Numerical Analysis on Non-Equilibrium Steam Condensing Flow in Rotating Machinery

Abstract

Flow of steam, different from other gas flows, involves droplet generation in flow expansion process. This phase transition affects not only the flow fields, but also machine performance including efficiency. In addition, it is totally harmful for machine structures as blades and casing. Therefore, prevention or preparation of droplet generation in steam flows is dreadfully important in stable machine operation. Nowadays, Computational Fluid Dynamics (CFD) is widely used in machine design and optimization process. Thus, simulation with CFD should consider this droplet generation phenomena to predict internal flows precisely. Many studies that analyze steam condensing flow in nozzles, cascades and steam turbines were carried out. Though, the flows of wet-steam which include non-equilibrium phase-transition phenomena are still difficult to predict, especially in the 3D rotating cases as steam turbines. Therefore, more studies are required to get comparable results with experiment. In this study, non-equilibrium wet-steam model was implemented on T-Flow to simulate realistic non-equilibrium steam condensing flow. In the cases of White cascade, characteristics of wet-steam flow were studied and pressure distributions were compared with experimental results for model validation. To use implemented wet-steam model for calculating flows in rotation, especially in steam turbines, a study of steam condensing flow in single stage steam turbine was conducted. Interaction between the stator and rotor using frozen rotor or mixing plane method in steady calculations were compared in order to find the effects of used interface on flow fields and steam condensation. As a result, condensing flows were predicted well even in the rotating cases by using non-equilibrium wet-steam model. The wet-steam parameters (nucleation, droplet size, wetness) are differed throughout the spans due to 3D effects and influenced by selection of interface as expected. In addition, droplet generation enhances entropy rise throughout the domain. The case using mixing plane seems to be overestimate the size of high wetness zone and it is recommended to use frozen rotor in multi-phase calculations. However, to apply this model in general cases, comparison with experimental data from real steam turbines should be conducted in further studies.

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

“…Therefore, wet-steam flow in a single stage steam turbine (Fig. 3) was simulated in the previous study [15]. As a result, droplet generation with condensation shock was also found in the steam turbine model.…”

“…(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.…”

“…4). The turbine model is a single stage steam turbine from ANSYS CFX tutorial with some modifications in geometry and is the used one in the previous study [15]. Since the stator domain possessed 6.0 degrees in circumferential direction, the rotor domain moved in counter-clockwise direction (=rotating direction) with 1.5 degrees in each case.…”

“…Since the stator domain possessed 6.0 degrees in circumferential direction, the rotor domain moved in counter-clockwise direction (=rotating direction) with 1.5 degrees in each case. The original (0.0 degrees) rotor position is the position which was tested with changing stator/rotor interface [15] and the leading edge (LE) of rotor blade is close to the trailing edge (TE) of stator blade at that position. In other words, rotor LE approaches to stator TE in the cases of -3.0 degrees to -1.5, 0.0 degrees, and rotor LE recedes from stator TE in the cases of 0.0 degrees to +1.5, -3.0 degrees.…”

“…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]. Resultingly, it was found that using mixing plane method can distort and overestimate wetness distribution in the rotor passage during mixing process, so use of mixing plane method is not recommended when simulating steady wet-steam flow.…”