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fuelRodGapFvPatchScalarField

Description

Coupled boundary condition for modeling the presence of an evolving gap conductance between two bodies. The gap conductance is based primarily on the FRAPCON 4.0 manual, making it most suitable for standard fuel rods (fuel pellet + cladding).

The total gap conductance \(h\) is calculated as the sum of:

  • \(h_{\text{gas}}\): Gas conductance, which depends on the gap width, surface roughness, jump distance and the properties of the gas mixture in the gap.
  • \(h_{\text{rad}}\): Radiative heat transfer, modeled based on the emissivity of the surfaces and the temperature difference.
  • \(h_{\text{contact}}\): Contact conductance, which depends on the surface roughness and contact pressure
Warning

This boundary condition assumes that the same boundary condition is applied to a pair of regionCoupledOFFBEAT patches, which are coupled using the AMI (Arbitrary Mesh Interface) mapping algorithm.

Warning

This boundary condition requires the presence of a gapGasModel in the simulation.

Options

The fuelRodGap fvPatchField can be selected in the patch subdictionary inside the boundaryField subdictionary of the temperature field.

Parameters in the patch subdictionary for fuelRodGap:

patchType Specifies the type of underlying fvPatch and must be set to regionCoupledOFFBEAT.
kappa The name of the conductivity field. By default, it is set to "k", which is the field name for conductivity used by the OFFBEAT material class.
emissivity The name of the emissivity field. By default, it is set to "emissivity", which is the field name for emissivity used by the OFFBEAT material class.
gapGasModel The name of the gapGasModel class. By default, it is set to "gapGas", which is the name for the OFFBEAT gap gas model class.
alpha Since this class is a gap-conductance model itself, it is not necessary to specify the gap conductance as it is calculated by the code. If alpha is specified, it will be used as the initial condition for the gap conductance.
roughness Specifies the roughness field for the surface, used to calculate the effective gap width.
relax Relaxation factor for the gap conductance. It is set to 1.0 by default, but values as low as 0.1 are often very useful to aid (and sometimes accelerate) convergence.
value Specifies the initial value of the patch temperature field.

Usage

Here is a code snippet of a typical patch field dictionary:

fuelOuter
{
    type            fuelRodGap;
    patchType       regionCoupledOFFBEAT;

    roughness       uniform 1e-6;
    relax           0.1;

    value           $internalField;
}