Hi I am trying to model a 3D shell and tube heat exchanger where heat is transferring through the tube wall. For some purpose i have taken the wall thickness to be negligible so i have to give the heat flux at the wall in addition to heat conduction. I have endothermic reaction inside the tube and exothermic reaction in the shell so it is difficult to calculate the heat transfer coefficients. As there are many options in comsol to give the heat flux at the wall. Is there anyone who can suggest how to give the heat flux at the wall?