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• Fire tests were conducted in model tunnel with different sealing ratios. • Self-extinction occurred at sealing ratio of 75% or above for medium or large fires. • A minimal sealing ratio of 75% was required to extinguish fires in short tunnels. Firefighting in a long tunnel with large fires is quite difficult as the hot smoke and radiation prevent firefighters from approaching the fire seat. Sealing the portals of the tunnel to prevent the access of fresh air provides an alternative approach and it can be implemented by blocking the two tunnel portals with sand bags to prevent fresh air supply from outside after all motorists are evacuated. However, it is difficult to completely seal both portals as high pressure arises from fires. Whether partial sealing could extinguish fires or not in tunnels is an interesting topic. A series of fire tests using propane with nominal heat release rates from 2.8 kW to 11.2 kW were carried out in a 1/20 reduced-scale model tunnel measuring 10 m long, 0.45 m wide and 0.23 m high to explore the impact of different sealing ratios, varying from 0%, 65%, 75%, 85% to 100%, on the self-extinction of fires. The experimental results showed that self-extinction occurred at a sealing ratio of 75% or above except for a heat release rate of 2.8 kW, where self-extinction occur only at a 100% sealing ratio. This finding demonstrates that the partial sealing strategy at a sealing ratio of 75% or above could extinguish medium or large fires in relatively short tunnels and provides a promising method for firefighting catastrophic fires in tunnels. [ABSTRACT FROM AUTHOR]

• Fire tests were carried out in a model tunnel made of adiabatic materials without heat lose to boundary. • The self-extinction of fires was observed in the tunnel unexpectedly. • An internally circulated flow was observed and persisted thorough the tests in the tunnel. • The makeup plays a significant role to make the self-extinguish the fires. It is widely accepted that temperature decay along tunnels in fire conditions is mainly due to the heat loss to the surrounding walls and the ceiling. A previous study found fires could self-extinguish in a 1:20 reduced-scale tunnel made of steel plates and fire-proof glass and measuring 20.8 m long, 0.45 m wide and 0.23 m high (Wang et al., 2019). Self-extinction was presumed to be caused by significant heat loss at the boundaries, which led to the descent of the smoke layer and subsequently blocked the supply of fresh air. A series of experiments with fires with heat release rates of 2.8 kW, 5.6 kW, 11.2 kW and 16.8 kW were conducted in a tunnel made of fireproof board and measuring 20.8 m long, 0.45 m wide and 0.23 m high. As the heat loss at the boundaries was negligible, it was expected that self-extinction of the fires could not occur. However, self-extinction of the fires was observed, contrary to the predictions made. Furthermore, the times to reach self-extinction for the fires at different heat release rates in the two tunnels made of materials with distinctive thermal properties were basically consistent. By analyzing the velocity at different distances and scrutinizing the flow patterns using a laser sheet, it was found that the smoke velocities decreased with an increase in distance away from the fire and reduced to approximately zero at 8–9 m, irrespective of the fire sizes. While the hot smoke moves toward the portal driven by buoyancy, the same amount of makeup air flows back to the fire seat as smoke or makeup air flowing through the portals are approximately zero. The flow in the tunnel is longitudinally bi-directional and internally circulated, which persists throughout the experiment and eventually leads to the self-extinction of the fires. [ABSTRACT FROM AUTHOR]