VOF Boiling Model Reference

Table 1. VOF Boiling Model Family Reference
Model Names	VOF Boiling
	Rohsenow Boiling
	Transition Boiling
Theory	See Boiling.
Provided By	[phase interaction] > Models > Optional Models
Example Node Path	[phase interaction] > Models > VOF Boiling
Requires	Physics continuum selections: Material: Multiphase (Automatically activates Multiphase Interaction) Multiphase Model: Volume of Fluid (VOF) (Automatically activates Segregated Flow, and Gradients) Optional Models: Segregated Multiphase Temperature and Gravity A liquid primary phase and a gas secondary phase that is the vapor of the liquid. For each phase: Material: Gas or Liquid A phase interaction is required, with a liquid primary phase and a gas secondary phase that is the vapor of the liquid. Phase interaction selections: Optional Models: VOF Boiling Boiling Models: Rohsenow Boiling or Transition Boiling
Properties	Key properties are: VOF Boiling: None Rohsenow Boiling: See Rohsenow Boiling Model Properties. Transition Boiling: See Transition Boiling Model Properties.
Activates	Physics Models	VOF Boiling: Boiling Models Rohsenow Boiling and Transition Boiling: None.
	Model Controls (child nodes)	VOF Boiling: None Rohsenow Boiling and Transition Boiling: HTCxArea. See HTCxArea Properties.
	Materials	Boiling Temperature (liquid phase only) Heat of Formation (both liquid and gas phase) Standard State Temperature (both liquid and gas phase) See Material Properties.
	Boundary Inputs	For wall boundaries only: Alpha_filmBoiling (for Rohsenow Boiling) Boiling Scale Factor (for Transition Boiling) See Boundary Settings.
	Field Functions	Boiling Rate of [phase interaction], Wall Boiling Heat Flux of [phase interaction] See Field Functions.

Rohsenow Boiling Model Properties

C_qw

The empirical coefficient

C_{q w}

in the Rohsenow expression for the wall heat flux, Eqn. (2671). Varies with the liquid-surface combination.

C_ew

The fraction

C_{e w}

of the wall heat flux due to boiling that is used in creation of vapor bubbles. Used in Eqn. (2672).

Sc_t

The turbulent Schmidt number that is associated with the turbulent diffusion of vapor bubbles. Used in Eqn. (1871).

Resolve Liquid-Vapor Interface

When this option is activated, the HRIC scheme is used to track the interface between the liquid and large bubbles of vapor, in a similar way to other VOF simulations.

In most VOF simulations, the HRIC scheme is used to mimic the convective transport of the VOF phases. The HRIC scheme tries to maintain a sharp interface between the phases. In boiling and cavitation simulations, it is assumed that the vapor bubbles are smaller than the mesh size and cannot be resolved. The HRIC scheme would produce unphysical results in such conditions. By default, the VOF Boiling model uses 1st-order convection instead.

However, if your simulation involves film boiling or large bubbles, and you know that the mesh resolution is sufficient to resolve them, activate this option.

n_p

The Prandtl number exponent

n_{p}

(1.73 by default) in Eqn. (2671).

URF_whf

Under-relaxation factor for heat flux due to boiling, at walls with prescribed heat flux.

Transition Boiling Model Properties

C_ew: The fraction $C_{e w}$ of the wall heat flux due to boiling that is used in creation of vapor bubbles. Used in Eqn. (2672).
Sc_t: The turbulent Schmidt number that is associated with the turbulent diffusion of vapor bubbles. Used in Eqn. (1871).
q_max: The positive empirical constant $q_{max}$ in the Transition Boiling model, Eqn. (2675) to Eqn. (2677).
k1: The positive empirical constant $K_{1}$ in the Transition Boiling model, Eqn. (2675) to Eqn. (2677).
delT1: The positive empirical constant $Δ T_{1}$ in the Transition Boiling model, Eqn. (2675) to Eqn. (2677).
k2: The positive empirical constant $K_{2}$ in the Transition Boiling model, Eqn. (2675) to Eqn. (2677).
delT2: The positive empirical constant $Δ T_{2}$ in the Transition Boiling model, Eqn. (2675) to Eqn. (2677).
phi: The constant $ϕ$ in the Transition Boiling model, Eqn. (2675) to Eqn. (2677).
Resolve Liquid-Vapor Interface: See Rohsenow Boiling Model Properties.
URF_whf: See Rohsenow Boiling Model Properties.

HTCxArea Properties

The HTCxArea node applies to both Rohsenow Boiling and Transition Boiling.

Specifies the heat transfer coefficient ( $W / m^{2} K$ ) between the vapor bubbles and the surrounding liquid, which is multiplied by the specific contact area (contact area per unit volume, $m^{2} / m^{3}$ ) between the two. This value is $H T C x A r e a$ in Eqn. (2678).

Material Properties

Unless stated otherwise, these material properties apply to both the liquid phase and the vapor phase.

Boiling Temperature: Applies to the liquid phase only.
The saturation temperature of the liquid phase. This value is $T_{s a t}$ in Eqn. (2671) and Eqn. (2678).
Heat of Formation: See Using the Heat of Formation.
Standard State Temperature: See Using the Standard State Temperature.

Boundary Settings

The following setting is available for each Wall boundary.

Alpha_filmBoiling

The vapor volume fraction that is associated with the start of film boiling. This value is the vapor volume fraction $α_{f i l m B o i l i n g}$ in Eqn. (2674).

Applies to Rohsenow Boiling only.

BoilingScaleFactor

The scale factor that allows you to scale the heat flux $q_{max}$ up or down for each boundary. This value is $S$ in Eqn. (2675), Eqn. (2676), and Eqn. (2677).

Applies to Transition Boiling only.

Field Functions

Boiling Rate of [phase interaction]: The rate of mass transfer from liquid to vapor due to boiling.; This field function is only available when the Temporary Storage Retained option is ticked for the Segregated VOF solver.
Wall Boiling Heat Flux of [phase interaction]: The heat flux passing from the wall into the liquid.