SPREAD_RATE t-squared fire growth rate

[enzo.d...@gmail.com]

Hi all,

About the SPREAD_RATE, the FDS User guide says "It should not be used to mimic a t-squared fire growth rate". 

However there is a CFD best practice paper that recommeds to use the spread_rate using the formula:

𝑆𝑃𝑅𝐸𝐴𝐷_𝑅𝐴𝑇𝐸=sqrt((𝛼/𝜋)∙(1/𝐻𝑅𝑅𝑃𝑈𝐴)) where α = Alfa–t2 growth rate (kW/s2)

 

So my question is : why the 𝑆𝑃𝑅𝐸𝐴𝐷_𝑅𝐴𝑇𝐸 should not be used to mimic a t-squared fire growth rate?

Thank you

[Salah Benkorichi]

Is the formula proposed validated? 

Do you have the full paper reference?

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[dr_jfloyd]

That statement refers to the RAMP_Q input not the SPREAD_RATE input.  In that particular example the RAMP_Q makes it so cells only burn for a short duration. As a result, the fire is not an expanding circle, but an expanding ring. This does not have t^2 behavior.  Without the RAMP, cells would keep burning and the spread rate would result in a t^2 fire.

[enzo.d...@gmail.com]

Thank you very much

[fds.sim...@gmail.com]

Am I right that with using the SPREAD_RATE it would not exactly result in at^2 fire because (depending of the size of the cells) the cells could never build exactly a circle?

[fde]

Correct. Another problem is that as the cell size in model get bigger, the HRR curve becomes like a ladder due to cells activate at once with full HRR. 

[blackarrow90]

It is actually a conference paper from Daniel Rosberg, there is also the whole presentation here: https://www.thunderheadeng.com/2014/10/femtc2014_d1-d-2_rosberg/

If the cells of the burner are modeled properly, I would say it is more realistic to use spread, otherwise you would have the activation of all fire cells immediately which is not hot fire behaves.

[Salah Benkorichi]

Thanks blackarrow90,

I didn't know it's that complex to share a published material.

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[blackarrow90]

Well, I always try to find and quote the source. What do you thing about implementing spread rate with t-squared fire?

On Tuesday, November 28, 2017 at 9:47:58 AM UTC+1, Salah Benkorichi wrote:

Thanks blackarrow90,

I didn't know it's that complex to share a published material.

On 28 Nov 2017 7:32 am, "blackarrow90" <bojan...@gmail.com> wrote:

It is actually a conference paper from Daniel Rosberg, there is also the whole presentation here: https://www.thunderheadeng.com/2014/10/femtc2014_d1-d-2_rosberg/

If the cells of the burner are modeled properly, I would say it is more realistic to use spread, otherwise you would have the activation of all fire cells immediately which is not hot fire behaves.

On Tuesday, November 21, 2017 at 6:34:37 PM UTC+1, enzo.d...@gmail.com wrote:

Hi all,

About the SPREAD_RATE, the FDS User guide says "It should not be used to mimic a t-squared fire growth rate". 

However there is a CFD best practice paper that recommeds to use the spread_rate using the formula:

𝑆𝑃𝑅𝐸𝐴𝐷_𝑅𝐴𝑇𝐸=sqrt((𝛼/𝜋)∙(1/𝐻𝑅𝑅𝑃𝑈𝐴)) where α = Alfa–t2 growth rate (kW/s2)

 

So my question is : why the 𝑆𝑃𝑅𝐸𝐴𝐷_𝑅𝐴𝑇𝐸 should not be used to mimic a t-squared fire growth rate?

Thank you

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[jacques Frezabeu]

Hello,

From my point of view it depends on what you are analyzing. I only use spreading rate when I need heat flux or flame height during the growing phase.

For other applications, when I need to explore steady state, I use a negative TAU_Q.

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[fde]

I found a workaround (in the userguide); introducing a certain size of TAU_MF(1) to the SURF line solves this problem.

In userguide p71, circular vents, TAU is 0.01 which gives this ladder increase. I think TAU should be a bit higher to follow the plot in Figure 10.1. 

[Steffen Sæle]

The user guide states : 

...Note that the RAMP_Q is used to turn the burning on and off to simulate the consumption of fuel

as the fire spreads radially. It should not be used to mimic a t-squared fire growth rate – the whole point of

the exercise is to mimic this curve in a more natural way. Eventually, the fire goes out as the ring grows past

the boundary of the rectangle...(end quote) 

The segment is refering to the use of the RAMP_Q, as I understand it. The method is otherwise sufficient to simulate a t-squared fire in a more natural way than fex. the time ramp of the HRRPUA over a constant area. 

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[fde]

Sorry I did not understand your point. 

My suggestion of using TAU_MF was to smooth the curve only.