Thunderhead Engineering Forum

Public Forum => PyroSim => Topic started by: Henning on February 17, 2009, 09:30:37 am

Title: Growing fire
Post by: Henning on February 17, 2009, 09:30:37 am

Hello

Is there a way to simlulate a growing fire, for example on a surface?

Example: I have a initial fire on 1 m2 with 250 kW/m2, and I want a "fast" growing fire (alpha). Its this possible??'

In FDS I think you have to define the size of the fire for different time. For example 1 m2 after 60 sec., and 2 m2 after 120 sec., and so on.

Title: Re: Growing fire
Post by: Charlie Thornton on February 17, 2009, 02:42:40 pm

If you want your fire to get hotter, but don't mind if it stays the same size, you can set the heat release rate per unit area using a ramp.

Another option might be to keep the HRR the same, but enable additional fire vents as the simulation progresses.

Or, if you know a lot about the material you are burning, you could have put your faith in the flame spread model.

It sounds like you want to use a normal, fixed-size, burner with a T² ramp-up time.  Is that correct?

Title: Re: Growing fire
Post by: Henning on February 18, 2009, 04:15:53 am

Yes, that is correct. I want to to use the same HRR (burner 250 kW/m2), but it must grow in size (m2).

Title: Re: Growing fire
Post by: Charlie Thornton on February 18, 2009, 09:14:03 am

If it absolutely must grow in area rather than HRR per unit area, then I suppose it comes down to using events to activate vents at prescribed times or to actually ignite the surface.  Do you have any data that you are trying to match against?

Title: Re: Growing fire
Post by: augustas83 on February 19, 2009, 05:23:45 am

Vent Properties -> Specification -> Fire Spread
I thing it should be the answer to your question, you just need to specify Origin and Spread rate. The problem is that it doesn't work as I aspect :) I cant make it spread too :) of course it could be due to my weak knowledge... If i am wrong then what Fire Spread menu stands for?

Title: Re: Growing fire
Post by: Charlie Thornton on February 19, 2009, 09:34:06 am

Ah, sorry, I didn't know we had that feature.  I'm glad you listed out how to get there.

I was able to put together a successful test problem to verify that I can spread a fire from the center of a vent.  Based on the documentation (FDS Users Guide Section 8.2.7), it looks like you can only specify a linear spread rate - they've got a little precautionary jab in there about trying to mimic a t² curve with spread rate alone.  On the bright side, you can control the burn of each cell of the vent with the ramp on the burner.

Sorry it took me so long to catch up!

Title: Re: Growing fire
Post by: Henning on February 23, 2009, 04:48:56 am

Thanx. This is very helpfull :)