Modeling Boiling

Boiling is a rapid vaporization of a liquid. It typically takes place when a liquid is heated to the boiling point (saturation temperature of the liquid T s a t ). Its saturation vapor pressure then becomes equal to or larger than the pressure of the surrounding liquid.

Vapor can form from a liquid in the following locations:

  • At a vapor-liquid interface away from surfaces
  • At a solid surface with pre-existing vapor or gas pockets
  • In the bulk of the liquid due to density fluctuations.

If you are only interested in modeling the improved heat transfer due to boiling, you can simulate boiling with a single-component liquid.

To model boiling in a VOF multiphase simulation:

  1. Create a VOF Multiphase simulation and activate the Gravity and Segregated Multiphase Temperature models.

    See Setting Up a VOF Multiphase Simulation.

  2. Create two Eulerian phases: a liquid phase and a vapor phase that represents the gaseous form of the liquid phase.

    Each phase must be single-component (that is, pure liquid and pure vapor). The VOF Boiling model does not support multi-component phases.

  3. Create a phase interaction. Select the liquid Primary Phase first and then select the vapor Secondary Phase.
  4. In the Phase Interaction Model Selection dialog, in the Optional Models group box, select VOF Boiling.
  5. In the Boiling Models group box, select one of the following:

    • Rohsenow Boiling

      A nucleate boiling model applicable for boiling at relatively low solid temperatures. A film boiling model is also included to facilitate its use with boiling at high solid temperatures.

      Nucleate boiling involves the creation and growth of vapor bubbles on a heated surface, which rise from discrete points on a surface. Film boiling occurs when the critical heat flux is exceeded and a continuous vapor film covers the heated surface.

    • Transition Boiling

      This model is applicable to both nucleate and transition boiling.

      Transition boiling occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling. It is an intermediate, unstable form of boiling with elements of both types.

  6. Close the Phase Interaction Model Selection dialog.
  7. Select the Rohsenow Boiling or Transition Boiling node and set the appropriate properties.

    See Rohsenow Boiling Model Properties and Transition Boiling Model Properties.

  8. Select the HTCxArea sub-node and specify the heat transfer coefficient between the vapor bubbles and the surrounding liquid.

    See HTCxArea Properties.

The boiling model has to know the boiling temperature for the liquid phase and the latent heat of vaporization. The latent heat of vaporization is the difference between the enthalpies of the vapor and the liquid at the saturation temperature.

  1. Set the material properties of the liquid and gas phases.

    The important properties are:

    • Boiling Temperature (liquid phase only)

      The boiling or saturation temperature T s a t of a substance is the temperature at which it can change its state from a liquid to a gas throughout the bulk of the liquid at a given pressure.

    • Heat of Formation (both liquid and gas phase)

    • Standard State Temperature (both liquid and gas phase)

    See Material Properties.

  2. For each wall boundary, expand the [boundary] > Phase Conditions > [phase interaction] > Physics Values node and set the appropriate values.

    • Alpha_filmBoiling

      Applies to Rohsenow Boiling only.

      Specifies the film-boiling vapor volume fraction α f i l m B o i l i n g , which is the volume fraction of the vapor phase that indicates full transition to film boiling.

      For values of the vapor volume fraction smaller than α f i l m B o i l i n g , nucleate boiling is assumed. For larger values, film boiling is assumed.

      A value of α f i l m B o i l i n g larger than 1 implies that there is no film boiling at the boundary.

      See Film Boiling Model.

    • BoilingScaleFactor

      Applies to Transition Boiling only.

      The scale factor that allows you to scale the heat flux q max up or down for each boundary.

      See Transition Boiling Model.