# How altitude effects gas flow rate or not?

## Main Question or Discussion Point

I am considering the effects on volumetric and mass flow rate for two conditions:

Condition 1) 1013mBar, 21C (Air density as per)
Condition 2) 700mBar (10,000ft) 21C (Air density as per)

Setup: A variable flow restrictor is connected to a constant source of oxygen at 50 psig. The variable flow restrictor is adjusted to give a volumetric flow rate of 10 L/min for condition 1.

Question: The same setup is now in condition 2. What happens to the flowrate?

Note 1: The variable restrictor is not adjusted when at condition 2. It remains as it was set for condition 1.
Note 2: It appears that the flow is 'choked' as the difference between upstream pressure and downstream pressure stays within the limits required to be in the choked state (Ratio1.9±0.2)

I'm unsure how the density ratio for the two conditions could affect the flow output?

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cjl
If the flow is choked (and that certainly appears to be), the downstream pressure will not affect the flow rate.

Gold Member
What cjl says is certainly true, though it will depend on the geometry of your exit. If it is simply a straight section or a converging section then your flow in both situations is choked (Mach 1 at the region of smallest area) and therefore lowering the downstream pressure is not going to change the mass flux. Only changing the area of your exit or your upstream pressure would change your mass flux.

If your exit is diverging, you would have additional work to do since at least a portion of the flow in the nozzle will be supersonic and the downstream pressure required to choke the flow will change.

I thought the restriction was choked, but experimental results suggest otherwise.

The geometry is as follows:

The restriction is created by an annulus made up by a tapered shaft in a plain hole.
The hole is 1.58mm in diameter.
The plate thickness is 0.85mm.
The shaft within the hole tapers from approx diameter 1.50mm to 1.49mm within the o.85mm thickness.

It would appear that the flow increases directly proportional to the absolute pressure ratio up to altitudes of 20,000ft (465mBar).

This surprised me because I was under the impression that as long as the abs pressure ratio was above about 2, the downstream pressure would not change the flow.

Can you explain a little more about the converging / diverging effects on choked flow? From the restriction description, is this considered a thin plate orifice?