HRR characteristic fire diameter

[xovennn]

Hello all,

I am trying to understand how the characteristic fire diameter works. In my case I am trying to simulate a fire of 30 000kW on a truck, I have the hrr curve from experimental work.

So far I tried to model my truc and set all its surfaces (203.76m2) on fire



My HRRPUA is equal to 147.23 kW/m2 (147.23*203.76 = 29 999.58 kW)
And it works well, my hrr curv is almost indetical to the one given by the experiment.

However I would prefer to split the fire surfaces. I calculated the characteristic fire diameter

Then I found the characteristic fire surface = 10.97016916m2 and I split this surface (according to my mesh resolution 0.30.30.3) on my truck using &VENTs that I set its SURF_ID on fire



By doing that my total fire surface is 10.98 m2, so I adapted my HRRPUA which is now equal to 2732.24 kW/m2.
(10.98*2732.24 = 29 999,99 kW).

But this time my hrr file gives me a hrr curve that doesnt match:
In gray is the hrr curv given by the VENTs method, in blue is the hrr curv given by the entire surface on fire and in orange is the experiemental hrr curv.

Do you have any idea why the hrr max is so low for the method using the VENTs ?
Thanks.

[drjfloyd]

You domain looks to small compared to the size of the burning object. You have increased the velocity at the burner by a factor of 20 after reducing the vent area. Mostly likely some fuel is not able to mix with air before it leaves the domain. In the HRR file compare the fuel mass loss rates. If those are the same, then you have unburned fuel leaving the domain.

[xovennn]

Indeed you were right, the fuel mass loss rate are the same for both methods