An experimental study on self-extinction of methanol fire in tilted tunnel

Abstract A 1/20 reduced-scale model tunnel with the sizes of 20.8 m long, 0.45 m wide and 0.23 m high and varying slopes from 0 to 5% were adopted to investigate the impact of slopes on the self-extinction of fire in the tunnel under natural ventilation. The mass loss rates of methanol fires with nominal heat release rates of 2.8, 5.6, 11.2 and 16.8 kW were recorded by a load cell. The presence of smoke was determined by both temperature rises measured by K-type thermocouples and gas concentrations measured by the gas analyzer throughout the experiments. The results showed that methanol fires with nominal heat release rate of 5.6, 11.2 and 16.8 kW self-extinguished, both in the horizontal tunnel and the 1% tilted tunnel, whilst the self-extinction was not observed in the 5% tilted tunnel. Different from the horizontal tunnel, more smoke in the 1% tunnel was driven to move in uphill than that in downhill. However, the longitudinal velocities developed in the 1% tunnel were indiscernible, irrespective of the heat release rate of fires, which were basically consistent with those in the horizontal tunnel. The cause of methanol fires to self-extinct, both in the horizontal tunnel and the 1% tunnel, was that the smoke layer descended to ground and blocked supply of fresh air to reach the fire seat. In the 5% tunnel, the stack effect in the tunnel was so strong that the airflow was unidirectional. The smoke moved in uphill direction while the make-up airflow from the downhill side reached the fire seat with sufficient oxygen for continuous combustion.