Experimental and Numerical Analyses of Relaxation Processes in LP Steam Turbines

Abstract

Flow fields in low-pressure (LP) steam turbines, starting expansion well above the saturation line, show four pronounced non-equilibrium processes (called relaxation processes). The first one above the saturation line is centred around heterogeneous nucleation/condensation, the second one around subcooling and the subsequent homogeneous nucleation/condensation (‘Wilson point’), and the third and the fourth ones are characterized by thermodynamic and mechanical effects in the established droplet-loaded part of the flow field. All these relaxation processes are interacting downwards in the progressive expansion in the turbine. In a multistage LP turbine, the most important ones are the second - because it is mainly responsible for the number of droplets (sizes) - and the third - because it creates most of the dissipation. In addition, the first and the fourth ones can damage the flow guiding geometry by corrosion and erosion, respectively. More than 40 years ago, Gyarmathy formulated the first strictly physical basis for these processes. Subsequently, many researchers have contributed to increase the physical understanding using both experimental and numerical methods. The intuition that the one-dimensional treatment of the flow field in multistage turbines cannot explain the measured droplet sizes behind the blading has been particularly relevant. It became clear that especially the temperature fluctuation within the blading has to be included in homogeneous nucleation/condensation considerations. Also of some importance has been the improved understanding of the first relaxation process, which was initially underestimated. This article highlights the current situation seen by a turbomachinery company that has been contributing to and supporting this discipline for many decades. A wide literature survey and a critical appraisal of published Baumann factors are included. High quality experimental tools and procedures are introduced on the basis of two generations of split-shaft model LP turbines and Damköhler numbers. Further, an overview of the in-house numerical tools and processes developed for wet steam applications is given. More recent experimental results on the influence of impurities and conditioning agents in the relaxation processes and newer numerical results on the influence of phase transition in the flow field around a blade row are presented.