Issue related to Constant heat flux Boundary condition
 

I want to specify constant heat flux Boundary condition as below and i have two questions
1. what are the difference between externalWallHeatFluxTemperature and fixedGradient BC?

2. I am getting some error for below BC

Wall
{
type externalWallHeatFluxTemperature;
mode flux;
q uniform 1446; // (q = W/m^2)
kappaMethod fluidThermo;
kappa none;
kappaMethod fluidThermo;
value $internalField;

// type fixedGradient;
// gradient uniform 55223; //{ DT/Dn = q (w/m^2) / K_air (W/m-K) = 1446/0.026 = 55223 }
}
It is showing below error in OF8

"--> FOAM FATAL ERROR:
Cannot find a fluidThermo or solidThermo instance

From function Foam::tmp<Foam::Field<double> > Foam::temperatureCoupledBase::kappa(const fvPatchScalarField&) const
in file derivedFvPatchFields/temperatureCoupledBase/temperatureCoupledBase.C at line 133."

FOAM exiting

Thanks

Maybe it doesn't matter since it should skip the second one, but try removing the duplicate "kappaMethod fluidThermo;" row?

You might be trying to use it with a solver that is not using solid or fluidThermo.