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Fire simulation in an underground car park.

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Message 1 of 2
jiaxinlimjx
532 Views, 1 Reply

Fire simulation in an underground car park.

Hi,

 

I have a problem simulating underground car park which is required to be run for 20 minutes after the fire souce catch fire. Some research was done and i also viewed a past example. Here. But the desired result still can not be achived.

 


Some unsolved question:

1. Massive sudden increase in velocity magnitude.

2. Divergent occour at around the 600th Time Step.

 

3. How does divergent happen? What causes it to happen?


4. Whether the parameter inputed below are appropriate.
 

Hope the below detail helps.

 

General details:
Height 3m, The car park was cut into 2 zone as it was too huge.

For 9 air changes needed, one free vent inlet.

 

Materials:

Ducts, Volume = Air (variable)

"Fire Source" = Resistance (Fire):

     Through Flow K = 0.5

     Normal Direction 1 K = 0

     Normal Direction 2 K = 0

     Conductivity = 1 W/m-k

     (all constant)

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

Jet Fan "Fan" = Custom Internal Fan ("Fan"), by sizes and preferences as manufactured:
     Flow = 630 m3/h

     Rotational Speed = 0 RPM

     Slip Factor = 0.7

     (all constant)

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

 

Boundary Conditions:

"Fire Source" =

     (Volume) Total heat generation 4 MW

        Steady state

        Temp dependent - Disabled

     (Surface) Scalar 1, Steady state.

 

Ducts:

     (Fresh Air Flow) (Surface) Volume Flow rate 6.825m/s each. Temperature 20DegC. Scalar 0.

     (Exhaust Air Flow) (Surface) Volume Flow rate 3.4125m/s each.

     Steady state.

 

Free Inlet:

     Pressure = 0

 

Initial Conditions:
Air Volume = Scalar 0

 

Mesh Size

Auto with adjustments in the fans, fire source and ducts.

 

Solver:

Control:        Solution Mode = Transient 
                       Time Step Size = 0.03

                       Stop Time = 1200
                       Inner Iterations = 1
                       Time Step to Run = 40000
Solution Controls:
                       
Intelligent solution control = Enable
                       Advection = 5
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

                       Radiation = off

Advanced:

                       General Scalar:

                       Diffusion coefficient: 0.12 cm2/s.

 

Sorry for the long post. This is for our project.

Thank you.

 

LJX,

1 REPLY 1
Message 2 of 2
Jon.Wilde
in reply to: jiaxinlimjx

Hi,

 

Here are my initial impressions:

  1. You cannot have internal Boundary Conditions - the way around this if you need them inside the model is to have them attached to a suppressed volume (the exception is volumetric boundary conditions, these can be applied to internal volumes without issue). So, your ducting needs to be a separate part - recess the boundary conditions for improved accuracy
  2. Your fans must have a shroud around the outer circumference, so that only the inlet and outlet faces are exposed to the air - to prevent flow leaking - same with the fire (no gaps)
  3. Should the fans have an rpm assigned?
  4. I would suggest you need to change air to variable, in order to capture the natural convection
  5. You need more mesh, the fire has only one element across it's depth, aim for a uniform mesh with 8-10 from inlet to outlet and continue this into the local region. Same for the fans.

I hope that helps.

 

Thanks,

Jon

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