Conjugate Heat Transfer (CHT) Capability
This summarizes the design and details of the conjugate heat transfer capabilities with the linear finite volume system through the SIMPLE executioner.
This capability is activated by specifying a boundary on the "cht_interfaces" parameter. Other cht-related parameters can control the iteration between the solid and fluid energy equations. Once the capability is activated it will check if the used boundary conditions are compatible or not. In general, we introduced CHT versions of common boundary conditions such as LinearFVRobinCHTBC and LinearFVDirichletCHTBC that are dedicated for CHT applications.
For coupling purposes several new functors are created under the hood:
heat_flux_to_solid_* (where * is the interface boundary name),
heat_flux_to_fluid_* (where * is the interface boundary name),
interface_temperature_to_solid_* (where * is the interface boundary name),
interface_temperature_to_fluid_* (where * is the interface boundary name),
where the first two describe the heat flux from one domain to the other, while the other two describe the interface temperature supplied to the named destination side. The fluid boundary condition must use interface_temperature_to_fluid_*, and the solid boundary condition must use interface_temperature_to_solid_*.
Energy Conservation Equations
The energy conservation equations for fluid and solid domains are:
Where and are the fluid and solid specific enthalpies, and are the thermal conductivities, and and are the external heat sources.
Boundary Conditions
The coupling of the solid and fluid domains is done through boundary conditions that ensure:
Interface Temperature Relation**
where the area-normalized thermal resistance is and is positive for heat flow from the fluid into the solid. Temperature continuity is recovered when .
Continuity of Conductive Flux at the Interface**
Interface Thermal Resistance
The "thermal_resistance" parameter specifies the area-normalized thermal resistance(s) in . Its default value is zero. It accepts either one functor that is used for every entry in "cht_interfaces", or one functor per interface in the same order as cht_interfaces. Every evaluated resistance must be nonnegative. A spatially varying functor may be used, but it must be defined on at least one side of every face on the corresponding CHT interface.
For each destination side, the handler applies the resistance drop to the boundary temperature computed on the source side:
The signed heat flux follows the outward-flux convention of the source side. Consequently, the two directional temperature functors represent the values seen by their destination regions and naturally produce the two sides of the temperature jump. The heat-flux continuity condition is unchanged.
Surface-to-Surface Radiation
Radiation exchanged between solid surfaces across a transparent fluid can be included by setting "surface_radiation_object_name" to a GrayLambertSurfaceRadiationBase user object, such as ViewFactorObjectSurfaceRadiation. The user object must contain at least one boundary listed in "cht_interfaces".
On every CHT interface that belongs to the user object, the net outward radiative heat flux density leaves the solid without being deposited in the fluid. The flux handed to the receiving domain is therefore the conductive flux leaving the source domain minus the radiative flux:
This applies to both the Dirichlet-Neumann and Robin-Robin coupling methods below. The CHT iteration is considered converged when the integrated fluxes satisfy on each interface within "cht_heat_flux_tolerance", where and are the outward conductive heat rates from the solid and fluid and is the net outward radiative heat rate.
The user object computes one averaged radiative flux per sideset, which is applied uniformly over the corresponding CHT interface. To resolve the spatial distribution of the radiative flux, split the walls into several sidesets, for example with PatchSidesetGenerator.
Coupling Methods
The methods currently recommended for CHT utilize LinearFVDirichletCHTBC and LinearFVRobinCHTBC in the two different ways listed below. The Robin BC can also emulate a Neumann BC by setting the "h" parameter to 0.
Neumann-Dirichlet Coupling
Robin-Robin Coupling
The Robin-Robin method introduces virtual heat transfer coefficients and to enhance stability and convergence.