24.7.6 Wet Steam Model

Boundary Conditions, Initialization, and Patching

When you use the wet steam model (described in this section in the separate Theory Guide and Section  24.6), the following two field variables will show up in the inflow, outflow boundary dialog boxes, and in the Solution Initialization task page and Patch dialog boxes.

• Liquid Mass Fraction (or the wetness factor)

  In general, for dry steam entering flow boundaries the wetness factor is zero.

• Log10(Droplets Per Unit Volume)

  In general this value is set to zero, indicating zero droplets entering the domain.

Solution Limits for the Wet Steam Model

When you activate the wet steam model for the first time, a message is displayed indicating that the Minimum Static Temperature should be adjusted to 273 K since the accuracy of the built-in steam data is not guaranteed below a value of 273 K. If you use your own steam property functions, you can adjust this limit to whatever is permissible for your data.

To adjust the temperature limits, go to the Solution Limits dialog box.

Solution Controls Limits...

The default maximum wetness factor or liquid mass fraction ( $\beta$) is set to 0.1. In general, during the convergence process, it is common that this limit will be reached, but eventually the wetness factor will drop below the value of 0.1. However, in cases where the limit must be adjusted, you can do so using the text user interface .

define $\rightarrow$ models $\rightarrow$ multiphase $\rightarrow$ wet-steam $\rightarrow$ set $\rightarrow$ max-liquid-mass-fraction

Note that the maximum wetness factor should not be set beyond 0.2 since the present model assumes a low wetness factor. When the wetness factor is greater than 0.1, the solution tends to be less stable due to the large source terms in the transport equations. Thus, the maximum wetness factor has been set to a default value of 0.1, which corresponds to the fact that most nozzle and turbine flows will have a wetness factor less than 0.1.

Solution Strategies for the Wet Steam Model

If you face convergence difficulties while solving wet steam flow, try to initially lower the CFL value and use first-order discretization schemes for the solution. If you are still unable to obtain a converged solution, then try the following solver settings:

1.   Lower the under-relaxation factor for the wet steam equation below the current set value. The under-relaxation factor can be found in the Solution Controls task page.

Solution Controls

2.   Solve for an initial solution with no condensation. Once you have obtained a proper initial solution, turn on the condensation.

To turn condensation on or off, go to the Solution Controls task page.

Solution Controls

In the Equations dialog box, deselect Wet Steam in the Equations list. When doing so, you are preventing condensation from taking place while still computing the flow based on steam properties. Once a converged flow field is obtained, turn the Wet Steam equation back on again and compute the mixture solution.