Verification of Similarity of Scaling Laws in Tunnel Fires with Natural Ventilation

Scaling laws based on the conservation of the Froude number have been widely used in both fire research and fire engineering design in tunnels. However, the applicability of scaling laws in tunnel fires with natural ventilation has not been systematically verified. In order to verify the scaling laws for tunnel fires with natural ventilation, two model tunnels were tested: Tunnel-L, measuring 20 m long, 0.9 m wide and 0.46 m high, representing the prototype tunnel, and Tunnel-S, measuring 10 m long, 0.45 m wide and 0.23 m high, representing a ½-reduced-scale tunnel. Both tunnels were made of material with a very low heat transfer coefficient, thus heat loss at the boundaries was negligible. The first series of fire tests, with heat release rates of 15.8 kW, 31.6 kW and 63.2 kW, was conducted in Tunnel-L and the second series of fire tests, with heat release rates of 2.8 kW, 5.6 kW and 11.2 kW, was conducted in Tunnel-S. The experimental results showed that compared to the physically measured data in Tunnel-L, the data deduced from Tunnel-S based on scaling laws could lead to a significant under-estimation of the peak temperature rises along the tunnel with errors as high as 50 ~ 70%, except those above the fire sources, and a remarkable over-estimation of smoke concentrations. The study demonstrated that scaling laws could be invalid for tunnel fires with natural ventilation and that results measured in reduced-scale tunnels should be further verified when applied to full-scale prototypes.