Smoke Concentration Outputs

[jamie]

Hello,

I am working on a project that has some history to it and someone many
years ago used a general purpose CFD code for fire modelling. The
previous set of results were reported in terms of smoke concentration,
defined as mass of products / mass of mixture (air + products). I
would like to compare apples to apples, so if I compare their results
of 1% smoke concentration, for example, with my FDS results is this
just simply the mixture fraction equal to 0.01? Or do I need to
adjust this to get my "apples"? Or would it be easier to add up the
mass fractions of all the product species? I've gone cross-eyed
thinking about it, so suggestions are greatly welcomed!

Thanks,
Jamie

[Jamie]

Ok, I think maybe my first post was a little misleading. The 1% smoke
concentration originally used is actually 1% by mass of what used to
be fuel. So I think the answer to my own question is that I can scale
of the soot density. So if I divide the soot density by the soot
yield, then convert to percentage (divide by 100) that should give my
the "smoke concentration" as previously defined.

Anyone out there have any thoughts? Does this seem reasonable?

[Kevin]

Smoke concentration in FDS is output as a mass fraction; that is, mass
of soot per total mass of everything. The preferred way of getting
this output is now

QUANTITY='MASS FRACTION', SPEC_ID='soot'

It sounds to me that the "smoke concentration" of the other model is a
mass fraction as well. Your second post seems to be talking about
"soot yield." This is the mass of soot produced per mass of fuel
burned. Because FDS just transport the soot along with everything
else, if you double the soot yield, you will double the soot mass
fraction throughout the space.

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[Jamie]

Right, let me try to clarify my earlier post a bit...I'm looking to
get an output from FDS for "smoke concentration" which, using your
clearer vocab, is mass of fuel burned per total mass of everything
else. So being that there is no one species for burned fuel (please
correct me if I'm wrong here), I plan on using the soot concentration
and scale from that. I can modify the soot concentration by dividing
by the soot yield to get the concentration of burned fuel (or "smoke
concentration" from the other model). For example if I have a soot
concentration of 0.005g/g in a given location and time, I can divide
that by my soot yield (say 0.1g/g) to get a "smoke concentration" of
0.05g/g. Basically it is (soot/everything)/(soot/fuel_burned) =
(fuel_burned/everything). I think this makes sense, but please let me
know if I am over simplifying.

Thanks,
Jamie

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[Kevin]

Actually, there is an output for "burned fuel". It is called
'MIXTURE_FRACTION_2'. FDS 5 splits the mixture fraction, Z, into Z1
and Z2. Z1 is the mass fraction of fuel, Z2 is the mass fraction of
combustion products. This is not included in the User's Guide, but
probably should be. You now only see 'MIXTURE FRACTION' as an output
option. This is the mass fraction of all the stuff that came out of
the burner divided by everything. That included burned and unburned
fuel.