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

Abstract

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.

Some Questions about the Safe Operation of Short Road Tunnels

“…Tunnels up to 700 m long, due to the traction induced by fire, have a greater probability of ventilation system collapse than longer tunnels. The fire traction in this case will increase due to the low aerodynamic resistance of the tunnel and hence the easy provision of an air supply sufficient for complete combustion (Lanchava et al, 2007;Lonnermark & Ingason, 2008). A similar opinion about fire intensification is given in (Ingason, 2010;Bajwa et al, 2009).…”

Can fire cause the collapse of Plasco Building: A numerical investigation

“…The velocity of the surrounding air influences HRR, it increases sharply with an increase in the velocities of the surrounding air. In an experimental study carried out by Lönnermark and Ingason, 51 the authors tested the wood cribs in a tunnel and found that the fire growth rate increased by 5 to 10 times by increasing the surrounding air velocity three times. In the current analysis, the maximum HRR recorded for Case 1 is 1.25 times higher than Case 2, as shown in Figure 10.…”

Numerical Study of the Effects of the Jet Fan Speed, Heat Release Rate and Aspect Ratio on Smoke Movement in Tunnel Fires

“…Moreover, Haukur Ingason et al (2008) [6] investigated the effect of air velocity on the heat release rate (HRR) and fire characteristics during a fire in the tunnel. The results showed that when increasing the ventilation rate, causing the maximum heat, the release rate increases.…”