4.10.10. Heat Flux

The heat flux boundary condition is no a complete fluid boundary condition, but can be used on top of another boundary condition to add a specified heat flux.

# Define an adiabatic (or fixed temperature) wall
begin wall boundary condition on surface sideset_4
  # ...
end

# And add a heat flux to it
begin heat flux boundary condition on surface sideset_4
  heat_flux = 500.0 # W/m2
end

The value specified should be in $\rm{W} / \rm{m}^2$ (or equivalent for your problem units) and use the convention that positive is a flux into the domain.

The complete list of available commands for the heat flux boundary condition and the syntax for the different options can be found in Heat Flux Boundary Condition On Surface.