Jet fan and Tunnel simulation

ahlo7 · April 6, 2009, 13:30

Hello,

I am simulating a jet fan in a tunnel.
The fan is set in a simple way: a cylinder with one end as entrance of the jet fan and another end as exit of the jet fan.
A tunnel air domain was cut by this fan cylinder and set the entrance of jet fan as an outlet and the exit of the jet fan as an inlet of the domain.
Two ends of the tunnel were set as openings with static pressure zero Pa.
The jet fan has 26m/s velocity with area 1.16m^2
The tunnel section is 45m^2 and 300m length.

The simulation went well and converged well, but the result looks not realistic.
The tunnel average velocity is 20m/s! It needs a huge power to drive this flow. The jet fan obviously has a small portion of such a power. Where it was wrong...?

Turbulence model used is k-e with 5% intensity

Thanks for any help

Ahlo

michael_owen · April 6, 2009, 17:44

I'm not sure I understand the problem set up, but it sounds like you may have a poorly defined problem that may admit multiple solutions. You say that you have 0 static pressure openings defined at both ends of the tunnel? The wind can blow through the tunnel at any speed in a case like this.

ahlo7 · April 9, 2009, 11:18

Quote:

Originally Posted by michael_owen

I'm not sure I understand the problem set up, but it sounds like you may have a poorly defined problem that may admit multiple solutions. You say that you have 0 static pressure openings defined at both ends of the tunnel? The wind can blow through the tunnel at any speed in a case like this.

Thank you, Michael

It is possible the BC were not well defined and it risks in having multiple solutions.
I assumed there is friction induced pressure drop along the tunnel, and this will somehow limit the air speed although with end of the tunnel have Pstatic=0. I added some back pressure at exit end of the tunnel, it improved, but velocity still looks high.

Any Suggestion to set better BC?

Tassi · April 15, 2009, 08:02

Hi

Didn't understand you description that well. I suggest you give outlet BC as velocity or mass flow and inlet as Static Pressure.

pratikmehta · April 16, 2009, 03:55

Hi,

I would suggest that you attach a .jpeg of your case describing your domain in that image . To me also it not clear how your problem is setup and the way you are defining your boundary conditions

Best regards
Pratik

abbasmh10 · June 10, 2010, 07:12

hi there

i was wondering if i can find help with my issue. i ve been using fluent in simulation of ventilation in rooms. my main task is to simulate a garage ventilation model using gambit and fluent. concerning the jet fans, i wonder wht kind of boundry condition shall i insert in the inlet and outlet. shall i treat the inlet as PRESSURE OUTLET, and outlet of the fan as VELOCITY INLET? or there is another boundary identification concerning the fan? thats for the normal ventilation, and soon i ll send a question concerning the smoke extraction

. i hope i ll find help with that. thank u in advance

ghorrocks · June 10, 2010, 09:04

Doesn't Fluent have a fan boundary condition. Not sure. You are asking the wrong forum, this is the CFX forum.

chvolk · March 7, 2012, 11:25

Hi, I want to make a jet fan simulation in a tunnel too, but I'm new in openFoam and a I really need some help. What solver do you use to get the simulation? How do you set the boundary conditions? Can you send me some tutorial or information about jet fan simulation ? or can you send me your work? I'll be very grateful!

Haga · November 13, 2019, 00:40

I happened to come across this thread while searching for some info regarding analysis of a jet fan in tunnel using CFX or Fluent. Here are some aspects that we need to consider for the same.

For simple setup, the finite volume domain can be easily modeled as Eulerian, not as Lagrangian since only the fluid (air) volume moves.
Pressure and temperature data at portals may be included real time to ensure natural air flow.
Fire can be setup as a STEP input for longer transient simulations.
Jet/axial fan exit velocity will always be higher than the tunnel air velocity whenever fan is operated.

I am also in search of steps to analyse tunnel ventilation system using CFX. If anybody have done some work in this regard, please share the knowledge.

ghorrocks · November 13, 2019, 05:54

Can you explain what your question is?

ANSYS support has some examples of tunnel ventilation. There are some literature and journal articles written by CFX/ANSYS people on this which might be useful for you. Contact ANSYS support to get them.

April 6, 2009, 13:30	Jet fan and Tunnel simulation	#1
ahlo7 New Member Join Date: Mar 2009 Posts: 8 Rep Power: 17	Hello, I am simulating a jet fan in a tunnel. The fan is set in a simple way: a cylinder with one end as entrance of the jet fan and another end as exit of the jet fan. A tunnel air domain was cut by this fan cylinder and set the entrance of jet fan as an outlet and the exit of the jet fan as an inlet of the domain. Two ends of the tunnel were set as openings with static pressure zero Pa. The jet fan has 26m/s velocity with area 1.16m^2 The tunnel section is 45m^2 and 300m length. The simulation went well and converged well, but the result looks not realistic. The tunnel average velocity is 20m/s! It needs a huge power to drive this flow. The jet fan obviously has a small portion of such a power. Where it was wrong...? Turbulence model used is k-e with 5% intensity Thanks for any help Ahlo

June 10, 2010, 07:12	jet fan boundary condition	#6
abbasmh10 New Member Join Date: May 2010 Posts: 4 Rep Power: 16	hi there i was wondering if i can find help with my issue. i ve been using fluent in simulation of ventilation in rooms. my main task is to simulate a garage ventilation model using gambit and fluent. concerning the jet fans, i wonder wht kind of boundry condition shall i insert in the inlet and outlet. shall i treat the inlet as PRESSURE OUTLET, and outlet of the fan as VELOCITY INLET? or there is another boundary identification concerning the fan? thats for the normal ventilation, and soon i ll send a question concerning the smoke extraction. i hope i ll find help with that. thank u in advance

November 13, 2019, 00:40	Some considerations in tunnel ventilation analysis using CFD	#9
Haga New Member Hareesh R Iyer Join Date: Dec 2010 Location: Thiruvananthapuram Posts: 21 Rep Power: 15	I happened to come across this thread while searching for some info regarding analysis of a jet fan in tunnel using CFX or Fluent. Here are some aspects that we need to consider for the same. For simple setup, the finite volume domain can be easily modeled as Eulerian, not as Lagrangian since only the fluid (air) volume moves. Pressure and temperature data at portals may be included real time to ensure natural air flow. Fire can be setup as a STEP input for longer transient simulations. Jet/axial fan exit velocity will always be higher than the tunnel air velocity whenever fan is operated. I am also in search of steps to analyse tunnel ventilation system using CFX. If anybody have done some work in this regard, please share the knowledge.

November 13, 2019, 05:54		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	Can you explain what your question is? ANSYS support has some examples of tunnel ventilation. There are some literature and journal articles written by CFX/ANSYS people on this which might be useful for you. Contact ANSYS support to get them. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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April 6, 2009, 17:44		#2
michael_owen Senior Member Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 17	I'm not sure I understand the problem set up, but it sounds like you may have a poorly defined problem that may admit multiple solutions. You say that you have 0 static pressure openings defined at both ends of the tunnel? The wind can blow through the tunnel at any speed in a case like this.

April 15, 2009, 08:02		#4
Tassi New Member Tassi Join Date: Mar 2009 Location: Mumbai, India Posts: 11 Rep Power: 17	Hi Didn't understand you description that well. I suggest you give outlet BC as velocity or mass flow and inlet as Static Pressure.

April 16, 2009, 03:55		#5
pratikmehta Member Pratik Mehta Join Date: Mar 2009 Posts: 32 Rep Power: 17	Hi, I would suggest that you attach a .jpeg of your case describing your domain in that image . To me also it not clear how your problem is setup and the way you are defining your boundary conditions Best regards Pratik

June 10, 2010, 09:04		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	Doesn't Fluent have a fan boundary condition. Not sure. You are asking the wrong forum, this is the CFX forum.

March 7, 2012, 11:25		#8
chvolk New Member christopher Join Date: Feb 2012 Posts: 1 Rep Power: 0	Hi, I want to make a jet fan simulation in a tunnel too, but I'm new in openFoam and a I really need some help. What solver do you use to get the simulation? How do you set the boundary conditions? Can you send me some tutorial or information about jet fan simulation ? or can you send me your work? I'll be very grateful!