Back to the Basics...what am I missing???
 

Problem:

I have a CHT problem. I have reduced it to a simple box of air with a heater inside. The problem is my air temperatures rise, as expected, but my density decreases as my pressure stays the same. I really expect temperature to rise. I did this in the lab by quickly evacuating air in a pressure vessel. The result was colder air than the vessel. As I monitored air pressure, as the air heated, the pressure begin to rise. What am I missing here? I'm solving steady state. If I solve transient, I get a pressure rise...??

Problem setup:
Steady state
Box: plastic
Air: ideal gas
Heater: metal sphere with volumetric heat gen
Heat transfer: total energy
BC: Convection on outer walls of Box.

Problem is very well converged and temperatures have steadied. Air at 25C shows absolutely no change in pressure. Air Ideal Gas shows about .1Pa change in pressure....not enough to cover the 50 degC heat rise.

Thanks for the input.

Lee Watson
TTi

Did you mistakenly use incompressible ideal gas (uses operating pressure and only changes density with temperature) instead of ideal gas?