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Active Member
Posts: 6
Registered: ‎10-24-2013

Distributed Resistance Loss coefficient

93 Views, 3 Replies
11-19-2013 01:18 PM

Hi,

I'm having problems deriving the constant K-loss coefficient for a protective vent specified by a flow of 16 l/min and dp = 12mbar.(PolyVent XL / M32x1.5)

Help pages indicate I need velocity and dP. Do I need to calculate the velocity using the vent size and what units should be used to arrive at dimensionless K?.I'm missing some theoretical background, I guess.

Help appreciated,

Gibz740 

 

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apolo.vanderberg
Posts: 263
Registered: ‎08-31-2011

Re: Distributed Resistance Loss coefficient

11-20-2013 06:53 AM in reply to: Gibs740
Gibs,
The flowrate you are stating is based on a given area. If you divide out the area you will have an 'average velocity' that will be approaching/entering the vent distribution.
From there you can back out and solve for K
K = (2 * dP) / (rho * V * V )

as long as all of this is in a consistent unit system (ex. Base SI units -convert from mBar to Pa, and to m/s for velocity, kg/m3 for density) you'll end up with a unitless number
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*Expert Elite*
Posts: 377
Registered: ‎10-02-2012

Re: Distributed Resistance Loss coefficient

11-20-2013 08:02 AM in reply to: Gibs740

Hope you are mindful of how you measure the DP. Immediately before the vent, static pressure is being converted to dynamic pressure.   Hence if you plot pressure across a line from your domain leading through the vent, the part of line before the vent will show sudden decrease of static pressure, since it is being converted into dynamic pressure. You are supposed to take inlet pressure value as the one before this fall had started. 

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Active Member
Posts: 6
Registered: ‎10-24-2013

Re: Distributed Resistance Loss coefficient

11-20-2013 10:00 AM in reply to: Gibs740

apolo - I did figure this out by myself (plus one more unit translation: Pa -> N/m2). You have just confirmed the procedure. Thanks.

omarK - these numbers are vent vendor-supplied. I guess they did it right. Thanks as well.

 

Again, thank you both. Gibs740

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