Different air volume passing through different flow paths of same cross sectional area

[D. J. LEE]

Hi everyone,

I needed to replace a duct with an orifice of the same size, so I did a simulation, as shown in the attached file.

There are only two states in the orifice: open and closed.

Upon reviewing the simulation results, I found a significant difference in the wind volume passing through the two flow paths.

This has piqued my curiosity, and I'm eager to understand the underlying reasons.
I thought more air volume would pass through the orifice than through the duct.

Please let me know what I don't understand.

Regards,

D. J. Lee

p.s. To Mr. Dolls, William Stuart

Are there any plans to increase the size of memory given to the names of control elements in programs provided by NIST, especially CONTAM?

I want more freedom in deciding variable names.

[Dols, William Stuart (Fed)]

Please review the equations for orifice and duct flow in the Theory section.

Your request is a bit tricky with CONTAM, because one needs to know the friction factor in order to accomplish this (you can estimate this with a Moody diagram).

The attached example shows nearly equivelent duct and orifice but in the opposite manner of what you requested.
That is, the duct has zero length and modified Sum of loss coefficient to match the 0.3 m² orifice according to the equations below:

 

 

Also, the loss coefficients for the terminals are set to the minimum value of 0.001.

 

- Stuart

 

W. Stuart Dols

National Institute of Standards and Technology
Engineering Laboratory
Indoor Air Quality and Ventilation Group

100 Bureau Drive

Gaithersburg, MD 20899-8633

 

 

From: con...@list.nist.gov <con...@list.nist.gov> On Behalf Of D. J. LEE
Sent: Thursday, June 6, 2024 12:18 AM
To: CONTAM <con...@list.nist.gov>
Subject: [contam] Different air volume passing through different flow paths of same cross sectional area

 

Hi everyone,

 

I needed to replace a duct with an orifice of the same size, so I did a simulation, as shown in the attached file.

There are only two states in the orifice: open and closed.

 

Upon reviewing the simulation results, I found a significant difference in the wind volume passing through the two flow paths.

This has piqued my curiosity, and I'm eager to understand the underlying reasons.
I thought more air volume would pass through the orifice than through the duct.

Please let me know what I don't understand.

 

Regards,

 

D. J. Lee

p.s. To Mr. Dolls, William Stuart

Are there any plans to increase the size of memory given to the names of control elements in programs provided by NIST, especially CONTAM?

I want more freedom in deciding variable names.

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[William Dols]

Whoops. I forgot to attach example to previous message.
- Stuart