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

Abstract

When 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. ARTICLE HISTORY

Effects of Relative Position between a Stator and a Rotor on Steam Condensing Flow in Rotating Machinery

“…Nucleation is a significant phenomenon of generating droplets in steam flow and it has a close relationship with supercooling level. Supercooling level is defined by the difference between saturation temperature and vapor temperature (Ts-Tc) and reaches to almost 30~40 Kelvin in most cases of non-equilibrium steam condensing flow [13,14]. Since similar distributions of supercooling level were observed throughout the span locations at constant rotor position, just the distributions near casing region were depicted in Fig.…”

“…(2) and 3, the source terms of each phase can be defined and two sets of governing equations are required for vapor and liquid phases in principle (inhomogeneous approach) [22]. However, since the main solver T-Flow is a density based solver which is used to calculate single-phase flow, two-phase flow can be modelled by assuming that the vapor and liquid are well mixed in a single mixture (homogeneous approach) [13][14][15]. The net amounts (vapor + liquid) of inter-phase transfer rates of mass and momentum are assumed to be zero owing to no external gain or loss.…”

“…Using implemented wet-steam model, T-Flow can predict steam condensing flows in various cases. At first, it was used to simulate the flow in a Moore nozzle C for validating the implemented code [13] and then steam condensing flows in a Moore nozzle B, a Moses and Stein nozzle and a White cascade were numerically studied [14]. In those studies, non-equilibrium phasetransitions with pressure, temperature increase (condensation shock) were observed and predicted pressure, droplet radius agreed well with the experimental data as in Figs However, calculation of steam condensing flow is required for not only nozzles and cascades, but also rotating machinery as steam turbines.…”

“…In POSTECH, non-equilibrium wet-steam model was implemented on in house code T-Flow and steam condensing flows in various Laval nozzles and blade cascades were numerically studied [13,14]. In addition, it was applied to steady calculations of a steam turbine model with changing stator/rotor interface [15].…”

On the application of isogeometric finite volume method in numerical analysis of wet-steam flow through turbine cascades

“…It should be noted that meshing the leading and trailing edges of airfoils is a challenging task in CFD problems. There may be some extra mesh refinements in the leading/trailing edge (see, e.g., [80,81]) or some trim/extension in the geometry of the airfoil (see, e.g., [40,82]) in order to deal with the issue of meshing those regions. In this paper, we used a practical way of geometry extension (see Fig.…”