Interaction Between Duct Fires and Ventilation Flow: An Experimental Study

Abstract

The influence of wood fires in a 0.3 x 0.3 x 10 m duct on forced ventilation flow was studied in a model tunnel network by measuring tunnel gas mass flow rate, velocity, pressure, and temperature. Comparison of these measurements obtained before and during the fires at various ventilation air velocities provides useful information on the interaction between duct fires and ventilation flow in terms of fire throttling effects and reverse flow phenomena. Results show that duct fires essentially increase flow resistances of the passageways of a tunnel network by virtue of volatile fuel mass injection and high combustion temperatures. In the present tunnel network, the flow resistance in the fire zone was increased by a factor of 6, and upstream and downstream of the fire by~1.5. The ventilation air velocity was thus throttled to less than half of its initial value before the fire. Reverse flow occurred when the throttled air velocity was~0.6 mis, which corresponds to a Froude number of~7 (based on duct height and ventilation air velocity). The experimental measurements were utilized in a flow network analysis to further elucidate the coupling process between duct fires and ventilation flow.

A Novel Multiscale Methodology for Simulating Tunnel Ventilation Flows During Fires

“…The analysis presented shows that the fire throttling effect is large and must be taken into account in tunnel flow calculations. This phenomenon was already observed experimentally in 1979 [36].…”

Investigating the Throttling Effect in Tunnel Fires

“…However, it is stressed that the throttling effect is not an artefact of this specific model, but a real effect that can be (and has been) observed in real tunnel fire situations and other numerical studies. The effect was clearly evident in the small scale experiments presented by Lee et al [13,14], who first described the phenomenon, and in the full scale Runehamar tunnel fire tests [23], where the flow was throttled by about 30 % (from about 3 m s -1 to just over 2 m s -1 ) as the fire grew to its maximum size. The effect was also identified numerically by Colella [19,20], using a different model, with very different geometry and input parameters to the simulations described here.…”

“…The throttling effect appears to have been known anecdotally in the mine ventilation industry since at least the 1960s [12], but does not appear to have been studied systematically until Lee et al in the late 1970s [13,14]. In essence, Lee et al found that a fire in a tunnel tended to change the resistance of the tun- …”

Fires in Ducts: A review of the early research which underpins modern tunnel fire safety engineering

“…This phenomenon was first investigated in the 1970s by Lee et al [70]. In this study, it was discovered that the larger the fire, the greater the tendency of the fire to resist this ventilation.…”

“…Throttling describes the tendency of a fire to resist the ventilation flow of the tunnel due to the additional volatile fuel gases and high temperatures that are generated by the fire [70]. This phenomenon, resulting from the interaction between the fire and the ventilation flow essentially caused a reduction the cross section of tunnel through which the ventilation flowed, resulting in this being reduced or "throttled".…”