E-coefficient

[Shivam Gupta]

Hello Everyone,

I am working on Fire Suppression through the generation of Mist using nozzle.

While going through FDS user guide, 

In section 17.6.2 Reduction of the Burning Rate

They have mention about E-Coefficient while using HRRPUA. 

I have read the various post in the group 

in which they perform the suppression of Fire without using E_Coefficient even though they have used HRRPUA as input.

I performed the Two case in FDS

Case-1: Using E_Coefficient and varied value from 100-10

Case-2: Simple Case without E_Coefficient.  

In both Case Fire was fully suppresed i.e HRR appraoches to zero.

Case-1 approach is long as it provides range of suppression time. 

Although for small diameter this range does vary significantly. 

Case-2 The generation of lot of mist which doesn't include the effect of pyrolysis of fuel. 

Which aproach is true one ? 

Kindly resolve the issue

  

[Kevin McGrattan]

E_COEFFICIENT works with water on a surface to reduce HRRPUA

Also, if the room oxygen decreases below about 15%, gas phase combustion will stop, even though fuel gas is still generated.

[dr_jfloyd]

The primary mechanism via which water mist systems suppress fires is not cooling of the fuel surface. E_COEFFICENT is essentially an empirical model of fuel surface cooling. I would not expect it to have any significant impact on a water mist system simulation. The fact that 1) and 2) have the same results should be an expected result. 

[Shivam Gupta]

Sir,

What I observed in the Simulation

If I use E-coefficient and varied the value from 100-10 the suppression time is almost half as compared to the result of case in which E-coefficient was not used.

Last I have one question

When we don't use E-coefficient. 

Does the suppression occurs when the oxygen concentration is below 15 % in the cell domain?

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[dr_jfloyd]

10 to 100 are extremely large values for the E_COEFFICIENT.  A value of 10 would predict that a standard sprinkler system discharging 0.07 kg/(m^2 s), 0.1 gpm/ft^2, would reduce a fire by a factor of 10 with a few seconds of discharge. NISTIR 5254 indicates that testing of office furniture with a sprinkler discharging 0.07 kg/(m^2 s) results in an E_COEFFICIENT for those tests in the range of 0.03 to 0.2. 

If there is no E_COEFFICIENT, then the only mechanism is the extinction model in FDS which is described in the Technical Reference Guide. The exact value of oxygen where extinction occurs in a grid cell is a function of the local temperature and species concentrations. It would be below 15 %. 

[Shivam Gupta]

Sir,

Thank you for giving me so much information on E- Coefficient. 

I have some more query. 

can we say suppression time observed in FDS will be over predicted  results ? 

As we are not considering the E coefficient. 

For some fuel, vaule of 10 has been also measured. 

If I assume that value to be true for my fuel than I am observing almost half of difference in suppression time.

Last question, 

In case oxygen concentration does not reach below 15%

How can I say fire is suppressed? 

Can I conclude on the basis of temperature ?

With time temperature will aslo come to certain value? 

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[dr_jfloyd]

Water mist systems do not generally extinguish fires by cooling of the fuel surface; maybe if you are lucky enough to have a nozzle very close to first item burning. E_COEFFICIENT=10 is an extremely fast suppression of a fire. Make sure that you understand exactly what the experiment measured, that it directly corresponds to the FDS E_COEFFICIENT, and that it is directly applicable to your case. I believe you are significantly over predicting the effect of a watermist system on the burning rate. 

If water mist systems cannot drop the O2 levels below those needed for extinction, they cannot be relied upon to put out a fire. This is seen in testing of water mist systems; see the US Coast Guard testing on water mist systems referenced in the FDS Validation Report. In this case they may still greatly reduce the hazard from the fire by reducing the gas temperature and/or reducing the rate of fire growth.