parameter for lack of oxygen

[Ashish bhigah]

Hi all,

I have a sort of discrepancy between HRR input and HRR output, as you can see in the graph. I suspect that this can be due to the lack of oxygen in that room, since I didn't activated on purpose the windows breakage.

Is there a parameter that I can measure with a device or a slice that can help me to address directly this problem to the lack of oxygen? 

Thanks in advance

[Randy McDermott]

Well, your O2 eventually climbs up to achieve 4.5 MW, so the room is capable of supporting it.  Are you saying that the window breaks at around 400 s?  Or are you saying you want to confirm the total O2 level in the room?  Lots of options for that: MASS_FILE, VOLUME INTEGRAL on a DEVC, just look at the user guide.

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[Ashish bhigah]

The windows didn't break. As you can see in the figure attached , at almost 400 s, it seems that the flame moves from the burner to the door of the room, searching oxygen in the corridor (I supposed it in this way, I am not sure). Do you think that placing a device of Oxygen with a volume integration in the room I can assess this behavior?

On Wednesday, January 22, 2020 at 12:26:36 PM UTC+1, Randy McDermott wrote:

Well, your O2 eventually climbs up to achieve 4.5 MW, so the room is capable of supporting it.  Are you saying that the window breaks at around 400 s?  Or are you saying you want to confirm the total O2 level in the room?  Lots of options for that: MASS_FILE, VOLUME INTEGRAL on a DEVC, just look at the user guide.

On Wed, Jan 22, 2020 at 4:42 AM Ashish bhigah <bhigah...@gmail.com> wrote:

Hi all,

I have a sort of discrepancy between HRR input and HRR output, as you can see in the graph. I suspect that this can be due to the lack of oxygen in that room, since I didn't activated on purpose the windows breakage.

Is there a parameter that I can measure with a device or a slice that can help me to address directly this problem to the lack of oxygen? 

Thanks in advance

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[Ashish bhigah]

I am trying to use this device. In order to deduce the oxygen concentration should I divide the value obtained with this device by the volume of the room?

&DEVC ID='Oxygen', QUANTITY='VOLUME FRACTION', SPEC_ID='OXYGEN', XB=1.25,7.25,8.5,12,6,9,SPATIAL_STATISTIC='VOLUME INTEGRAL' /

[Randy McDermott]

What units do you want for concentration?  The volume integral will take the volume fraction and integrate it over the domain volume.  So, you are left with the volume of O2 in m^3.  If you want mass, then you need to integrate DENSITY with SPEC_ID='OXYGEN' and divide by the room volume.

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[Ashish bhigah]

I have never did this. But I was supposing that maybe the oxygen concentration can be a good parameter, since I know that, in the air, the oxygen concentration 20.9%. 

So if I divide the volume fraction obtained with that device and I divide it by the room volume I should obtain the oxygen concentration isn't it? 

But I don't know which can be the best indicator for the lack of oxygen

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[Ashish bhigah]

Using this parameter and dividing by the room volume I see that, at the beginning of the simulation, the oxygen concentration is 20.7% instead of 20.9% as I was expecting. Why?

[dr_jfloyd]

FDS has a default humidity of 40 %. 20.9 % is for dry air.

[Ashish bhigah]

Thank you for your answer Dr Floyd, I will run a test case to know which is the behavior of the oxygen concentration in the room. 

But I still have a doubt, the Heat release rate reaches the maximum prescribed, so I was wandering if the discrepancy in the hrr can be related to the lack of oxygen in the room and to consequent movement of the flame through the corridor or if there can be other explanations? Do you have any suggestion? I was sure that cannot be other explanation in this kind of discrepancy in the hrr.

[dr_jfloyd]

I do not see a discrepancy in your plot. You are using a CFD code not a zone model. The heat release rate is determined by the mixing of fuel and air due to the turbulent flow field that develops. That rate of mixing is not constant.

[Ashish bhigah]

So the fact that after 400 the curve seems to be, during the hrr growth, underestimated, can be due to the fact that the rate of mixing is changing and maybe it is changing location where they mixes, isn't it? Is there a parameter that can help me to assess this rate of mixing?? 

[dr_jfloyd]

Yes, it takes time for the fuel leaving the burner to make its way to a location where it can mix with sufficient oxygen to burn. As you consume the oxygen in the room and the flame sheets moves towards the door, the time it takes fuel to leave the burner and travel to where the oxygen is will increase.

You can output mixing time, concentrations of fuel and oxygen, reaction rates, plus a number of other outputs related to combustion and the fluid dynamics. Review the options in the User's Guide and determine what outputs best make sense for your particular needs. 

[Ashish bhigah]

Thank you. You really helped me. Now I have a lot to read and to try. 

[Khalid Moinuddin]

This is an old post, but I am interested to calculate oxygen concentration at different locations of within a room. We want to compare with the experimental data. The measurement unit is % by volume. 

If we are using the below command:

&DEVC XYZ=1.18,0.6,0.6,QUANTITY='VOLUME FRACTION',SPEC_ID='OXYGEN', ID='O2_Door'/

The results we get are in mol/mol. However, the range is from 0 to 0.20797 which is the similar range for  % by volume. Should we continue to use the above command or set the command differently?

Many thanks in advance.

[fde]

Note that the concentration from the experiment might be reported as "dry", i.e. excluding the water vapor in the air. Make sure to account this. FDS also has a parameter to account this. See  22.10.21 Dry Volume and Mass Fractions

[Khalid Moinuddin]

Thank you. We will compute both dry and wet.

 

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[Kevin McGrattan]

mol/mol is a volume fraction. It differs from volume % by a factor of 100.

[Ed Wynn]

I am not affiliated with NIST or FDS developers, so my comments may not be as expert as others.

It seems to me that your original question is not about the details of oxygen content, or not only about that. Instead, you are really asking about the extinction model and where it is active. For one way to postprocess this, search for "output quantity extinction" in the FDS User Guide. In the 6.8.0 version, this sends you to section 22.10.28, where you find that you can plot slices of 'EXTINCTION' to see where combustion is active and unrestricted (EXTINCTION==0) or active and restricted by the extinction model (EXTINCTION==1). This works with a command such as:

&SLCF PBY=0.0, QUANTITY='EXTINCTION' /
A typical slice would be through the middle of the fire bed. No doubt you could also generate quantitative measures, such as the volume where EXTINCTION==1.

Another route is to postprocess where heat release is occurring. If it is all (or mostly) near the fire bed, then that is a sign that the fire is well ventilated. You can visualize heat release locations in 3D -- in Smokeview, right-click and Load... 3D Smoke... HRRPUV.  This looks different (and probably better, when available) if you activate "Use GPU" in the Files/Data/Coloring dialog. Maybe also change the HRRPUV cutoff parameter: in that dialog, insert a lower value such as 20 into the box in "Data...3D Smoke...smoke/fire color...Color as fire when HRRPUV (kW/m3) >". For scripting, this is now available in the .ini file as HRRPUVCUTOFF.

You can make this quantitative with commands such as:

&DEVC ID='HRR__exterior',  XB=-1.8,0.0,-1.2,1.2,0.0,2.4, STATISTICS='VOLUME INTEGRAL',  QUANTITY='HRRPUV' /

to find how much of your expected fire size (in kW) is happening inside the specified coordinates. It can be useful to define the coordinates to cover the fire compartment (rather than the corridor, for example), or within a few metres of the fire bed.

Note that there are some potentially significant changes to the defaults between versions 6.7.9 and 6.8.0 in this area, relating to the new default 'EXTINCTION 1' for EXTINCTION_MODEL when SIMULATION_MODE='VLES' (which is also the default). This can change the results considerably in real cases.