Reply To: Volume Fraction too Low. How to Improve?

[Rob]

You're setting the HTC on the "outside" of the model to avoid having to model another fluid zone. If you also have tubeside condensation then it's not a simple boundary and you need to do some more thinking. 

The +/- is flow/flux in/out of the cells to "outside" with outside being where there's no mesh. So on an inlet a +ve flux is fluid entering the flow domain, a -ve value is flow leaving the modelled domain.