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.