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.

The Mixture Multiphase (MMP) model uses the nucleate boiling methodology derived for the Eulerian Multiphase (EMP) model (see Boiling). The Mixture Multiphase (MMP) model solves only one mixture temperature. The nucleate boiling model is driven by the super heat at the wall. As such, for wall boiling the mixture is always in contact with the wall, and the convective heat flux of liquid and vapour are lumped together.

To set up a basic bulk boiling simulation, first verify that your application is suitable for modeling as a subcooled boiling process. Base your model on real experimental or industrial conditions, or at least on an energy balance confirming that subcooled boiling is expected. Then, start from the default model selections and default model calibrations.

To set up a wall boiling simulation, you activate the Boiling/Condensation model before activating the Wall Nucleate Boiling model. The Wall Nucleate Boiling model is intended to cover forced-flow, subcooled boiling up to pressures of around 155 bar, with as little calibration of model constants as possible. Therefore, it is built up mechanistically from many simpler submodels:

  • Wall Dryout Area Fraction
  • Nucleation Site Number Density
  • Bubble Departure Diameter
  • Bubble Departure Frequency
  • Wall Transient Conduction
  • Bubble Influence Wall Area Fraction
  • Bubble Induced Quenching Heat Transfer Coefficient

The output from each of these submodels can be inspected graphically at run time. If suitable experimental data is available, these submodels can also be calibrated or replaced with user-defined relationships. The wall boiling submodels are implemented to use all of the phase and interface properties you specify (such as density, saturation enthalpy, surface tension).

The primary phase is the liquid phase and the secondary phase is the vapor phase.

To set up a Mixture Multiphase (MMP) boiling simulation:

  1. For the physics continuum that represents the Eulerian multiphase flow, select the following model in addition to the models that you previously selected.
    Group Box Model
    Optional Models Segregated Multiphase Temperature
  2. In the phase interaction, select the following models in addition to the models that you previously selected:
    Group Box Model
    Optional Models Boiling/Condensation
  3. Expand the [phase interaction] > Models > Boiling/Condensation node and specify a value or correlation for the Nusselt number to control the heat transfer rate to the boiling interface:
  4. Select the Interaction Length Scale node and specify the First Dispersed Regime Interaction Length Scale and the Second Dispersed Regime Interaction Length Scale.

If you want to model wall boiling, continue with the following steps.

  1. Reopen the Phase Interaction Model Selection dialog and, in the Optional Models group box, select Wall Nucleate Boiling.
    The Wall Bubble Nucleation and Wall Transient Conduction (Quenching) models are selected automatically. See Wall Boiling Model Reference.
  2. Select the [phase interaction] > Models > Wall Nucleate Boiling node and set the Wall Dryout Area Fraction.

    Specifies the amount of heat flux applied at the wall that goes towards vapor convection, as opposed to liquid convection and evaporation.

    See Wall Dryout Area Fraction Properties.

  3. Set the appropriate simulation properties.

    The Wall Boiling model requires that you set valid and accurate data for the following:

    • For Reference Values > Gravity, make sure that acceleration due to gravity is acting in the correct direction.
    • For Reference Values > Reference Pressure, set the value to the expected outlet pressure, if setting zero relative pressure conditions at the outlet. Otherwise, set the value to the design operating pressure.
    • For Phase Interaction > Multiphase Material > Surface Tension, set the surface tension coefficient, at saturation temperature, for the liquid-vapor interface.
    • For Eulerian Phases > Gas > Molecular Weight, set an appropriate material property for the vapor phase.

The Wall Boiling model has submodels that capture various aspects of the boiling process. It is recommended that you do not change the default properties for these submodels, unless you have supporting experimental data.

Some of the original coefficients are based upon water as the working fluid. Therefore, some adjustments are required when using a working fluid other than water.

The wall contact angle for the Hibiki Ishii Nucleation Site Number Density and Kocamustafaogullari Bubble Departure Diameter models is specific to the combination of working fluid and boiling surface.

The wall contact angle is a nominal value that is based on room temperature, rather than a value measured under boiling conditions.

Adjust the following options, from older correlations for use with other working fluids:

  • Kurul Podowski Interaction Length Scale
  • Lemmert Chawla Nucleation Site Number Density
  • Tolubinsky Kostanchuk Bubble Departure Diameter
  1. Select the Wall Bubble Nucleation node, and set the appropriate properties.

    • Nucleation Site Number Density

      The nucleation site number density determines the number of locations on the heated surface where bubbles form, per unit area. Nucleation Site Number Density Properties

      See Nucleation Site Number Density Properties.

    • Bubble Departure Diameter

      The bubble departure diameter determines the diameter of the bubble at the instant it leaves the nucleation site.

      See Bubble Departure Diameter Properties.

    • Bubble Departure Frequency

      The bubble departure frequency determines how many bubbles leave a nucleation site per second.

      See Bubble Departure Frequency Properties.

    • Lift Off Diameter (only if S-Gamma Model is selected in the dispersed phase)

      The lift off diameter determines the diameter of the bubbles as provided to the respective particle size distribution model at the wall.

      See Lift Off Diameter Properties.

  2. Select the Wall Transient Conduction node, and set the appropriate properties.

    This model corrects the Bubble Induced Quenching Heat Flux so that it uses the temperature of the liquid brought to the wall by the action of the departing bubble.

    See Wall Transient Conduction Properties.

  3. Specify the boundaries on which wall boiling is allowed. For each wall boundary, do the following:
    1. Select the Regions > [region] > Boundaries > [wall boundary] > Physics Conditions > Wall Interphase Mass Transfer Option node and set Method to Active.
      To prevent wall boiling from occurring on this boundary, set Method to None. For example, use this option when a boundary is adiabatic.
    2. Select the [wall boundary] > Physics Conditions node and for each phase in the phase interaction, set the User Wall Heat Flux Coefficient Specification.