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Determination by a CFD code of the heat release rate in confined and mechanically-ventilated compartment fire

Ayoub Nasr has defended his thesis on 12th December 2011 in Poitiers.

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/DPAM/SEMIC/LIMSI

Authors > NASR Ayoub

Publication Date > 12/12/2011

Summary

For several years, many experimental/numerical research programs have been carried out at IRSN in order to provide sufficient data on the burning process and understand the behavior of a pool fire in an confined and mechanically ventilated compartment. Several experimental tests have shown that in some cases, the oxygen concentration in the local decreases then stabilizes until fire exctinction. The fuel mass loss rate is instantaneously adjusted according to the ventilation in the local, which may leads to a lower fuel consumption rate as compared to that in free atmosphere. The fire duration is then 2 to 3 times greater that that obtained in free astmosphere, which may damages some specific safety equipment used to reduce the spread of fire between compartments such as fire doors.

 

The objective of this work is to propose a theoretical approach that allows the determination of burning rate fuels for pool fires in a closed compartment. Fuel response to vitiated air as weel as burning enhancement due to hot gases and confinement should be taken into account. Thus, a theoretical formulation, based on an energy balance equation at hte pool fire surface, was developped and compared with the empirical correlation of Peatross and Beyler before being implemented in a CFD code "ISIS" developped at IRSN and validated against PRISME fire test results. The main adavntage of this global approach is that no assumptions were made on the relative importance of each mode of heat transfer form the flame. In fact, the convective and the radiant components of the heat flux from the flame to the fuel surface were determined taking into account the air vitiation effect.

 

In addition to this theoretical approach, an experimental work was conducted at the Institut Pprime to study heptane pool fires in a reduced-scale fire compartment, in the aim to investigate the effects of vitiated air on fire parameters. These results were used to validate the theoretical formulation developed earlier to dtermine the burning rate of fuels for pool fires in a closed environment. An original method to separate effects of the radiant heat flux of the flame and the external heat feedback to the fuel surface was developed. These tests also contributed to a better understanding od the effects of ventilation and pan diameter on the fuel pyrolisis at the pool fire surface.

 

As a perspective, the theoretical approach should be extended to adress the problem of estimating the flame duration. In this context, the flame extinction process should be treated in more detail by taking into account the detailed kinetics of the chemistry occuring in a flame. Similar experimental works should also be conducted in the aim to study the burning process of a poll fire using different types of fuel (alkenes, ketones, solid fuels,...).


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