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The Effect of Air Velocity on Heat Release Rate and Fire Development during Fires in Tunnels

Lönnermark, A. and Ingason, H., 2008. The Effect of Air Velocity on Heat Release Rate and Fire Development during Fires in Tunnels. Fire Safety Science 9: 701-712. doi:10.3801/IAFSS.FSS.9-701


Model scale fire tests using wood cribs of different porosity in a ventilated tunnel are presented. The study focuses on the effect of air velocity on maximum Heat Release Rate (HRR) and fire growth rate. To study the influence of different parameters, free burn tests and fire tests inside a model-scale tunnel were performed. The tunnel was 10 m long, the widths used were 0.3 m, 0.45 m, and 0.6 m and the heights used were 0.25 m and 0.4 m. If compared to a normal traffic tunnel, these measures can correspond to a tunnel with a scale of 1:20. The tests show that for a higher porosity wood crib and higher velocities than 0.45 m/s an increasing ventilation rate increases the maximum HRR in the range of 1.3 to 1.7 times the value measured outside the tunnel under ambient conditions. For the lower porosity wood crib and higher velocities, the corresponding increase in the maximum HRR was 1.8 and 2.0, respectively. When compared to ambient conditions inside the tunnel based mass loss rate, this increase was found lower. For all cases when the velocity was 0.22 m/s and the low ceiling height was used, the ratio was found to be lower than one. This was not the case when high ceiling height was used. For the case with a velocity of 0.67 m/s, the fire growth rate increased by a factor of 5–10 times the free burn case. The value depends on the dimensions of the tunnel cross section.

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