FDS layer zone Device

[Ahmad Orabi]

Hello Dears,

To plot the smoke layer height in FDS we need to use layer zone device, the device works when the fire spread is high  but in my case the fire spread is low, so the device doesn't recognize the smoke layer. Is there anyway to solve this problem and make the device  works well, or is there another way to plot the smoke  layer height?

[dr_jfloyd]

If your fire size is so small that you don't create enough stratification for the layer device to return a meaningful result, then you may not really have an upper layer. You can look a putting a vertical column of temperature measurements (see 21.2.2 in the guide) and then visually examining the temperature profile. With a small fire there may not be a sharp change in the gradient of the temperature profile to indicate the presence of a layer.

[Ahmad Orabi]

Thanks for your response,
Actually, the fire size is not so small. But anyway that's exactly what is happening, and I can't change the fire features because these features are recommended by the Standard to be used in the simulation.
Now the question is: Can we set the device in a way to solve this problem without changing the fire features?

[Kevin McGrattan]

"Layer Height" is a concept that assumes that the fire has produced enough smoke to create a relatively uniform layer over the span of the ceiling. If you do not have this in your scenario, then do not use the layer concept or that device.

[Ahmad Orabi]

Thanks Mr Kevin for your response. My study is all about the smoke layer and the fire produce a smoke layer, but it seems like the device has defined sensitivity I am not able to define it, but clearly it need a limit of temperature or soot visibility to produce data plot. Anyway I have no problem with the layer height concept but it is definitely required in my case, so my aim is to find a solution to make the device perform properly or find another method to measure the smoke layer height.

[dr_jfloyd]

You can look a putting a vertical column of temperature measurements (see 21.2.2 in the guide) and then visually examine the temperature profile. 

[tb chan]

Dear dr_jfloyd

Would you please help for :

1.   If the fire load is 25MW, I found  the dimensional of D* calculated was very small than I expected.  HRRPUA x sizes can archive as more realistic.  However, how come to calculate the mesh?  

2.  Propane is effective to heat combustion 

      (radiative fraction) which Qc = Q x.      

      Xr =0.3          X =0.7     ?

3.   How to know which type of fire material 

       in FDS configure with limited HRRPUA?

tb 

dr_jfloyd <drjf...@gmail.com>於2021年6月18日 上午5:14寫道：



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[dr_jfloyd]

1. Setting a grid size based on D*/dx is just a rule of thumb for simulations where the quantities of interest are related to the fire plume (plume temperature, layer growth, etc.). It is up to you to determine if this grid size is appropriate for your specific simulation.

2.  Assuming that you intend Qc to be the convective heat release rate then yes, Qc = Q * (1- Xr)

3. It is not clear what you are asking. You should be setting inputs such as boundary conditions, material properties, and fuel species + reaction definitions to represent as best  you can the specific scenario you are modeling.