topic Re: Smoke Visibility and exit signs in CFD Forum

Smoke Visibility and exit signs

Anonymous — Thu, 25 Feb 2016 13:11:39 GMT

Hi!

I would like to ask something regarding the Smoke Visibility capability of CFD Simulation.

The tutorial states that the application "computes the distance a lighted sign can be seen from within a smoke-filled environment" and later defines the sign visibility constant, with a constant of 8 for an illuminated sign. The smoke visibility output quantity indicates how far away a sign is visible at every location in the model.

Since we do not define the location of the exit signs during any stage of the design, how do we know if and when they are visible through the smoke?

Should I position the exit signs in the design and assign the material as an LED Device?

Or does the program assumes that by assigning the constant for an illuminated sign, all iso surfaces with the required visibility distance can be locations for an exit sign?

Hopefully this makes sense

Thanks,

Panagiotis.

Re: Smoke Visibility and exit signs

Jon.Wilde — Mon, 29 Feb 2016 08:53:44 GMT

CFD is telling you how far you could see from each point in the model, it is not related to any physical signage within the model. There is a page in our help about this here: Smoke Visibility

Does that answer your question?

Re: Smoke Visibility and exit signs

Anonymous — Mon, 29 Feb 2016 10:20:06 GMT

Yes, thank you for the reply.

What I am trying to do is simulate a fire in a corridor and calculate the visibility due to smoke, with one inlet simulating a window from where the air comes in and one outlet for the smoke to go out (question - is the outlet necessary?)

If I set for the inlet the boundary conditions: temperature=20oC, pressure=0 and scalar=0, somehow it is identified as an oultet (for the proper outlet I set only pressure=0). Using a small positive number for volume flow tricks the program into accepting the inlet as an inlet, but does this affect the credibility of the simulation?

Thank you,

Panagiotis.

Re: Smoke Visibility and exit signs

Anonymous — Wed, 02 Mar 2016 10:59:09 GMT

Hi, I have tried running the simulation, but the results I get are quite strange (the velocity vectors at the fire source are upwards and downwards) and scalar distribution is also strange. I have attached the CFZ and the main settings of the parameters, I would really appreciate it if you could help identifying what the problem is.

The settings are:

Materials:
Air = variable
fire = Resistance:
Through Flow K = 0.85
Normal Direction 1 K = 0.85
Normal Direction 2 K = 0.85
Conductivity = 200 W/m*k
Flow direction = Global Z

Boundary Conditions:
fire
Volume: Total heat generation 6000 W, Steady state
Top Surface: Scalar 1, Steady state

outlet:
Surface pressure = 0 pa (constant)

inlet:
Scalar = 0 (constant)
Surface pressure = 0 pa (constant)
Temperature = 20 Celsius (constant)
Volume Flow Rate = 0.01 m3/s

Initial Conditions:
Air Volume
Scalar = 0
Temperature = 20 Celsius

All Volumes:
Temperature = 20 Celsius

Mesh Size
Auto (for starter, will adjust near fire later)

Solver:
Control:
Transient
Time step: 0.1
Stop time 600
Inner Iterations 1
Time steps to run -1

Solution Control:
Intelligent Solution Control off
Advcetion scheme Adv5

Result Quantities
Scalar = on
Smoke Visibility = On; Parameters:
Extinction Coefficient = 21000 ft2/lb
Sign Visibility Constant = 3
Combustion Particulate Yield: 0.05.

Physics:
Flow = on
Incompressible.
Heat Transfer = on
Auto forced convection = off
Gravity Direction = 0,0,-1
Radiation = off

Advanced:
General Scalar:
Diffusion coefficient: 1*10^-10 m2/s.

Thank you very much,
Panagiotis.

Re: Smoke Visibility and exit signs

Jon.Wilde — Wed, 02 Mar 2016 11:49:37 GMT

It is fine for CFD to say only 'outlets', this just means that they have no flow rate assigned.

Can you share an image of the strange results and also of the mesh in this location (use a cut-plane). I suspect that this might be the only issue as your setup sounds good.

Re: Smoke Visibility and exit signs

Anonymous — Thu, 03 Mar 2016 14:26:02 GMT

Hello,

I am attaching a few screen shots,

the first shows the mesh around the fire (I have created a suppresed wall around the resistance)

, the second the fact that on the top left hand corner there is no air volume (scalar slider is at zero) plus a strange spike in the scalar plot, which then converges to a steady low state

and the third the velocity vectors around the fire (both upwards and downwards), as well as the fact that the smoke plume does not rise vertically but rather drifts toward the wall.

Thanks you,

Panagiotis.

Re: Smoke Visibility and exit signs

Jon.Wilde — Fri, 04 Mar 2016 10:08:32 GMT

I think your mesh is too coarse, did you see this webinar already?

Can you show the mesh on a cut plane through the fire? I can't really see what is looks like right now.

Thank you

Jon