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# Humidity

Autodesk Simulation CFD can simulate the condensation of liquid from a moist gas. Evaporation, however, is not supported.

Boundary Conditions

To model the effect of moisture on a gas flow, specify a Humidity and a Temperature boundary condition at every inlet of the model.

Initial Condition

It is often helpful to initialize the temperature and humidity throughout the model. A good value is often one of the inlet boundary condition values.

Materials

Apply the Moist Air material. Alternatively, create a new material with a density that varies using the Moist Gas variation.

Select Variable on the Material Environment dialog. This is needed to allow the material properties to vary.

Solve

On the Physics tab of the Solve dialog, enable Heat Transfer. Click the Advanced button, and select Humidity.

To view humidity as a results output quantity: click the Control tab, click the Results quantities button. Select Scalar.

For incompressible flows, only the temperature affects the fluid properties (including relative humidity). If pressure effects are to be considered, select Subsonic Compressible.

Analysis Continuation

When continuing an analysis from existing results, there may be a blip in the convergence monitor for temperature and scalar due to some internal conversion variables.

Results Visualization

The amount of liquid condensed and the calculated field values of relative humidity can be viewed as results. The condensed liquid is calculated as a mixture fraction, i.e., the mass of the condensed liquid divided by the total mass of the liquid, vapor and carrier gas.

The available results quantities for a humidity analysis are Relative Humidity and Percent Liquid.

The Percent Liquid is plotted over a range from 0 to 1. A value of 0 corresponds to completely dry air. A value of 1 indicates that liquid is condensing. Physically, a value of 1 indicates a liquid mist within the carrier gas.

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