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Message from discussion Modelling a fire
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dr_jfloyd  
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 More options Mar 10 2008, 10:05 pm
From: dr_jfloyd <drjfl...@gmail.com>
Date: Mon, 10 Mar 2008 19:05:52 -0700 (PDT)
Local: Mon, Mar 10 2008 10:05 pm
Subject: Re: Modelling a fire
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Have you reviewed the many example files and have you read the theory
and user manual?

1) The details of the combustion model are in the theory manual.  You
have two choices: multi-parameter mixture fraction with one or two
step chemistry or finite rate chemistry (you must specify all species,
reactions, reaction rates, etc).  In the mixture fraction approach if
you use HRRPUA, then yes fuel is inserted into the domain.  How it is
decided whether or not to burn it is in the theory manual.

2) Why do you expect you would see adiabatic flame temperatures?  What
are your grid sizes?  1 mm, 1 cm, 10 cm, 1 m ?The theory manual states
that temperature is extracted via the equation of state from the
conservation of mass and species.  If I have a 1 mm wide grid cell
(the width of a flame) then I would expect to see very high
temperatures where combustion occurs as the entire grid cell is a
flame.  If I have a 10 cm wide cell containing a 1 mm wide flame, then
I would not at all expect the temperature to reflect the flame
temperature as the bulk of the grid cell is not a flame.

3) Depends on what you are trying to do.  If you are interested in far
field quantities, the inlet fuel temperature is not very important.
c_p ~ 1 kJ/kg-K and deltah_combustion is ~ 20 - 50 MJ/kg.  So the
amount of energy added to by combustion is going to be much larger
than any energy change resulting from the inlet temperature.  However,
I have observed that near field fluxes for some configurations  (flux
to the wall when the burner is up agaisnt a wall) are impacted by the
burner temperature.

On Mar 10, 1:45 pm, "Ferrara Gabriele"

<Gabriele.Ferr...@snamprogetti.eni.it> wrote:
> Hello everybody.

> I work in the HSE department of a big European Engineering Company (Snamprogetti-Saipem, ENI Group).
> We are currently running some FDS simulations of some well characterised experimental configurations.

> So far I met a lot of difficulties in properly understanding how to simulate a fire (with a prescribed Heat Release Rate, HRR ). More specifically the questions are:

> 1) Is the fire modelled by injecting fresh fuel in the domain (by means of the vent B.C.s) and then igniting it?

> 2) Why do I not have any evidence of high temperatures of the burned gases (i.e. a T close to the adiabatic temperature of the fuel). In other words: why do I have Temperatures no higher than 200°C?

> 3) When setting the HRR for the burner-vent, what is the meaning of the surface temperature (i.e. the TMP_FRONT value)? Can I set both variables ( HRRPUA and TMP_FRONT) independently? Does it make sense to leave the TMP_FRONT of a burner at the ambient temperature?

> I thank you all for any help you'll give me.

> Regards

> Gab  

> Gabriele Ferrara

> Health, Safety & Environment - Design (HSED)
> tel.:      +39 02 520 43684
> fax:      +39 02 520 53745
> e-mail: gabriele.ferr...@snamprogetti.eni.it

> Snamprogetti S.p.A.
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