost ineffective for alleviating erosion caused by tiny droplets; the slight decrease in mass flow rate will reduce ejector performance to some extent, while the sharp increase of entropy generation results in more energy wastes. Therefore, nozzle area ratio and surface roughness should be carefully designed to alleviate potential condensation and enhance ejector performance at the same time.

Optimum Tilt Angle of Flow Guide in Steam Turbine Exhaust Hood Considering the Effect of Last Stage Flow Field

Heat transfer and vacuum in condenser are influenced by the aerodynamic performance of steam turbine exhaust hood. The current research on exhaust hood is mainly focused on analyzing flow loss and optimal design of its structure without consideration of the wet steam condensing flow and the exhaust hood coupled with the front and rear parts. To better understand the aerodynamic performance influenced by the tilt angle of flow guide inside a diffuser, taking a 600 MW steam turbine as an example, a numerical simulator CFX is adopted to solve compressible three-dimensional (3D) Reynolds time-averaged N-S equations and standard k-e turbulence model. And the exhaust hood flow field influenced by different tilt angles of flow guide is investigated with consideration of the wet steam condensing flow and the exhaust hood coupled with the last stage blades and the condenser throat. The result shows that the total pressure loss coefficient and the static pressure recovery coefficient of exhaust hood change regularly and monotonously with the gradual increase of tilt angle of flow guide. When the tilt angle of flow guide is within the range of 30° to 40°, the static pressure recovery coefficient is in the range of 15.27% to 17.03% and the total pressure loss coefficient drops to approximately 51%, the aerodynamic performance of exhaust hood is significantly improved. And the effective enthalpy drop in steam turbine increases by 0.228% to 0.274%. It is feasible to obtain a reasonable title angle of flow guide by the method of coupling the last stage and the condenser throat to exhaust hood in combination of the wet steam model, which provides a practical guidance to flow guide transformation and optimal design in exhaust hood.

Numerical analysis of the oscillation frequency of the shock wave and the evaporation level on the Mach disc in the IMP PAN nozzle

In the present paper, we have focused on the phenomena occurring in the supersonic part of the de Laval nozzle, characterised by oscillation of the shock wave in steam flow. The effect of shock oscillation was observed by using the Topler optical system in the IMP PAN experiment carried out on a symmetrical planar de Laval nozzle. Having observed the shock fluctuation in wet steam flow, according to Puzyrewski's observations, we analysed the oscillation frequency in the IMP PAN nozzle depending on the pressure conditions. Additionally, we analysed the evaporation level of condensate droplets during passage through the Mach disc depending on the inlet conditions. The model of a single continuum wet steam model with a special microstructure growing up during phase transitions was validated on an experiment carried out by Dykas et al. in 2013 on the half arc nozzle. The present work includes simulations results of oscillation frequency of the shock wave and the evaporation level of the liquid phase on the Mach disc.

Numerical analysis of non-equilibrium steam condensing flows in various Laval nozzles and cascades

ABSTRACTWhen steam is used in fluid machinery, phase transition can occur that affects not only the flow fields but also machine performance. Therefore, to achieve an accurate prediction of steam condensing flow using computational fluid dynamics (CFD), phase-transition phenomena should be considered and a non-equilibrium wet-steam model is required. Such a model is implemented in this study using the in-house code T-Flow, and the flow fields – including phase-transition phenomena – in various Laval nozzles are examined. The results for multi-phase flows can be obtained in relatively short time by using mixture assumption and an inner-iteration method. The calculated results reflect the characteristics of the condensing flows well and are comparable with those obtained experimentally. Also, it was found that the superheating level of incoming steam can explain the tendency of condensation in the nozzles considered in a simple way. In addition, steam condensing flows in the blade cascades were simulated. As a result, the predicted blade loading agreed well with the experimental data and the superheating level at inlet was responsible for the condensation trend not only in the nozzles but also in the cascades. In future work, the characteristics of steam condensing flow in a steam turbine where complex flows and phase transition occur can be investigated using the presented model.

CFD Modeling of the Supersonic Condensation Inside a Steam Ejector

Abstract Supersonic steam ejectors are commonly used in many applications, for example suction of non-condensable gases in steam power plants or heat-powered chillers. In the specific case of ejector chillers, CFD studies of the ejector are necessary for optimization of this device because ejector performances have a direct impact on the COP of the chiller. The complex ejector flow and the high Mach numbers make characterization of the phase change inside this component difficult. Metastability effects also have to be accounted for and some CFD commercial software provide built-in wet-steam models for this purpose. A simpler approach for numerical modeling of multiphase ejector is the Homogeneous Equilibrium Model (HEM) in which the phase change occurs in equilibrium conditions, i.e. metastability is neglected. These second kinds of models are still important tools for preliminary analysis of condensing ejectors. In the present paper a comparison between commercial software wet-steam models and an in-house developed model based on HEM is presented.

Numerical Investigation of Boundary Layers in Wet Steam Nozzles

Condensing nozzle flows have been used extensively to validate wet steam models. Many test cases are available in the literature, and in the past, a range of numerical studies have dealt with this challenging task. It is usually assumed that the nozzles provide a one- or two-dimensional flow with a fully turbulent boundary layer (BL). The present paper reviews these assumptions and investigates numerically the influence of boundary layers on dry and wet steam nozzle expansions. For the narrow nozzle of Moses and Stein, it is shown that the pressure distribution is significantly affected by the additional blockage due to the side wall boundary layer. Comparison of laminar and turbulent flow predictions for this nozzles suggests that laminar–turbulent transition only occurs after the throat. Other examples are the Binnie and Green nozzle and the Moore et al. nozzles for which it is known that sudden changes in wall curvature produce expansion and compression waves that interact with the boundary layers. The differences between two- and three-dimensional calculations for these cases and the influence of laminar and turbulent boundary layers are discussed. The present results reveal that boundary layer effects can have a considerable impact on the mean nozzle flow and thus on the validation process of condensation models. In order to verify the accuracy of turbulence modeling, a test case that is not widely known internationally is included within the present study. This experimental work is remarkable because it includes boundary layer data as well as the usual pressure measurements along the nozzle centerline. Predicted and measured boundary layer profiles are compared, and the effect of different turbulence models is discussed. Most of the numerical results are obtained with the in-house wet steam Reynolds-averaged Navier–Stokes (RANS) solver, Steamblock, but for the purpose of comparison, the commercial program ansys cfx is also used, providing a wider range of standard RANS-based turbulence models.

Nonrealizability Problem With Quadrature Method of Moments in Wet-Steam Flows and Solution Techniques

The quadrature method of moments (QMOM) has recently attracted much attention in representing the size distribution of liquid droplets in wet-steam flows using the n-point Gaussian quadrature. However, solving transport equations of moments using high-order advection schemes is bound to corrupt the moment set, which is then termed as a nonrealizable moment set. The problem is that the failure and success of the Gaussian quadrature are unconditionally dependent on the realizability of the moment set. First, this article explains the nonrealizability problem with the QMOM. Second, it compares two solutions to preserve realizability of the moment sets. The first solution applies a so-called “quasi-high-order” advection scheme specifically proposed for the QMOM to preserve realizability. However, owing to the fact that wet-steam models are usually built on existing numerical solvers, in many cases modifying the available advection schemes is either impossible or not desired. Therefore, the second solution considers correction techniques directly applied to the nonrealizable moment sets instead of the advection scheme. These solutions are compared in terms of accuracy in representing the droplet size distribution. It is observed that a quasi-high-order scheme can be reliably applied to guarantee realizability. However, as with all the numerical models in an Eulerian reference frame, in general, its results are also sensitive to the grid resolution. In contrast, the corrections applied to moments either fail in identifying and correcting the invalid moment sets or distort the shape of the droplet size distribution after the correction.

Effects of superheated steam on non-equilibrium condensation in ejector primary nozzle

In order to reduce the non-equilibrium condensation occurring in ejector primary nozzle, wet steam model was adopted to investigate the relationship between steam superheated level and non-equilibrium condensation within ejector primary nozzle. Simulation data of axial static pressure along primary nozzles were validated with experimental data reported in literature. The non-equilibrium condensation process from homogeneous nucleation to droplet growth stage and the resulting products were carefully studied. Moreover, six inlet superheated levels from 5 K to 30 K with the increment of 5 K were compared, and simulation results showed that the increase of superheated level from 5 K to 30 K causes 40.22% delay in the location and 43.92% reduction in the intensity of the condensation shock. Furthermore, there is about 24.30% liquid mass fraction decrease when the superheated level raises to 30 K and total entropy generation increases slowly with the increase of superheated level.

Effect of mixing on the performance of wet steam ejectors

Steam ejector computational simulations using a wet steam model give higher entrainment ratios and higher critical back pressures for the ejector compared with the ideal gas model. This paper identifies the origin of these differences. Simulation results show that the wet steam model predicts an entrainment ratio for the choked flow ejector operation that is 10% higher than that for the ideal gas model. The wet steam model also gives a higher critical back pressure by about 7% relative to the ideal gas model with a closer agreement to experimental data for the unchoked ejector operation. Enhanced mixing layer growth which arises due to steam condensation in the primary nozzle is identified as the main reason for higher entrainment ratio of the ejector simulations using the wet steam model. The difference in the mixing layer growth rate between ideal gas and wet steam simulations is 21%, indicating enhanced entrainment for the wet steam model. Furthermore, the mixture at the start of the diffuser is shown to have a higher pitot pressure than in the ideal gas simulations and these elevated pitot pressures allow the ejector to operate in a choked mode to a higher critical back pressure.

Numerical study of primary steam superheating effects on steam ejector flow and its pumping performance

The effects of primary steam superheating on steam condensation in nozzle and the performance of steam ejector were investigated using CFD (computational fluid dynamics) method. Using a wet steam model being proposed in our previous study, simulations based on the primary steam with five superheated levels were performed, and the results demonstrate the superheating operation of the primary steam weakens the spontaneous condensation intensity and postpones its occurrence within the nozzle vicinity. Due to the droplets nucleation refinement for the condensation of superheated steam, the mixing process between the primary and the secondary fluids is improved. Consequently, a higher entrainment ratio is achieved. However, the superheating operation may not exceed 20 K, as its contribution on entrainment ratio improvement is not as significant as 0 K–20 K superheating, and too much superheating will requires more energy as input, which is not a practical solution to further improve the steam ejector pumping performance.

Two-Phase Flow Modeling and Measurements in Low-Pressure Turbines—Part I: Numerical Validation of Wet Steam Models and Turbine Modeling

In this publication, an overview of the current state of wetness modeling at the Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM) is given. For the modeling, an Euler–Euler method implemented in the commercial flow solver Ansys CFX is used. This method is able to take into account the nonequilibrium state of the steam and models the interactions between the gaseous and liquid phases. This paper is the first part of a two-part publication and deals with the numerical validation of wet steam models by means of condensing nozzle and cascade flows. A number of issues with regard to the quality of the computational fluid dynamics (CFD) code and the applied condensation models are addressed comparing the results to measurements. It can be concluded that a calibration of the models is necessary to achieve a satisfying agreement with the experimental results. Moreover, the modeling of the low pressure model steam turbine operated at the ITSM is described focusing on the asymmetric flow field in the last stage caused by the axial–radial diffuser. Different simplified axisymmetric diffuser models are investigated in steady state simulations, and the results and the arising issues for part-load, design-load, and over-load conditions are discussed. Thereafter, a comparison between the equilibrium and nonequilibrium steam modeling approaches is performed and the advantage of the nonequilibrium model is highlighted. The second part of the publication focuses on experimental investigations and compares the numerical results to wetness measurement data. For this purpose, different loads are also considered.

Two-Phase Flow Modeling and Measurements in Low-Pressure Turbines—Part II: Turbine Wetness Measurement and Comparison to Computational Fluid Dynamics-Predictions

The correct computation of steam subcooling, subsequent formation of nuclei and finally droplet growth is the basic prerequisite for a quantitative assessment of the wetness losses incurred in steam turbines due to thermal and inertial relaxation. The same basically applies for the prediction of droplet deposition and the resulting threat of erosion. Despite the fact that there are many computational fluid dynamics (CFD)-packages that can deal with real-gas effects in steam flows, the accurate and reliable prediction of subcooling, condensation, and wet steam flow in steam turbines using CFD is still a demanding task. One reason for this is the lack of validation data for turbines that can be used to assess the physical models applied. Experimental data from nozzle and cascade tests can be found in the open literature; however, these measurement results are only partly useful for validation purposes for a number of reasons. With regard to steam turbine test data, there are some publications, yet always without any information about the turbine stage geometries. This publication is part of a two-part paper; whereas Part I focuses on the numerical validation of wet steam models by means of condensing nozzle and cascade flows, the focus in this part lies on the comparison of CFD results of the turbine flow to experimental data at various load conditions. In order to assess the validity and reliability of the experimental data, the method of measurement is presented in detail and discussed. The comparison of experimental and numerical results is used for a discussion about the challenges in both modeling and measuring steam turbine flows, presenting the current experience and knowledge at Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM).

Ejector primary nozzle steam condensation: Area ratio effects and mixing layer development

Recent ejector simulations based on wet steam modeling give significantly different performance figures relative to ideal gas modeling, but the origins of such differences are not clear. This paper presents a numerical investigation of flow in the primary nozzle of a steam ejector to further explore the differences between ideal gas and wet steam analysis of ejector flows. The wet steam modeling was first validated using primary nozzle surface pressure data from three experiments reported in the literature. Ejector primary nozzles with area ratios (AR) of 11, 18 and 25 were then simulated using wet steam and ideal gas models. The wet steam simulations show that nozzle static pressures are higher than those for ideal gas model, and in the AR = 25 case, the static pressure is larger by a factor of approximately 1.7. In contrast, no significant difference exists between the nozzle momentum flux for both ideal gas and wet steam models, except the vicinity of the nozzle throat where nucleation occurs. Enhanced mixing between primary and secondary streams, which arises because primary stream condensation reduces compressibility in the mixing layer, is proposed as an explanation of the increased entrainment ratio observed in recent wet steam ejector simulations.

The spontaneously condensing phenomena in a steam-jet pump and its influence on the numerical simulation accuracy

A wet steam model for transonic flow was proposed to investigate the flow behaviours in a steam-jet pump. Contrary to the general ideal gas assumption adopted by the most of the researches focusing on steam-jet pump investigation, the numerical simulation using this wet steam model that was implemented based on a commercial computational fluid dynamics code, and the simulation results demonstrated the existence of the spontaneous condensation as the supersonic flow passing through the nozzle. Our results also gave good agreement with the referenced experimental data, and with higher accuracy when comparing with the results generated from ideal gas modelling. The present study confirms that the simulation accuracy can be improved by the wet steam modelling, and gives a proper prediction of pump operating status.

Numerical simulation of a new hollow stationary dehumidity blade in last stage of steam turbine

As a result of adopting saturation steam and long blade, problems of water erosion of last stage blade for steam turbine become more prominent. In order to improve the operation reliability and efficiency of steam turbine, it is necessary to investigate the nonequilibrium condensing wet steam two phase flow and the dehumidity method. A wet steam model with user defined function based on FLUENT software was investigated to simulate the steam condensing flow in the cascades. The simulation consequences show that the pressure variations in simulation depict a good agreement with the experiment data. On the basis of the discrete phase model simulation results and experiment data, the efficiency of existing dehumidity blade with suction slot was calculated. A new stationary dehumidity blade was designed to elevate the dehumidity efficiency: the efficiency in the suction surface was increased by 21.5%, and that in the pressure surface was increased by 12.2%.

Numerical analysis of spontaneously condensing phenomena in nozzle of steam-jet vacuum pump

The primary fluid in a steam-jet vacuum pump is not assumed as a perfect gas as general research in the present study. A mathematic model based on the wet steam model for transonic flow is proposed to investigate the flow behaviours of primary fluid in the nozzle of a steam-jet vacuum pump. The simulation based on a wet steam model was carried out to predict the flow characteristics of primary fluid along the nozzle axis by a commercial computational fluid dynamics (CFD) code (FLUENT6.3). The simulation results showed that there was spontaneous condensation as the supersonic flow passing through the nozzle and the simulation results had a good agreement with the experimental data. It is found from the numerical simulation results that the steam flow characteristics in nozzle are quite different from a wet steam model and a perfect gas assumption: the outlet pressure of the nozzle predicted in the present study is higher than that as the primary fluid assumed as perfect gas, the outlet velocity is about 10% lower than that as the primary fluid assumed as a perfect gas, and the temperature at the outlet of the nozzle is much higher than that as the primary fluid assumed as a perfect gas. The simulation results demonstrate that the thermo-positive process due to steam condensation would hinder the supersonic expanding flow process in nozzle and depress the efficiency of the nozzle which would affect the pumping performance of steam-jet pump.

Numerical Simulation of Water Droplets Deposition on the Last-Stage Stationary Blade of Steam Turbine

Based on the method of discrete phase, the law of droplets’ deposition in the last stage stationary blade of a supercritical 600 MW Steam Turbine is simulated in the first place of this paper by using the Wet-steam model in commercial software FLUENT, where the influence of inlet angle of water droplets of the stationary blades is also considered. Through the calculation, the relationship between the deposition and the diameter of water droplets is revealed. Then, the amount of droplets deposition in the suction and pressure surface is derived. The result is compared with experimental data and it proves that the numerical simulation result obtained in this paper is reasonable. Finally, a formula of the relationship between the diameter of water droplets and the inlet angle is fit, which could be used for approximate calculation in the engineering applications.