Abstract
Four 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 were observed. These pulsations were registered only when the measured HRR was higher than 125–135 MW. Two different periods of pulsations were registered, short periods of approximately 4 s and longer periods of approximately 18 s. In the article, the pulsations are presented and explanations are given, using a frequency response analysis based on an impedance approach. Using this approach, the authors were able to find the intrinsic resonances of the system, which were close to the periods of 4 and 18 s found in the experiments. Several factors can affect the pulsations, but the calculations show that the oscillation periods are properties of the system. The analyses show further that at certain frequencies (close to those found during the tests), a small disturbance in the flow can create large amplitudes in the pressure. It is proposed that this phenomenon should be studied in conjunction with future full scale tunnel tests.
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