[sjosh90]

I have selected segregated fluid enthalpy model for energy to simulate fluegas flow inside boiler tubes. Is the selected model correct? Also explain how to specify the direction of heat flux (positive/negative) if the heat is going out of the walls of the geometry. I have attached the image of my geometry which is of fluegas path only.

[abdul099]

Why don't you create a simple block with just a few 100 cells and apply a positive heat flux to a boundary? Then you can see if heat is taken out of your body or put in, and you immediately know what a positive or negative heat flux means...