Experimental parametric study on low-frequency oscillating behaviour of pool fires in a small-scale mechanically-ventilated compartment

Abstract

The purpose of this work is to study the unstable oscillatory behaviour, with frequency in the order of few mHz, that has been occasionally observed in mechanically-ventilated compartment fires. To address this challenge, a series of experiments using a small-scale compartment was conducted using heptane and dodecane as fuels. Results show that, after the fire is fully-developed, unstable and stable combustion regimes can occur depending on the fuel type, the pool size and the air renewal rate of the compartment (ARR). A special regime of unstable oscillatory combustion with low frequencies (LF), 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 results from the competition between oxygen supply and fuel vapor supply due to the heat feedback from the flame and enclosure to the fuel tray. The LF oscillation period depends mainly on time required for storing enough fuel vapor for strong combustion. 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.