16.6.1 Overview and Limitations of the Wet Steam Model

Overview

During the rapid expansion of steam, a condensation process will take place shortly after the state path crosses the vapor-saturation line. The expansion process causes the super-heated dry steam to first subcool and then nucleate to form a two-phase mixture of saturated vapor and fine liquid droplets known as wet steam.

Modeling wet steam is very important in the analysis and design of steam turbines. The increase in steam turbine exit wetness can cause severe erosion to the turbine blades at the low-pressure stages, and a reduction in aerodynamic efficiency of the turbine stages operating in the wet steam region [ 237].

ANSYS FLUENT has adopted the Eulerian-Eulerian approach for modeling wet steam flow. The flow mixture is modeled using the compressible Navier-Stokes equations, in addition to two transport equations for the liquid-phase mass-fraction ( $\beta$), and the number of liquid-droplets per unit volume ( $\eta$). The phase change model, which involves the formation of liquid-droplets in a homogeneous nonequilibrium condensation process, is based on the classical nonisothermal nucleation theory.

This section describes the theoretical aspects of the wet steam model. Information about enabling the model and using your own property functions and data with the wet steam model is provided in this section in the separate User's Guide. Solution settings and strategies for the wet steam model can be found in this section in the separate User's Guide. Postprocessing variables are described in this section in the separate User's Guide.

Limitations

The following restrictions and limitations currently apply to the wet steam model in ANSYS FLUENT:

• The wet steam model is available for the density-based solver only.

• Pressure inlet, mass-flow inlet, and pressure outlet are the only inflow and outflow boundary conditions available.

• When the wet steam model is active, the access to the Create/Edit Materials dialog box is restricted because the fluid mixture properties are determined from the built-in steam property functions or from the user-defined wet steam property functions. Therefore, if solid properties need to be set and adjusted, then it must be done in the Create/Edit Materials dialog box before activating the wet steam model.