Is the helix a solid body, or do you have a second fluid domain inside the coil that you are trying to heat/cool? If it is the former, then the 2D quasi-"axisymmetric" model may very well be sufficient for what you are looking for. You could even create a nice pure quad mesh with an o-grid looking block around each tube. Boundary Conditions: Do you know the temperature at the surface of the helix? Then just apply this. Or does the coil have heat generation? Then you can divide out what the heat generation would be and apply the flux on the surfaces.