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Choked Flow with altitude.

Choked Flow with altitude.

Choked Flow with altitude.

(OP)
I am trying to understand the effects of altitude on a choked flow.

I have, for example, a charged cylinder of oxygen at 100% concentration. This is pressurised to 2000 psig. The outlet pressure from the charged cylinder is regulated to 50 psig. The outlet from the pressure regulator goes to a flow restrictor which controls the outlet flow to a fixed 10 L/min.

The starting condition is that the setup above is in air at sea level with normal ambient conditions of 1013.25 and 21°C.

My comparison condition is that the exact same setup is now raised to 10,000 feet in air. My question is what is the flow at this altitude?

Assumptions: The source regulated pressure remains unchanged and the physical geometry of the flow restrictor does not change.

RE: Choked Flow with altitude.

Sounds somewhat academic to me. By "controls the outlet flow to a fixed 10 L/min", what exactly do you mean? If it truly controls the outlet flow to 10 L/min, isn't the answer obvious?

What about temperature at 10,000 feet? Me thinks it is somewhat colder up there.

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

RE: Choked Flow with altitude.

As long as upstream pressure/downstream pressure >= about 1.893 flow will be choked. Any further drop in downstream pressure will not increase flow.

Ted

RE: Choked Flow with altitude.

(OP)
Thnank you for your replies.

There is no control mechanism, that was just my bad description! Sorry.

The flow is set to 10L/min using a flow restrictor. There is no closed loop control. The restrictor could be described in its simplest form as a hole. Its size will not change to compensate for ambient conditions. The hole is sized to give 10 L/min flowing to atmosphere with an upstream pressure of 50 psig.

I understand that the flow is not affected by downstream pressure variations if the ratio is >= about 1.893 if all other conditions remaining the same.

For the scenario in question, the temperature is the same at 10,000 feet (approx 700mBar) as it is at sea level 1013.25, i.e. 21C. This could be, for example, a pressurised aircraft cabin where the pressure is kept at between 700 & 800 mBar, but the temperature stays close to room temperature.

I'm not clear if the air density ratio at the Absolute Pressure of 700mBar, keeping temperature constant, will change (increase) the 10 L/min.

Thanks for your time in advance........



RE: Choked Flow with altitude.

Please describe the restrictor. Can it be modelled as a "thin plate orifice" or a "thick plate orifice" or something else? I.e. what is diameter/thickness ratio? This will help decide what will happen to the volumetric flow rate.

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

RE: Choked Flow with altitude.

(OP)
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.

RE: Choked Flow with altitude.

I'd assume it's like a thin plate orifice. In a thin plate orifice, the flow increases as downstream pressure falls below the corresponding critical pressure ratio. Technically, it doesn't choke like a nozzle or thick plate orifice. So, the flow will increase.

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

RE: Choked Flow with altitude.

(OP)
Thanks Latexman,

So are you saying:

Upstream Abs Pressure: 4 Bar
Downstream Abs Pressure: 1.013 Bar
Pressure Ratio: 3.94

with a fixed, thin plate orifice and the conditions changing to:

Upstream Abs Pressure: 3.687 Bar
Downstream Abs Pressure: 0.7 Bar
Pressure Ratio: 5.27

That the flow will increase?

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