Jörg Starzmann

Two-Phase Flow Modeling and Measurements in Low-Pressure Turbines: Part 1 — Numerical Validation of Wet Steam Models and Turbine Modeling

In this publication an overview of the current state of wetness modeling at the Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM) is given. For the modeling an Euler-Euler method implemented in the commercial flow solver ANSYS CFX is used. This method is able to take into account the non-equilibrium state of the steam and models the interactions between the gaseous and liquid phases.This paper is the first part of a two-part publication and deals with the numerical validation of wet steam models by means of condensing nozzle and cascade flows. A number of issues with regard to the quality of the CFD code and the applied condensation models are addressed comparing the results to measurements. It can be concluded, that a calibration of the models is necessary to achieve a satisfying agreement with the experimental results.Moreover, the modeling of the low pressure model steam turbine operated at the ITSM is described focusing on the asymmetric flow field in the last stage caused by the axial-radial diffuser. Different simplified axisymmetric diffuser models are investigated in steady state simulations and the results and the arising issues for part-load, design-load and over-load conditions are discussed. Thereafter, a comparison between the equilibrium and non-equilibrium steam modeling approaches is performed and the advantage of the non-equilibrium model is highlighted.The second part of the publication focuses on experimental investigations and compares the numerical results to wetness measurement data, see Schatz et al. [1]. For this purpose, also different load conditions are considered.Copyright

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

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

Copyright

Experimental study of the effects of temperature variation on droplet size and wetness fraction in a low-pressure model steam turbine

The three-stage low-pressure model steam turbine at the Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM) was used to study the impact of three different steam inlet temperatures on the homogeneous condensation process and the resulting wetness topology. The droplet spectrum as well as the particle number concentration were measured in front of the last stage using an optical-pneumatic probe. At design load, condensation starts inside the stator of the second stage. A change in the steam inlet temperature is able to shift the location of condensation onset within the blade row up- or downstream and even into adjoining blade passages, which leads to significantly different local droplet sizes and wetness fractions due to different local expansion rates. The measured results are compared to steady three-dimensional computational fluid dynamics calculations. The predicted nucleation zones could be largely confirmed by the measurements. Although the trend of measured and calculated droplet size across the span is satisfactory, there are considerable differences between the measured and computed droplet spectrum and wetness fractions.

Numerical Investigation of Boundary Layers in Wet Steam Nozzles

Copyright

Two-Phase Flow Modeling and Measurements in Low-Pressure Turbines - Part II: Turbine Wetness Measurement and Comparison to Computational Fluid Dynamics-Predictions

Copyright

Results of the International Wet Steam Modeling Project

The purpose of the “International Wet Steam Modeling Project” is to review the ability of computational methods to predict condensing steam flows. The results of numerous wet-steam methods are compared with each other and with experimental data for several nozzle test cases. The spread of computed results is quite noticeable and the present paper endeavours to explain some of the reasons for this. Generally, however, the results confirm that reasonable agreement with experiment is obtained by using classical homogeneous nucleation theory corrected for non-isothermal effects, combined with Young’s droplet growth model. Some calibration of the latter is however required. The equation of state is also shown to have a significant impact on the location of the Wilson point, thus adding to the uncertainty surrounding the condensation theory. With respect to the validation of wet-steam models it is shown that some of the commonly used nozzle test cases have design deficiencies which are particularly apparent in the context of two- and three-dimensional computations. In particular, it is difficult to separate out condensation phenomena from boundary layer effects unless the nozzle geometry is carefully designed to provide near-one-dimensional flow.

Two-Phase Flow Modeling and Measurements in Low-Pressure Turbines: Part 2 — Turbine Wetness Measurement and Comparison to CFD-Predictions

Copyright

Numerical investigation of the impact of part-span connectors on aero-thermodynamics in a low pressure industrial steam turbine

A numerical study on the flow in a three stage low pressure industrial steam turbine with conical friction bolts in the last stage and lacing wires in the penultimate stage is presented and analyzed. Structured high-resolution hexahedral meshes are used for all three stages and the meshing methodology is shown for the rotor with friction bolts and blade reinforcements. Modern three-dimensional CFD with a non-equilibrium wet steam model is used to examine the aero-thermodynamic effects of the part-span connectors. A performance assessment of the coupled blades at part load, design and overload condition is presented and compared with measurement data from an industrial steam turbine test rig. Detailed flow field analyses and a comparison of blade loading between configurations with and without part-span connectors are presented in this paper. The results show significant interaction of the cross flow vortex along the part-span connector with the blade passage flow causing aerodynamic losses. This is the first time that part-span connectors are being analyzed using a non-equilibrium wet steam model. It is shown that additional wetness losses are induced by these elements.Copyright

Nucleation and wake-chopping in low pressure steam turbines

While wetness formation in steady flows such as nozzles and cascades is well understood, predicting the polydispersed droplet spectra observed in turbines remains challenging. The characteristics of wetness formation are affected by the expansion rate at the Wilson point. Because the expansion rate varies substantially both axially and circumferentially within steam turbines, the location of the Wilson point within a blade row is a primary factor determining the droplet spectrum and phase change losses. This effect is first investigated using a single streamline with a varying expansion rate, and it is shown that the phase change losses during spontaneous condensation are highest when a large region of high subcooling precedes the Wilson point. The conditions resulting in the highest wetness loss in the nucleation zone do not correspond to those that produce the largest downstream droplets. The effect of nucleation location is then assessed using a non-equilibrium RANS calculation of a realistic low pressure (LP) steam turbine geometry. A quasi-three dimensional (Q3D) flow domain is used to simplify the analysis, which is performed both steadily and unsteadily to isolate the effects of wake-chopping. The inlet temperature is varied to investigate the impact of the Wilson point location on the steady and unsteady wetness loss and droplet spectra. The trends observed in the 1D analysis are repeated in the steady RANS results. The unsteady results show that the Wilson zone is most sensitive to wake-chopping when located near a blade trailing edge and the following inter-row gap. The predicted wetness losses are compared to those predicted by the Baumann rule.