[Venk]

Hi,
I am modelling (using ANSYS Fluent) a vertical helical coil heat exchanger with primary and secondary flows (gas to gas heat exchange). Secondary flow enters via 96 helical coil tubes and gets heated up by the primary flow. It is not practical to mesh all the helical coil tubes explicitly and hence I am assuming only 4 helical tubes with equivalent flow area for flow. I have not included any solids, so currently it is fluid only simulation.

My issue is how do I account for heat transfer area without explicitly modelling the area in my CFD model. I am using convection augmentation factor via text user interface with the assumption that it is simply the ratio of actual heat transfer area to the model heat transfer area. But the results are not what I am expecting, it is definitely increasing the flux on the walls, consequently temperature changes. My heat transfer area from the coils is around 17 times smaller than the actual heat transfer area. I am no where near the measured values.

Can someone who attempted similar problems throw some light on this approach?

Basically, if we are not able to represent the actual geometry, for flows, we can still give pressure drop/resistance via porous approach. What should be done for convective heat transfer if the model area is not same as the actual area of heat transfer.
Thank you and looking forward to hearing from heat transfer experts.
Regards,
Venkat