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.Â
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.Â
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