Stratification and air temperature changes near to hot surface

[Fath Elrahman Al-Asheer]

Hi users I hope someone can help me .. I'm trying to simulate stratification study of an atrium with hot ceiling (Hot Wall) and burner ( Ethanol ) at the center of the building. The main idea is to study at which ceiling temperature the stratification of the smoke will happen. I have tried to add devices at the ceiling but it doesn't show the ceiling temperature which set to be 50C it keeps resulting in 20C. And when I tried to add devices as  WALL TEMPERATURE an error occur because it says to which cell mesh should the device be attached. I believe my mistake is about IOR maybe !!  

My questions are:

1/ how to add a device or slice file that shows the air temperature around the ceiling ?
  

2/ how to add a device to the ceiling that shows the ceiling temperature ?

I have added the input file.

Thank you

[Kevin]

If you want help, post a much shorter input file that includes only those features about which you have questions. Nobody has the time (especially on a Saturday) to work through such a long file.

[Fath Elrahman Al-Asheer]

Sorry Kevin. Here a shorter input file. I hope it is ok now ?

[Kevin]

Use 'TEMPERATURE' in a gas phase cell, and use 'WALL TEMPERATURE', IOR=-3 on the ceiling.

[Fath Elrahman Al-Asheer]

Thanks  Kevin. The input file now is correctly structured and running but still no changes in the air around the hot surface.


Is this line is correct to add a hot surface that can change the air specification near to it ? anything to add ?

&SURF ID='HOT WALL', COLOR='BLUE',TRANSPARENCY=0.2, TMP_FRONT=250./

&OBST XB = 0.0,20.0,0.0,20.0,24.84,25.0, SURF_ID ='HOT WALL'/ atrium ceiling

Thank you

[Fath Elrahman Al-Asheer]

I hope this photo shows the idea .

[Randy McDermott]

Your XB = 0.0,20.0,0.0,20.0,24.84,25.0 is a line, not a plane.  Give your OBST some volume.  Then it should show 250 C on a BNDF.

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[Fath Elrahman Al-Asheer]

Sorry sir .. It would be kind of you if you explain further. I'm new to FDS.

The ceiling is defined by three points  &OBST XB = 0.0,20.0,0.0,20.0,24.84,25.0, SURF_ID ='HOT WALL'/ atrium ceiling  .. which makes it as a plane.

What do you mean by adding volume ?

Thank you. 

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[Randy McDermott]

Sorry, I had misread the line, you are right.  Try adding THICKEN=T to the OBST line.  It is possible your grid resolution is not able to capture it.

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[Fath Elrahman Al-Asheer]

Thank you so much .. Very true .. Now it is working fine and the device is showing the ceiling temperature 250 C.

Now the issue is the ceiling is heating up the air above it as the attached photo shows and the change in the air under it is not significant which not enough to make changes in the air density.

I would like to design the ceiling with a temperature to transfer the heat to downward surface so the air density would change and air become much lighter than the smoke which leads to stratified air layers which prevents the fire smoke from reaching the smoke detectors.

What is your advice about these lines:
&SURF ID='HOT WALL', COLOR='BLUE',TRANSPARENCY=0.2, TMP_FRONT=250, EMISSIVITY=0.9/

&OBST XB = 0.0,20.0,0.0,20.0,24.84,25.0, THICKEN=.TRUE., SURF_ID ='HOT WALL'/ atrium ceiling

To which section in the user guide i can refer to ?

Thank you.

I have added a very simple input file that address the problem directly.

[Randy McDermott]

If you want different temperatures on difference surfaces of and OBST, search on SURF_IDS or SURF_ID6 in the user guide.  But I am doubtful that a design is possible for a turbulent plume.  If the flows could somehow remain laminar you might be able to accomplish this, but your Rayleigh numbers should be far too high to expect for complete stratification, that's my guess anyway.

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[Fath Elrahman Al-Asheer]

Noted sir. 

Thank you so much for the help.

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[Fath Elrahman Al-Asheer]

Dear Mr.Randy,


I have been reading the user guide and trying many ways to add the temperature to bottom surface using SURF_IDS as you suggested but still couldn't manage to get the hot surface (the ceiling) to transfer the heat flux downwards  to heat up the gas below the ceiling (instead of convicting the heat upward as shown in the attached picture):

I attached the updated input file. Pasted below are the modification I have done on the ceiling lines in updated file. Your advice on the matter is highly appreciated: 

&MATL ID='GLASS',

      SPECIFIC_HEAT=0.84,

      CONDUCTIVITY=0.76,

      DENSITY=2700.0/

&SURF ID='WINDOW',

      RGB=102,255,255,

      TRANSPARENCY=0.2,

      MATL_ID(1,1)='GLASS',

      MATL_MASS_FRACTION(1,1)=1.0,

      THICKNESS(1)=0.005,

      TMP_INNER=250,

      EMISSIVITY=0.93,

      CONVECTIVE_HEAT_FLUX=10,

      BACKING = 'EXPOSED'/ 

&OBST XB = 0.0,20.0,0.0,20.0,24.84,25.0, SURF_IDS ='INERT','INERT','WINDOW'/ atrium ceiling

 

Thank you.

[Randy McDermott]

I am not sure what you would like me to say.  I don't quite understand the problem.  Are you trying to design something?  Or do you have an experiment that you are trying to model?

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[Kevin]

Your inputs are not appropriate. If you just want a hot wall, do not include the MATL line and just set TMP_FRONT or NET_HEAT_FLUX on the SURF line. Do not set both.

[Fath Elrahman Al-Asheer]

I'm very sorry sirs for confusing you. English is not language.

I would like to simulate the sun effect on the glazing roof of a shopping mall. where the sun is heating up the air layers underneath the ceiling to a specific temperature which make the air layers less dense than the smoke of the fire at the ground floor. causing the smoke to stratify under that air hot layers due to density difference so that it cannot penetrate to the smoke detectors attached to the ceiling.

I have done many trails using using several lines such as TMP_FRONT, TMP_BACK and TMP_INNER based on the user guide and also tried using SURF_IDS.

So far using TMP_FRONT is best but the heat generated from the hot surface is transferred/ directed upwards (+Z axis face of the ceiling) outside of building instead of inside building (-Z axis face). 

My question is about the correct input file parameters:

How to make the heat to be directed to negative direction on the -Z axis so that the air temperature increased more than just 21C ? 

The photo attached below generated using these lines:
&SURF ID='WINDOW', COLOR='BLUE',TRANSPARENCY=0.2,TMP_FRONT=250/

&OBST XB = 0.0,20.0,0.0,20.0,24.84,25.0, SURF_ID ='WINDOW'/ atrium ceiling

The boundary file shows the ceiling temperature is 250.

The slice file shows that the heat generated from the ceiling is directed outside of building and no heat directed inside the building.

Thank you.

[dr_jfloyd]

You have a hole in the roof.  The heat is going to rise out of the hole and you will not form a deep layer inside.

You have applied the 250 C surface to all sides of the OBST for the ceiling, this is heating  the gas above the roof which is rising up.

You haven't defined material properties for any of the walls, so they are the default INERT surface with is a fixed 20 C wall. In a real building, the surfaces will heat up and not remove as much heat from the gas.

Sunlight entering the glass roof will add heat to surfaces below the roof

[Fath Elrahman Al-Asheer]

Thank you Dr. Floyd for the advice. I'll be working based on it and update you soon.

[Fath Elrahman Al-Asheer]

I would like to thank all you sirs.

I made the required changes (defining materials for walls and adding 250C only to the bottom surface of the ceiling)  and it is working correctly.

On Friday, February 16, 2018 at 8:44:06 PM UTC+8, dr_jfloyd wrote:

[dr_jfloyd]

Where are you getting 250 C from? Peak solar irradiance is about 1 kW/m^2.  250 C would take about 6 kW/m^2 to sustain.

[Fath Elrahman Al-Asheer]

I'm using the ceiling as a heater to heat the air up to 40C - 60C then it create stratified air layers. Then after sometime fire will be started.
The main goal of the project is to study the smoke behavior under several stratified temperature air layers.

I'm new to FDS just few weeks into learning it through reading the user guide .

[dr_jfloyd]

It is going to take a while to form a stratified layer of any depth just by heating from the top surface.  You may want to consider using INIT and SURF to set the stratification.  For example, if you want a 1 m deep layer that is 50 C, set a 1 m deep region to 50 C with INIT and set TMP_FRONT=50 for the surfaces adjacent to that region.

[Fath Elrahman Al-Asheer]

Thank you for the suggestion. 

I will read about it and update you soon.