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23.6.1 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 [ 251].

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 Section  23.13. Solution settings and strategies for the wet steam model can be found in Section  23.14.5. Postprocessing variables are described in Section  23.15.1.



Limitations


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


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