I'm encountering a few issues while trying to run a thermal test on a flask with an internal heat load. I've started with a steady state at a low ambient and the natural convection patterns seem to very sensible (see below):
However, when I apply an 800C ambient temperature (under thermodynamic parameters and enivornment radiation) for 30 minutes I seem to generate a downdraught in the main body of air of around 6m/s (see below). As you can see, the flow is entering the domain at the top at around this velocity. I have applied no velocity component to the air, so I am at a loss as to what is causing this. The temperature profile does not look unreasonable, but the only downward flow I would expect would be close to the flask where it is cooling the ambient air. Is there a setting I've missed that might be causing this?