Acoustic Considerations Regarding Pulsations During Large-scale Fire Tests In A Tunnel

Abstract

Large-scale fire tests have been carried out with Heavy Goods Vehicle (HGV) cargos in the Runehamar tunnel in Norway. During two of the fire tests, large pulsations of the gas flow inside the tunnel occurred. These pulsations were registered only when the measured heat release rates (HRR) were higher than 125 MW -135 MW. Two different periods of the pulsations were registered, short periods of about 4 s and longer periods of approximately 18 s. In the article, the pulsations are presented and some plausible explanations are given, using an acoustic approach. There are several factors that can affect the pulsations, e.g., the size of the experimental set-up, the size of the fire, the thermal expansion at the site of the fire, the slope of the tunnel, the wind outside the tunnel, etc., but the calculations show that the oscillation periods are properties of the system, and can be what is referred to as thermoacoustic instabilities.

Pulsations during large-scale fire tests in the Runehamar tunnel

“…Some of these factors may affect this phenomenon and be important for the starting and maintaining of the pulsation, but the analyses show that the main reasons for the pulsation and their properties depend on the properties of the tunnel as a system. Lo¨nnermark and Ingason discussed these observations previously and indicated that a phenomenon called thermoacoustic instability could be the reason for this [10].…”

“…The authors have previously shown that the described phenomenon can be explained as thermoacoustic instabilities [10]. Other investigators have observed oscillations in the HRR in different types of combustion chambers [16][17][18].…”

“…Therefore, the different parts of the tunnel have been assigned impedances and equations developed for unsteady flow have been used to study the tunnel as a system in more detail than was done in the previous analysis [10]. The equations are used together with a frequency response analysis where it is assumed that a forced vibration takes place in the tunnel, even if the excitation source of the vibration is unknown.…”

“…a Observe that the period of the long oscillations is measured from one maximum to the next and is actually half the modulation period [10]. b The pressure at the downstream measurement station were sampled only every 5 s instead of every second.…”

“…The pulsations of the air mass in the tunnel are described and theoretical analyses are carried out. An acoustic explanation of the oscillations has been presented by Lo¨nnermark and Ingason previously [10]. The aim of this paper is to describe these pulsations using an alternative theoretical approach, i.e.…”