Jet Fans in tunnel

[Ronny Olbert]

Hi together,

I currently have a problem with the simulation of some jet fans in a tunnel.
In a previous simulation I have blown air with a velocity of about 5 m/s into the tunnel by setting a VENT to the entrance of the tunnel. The smoke was blown out of the other side of the tunnel, there was no backflow or backlayering effect.

Now I want to simulate this case with two triplets of jetfans. Each jet fan has a velocity of 5.95 m/s. The problem is, that the smoke flows back against the jet fans and exits at both end of the tunnel.

What is the problem or what is wrong in my simulation?

Thanks a lot!

Greetings
Ronny

[dr_jfloyd]

The two are not at all equivalent. When you specify a 5 m/s inlet on the tunnel, you are saying that no matter what back pressure and ceiling jet the fire creates, that there will always be 5 m/s entering the tunnel. This is not a realistic boundary condition. With the jet fans you are imposing a pressure condition. Each fan can create some amount of pressure and if the fire is more, then back flow will happen.

[Ronny Olbert]

Thanks for your answer!

The situation you described makes sense. I tried it again with a velocity of 15 m/s (exemplary) and there was no back flow or back layering effect.
But in other simulations of my case and in literature disclosures it is said, that back flow and backlayering effect will be avoid by velocities of about 2 - 3 m/s. From these values I am far away.

Is there anything else I'm doing wrong?

Greetings
Ronny

[dr_jfloyd]

What have you done to validate that your grid size and method of implementing the jet fans results in an adequate model of the tunnel conditions?

[Ronny Olbert]

Currently I placed the jet fans according to the information from the real fire experiment. The coarse grid size of 0.5 m I have chosen to minimize the computational capacity.

Am Dienstag, 9. Juli 2013 16:04:49 UTC+2 schrieb Ronny Olbert:

[dr_jfloyd]

You put in some inputs that are supposed to represent the behavior of the jet fans. What have you done to determine if those inputs do indeed represent the behavior of the fans? Have you looked to see if jet fans entrain the correct amount of air? Have you done a grid study to determine if an 0.5 m grid is sufficient?

[Nguyen Tuan Ngoc]

Hello,
I also used FDS for back layering canculation of one tunnel. the air velocity of jet fan is hight, about 20m/s but it is only correct around the fan area. Far from fan, the air velocity is lower so the smoke back layering could happend. So the position of fans must be appropriate to create the air velocity in every points of cross section that are higher than the critical velocity to prevent the back layering!

On 10-07-2013, at 21:38, dr_jfloyd <drjf...@gmail.com> wrote:

> You put in some inputs that are supposed to represent the behavior of the jet fans. What have you done to determine if those inputs do indeed represent the behavior of the fans? Have you looked to see if jet fans entrain the correct amount of air? Have you done a grid study to determine if an 0.5 m grid is sufficient?
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[Mohamed ASSAL]

Hi Ronny,

Increasing jet fan air velocity may solve your problem, but you should
think that there are standards for that, and also the human comfort
inside the tunnel is an important factor.

Cheers Mohamed.

2013/7/10, Nguyen Tuan Ngoc <tuann...@gmail.com>:

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[Ronny Olbert]

Thanks to all for your answers!

After your tips and some reflection, I realized that this can not work. I ignored the degradation of the velocity. If i set a velocity of about 5 m/s, this value will not be constant over the entire tunnel. I think this is what you mean? The 5 m/s I only have close to the jet fans.

Am Dienstag, 9. Juli 2013 16:04:49 UTC+2 schrieb Ronny Olbert: