### 7.15.6. Droplet Condensation Model

To activate the Droplet Condensation Model (or Nonequilibrium Steam Model when steam is the working fluid), use the following steps:

1. Choose your vapor and liquid materials, as well as a homogeneous binary mixture to define saturation properties. For each condensed phase, there should be a binary mixture defined between each droplet phase and the continuous phase. For steam calculations, the IAPWS library is recommended.

2. Choose a multiphase simulation with one continuous vapor phase and as many dispersed liquid phases as appropriate. (Each droplet phase is assumed to be monodispersed at each point in space, although the diameter can differ through the domain.)

3. For many condensing droplet applications, the homogeneous multiphase model is appropriate. When the droplets become bigger (for example, 1 or larger), then the inhomogeneous multiphase model should be considered.

4. For the heat transfer model, ensure that Homogeneous Model is deactivated and select Fluid Dependent.

5. Set the droplet phase morphology to `Droplets (Phase Change)`.

6. Set the continuous phase heat transfer model to `Total Energy`.

7. Subsequent modeling choices depend on whether the droplets are small or not.

   For small (less than 1 ) droplets, the following recommendations apply:

   1. Select `Small Droplet Temperature` (recommended) or `Total Energy` as the heat transfer model. Note that the droplet temperature setting implies that no transport equation is solved for the energy state of the droplet, but is determined from an algebraic relationship valid for sub-micron droplets.

   2. Activate the homogeneous nucleation model if you expect nucleation to occur in the domain. For multidomain applications (for example, multistage turbines), the nucleation model can be controlled independently for each domain. This allows one to model polydispersed droplets where a new set of droplets nucleates in each stage component (rotor or stator). To do this, one droplet phase can be defined for each turbine component (or group of components), and for a particular component (group) the nucleation model activated only for the corresponding droplet phase. Each droplet phase will continue to grow through the entire machine.

   3. On the Fluid Pairs tab, select `Small Droplets` for Interphase Heat Transfer Model. For mass transfer, select `Phase Change` and choose the small droplets option

   For larger droplets, the following recommendations apply:

   1. Select `Total Energy` (recommended) or `Thermal Energy` as the heat transfer model

   2. On the Fluid Pairs tab, select `Two Resistance` as the interphase heat transfer model. On the continuous phase side, select the Ranz Marshall correlation, while on the droplet side, a Nusselt number of 6 is appropriate. For mass transfer, select `Phase Change` and choose the thermal phase change model

   In addition, keep the following in mind when setting boundary conditions and initial conditions:

   - Droplets may or may not appear through inlets and openings. If they do appear, choose the droplet volume fraction and droplet diameter (or droplet number) as appropriate. If they do not enter the boundary, choose zero for the volume fraction and droplet number; the droplet diameter is irrelevant

   - The gas temperature at inlets, openings, and for initial conditions should not extend too far in the supercooled (or superheated) region when a second phase is present. A good rule of thumb is that these temperatures should not be more than 5 [K] below (or above) the saturation temperature, or else nonphysical mass transfer rates will develop, which could cause divergence.

The following procedure is often helpful to solve droplet condensation problems:

1. Obtain a solution with the nucleation model turned off. This allows the supercooling to develop naturally in the flow, sets up a realistic expansion rate, and provides a very good initial guess for the full calculation.

2. Activate the nucleation model.

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