Modeling Boiling

The boiling models can be used for forced-flow, subcooled boiling applications.

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 Mass Transfer Rate model before activating the Wall Boiling model. The Wall 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).

High heat flux wall boiling cases can sometimes fail to converge. The two main areas of concern are the interphase mass transfer rates and the nucleation site number density. For more information, see Managing Convergence Issues in Wall Boiling Simulations.

For a Continuous-Dispersed phase interaction, the dispersed phase is the vapor (gas) phase of the continuous liquid phase. For a Multiple Flow Regime phase interaction the primary phase is the liquid phase and the secondary phase is the vapor phase.

The steps in this procedure are intended to follow on from one of the following:

To set up a 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 Phase Coupled Fluid Energy
  2. In the phase interaction, select the following models in addition to the models that you previously selected:
    Group Box Model

    Energy

    One of:

    • Segregated Fluid Enthalpy
    • Segregated Fluid Temperature

    Reaction Regime

    Non-reacting (applies to multi-component phases only)

    Optional Models

    		Interphase Mass Transfer
  3. In the Interphase Mass Transfer Rate group box, select one of the following:
  4. Expand the [phase interaction] > Models node and do one of the following:
    • For a Continuous-Dispersed phase interaction, select the Boiling Mass Transfer Rate node or Multicomponent Boiling Mass Transfer Rate node and set the following:
    • For a Multiple Flow Regime phase interaction, select the Boiling/Condensation node and set the following:

The following step applies to the Multicomponent Boiling Mass Transfer Rate node only.

  1. Set the Phase 0 Components property to specify the relationship between the phase components.

    You match the components of the continuous phase (in the left-hand column) with the components of the dispersed phase in the right-hand column.

    For each transfer activated, the continuous phase component name appears under the Equilibrium Coefficient > Composite node.

  2. Select the Interaction Length Scale node and do one of the following:

    • For a Continuous-Dispersed phase interaction, select the appropriate method for the Interaction Length Scale.

    • For a Multiple Flow Regime phase interaction, select the appropriate methods for the First Dispersed Regime Interaction Length Scale and the Second Dispersed Regime Interaction Length Scale.

    See Interaction Length Scale Model Reference.

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 Boiling.
    See Wall Boiling Model Reference.
    The Wall Bubble Nucleation and Wall Transient Conduction models are selected automatically.
  2. Select the [phase interaction] > Models > Wall 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 and Li 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.
  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] > Phase Conditions node and for each phase in the phase interaction, set the User Wall Heat Flux Coefficient Specification.

Return to the appropriate workflow: