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.

A 3D method to evaluate moisture losses in a low pressure steam turbine: Application to a last stage

“…However, Kreimeier et al [28] argued that this representation of the real expansion based on a dry reference process is misleading concerning the deduction of the moisture losses. The reason is that even without moisture losses, the difference still exists between the isentropic efficiency of dry and wet expansion processes due to the difference of isentropic exponent between wet steam and dry steam.…”

Study on water extraction of hollow stationary blade under wet steam flow conditions

“…Many experimental and numerical investigations on nonequilibrium condensing flow have been conducted for years, where the classical condensation theory and the growth rate of water droplets have been validated and studied in Laval nozzles [3][4][5] and turbine cascades [6][7][8] . Moreover, experiments have been carried on in model or full-scale LP steam turbines [9][10][11][12][13] although the actual wet steam flow is more complex, and related numerical simulations [14][15][16][17] have been undertaken to provide more details. A full understanding of the wetness effect on turbine performance is helpful for the optimization of turbine flow path design.…”

The assessment of the effect of binary homogeneous nucleation on wet steam energy loss in a low pressure steam turbine

“…The wetness energy loss covers the thermodynamic loss, drag loss of fine and coarse droplets, impact loss, collected water loss, centrifuging loss, and exit loss. The related Baumann factor can then be evaluated and compared with the mean value of order B ¼ 0.6 (covering the range 0.45-0.75 in the turbine tests 4,7 ).…”