Difference between HF and Temperature BCs
 

Hi guys,

I've been having a problem lately with a 3D CFD model of a solar Stirling engine.

I am basically running 2 set-ups, 1 where I use HF BCs for the surfaces of the heater and 1 where I use constant temperature BCs.

The 1st way to check if such models are "valid" is by calculating the indicated power output from the pressure and volume monitors on the piston surfaces. Then, the pressure and temperature variations inside the working spaces need to be checked against experimental data etc.

Both these models have very similar temperature and pressure variations.
However, for the model with the temperature BCs I get a power output of aprox. 2 kW which is what I expect, whereas for the one with HF BCs I get around 1 kW.

Is there any physical/numerical implication which I am missing that would explain this?

I had run a 3D model which included shell conduction for which I had actually meshed the wall thicknesses of the engine and had applied HF BCs and it had worked fine.

Is there any particular boundary surface set-up for which HF BCs are inapplicable?

Please let me know.

Brgrds,
Thanos