On domains the total heat flux, tflux, corresponds to the conductive and convective heat flux. For accuracy reasons the radiative heat flux is not included.

For fluid domains (for example, Fluid), the total heat flux is defined as:

The conductive heat flux variable, dflux, is evaluated using the temperature gradient and the effective thermal conductivity:

In the general case keff is the thermal conductivity, k.

For heat transfer in fluids with turbulent flow, keff = k + kT, where kT is the turbulent thermal conductivity.

For heat transfer in porous media, keff is the effective conductivity computed from the solid and fluid conductivities.

The turbulent heat flux variable, turbflux, enables access to the part of the conductive heat flux that is due to turbulence.

The convective heat flux variable, cflux, is defined using the internal energy, Ei:

The internal energy, Ei, is defined as:

where H is the enthalpy defined in Equation 4-5.

The convective heat flux may be oriented in the opposite direction of the velocity field’s direction, when the internal energy, Ei, has a negative value. This happens when the sensible enthalpy (variation from reference enthalpy Href) is negative. Href is set to 0 J/kg at pref (1 atm) and Tref (293.15 K) in COMSOL Multiphysics. See Thermodynamic Description of Heat Transfer for details.

The total energy flux, teflux, is defined when viscous dissipation is enabled:

where the total enthalpy, H0, is defined as

In participating media, the radiative heat flux, qr, is not available for analysis on domains because it is more accurate to evaluate the radiative heat source .