boundary conditions for water boiler

Dear Sirs,
I'm evaluating Caedium for use in industrial heat exchangers environment.
My application is a steady state regime with water heating as it flows into a custom forced circulation boiler (no phase change). We need to optimize the geometry.
The boiler is part of a closed system (of course only the boiler has been modelled), so the mass flow is constant and the water is pressurized at 1.5 bar.
Which boundary conditions would you suggest at inlet/outlet? I'm thinking about:
1) Inlet: "Default" (with reference velocity derived from the mass flow), or "Volume Flow Rate";
2) Outlet: "Default" or "Extrapolated" but from the tutorials I cannot understand the difference.
(As to heat transfer, we will apply fixed temperature/heat flux to the "active" walls of the fluid-volume).
Thanks a lot for your help.

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Default boundary conditions good starting point

Often for boundary conditions Type = Default is a good starting point. If you have a mass flow rate then yes, consider using Physics:Inlet->Type = Volume Flow Rate, i.e., Mass Flow / Density. However, it is still advisable to specify a good estimate of your velocity, based on the mass flow rate for Substance->Properties->References->U.

For your outlet I'd suggest using Physics:Outlet->Type = Default if it is positioned away from strong flow gradients (i.e., little change in velocity, pressure), otherwise Type = Extrapolated should be a good choice.

For heat transfer simulations, check that Substance->Properties->References->g (gravity) is in the correct direction for your geometry orientation.

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