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

Abstract

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.

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

“…where Kn is the Knudsen number defined by the ratio of molecular mean free path and droplet diameter, k c is the thermal conductivity of the vapor and c is an empirical factor set to 3.18 [21]. The equations of International Association of Properties of Water and Steam (IAPWS-IF97) are utilized to model the thermodynamic properties of water and steam and the surface tension of water.…”

Second Law Analysis of Condensing Steam Flows

“…Subsequent to the International Wet Steam Modelling Project [17], the real geometry of the blade trailing edge was made available. Hence, in the present study a sharp trailing edge was used in contrast to previous studies [18,19].…”

“…(13) and (15), respectively. Due to the fact that the entropy production within shocks cannot be calculated directly, the shock loss coefficient is obtained by subtracting the other loss coefficients from the total loss based on the inlet and outlet states: (19) Considering the results of the grid independency study shown in Tab. 2, it becomes clear that by using Eq.…”

Numerical analysis of non-equilibrium steam condensing flows in various Laval nozzles and cascades

“…The droplet size is strongly affected by the equation of state in the wet-steam model, as in Halama and Fort (2013) and Grubel et al (2014). However, the droplet radius has been underestimated in many numerical studies (e.g., Dykas & Wroblewski, 2012;Yang & Shen, 2009) and measurement uncertainty should be taken into account due to its small size.…”

“…Chandler, White, and Young (2014) analyzed unsteady low pressure (LP) turbine flows with non-equilibrium wet-steam calculations. Grubel et al (2014) and Schatz et al (2014) conducted experiments and simulations with LP turbines, and compared them in detail. However, wet-steam flow remains still difficult to predict accurately, because it includes non-equilibrium phase-transition phenomena of which the physics are not fully understood.…”