Discussion Groups

Simulation CFD

Reply
Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Smoke ventilation from parking lots

450 Views, 20 Replies
12-17-2013 12:53 AM

Hi!

I have a problem with setting transient solver for the expansion of fire in a parking lot.

I reviewed I think any smoke tutorial and guidelines you have and still have some questions:

1. How do I set the transient solver correct and how can I know how much time passes?

For this example lets assume I need to see what happens in 10 minutes after the "car" catches fire.

 

2. What is the correct wattage needed for the boundary condition of the "car" volume?
Assuming the "car" is a cubic volume as shown at the file attached, who catches fire.

 

3. Whatever you can add to smoke ventilation and visualization

 

4. Can you please check the other inputs and give your opinion on the design.

The details for the design (for others also :smileyhappy: ) :

 

General details:
2000 m^2 parking lot, avrage. Height 3.2m, total volume: 6400 m^3.

For 8 air changes needed -> 2 outlet of 15000 cfm each, one free vent inlet.

 

Materials:

Ducts, Volume = Air (variable)

"Car" = Resistance (Fire):

     Through Flow K = 0.85

     Normal Direction 1 K = 0.85

     Normal Direction 2 K = 0.85

     Conductivity = 200 W/m*k

     (all constant)

   Flow direction of Fire material picked towards up (Z vector)

Jet Fan Case = Metal (Suppressed)

Jet Fan "Fan" = Custom Internal Fan ("Typhoon"), by sizes and preferences as manufactured:
     Flow = 2.7 m3/s

     Rotational Speed = 0 RPM

     Slip Factor = 1

     (all constant)

   Flow direction towards the front of the jetfan (x and y vectors depend on location in the parking lot)

 

Boundary Conditions:

"Car" =

     (Volume) Total heat generation 5000 W

        Steady state

        Temp dependent - Disabled (What does it mean???)

     (Surface) Scalar 1, Steady state.

 

Ducts outlets:

     (Surface) Volume Flow rate 15000 ft3/min each.

     Steady state.

     direction - towards the outside of the volume.

 

Free Inlet:

     Scalar = 0.

     Pressure = 1.01325e+06 dyne/cm2 (As Environment)

     Temperature = 30 Celsius

 

Initial Conditions:
Air Volume = Scalar 0

 

Mesh Size

Auto with adjustments in the fans and ducts (0.6 with uniform and spread)

 

Solver:

Control:

Main = need your help :smileyhappy:
Result Quantities =

    Scalar = on

    Smoke Visibility = On; Parameters:

        Extinction Coefficient = 37000 ft2/lb

        Sign Visibility Constant = 3

        Combustion Particulate Yield: 0.008.

Physics:


Flow = on

     Incompressible.

Heat Transfer = on

Auto forced convection = off

Gravity Direction = 0,0,-1 (-Z here)

Radiation = off

Advanced:

     General Scalar:

            Diffusion coefficient: 0.12 cm2/s.

 

 

Sorry for the long question, I just thought others would appreciate it , I looked in many furors and no one really solve this problem...

Thank you SOOOO MUCHH!!!!!!

 

Sahar Rubin,

Mechanical Engineer.

 

Please use plain text.
Product Support
wildej
Posts: 731
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-17-2013 01:43 AM in reply to: sahar.rubin

Hi Sahar,

 

It sounds like you are just about there to be honest.

 

You do need a gap beneath the fire, for the air to flow from the room, up into the resistance and out of the top. Your resistance is about the right size, just have it open to the air beneath rather than closed off - does that make sense?

I would just leave the inlets as P=0, this is gauge pressure so just standard atmospheric conditions.

 

To answer your questions:

 

1) I would start with 0.5s and see if that is stable. With scalars we recommend just 1 inner iteration. If this is not stable, you might require a smaller step.

Set the stop time to 600 (seconds are the default unit)

Set the number of iterations to run to -1 (this means it is ignored)

Intelligent Solution Control is off, which is good (this ensures we run to 10 mins and do not stop at a false convergence)

 

2) This is hard to say, a number of kW most likely. Do you not have any sort of design requirements here? 5000W is a good start.

 

3) Use Planes and ISO surfaces to investigate scalars. 0 will be clean air and 1 will be 100% smoke. The numbers inbetween will be a mix of the two

 

4) I would add a uniform mesh to the fans - we need 5 elements from inlet to outlet (of the fan material). Then you will also need mesh regions to capture the jet - did you see my pdf?

 

Have you had training with CFD? Your setups are very good :smileyhappy:

I hope that helps.

 

Kind regards,

Jon



Jon Wilde
Please use plain text.
Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-17-2013 02:30 AM in reply to: wildej

Hi Jon!

Thanks for the quick reply!!!

I did put a gap beneath the fire, as you mentioned in another conversation I saw here :smileyhappy:

about 0 gage, if I put the 0 gage for the inlet, wouldn't it change the inlet to outlet? What I mean is, if I put the inlet to 0 gage and I want to keep suction of 30000 cfm from the room wouldn't it harm my calculation?
As I mentioned, I have 2 outlet (extraction from fans by ducts) of 15000 cfm each and one free inlet (means the the air comes naturally through the simulated duct/hole in the ceiling - I do this to simulate air infiltration while smoke extraction is needed, if you have a different approach I would love to hear it :smileyhappy: )

 

about the solver:

     would start with 0.5s (time step size?) and see if that is stable. With scalars we recommend just 1 inner iteration. If this is not stable, you might require a smaller step. 

     Set the stop time to 600 (seconds are the default unit)

     Set the number of iterations to run to -1 (this means it is ignored) (time steps to run?)

     Intelligent Solution Control is off, which is good (this ensures we run to 10 mins and do not stop at a false convergence)

     what about Save intervals? How do I use it and what is it basically?

 

I've seen you PDF and it really helped! 

what do you mean by: "we need 5 elements from inlet to outlet (of the fan material)"?

 

never had any training, just reading a lot from furors, asking question (to be precise, asking you questions :smileyhappy: ), and trying tutorials and reading material in Autodesk's wikihelp, 
I've been using the software for only 2 weeks now!

anyways, thanks for the compliment :smileyhappy: and moreof, thanks for reply so fast and helping so much, you are the best!


Sahar

Please use plain text.
Product Support
wildej
Posts: 731
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-17-2013 02:57 AM in reply to: sahar.rubin

Hi Sahur,

 

No problem, it is a pleasure to help. You are learning very fast with no training, I am impressed!

 

The inlet pressure should not actually matter, as the dP will be the same regardless. But sure, leave it as it is, no problem at all.

I can see where you are coming from and I think you are correct, it will certainly help wih interpreting the results and may well be very helpful in creating a positive pressure.

 

Time Step Size: 0.5s yes. We may well need to reduce this if you face any divergence, time to test it and see.

The -1 is for the Timesteps To Run yes. This means that we do not need to think about how many timesteps we will run as we tweak the timestep size to improve the stability of the analysis. It will just always run to 10 mins.

Save intervals: These are used if you wish to backtrack at any point and look at the results at a point between zero and ten minutes. We recommend saving no more than about 40-50 steps in one analysis (just incase the file becomes too large to open, rare but it can happen). So you could set this to save every 30 seconds, and you would then have 20 results over the course of the analysis that you could animate. Something like this.

 

Regarding the fan material - this is a before and after of what the mesh should look like. I selected the volume, gave it a uniform mesh and refined it to the level we need. From the inlet face to the outlet face we now have 5 elements (not that it really helps here but you can right click the part and select preview to see the mesh on the surface). You will likely also need the refined mesh regions to capture the jet. As these are a little tricky to manipulate in CFD, you could always draw them in CAD. They would still have an air (variable) material assigned to them but you would also be able to select them for additional mesh refinement.

 

Mesh on a fan.png

 

Regarding the fire, I would remove the solid part here so that the air can enter around the edges beneath the fire. The solid around the edge of the fire can stay though, that often helps. Does that make sense?

 

Gap Here.png

 

Please let me know if I have not answered all of your quesions, I think I have :smileyhappy:

 

Best regards,

Jon



Jon Wilde
Please use plain text.
Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-17-2013 03:09 AM in reply to: wildej

thanks again!

 

only one last thing:

"Car" =

     (Volume) Total heat generation 5000 W

        Steady state

        Temp dependent - Disabled (What does it mean???)

     (Surface) Scalar 1, Steady state.

 

and in the boundary conditions everytime i put it on steady state, and i don't really know how will it change if i'll adjust it to transient?

 

i'm running a simulation now, i'll upload the result to help others and for your review :smileyhappy:

 

Thank for everything!

 

Sahar

Please use plain text.
Product Support
wildej
Posts: 731
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-17-2013 03:16 AM in reply to: sahar.rubin

Hi Sahar,

 

That just means that we can have heat loads (or heat losses) which switch on and off depending on the temperature at a particular location. Here, the lead load will remain at a constant 5kW whether you run steady state or transient.

We only use transient BC's if we wish them to change over time.

 

Kind regards,

Jon 



Jon Wilde
Please use plain text.
Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-17-2013 05:16 AM in reply to: sahar.rubin

Hi again.

 

ok, after a lot of time calculating, i stopped it after 147 iterations (which means about 1 minute) and the results aren't what i thought would be,

 

1. the temperature hardly changed, which isn't likely to happen when fire bursts.

2. in conticuation with 2, hardly any smoke, and something funny happend, look at the inlet, there's some smoke gathering there, Why???

3. the velocity magnitude looks different than in the steady state mode, it looks round rather than long in steady state solver..

what am i doing wrong???

Please use plain text.
Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-17-2013 05:17 AM in reply to: sahar.rubin

just some more attachments :smileyhappy:

 

thanks!!!

Please use plain text.
Product Support
wildej
Posts: 731
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-17-2013 05:19 AM in reply to: sahar.rubin

Hi Sahur,

 

This is likely down to meshing/timesteps. Do you have the CFZ to hand?

 

I do have a few support cases to run through first but will be onto this as soon as I can be.

 

Thanks,

Jon



Jon Wilde
Please use plain text.
Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-17-2013 05:31 AM in reply to: wildej

The .CFZ is attached here,

whenever you can it'll be amazing

 

thanks for everything Jon!

 

Sahar

Please use plain text.