Experimental study on low-frequency oscillating behavior in mechanically-ventilated compartment fires

Abstract

This study focuses on fire behavior in a mechanically-ventilated compartment, with a special emphasis on the low-frequency (LF) oscillatory behavior that has been occasionally observed. LF oscillations, typically in the order of a few mHz, can lead to large thermodynamic pressure variations, which in turn can cause fire safety issues (e.g. loss of confinement, mechanical damages). To address these issues, small-scale experiments are conducted varying the air renewal rate (ARR) in the compartment from 8 to 20 h-1 and the ventilation configuration. The fire source consists of heptane fuel loaded in a pan with a diameter of 18 cm. Depending on the ARR, LF oscillations are observed on the time evolution of the burning rate, and thus of all the other variables, with a frequency in the range of 16-26 mHz. As the ARR increases, there are three distinct regimes of burning behavior observed in this study: (1) rapid extinction due to smoke filling; (2) LF oscillating burning, followed by blow-off extinction after a few oscillations; and (3) LF oscillating burning, where extinction occurs because of the burning rate "runaway" due to an intensification of the heat transfer through the rim of the container. The oscillatory behavior and fire extinction result from the competition between oxygen supply and fuel vapor supply due to the heat feedback to the fuel tray. Both regimes (2) and (3) are accompanied by displacements of the flame out of the pan towards regions where oxygen is present. The influence of ARR and ventilation configuration (i.e. air inlet location and blowing direction) on the burning rate and LF oscillation properties (frequency, amplitude) is examined and discussed.

Effect of environmental conditions on fire combustion regimes in mechanically-ventilated compartments

“…First, low frequency oscillatory behavior of the burning rate is reported for renewal rate between 15 h-1 and 18 h-1. The phenomenon is typical for configurations in which the fire environment is characterized by an oxygen concentration near to the LOI and a high enough temperature such that unstable combustion can take place [30], [28], [19]. The second specific behavior reported with 0.24 m diameter is that the test with the highest renewal rate (42.3 h-1) does lead to extinction by lack of fuel, but with very atypical unsteady burning rate before extinction.…”