CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > FLUENT

negative or positive pressure jump at exhaust fan

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Search this Thread Display Modes
Old   March 26, 2007, 12:35
Default negative or positive pressure jump at exhaust fan
Selo melo
Posts: n/a
I have an exhaust fan in physical model which pumps the air out of the system. In fluent i could not decide whether it should have a negative or positive pressure jump. To make it act as an exhaust (not the opposite, an intake) which one should i choose? (In documentation it says "You must be careful to model the exhaust fan so that a pressure rise occurs for forward flow through the fan". )
  Reply With Quote

Old   March 28, 2007, 16:53
Default Re: negative or positive pressure jump at exhaust
Seyed Farid Hosseinizadeh
Posts: n/a
Hey: In my mind just check it two times. Once with positive and next with negative. If you had flow out it means you have applied negative and in next it is positive. I had some experience but I'm not sure, maybe you can apply other boundary conditions instead of this type. I'm pretty sure you can. Thanks/Farid
  Reply With Quote

Old   March 28, 2007, 22:23
Default Re: negative or positive pressure jump at exhaust
Selo melo
Posts: n/a
I built a small grid to make some tests. A small rectangle with an entrance, a mid boundary and an exhaust. I used laminar flow eq.

For an intake and an exhaust with positive values both, (mid assigned as interior) there is a net flow from inlet to outlet. I noticed that the average velocity is close to the value below;

According to Bernoulli eq. p/rho + (v^2)/2=c and if v1=0 then 2*(p2-p1)/rho=v^2 and v=sqrt(2*(p2-p1)/rho). The pressure difference between two points decides the magnitude of the velocity and it changes with friction (laminar and turbulent different).

If we come back to my case the average velocity (by fluent) is close to the value i estimated with bernoulli.

Further, i put a fan boundary in the middle, which has the same direction with the flow. I gave a constant pressure jump. I found an average velocity with fluent analysis. Guess now, how i estimated the velocity. I added the pressure jump value to my total pressure difference and found the velocity very close to the average velocity. I concluded that the velocity is found using "net" pressure difference value. (If you have a fan in reverse direction you will substract it.)

Besides, i assigned velocity inlet to entrance, fan to mid and outflow to the exit. My observations are such that the velocity boundary is dominant and it is not affected. If you say 10 m/s, whatever you give for fan pressure does not change the amount of flowrate decided with 10m/s.

If you want your flowrate to be affected by pressure, then all your boundaries should be pressure type boundaries.

These are my observations but i can not assure that they are 100% right.
  Reply With Quote


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
[blockMesh] Axisymmetrical mesh Rasmus Gjesing (Gjesing) OpenFOAM Meshing & Mesh Conversion 10 April 2, 2007 14:00
negative pressure Mark Render Main CFD Forum 8 April 23, 2002 06:02
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 05:13
Hydrostatic pressure in 2-phase flow modeling (CFX4.2) HB &DS CFX 0 January 9, 2000 13:19
Hydrostatic pressure in 2-phase flow modeling (long) DS & HB Main CFD Forum 0 January 8, 2000 15:00

All times are GMT -4. The time now is 04:32.