modelCondensation

Information

This components allows to model the mass flow rate and the energy transfer induced by the condensation of a condensable species over a colder surface.

Remark: Condensation is scientifically defined as the transition from gaseous to solid state. However, it is commonly used today for the passage from the gaseous to liquid state. The term “condensation” is therefore used in this report for a transformation corresponding to liquefaction.

The mathematical beyond this module uses a different approach than the filmwise or dropwise condensation where the main assumption remains that the limiting phenomena is the capability of the film to evacuate the heat from condensation. Here the mains assumptions is that the model for condensation is limited by the capability of the flow of moisture laden vapour to be brought into contact with the cold surface.

During condensation, the heat transfer and mass transfer mechanism can be decomposed by the following steps.

1. A mass of moist air from a control volume is brought into contact with the wall by the natural convection.
2. The gas exchanges heat with the wall until the temperature of the wall and the gas are equal (hypothesis required to computed the mass flow rate induced by natural convection: see FreeConvection module for details). The Heat tranfer induced by this cooling phase is not counted in this module since it is the role of the convection modules. Here only the Heat from condensation is exchanged
3. The condensable liquid contained in this mass of gas is then totally drained until the density of the condensable species equates to that of saturation at the temperature of the wall. Since the characteristic time of condensation is much less than the convective time, it can be considered that the whole potentially condensable material quantity has condensed.

The element limiting the rate of condensation is the ability to bring the mass of moist air into contact to a wall. This is represented by a coefficient βv determined by the following equation:

Where:

• V_fow is the volume flow rate induced by the convection phenomenon
• β is the mass transfer coefficient
• A is exchange surface area between the fluid and the wall
• m_fow is the mass flow rate of condensation
• d is the density of the condensable species
• d_sat is the saturation density of the condensable species at the wall surface temperature heatPort.T

As explained at the begining of this description, the fluid of the control volume is first cooled before condensation occurs. For the energy balance, this phase of sensible cooling is not counted as heat exchange by condensation because it has already been in the convective exchange model. The additional energy transferred from the fluid into the wall is the latent heat from condensation.

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