[Bldg-sim] FW: IES CFD Modeling Issue: Reverse Flow through Diffusers

Keith Swartz kswartz at ecw.org
Mon Jan 14 12:51:05 PST 2008



If the velocity in the main duct is extremely high, it could be sucking
air into the duct. The high velocity pressure of the air in the main
duct can lower the static pressure in the duct below the room static
pressure. This aspirating effect is used in chemistry labs to produce a
vacuum in a system by hooking a hose as a side tap to another hose with
water running through it. It can also be demonstrated with a simple
experiment. Hold the short end of a dollar bill directly below your
lower lip, with the other end of the dollar bill hanging down. Now blow.
The dollar bill will fly up like a flag. Your high-velocity air is like
the air in the duct, and the low pressure in the main air stream is
sucking up the dollar bill like the air in the duct is sucking air out
of the room. I recommend estimating the velocity pressure and the static
pressure at the problem points to see if aspiration is causing the





Keith Swartz, PE, LEED(r) AP


Energy Center of Wisconsin

455 Science Drive

Suite 200

Madison, WI 53711


Phone: 608-238-8276 ext. 123

Fax:     608-238-0523


The Energy Center of Wisconsin is an independent, non-profit
organization that seeks solutions to energy challenges.


From: Brain, Joshua [mailto:JBrain at Vanderweil.com] 
Sent: Friday, January 11, 2008 7:44 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] IES CFD Modeling Issue: Reverse Flow through




I am using IES to conduct a CFD model on a long and narrow room (283ft
by 60ft) with three large ducts (square 4ft by 4ft) running along the
ceiling (280ft long, 28ft off the floor). I have 18 diffusers equally
spaced along each of the three 280ft ducts for a total of 54. Each
diffuser has a CFM of 1110, an area of 3.75 sf, and a discharge velocity
of 300 fpm. There is a single return grill at one end of the building. 


My problem is: when I look at the velocity vectors in the MicroFlo
Viewer some of the diffusers have the air flowing backwards or into the
diffuser, not flowing out as it is supposed to. This occurs across all
three duct lengths at the same diffuser location. 


Has anyone else run into this and have they resolved the issue?  IES
took my model and made changes to it only to find that they could not
fix it. 


Thank you in advance for any advice, 



Joshua Brain

Mechanical Group 

R.G. Vanderweil Engineers, LLP 

274 Summer Street     Boston, MA  02210 

Tel: (617) 423-7423 

Fax: (617) 423-7401 

Direct: (617) 956-4565 





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