Visibility criterion

[Victor Shestopal]

Dear authors of the FDS and CFAST software,

 

Attached is an email from D Gottuk and below from AW Heskestad.

What is your opinion - how should we interpret visibility obtained in computation?

 

Victor Shestopal
Australia

 

[dr_jfloyd]

We, the developers, are not going to tell you what soot yield to use.  That is a determination for the user.  It is up to you to determine what soot yield is appropriate for your application.  We will merely produce and transport it per your inputs.

As far as the deposition issue goes.  I think this depends on the application and I am not sure that we know enough to determine how significant the impact is in all cases.  Very little research has quantified the quantity of soot deposited under various conditions of geometry, temperature, and flow speed.  The work mentioned by Dan was mostly an attempt to explain why we were having difficulties getting detector activation for fires we felt should have and we attributed it to a combination of soot yield data in handbooks not being applicable to our fire and deposition.  In our particular application it appeared to be significant.  These, however, were steady-state, small fires and the impact of deposition may very well be different from that for a growing fire where soot concentrations will be increasing over time or for an atrium type of geometry where impact deposition is probably less important and other mechanisms (like thermophoresis) will be smaller due to ceiling heights and free air volume.  I would be very leery of anyone trying to state some de-rating factor for visibility based on the limited state of our knowledge.

[Victor Shestopal]

The issue is not soot deposition. The issue is the "specific extinction coefficient", also termed "smoke extinction area". FDS and CFAST assume it 8.7 m2/g, based on Mulholland and Croakin's tests. In Heskestad's tests it is approximately 1 m2/g. There is no word of warning in the FDS documentation regarding this, and we, engineers, arrive at a situation where computed temperature and toxicity are low, but the computed visibility would not allow for safe evacuation. This results in unnecessary smoke exhaust and other fire safety precautions. Millions of dollars throughout the industry.

 

I would expect the authors of the software to pay attention to this issue.

 

Victor Shestopal

Australia

 

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[dr_jfloyd]

Dan's email refers to soot deposition and smoke production.  Nowhere in the email discussion that you posted is there any reference to the number 1m2/g or 8.7 m2/g.  If you wish to raise an issue about specific numbers be clear about what you are trying to ask and provide references otherwise how do you expect us to respond in meaningful manner? 

It is not our responsibility to dictate what inputs are or are not appropriate for you the model user.  That is your responsibility.  If you want to use another extinction coefficient and can justify it to your AHJ, you are free to do so. 

[dr_jfloyd]

A thought has occurred.  Are you comparing the same numbers? In the 8.7 m^2/g, the g is grams of soot.  If the 1 m^2/g number comes from a large scale fire (where measuring the soot mass would not be easy) is that g grams of fuel (as in soot obscuration produced per mass of fuel)?  If that is the case and one assumes something like a 10 % soot yield then 1 m^2/g of fuel would become 10 m^2/g of soot.

[Victor Shestopal]

The following calculations are relating to the eattached example of FDS5 output.

 

Taking the largest SEA from Interflam '93 publication of AW Heskestad SEA = 1000 m2/kg, I compute as following.

 

The entry file specifies for polyester as fuel:

 SOOT_YIELD  0.08 kg/kg

 CO_YIELD      0.075 kg/kg

 

The program produces the results of visibility in tems of meters based on equation k = 3 / Visibility where k is extinction coefficient, 1/m.

 

The attached example shows results of computation. In the same areas:

CO = 3x10-5 mol/mol

Visibility = 10 m and accordingly k = 0.3 m-1.

 

Molecular mass of CO and air are close, so that CO = 3x10-5 kg/kg. 

However, SOOT = k / SEA = 3x10-4 kg/m3 = 2.5x10-4 kg/kg.

 

The difference is an order of magnitude.

 

Then i have found in in SFPE Handbook 4-th Edition, Page 2-297, the following:

 

"More recently Mulholland and Croarkin reviewed the modern data for flaming fires and concluded that the best value of Km is 8.7 m2/g... This value is currently accepted specific extinction coefficient used in models like CFAST and FDS".

 

And eventually the explanations by AW Heskestad sent you previously clarify what has happened: the software is using small-scale results, and we, engineers, apply them to warehouses and the like.

 

May be, now I have explained myself clearly.

 

----- Original Message -----

From: dr_jfloyd

To: fds...@googlegroups.com

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[Victor Shestopal]

These questions are not to me: I refer to the publications by people whose emails I sent you. One fire fighter also confirmed in the discussion that the computed visibility is too low (email attached). In our engineering experience, only computed visibility defines the capacity of fire safety systems. However, if to make a 2 - 5 times correction - what a relief !

 

Regards,

Victor Shestopal

 

----- Original Message -----

From: dr_jfloyd

To: fds...@googlegroups.com

Sent: Wednesday, August 10, 2011 1:43 PM

Subject: Re: [fds-smv] Re: Visibility criterion

A thought has occurred.  Are you comparing the same numbers? In the 8.7 m^2/g, the g is grams of soot.  If the 1 m^2/g number comes from a large scale fire (where measuring the soot mass would not be easy) is that g grams of fuel (as in soot obscuration produced per mass of fuel)?  If that is the case and one assumes something like a 10 % soot yield then 1 m^2/g of fuel would become 10 m^2/g of soot.

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[dr_jfloyd]

In that paper the smoke extinction is defined as obscuration / unit mass burned:

Section 2.2.: " 'smoke extinction area' (SEA; [m^2/kg]) (expressed in based-e units.) This parametre is an expression of the extent of obscuration produced smoke per unit mass burned".

That is a different parameter than the FDS input which is obscuration / unit mass of soot.  You are not comparing the same unit.  To go from mass burned to mass of soot you need to multiply the fuel mass by the soot yield or in this case divide the extinction coefficient by the soot yield. 

Your report from the fire fighter is anecdotal evidence at best.  On what basis is the firefighter making this comparison?  How "calibrated" is that person's vision on visibility distance?

[Victor Shestopal]

Dear Sir,

 

You somehow stop at the crucial point and are not moving where you do not want.

FDS computes visibility as 3/k - extinction coefficient. Now - how it computes extinction coefficient? - Obviously upon the amount of soot in volume, kg/m3. How to come to the extinction coefficient, k 1/m? Apparently, using some assumption regarding SEA, which is in m2/kg. What SEA is FDS using? The SFPA Handbook says you are using 8700 m2/kg. Is it right or wrong?

 

Regards,

 

Victor Shestopal

Australia

 

Sorry, I do not know your name and have to use a "Sir" salutation.

 

----- Original Message -----

From: dr_jfloyd

To: fds...@googlegroups.com

Sent: Wednesday, August 10, 2011 10:39 PM

Subject: Re: [fds-smv] Re: Visibility criterion

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[dr_jfloyd]

If you are going to make accusations of unethical actions on my part, our conversation will be over.

As I stated you are not comparing the same measurements. FDS uses 8700 m2/g of SOOT. The Heskestad number is per g of FUEL. Yes the numbers are different, but they are measuring TWO different quantities. In your example convert the 1 m2/g of fuel to g of soot by using the 0.075 soot yield which by the way would make it 13 m/g of soot an even larger value than we use in FDS

[Victor Shestopal]

Dear all,

In the coarse of these discussions, one may conclude that there is an
evidence that the popular software uses small-scale measurements to compute
visibility. Namely, in some tests of Mulholland and Croakin's work the
chamber was 1 m3. In Gottuk's tests measurements were made at 4.6 m from the
fire, and the discrepancy with the computed prediction was large.
Mulholland reports on the considerable agglomeration of soot clusters. Can
this agglomeration be the reason for the deviation of the large-scale test
data from the computed results? The agglomeration of smoke particles may
result in decrease of the extinction coefficient. Otherwise, it is difficult
to understand: why smoke changes while floating.

It would be of great assistance to get the opinions on this issue.

Victor Shestopal
Australia

----- Original Message -----
From: "Bogdan Dlugogorski" <bogdan.dl...@newcastle.edu.au>
To: <ia...@newcastle.edu.au>
Sent: Wednesday, August 10, 2011 6:11 AM
Subject: IAFSS> Re: Visibility criterion

>>>> Dan Gottuk <DGO...@haifire.com> 10/08/2011 5:32 am >>>
> We concur with the smoke production issue mentioned below. We have
> conducted tests and compared published small-scale smoke yields to larger
> scale fires. The measured soot yields for 100 kW fires range from 2 to 5
> times smaller than the reported small-scale values. This finding was
> reported in our 2008 IAFSS paper:
>
> Gottuk, D., Mealy, C., and Floyd, J., "Smoke Transport and FDS
> Validation," Fire Safety Science - Proceedings of the 9th International
> Symposium, International Association of Fire Safety Science, Karlsruhe,
> Germany, September 21-26, 2008, pp. 129-140.
>
> We also found that for some situations there can also be up to 40% soot
> loss due to deposition on a ceiling above the plume. This type of soot
> loss has not generally been taken into account in models.
>
> Regards,
> Dan Gottuk
>
> Daniel T. Gottuk, Ph.D., P.E. | Senior Engineer | Hughes Associates, Inc.
> 3610 Commerce Drive | Suite 817 | Baltimore, Maryland 21227
> Tel: 410.737.8677 x 217 | Cell: 443-310-5558 | www.haifire.com
>
>
>
>
> -----Original Message-----
> From: iafss-...@newcastle.edu.au
> [mailto:iafss-...@newcastle.edu.au] On Behalf Of Bogdan Dlugogorski
> Sent: Tuesday, August 09, 2011 2:54 PM
> To: ia...@newcastle.edu.au
> Subject: IAFSS> Re: Visibility criterion
>
>>>> Atle William Heskestad <AtleWillia...@sintef.no> 9/08/2011
> 9:53 pm >>>
> Dear all
>
> There has been some internal communication on the issue between Victor
> and me, and which can be summarized to that small scale test data for
> smoke might be as much as 10 times too high compared to experimental
> data from large scale fire testing.
>
> In my thesis there was shown that small scale tests might produce far
> more smoke than what is measured in large scale test. This was revealed
> by comparing experimental smoke data normalized to heat release (i.e. to
> oxygen consumption).
>
> Thus references have to be used with care, especially when several
> references are combined in simulations. The crucial issue is to use
> consistent input data, which means that input data have to originate
> from the same fire conditions.
>
> For those further interested in the issue, there was made a paper by
> Anne Steen-Hansen and me on the Human behavior fire symposium in 2001.
> The paper shows the visibility conditions compared to the toxic
> conditions by use of experimental data made by SINTEF NBL in connection
> with the investigation on the Scandinavian ferry disaster. The paper is
> also available through the PhD-thesis to Anne, which can be downloaded
> from the home pages to NTNU.
>
> The mentioned reference deals only with experimental data. I am not
> aware of any publications on simulations of smoke atmospheres including
> the visibility conditions, and where the simulations are verified with
> large scale experimental data. Some CFD computational tools might be
> capable on such computations, but I have not seen such simulations
> compared with experimental data originating from large scale fires.
> Please let me know if such references are available.
>
> Best regards, AtleW Heskestad
>
>
>
> From: Victor Shestopal [mailto:fire...@optusnet.com.au]
> Sent: 5. august 2011 01:32
> To: iafss...@newcastle.edu.au; ia...@newcastle.edu.au;
> ia...@cc.newcastle.edu.au
> Cc: Atle William Heskestad
> Subject: Visibility criterion
>
> Dear Dr Heskestad,
>
> Your message has very important qualitative information.
> To use this information in our practical engineering work, some
> quantitative hints would be also important. Below are several lines of
> one of my report summarizing results obtained using FDS5 (fuel is
> polyurethane) :
>
> The maximum temperature at the height of 2.1 m in 25 minutes is
> below 43*C everywhere...
> The maximum concentration of carbon monoxide at the height of 2.1 m
> above the floor in 25 minutes after ignition is below 350 ppm
> everywhere...
> The visibility of 10 m at the height of 2.1 m will be maintained for
> not more than 2.5 minutes. However, at the height of 1.4 m above the
> floor the visibility of 5 m will be maintained for 8 minutes.
>
> I do not believe that such results might be realistic. The requirement
> to maintain visibility causes extra spending to our clients. For the
> entire industry this is millions of dollars.
>
> Victor Shestopal
> Australia
>
>
>
> Iafss mailing list
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[dr_jfloyd]

You are misconstruing the Gottuk tests. Measurements of soot yield in the large scale did not match measurements of soot yield from the small scale. Once we had a fuel with a similar yield to the fds simulations, we saw comparable results of visibility in the plume.

I am not sure if agglomeration would be significant over the time range we are dealing with. Have you computed an agglomeration kernal to see what rate would be expected?

[TimMcD]

Hi Victor,

You raise an interesting point, however, I'm a little confused about
some of the "evidence" here.

1. Dr Gottuk mentions a paper (which he incidently published with the
same Dr Floyd you've been questioning) in which: "The measured soot

yields for 100 kW fires range from 2 to 5 times smaller than the

reported small-scale values". What exactly are you/we calling small
scale and large scale? If 100 kW is large scale, then large scale
experiments were certainly covered in the Mulholland & Croarkin
review. They list a range of fire sizes which are larger than 100 kW
(up to 350 kW) from one study.

2. Mulholland and Croarkin also mention one study from Factory Mutual
that included "Small to Large" scale experiments (see Table 1). Of
interest is that the average mass specific extinction coefficient from
this set of experiments was 10.2 m²/g. I can unfortunately not get
access to this study to find out what constitutes as large.

3. In your last post you mention that there is a deviation of computed
results from large-scale test data. Where is this large scale test
data? How big is it - 1 or several MW? Could you please share it with
the FDS community so we too can quantify any difference?

4. Unless a better value for a general mass specific extinction
coefficient exists, there is no need for the FDS Developers to change
the default. Fortunately, they have provided a path for users to
change this value if they feel it is necessary (see FDS User Guide).

Personally, I am not aware of (or don't have access to) any better
large scale experimental results for the mass specific extinction
coefficient, so I will trust the default for now because I have
confidence defending its use (based on the nothing better argument).

Cheers
Tim


On Aug 12, 8:16 am, "Victor Shestopal" <firec...@optusnet.com.au>
wrote:

> Dear all,
>
> In the coarse of these discussions, one may conclude that there is an
> evidence that the popular software uses small-scale measurements to compute
> visibility. Namely, in some tests of  Mulholland and Croakin's work the
> chamber was 1 m3. In Gottuk's tests measurements were made at 4.6 m from the
> fire, and the discrepancy with the computed prediction was large.
> Mulholland reports on the considerable agglomeration of soot clusters. Can
> this agglomeration be the reason for the deviation of the large-scale test
> data from the computed results? The agglomeration of smoke particles may
> result in decrease of the extinction coefficient. Otherwise, it is difficult
> to understand: why smoke changes while floating.
>
> It would be of great assistance to get the opinions on this issue.
>
> Victor Shestopal
> Australia
>
>
>
> ----- Original Message -----
> From: "Bogdan Dlugogorski" <bogdan.dlugogor...@newcastle.edu.au>
> To: <ia...@newcastle.edu.au>
> Sent: Wednesday, August 10, 2011 6:11 AM
> Subject: IAFSS> Re: Visibility criterion
>

> >>>> Dan Gottuk <DGOT...@haifire.com> 10/08/2011 5:32 am >>>

> > We concur with the smoke production issue mentioned below.  We have
> > conducted tests and compared published small-scale smoke yields to larger
> > scale fires. The measured soot yields for 100 kW fires range from 2 to 5
> > times smaller than the reported small-scale values. This finding was
> > reported in our 2008 IAFSS paper:
>
> > Gottuk, D., Mealy, C., and Floyd, J., "Smoke Transport and FDS
> > Validation," Fire Safety Science - Proceedings of the 9th International
> > Symposium, International Association of Fire Safety Science, Karlsruhe,
> > Germany, September 21-26, 2008, pp. 129-140.
>
> > We also found that for some situations there can also be up to 40% soot
> > loss due to deposition on a ceiling above the plume. This type of soot
> > loss has not generally been taken into account in models.
>
> > Regards,
> > Dan Gottuk
>
> > Daniel T. Gottuk, Ph.D., P.E. | Senior Engineer | Hughes Associates, Inc.
> > 3610 Commerce Drive | Suite 817 | Baltimore, Maryland 21227

> > Tel:410.737.8677begin_of_the_skype_highlighting            410.737.8677      end_of_the_skype_highlightingx 217 | Cell:443-310-5558begin_of_the_skype_highlighting            443-310-5558      end_of_the_skype_highlighting|www.haifire.com

>
> > -----Original Message-----
> > From: iafss-boun...@newcastle.edu.au
> > [mailto:iafss-boun...@newcastle.edu.au] On Behalf Of Bogdan Dlugogorski
> > Sent: Tuesday, August 09, 2011 2:54 PM
> > To: ia...@newcastle.edu.au
> > Subject: IAFSS> Re: Visibility criterion
>

> >>>> Atle William Heskestad <AtleWilliam.Heskes...@sintef.no> 9/08/2011

> > iafssmemb...@dial.pipex.com FOR MEMBERSHIP SUBSCRIPTION.

> > **********************************************************************
>
> > **********************************************************************
> > Views, opinions and information presented here are not necessarily
> > those of the International Association for Fire Safety Science and/or
> > the Society of Fire Protection Engineers.  To subscribe visit
> >http://mailman.newcastle.edu.au/mailman/listinfo/iafss (for IAFSS
> > Discussion Forum)

> > orhttp://mailman.newcastle.edu.au/mailman/listinfo/sfpe(for SFPE

> > Discussion Forum).  At present, both Fora co-share their postings.
> > **********************************************************************
>
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> > **********************************************************************
>
> > **********************************************************************
> > Views, opinions and information presented here are not necessarily
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> > the Society of Fire Protection Engineers.  To subscribe visit
> >http://mailman.newcastle.edu.au/mailman/listinfo/iafss (for IAFSS
> > Discussion Forum)

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> > Discussion Forum).  At present, both Fora co-share their postings.

> > **********************************************************************- Hide quoted text -
>
> - Show quoted text -

[Victor Shestopal]

Dr Floyd,

You write "Once we had a fuel with a similar yield to the fds simulations,
we saw comparable results of visibility in the plume". Where these results
can be seen? Are any publications or reports? Please provide an evidence.

So far, in your and Daniel Gottuk's paper there is something different (page
137, bottom): "it is important that the model input was a fire with a soot
yield of 2.2 percent and the experimental tests used a fire with a soot
yield of 4.8 percent... it became apparent that soot loss was a substantial
phenomenon that had to be considered".

Further clarification would be very helpful.

Regards,

Victor Shestopal
Australia

.

----- Original Message -----
From: "dr_jfloyd" <drjf...@gmail.com>
To: <fds...@googlegroups.com>
Sent: Friday, August 12, 2011 10:53 PM
Subject: [fds-smv] Re: Visibility criterion

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[Sandeep Upadhya]

Hi all,

On the subject of mass extinction co-efficient (Km) and visiblity, I am currently doing an exercise where I have a specified mass optical density of 0.40 m2/g or 400m2/kg (for PVC). The mass optical density relates to the amount of light obscuration over the quantity of fuel burnt.

 

The amount of light obscuration over the quantity of soot produced  (soot yield of 0.14) would then equate to 400/0.14= 2857.2 m2/kg. Is this qunatity called the particulate optical density and therefore to convert it into a mass extinction co-efficeint(Km), you need to multiply by a factor or 2.3? This would give a value of 6571.5 m2/kg.  

 

I know that the default value used by FDS for mass extiction co-effecient is 8700 m2/kg.

Since S(visibility) = c/k and where K=  (pYS) * Km,  do i therefore need to reduce my value of Km on FDS?

 

I feel that in my case, using the default value of Km would give me a much reduced visibility wherelse in reality i could expect to have a higher visibility for my test case?

 

Please help. I have gone over Mulholands paper and also referred to the SFPE handbook and my understanding of this is still very vague.

 

Regards

San