Wetness loss prediction for a low pressure steam turbine using computational fluid dynamics

Abstract

Two-phase computational fluid dynamics modelling is used to investigate the magnitude of different contributions to the wet steam losses in a three-stage model low pressure steam turbine. The thermodynamic losses (due to irreversible heat transfer across a finite temperature difference) and the kinematic relaxation losses (due to the frictional drag of the drops) are evaluated directly from the computational fluid dynamics simulation using a concept based on entropy production rates. The braking losses (due to the impact of large drops on the rotor) are investigated by a separate numerical prediction. The simulations show that in the present case, the dominant effect is the thermodynamic loss that accounts for over 90% of the wetness losses and that both the thermodynamic and the kinematic relaxation losses depend on the droplet diameter. The numerical results are brought into context with the well-known Baumann correlation, and a comparison with available measurement data in the literature is given. The ability of the numerical approach to predict the main wetness losses is confirmed, which permits the use of computational fluid dynamics for further studies on wetness loss correlations.

Two-Phase Flow Modeling and Measurements in Low-Pressure Turbines—Part I: Numerical Validation of Wet Steam Models and Turbine Modeling

“…For this purpose, the slip velocity between the droplets and the steam had to be taken into account, which enables a more detailed analysis of wetness losses. The effect of droplet size on thermodynamic and kinematic relaxation losses as well as on the braking loss has been studied by Starzmann et al [5], who confirm the conclusion of Gyarmathy [6], that for all investigated cases, the thermodynamic wetness loss contributes the most to the overall wetness losses and work should be done to consider the influence of droplet size in wetness loss correlations.…”

“…This is due to the fact that this kind of calibration can be interpreted as an increase of the critical radius which requires a higher subcooling for the formation of sta ble nuclei. A more detailed loss analysis for the White cascade was performed by Starzmann et al [5],…”

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

“…The volumetric mass concentration of coarse droplets is taken at a distance of y + = 30 away from the wall as recommended by Starzmann et al [7]. Fig.…”

“…The impinging of coarse droplets on the moving blade causes a reduction of blade work, which is usually called impact loss. The impact loss can be determined from the momentum acting on the blade surfaces [7]. This momentum is evaluated for each grid point i with the associated surface area and the summation over all grid nodes on the blade surface gives the impact loss as follows:…”

“…In recent years, the advances in computer hardware and CFD technology have made it possible for a 3D calculation of condensing flow-field in steam turbine and thus a 3D analysis of the moisture losses. Starzmann et al [7] made a tentative prediction of the moisture losses in a three-stage model low pressure steam turbine by a 3D two-phase CFD modeling, but only thermodynamic loss, kinematic and braking losses were considered. Petr and Kolovratnik [8] employed a statistical two-dimensional (2D) evaluation method to compute the moisture losses based on the 3D computational flow-field in a 1000 MW nuclear and a 210 MW fossil-fired low pressure steam turbine.…”