If you're fixing (via the profile) the flow field I'd expect some flux balance issues due to how data is stored, transferred and calculated. The amount of difference is very likely model and material property related, and as I've never tried what you're doing (I'd have used Fluent rather than another code) I don't have anything to compare with.Â
Once you've got the first iteration or few done you can use Solve>Controls>Equations to turn off the flow equations as you only need to solver the scalar: the remainder of the flow is fixed by the profile. That'll speed up the scalar calculation.Â
Overall I can't give detailed advice, and would be checking results very carefully having transferred data in that way.Â
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