I'm sure this is a stupid question and maybe it's due to lack of enough coffee, but I'm having a problem defining the pressure in a flow simulation. We're trying to simulate a portion of a vacuum chamber.  Just a tube, .093" diameter x 2" long.  On inside face of the lid at one end of the tube I set the inlet static pressure to 1 Torr (that's what we measure on the real system) and I have a 12 lpm outlet flow on the inside face of the lid at the other end.  When my environmental pressure is set to 1 Torr (in General Settings > Initial Conditions > Thermodynamic Parameters) everything is fine.  If I set that to 760 Torr, the flow goes backwards and the pressure increases to 340 Torr at the inlet.

I thought that by defining the pressure at the inlet, I'm forcing it to use 1 Torr.  But apparently Solidworks assumes that my inlet is open to atmosphere, and completely ignores my 1 Torr boundary condition?  Why even have a boundary condition on the inlet then?  And what pressure is it assuming at the exit?  I'm just confused in how to set up a model that isn't open to atmosphere.

Any help would be appreciated.  Thanks a lot.

Michael