I have a cylindrical geometry with fluid flowing inside. The inside surface has a layer of deposits. Then I have a metal wall. The metal wall is insulated. So I have three resistances. Fluent does not offer heat loss through a composite wall so I have to use UDF to calculate heat loss through it. I can do it by defining constant wall heat flux and using a profile defined by UDF. But I don't know much about UDF coding.
Can someone help me with this?
See below the composite wall figure attached. The geometry is cylindrical.
If you define a constant wall heat flux boundary condition then Fluent calculates the wall temperature. The inside wall temperature will be calculated by fluent. You can use that temperature to calculate the heat loss through the composite wall using q = U*(Tw-Tamb)
You will first need t calculate the overall heat transfer coefficient, which will be the sum of the resistances of the deposit, wall, insulation and outside film coefficient. You will also need to calculate/estimate the outside film coefficient (will be very small typically in the range of 10-50 W/m2K, negligible in comparison to the insulation if the insulation is thick enough).
This will be the code for the wall heat flux profile:
Remember that WALL_TEMP_INNER is the wall temperature at the outer surface of the wall. WALL_TEMP_OUTER is the inner face temperature, i.e., the cell that faces the fluid.
The negative sign means that the heat is flowing out from the system to the surrounding.
Hope that helps.