Use this node with 1D components to model domains with another cross sectional area or another cross sectional perimeter than the global one that is used in the Heat Transfer interface Physical Model section. In 1D geometries, the temperature is assumed to be constant in the radial direction, and the heat equation is modified to account for that.

Enter values for the Cross sectional area Ac and the Cross sectional perimeter Pc to set the cross section of the domain in the plane perpendicular to the 1D geometry.

Physics Tab with interface as Heat Transfer in Solids or Heat Transfer in Fluids selected:
Domains>interface>Change Cross Section