If the pressure drop in each zone is minimal, do you need to have a variable density? If not, you can set (for example) air to have a density of 4.5 kg/m3 if that's correct for 8 bar, and another air material in the tube at 1.18 kg/m3 to mimic atmospheric conditions. Pressure is just a driver for flow in most cases so we're only really worried about the difference over the domain.
In CFX there's a flag somewhere to allow you to set two sets of operating conditions. I think it's detached zones or similar.
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