Experimental and numerical study on low-frequency oscillating behaviour ofliquid pool fires in a small-scale mechanically-ventilated compartment
Résumé
The unstable oscillatory behaviour, with frequency in the order of few mHz, that has been occasionally observed
in mechanically-ventilated compartment fires, is studied experimentally and numerically. First, a series of experiments
using a small-scale compartment have been conducted using heptane and dodecane as fuels. Results
show that unstable and stable combustion regimes can occur depending on fuel type, pool size, air renewal rate
of the compartment (ARR), and ventilation conditions. For a certain range of these factors, unstable low-frequency
(LF) oscillatory combustion, accompanied by thermodynamic pressure and ventilation flow rate variations
and displacement of the flame outside the pan, is observed. The occurrence and persistency of LF oscillations
result from the competition between oxygen supply and fuel vapor supply due to the heat feedback from
the flame and enclosure to the fuel tray. Whatever the fuel type, it is found that i) the range of ARR where LF
oscillations appear and the oscillation amplitude increase with the pool size, and ii) the frequency increases,
while amplitude decreases, with increasing ARR, independently of the pool size. It is also found that the more
flammable the fuel, i) the smaller pool size for which LF oscillations appear and the higher the frequency for the
same ventilation conditions, and ii) the wider the range of ARR where LF oscillations appear for a given pool
size. The effects of air inlet position and blowing direction on the oscillations properties is also investigated.
Second, predictive CFD simulations have been performed using the in-house SAFIR software. Although SAFIR
does not correctly describe the displacement of the flame outside the fuel pan, it satisfactorily reproduces the LF
oscillatory fire behaviour, especially its dominant frequency. Information about inaccessible or difficult-tomeasure
local quantities, such as the local evaporation rate, temperature and heat flux at the liquid surface, and
species concentrations, are provided from the numerical simulation.
Domaines
Sciences de l'ingénieur [physics]
Origine : Fichiers produits par l'(les) auteur(s)
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