FDS 5 capabilities in water mist fire suppression system.

[Iqbal]

Hi All,

I want to model water mist fire suppression in FDS. Whatever, i have
understood from my literature review that--
In earlier studies, done by using FDS 3 or FDS 4, it was seen that the
numerical models were well enough to predict different parameters in
the combustion model without fire suppression (e.g. temperature, heat
release rate, gas mole fraction etc.). But, the problem is that,
those numerical models were not well enough to simulate the
suppression model with water mist.

Now, I want to know that --

1. Is FDS 5 capable of simulating fire suppression model? Are there
any validation work or numerical study on water mist fire suppression
using FDS 5? If yes, would you please mention the paper title so that
i can get them?

2. What is the criterion for fire extinguishment in FDS ...??? Only
the limiting oxygen mole fraction or else...??? Because, in the real
situation, there are many factors that can affect the fire
extinguishment by water mist system e.g. cooling of fire source,
oxygen displacement, and so on.

3. Is there any secondary water droplets break-up model in FDS5? Kim
and Ryou (2003), in his water mist model by FDS 3, used Reitz and
Diwakar model for secondary break-up of water droplets. Is this
already in FDS5 or this model was added by the author?

4. When FDS 6 will be released? Will it include water mist fire
suppression mechanism?

I will be glad if i can get sufficient information in regard of those.

Regards

[Kevin]

1. Try the literature review section in the FDS Validation Guide?

2. FDS Technical Reference Guide, Combustion?

3. No break up model

4. FDS 6 is planned for release this year. What do you mean by a
"water mist fire suppression mechanism"?

[Iqbal]

Hi Kevin,

Thanks a lot for your reply. According to your advice, i have gone
through the technical and validation guide. And still i have some
confusion about FDS5 and water mist.

On the basis of your reply, i have some more questions. I will be glad
to get help from you.

In response to your answer to my Question 1:
The validation works on water mist system, mentioned in the FDS5
validation guide, were conducted around 2003 [Kim and Ryou (2003),
Hume (2003) and Kim and Ryou (2004)]. So, certainly, those validation
work were not on FDS5. I want to know that are there any validation
work on water mist fire suppression using FDS 5.

In response to your answer to my Question 2:
According to the FDS5 Technical Reference Guide (pg.-45&46), the
extinction of gas fire is judged by a simple empirical rule based on
oxygen concentration and temperature of the gases in the vicinity of
the flame sheet. As shown in Figure 6.2, fire will burn if the oxygen
limit is above 15% by volume fraction even the temperature is 0oC.
But, in the real situation, is it feasible?

In response to your answer to my Question 4:
Water spray and sprinkler systems fight fires primarily by wetting the
surfaces of the fuel and the surrounding structures to cool the
structures and the fuel surfaces, and thereby suppressing the fires.
Whereas, water mist systems fight fires primarily by evaporation of
water in and around the fire plume. The evaporation of water takes
large quantities of energy which cools the flames and blocks the
transfer of radiant heat and reduces the net heat output of fire. It
also includes oxygen displacement form fire burning area and oxygen
dilution in the burning area, and finally suppression of fires.
So, there is some additional features in water mist suppression system
compared to conventional water sprinkler system, i.e. droplet
evaporation, oxygen displacement, oxygen dilution etc.

Now, I want to know that will those be included in the future version
of FDS (ver. 6).


I will be glad to get reply from you.

Regards

Iqbal

[Kevin]

> In response to your answer to my Question 1:
> The validation works on water mist system, mentioned in the FDS5
> validation guide, were conducted around 2003 [Kim and Ryou (2003),
> Hume (2003) and Kim and Ryou (2004)]. So, certainly, those validation
> work were not on FDS5. I want to know that are there any validation
> work on water mist fire suppression using FDS 5.

There's nothing in the Validation Guide. I have heard of various
people doing validation work with mist and sprinklers, but I don't
have a reference for you.

>
> In response to your answer to my Question 2:
> According to the FDS5 Technical Reference Guide (pg.-45&46), the
> extinction of gas fire is judged by a simple empirical rule based on
> oxygen concentration and temperature of the gases in the vicinity of
> the flame sheet. As shown in Figure 6.2, fire will burn if the oxygen
> limit is above 15% by volume fraction even the temperature is 0oC.
> But, in the real situation, is it feasible?

FDS is basically a mixed-is-burned model. Assuming there is enough
oxygen present, combustion takes place. It is assumed that there is
always a source of ignition. In other words, if you mix fuel gas and
oxygen at 0 C and create a spark or light a match, combustion will
occur. If we are to simulate a real fire, it is almost impossible to
know whether or not there is a "hot spot" at a given point and at a
given time. FDS errs on the side of assuming that there is.

>
> In response to your answer to my Question 4:
> Water spray and sprinkler systems fight fires primarily by wetting the
> surfaces of the fuel and the surrounding structures to cool the
> structures and the fuel surfaces, and thereby suppressing the fires.
> Whereas, water mist systems fight fires  primarily by evaporation of
> water in and around the fire plume. The evaporation of water takes
> large quantities of energy which cools the flames and blocks the
> transfer of radiant heat and reduces the net heat output of fire. It
> also includes oxygen displacement form fire burning area and oxygen
> dilution in the burning area, and finally suppression of fires.
> So, there is some additional features in water mist suppression system
> compared to conventional water sprinkler system, i.e. droplet
> evaporation, oxygen displacement, oxygen dilution etc.
>
> Now, I want to know that will those be included in the future version
> of FDS (ver. 6).

These features are already in FDS 5. But there are several problems
associated with mist -- characterization of the discharge, evaporation
of very fine droplets, wetting of surfaces, dripping, etc. All these
mechanisms are in place, but there has not been enough V&V work to
ensure that they can reliably predict mist suppression.