Influence of plugging ratio and ignition position on unsteady performance of flame spread over n-butanol in circular cross section tunnels

The shield tunnel with the circular cross section can be built without destroying the surrounding rock and ground buildings. Oil fires are particularly dangerous commonly existing from oil leaks caused by vehicle collision accident in circular cross section tunnel. The structural strength of tunnel’s ceiling and side walls should be significantly reduced by the fire. In addition, once a tunnel fire occurs, a common disposal method is to block one or two entrances of the cross-sectional bending tunnel, isolate the oxygen and make the fire self-extinguishing. In this work, the performances of flame spread over n-butanol fuel in two cross-sectional bending tunnels are revealed with the plugging ratios in range of 2 %−100 % and the curvature radius of 50 and 100 mm respectively. The plugging ratio is labeled as the ratio of the arc area sealed by the blocking sheet to the cross-sectional area of circular tunnel. The flame spread tests were conducted under three plugging methods (ignition-end plugging, non-ignition-end plugging and two-ends plugging). The tests are conducted under two fuel depths of 5 and 10 mm, which represent thin and thick pools of flame spread. The phenomena and classifications of flame spreading under the three plugging methods are summarized. The frequency of flame spread in pulsation regime increases from 2.5 to 4.5 Hz as the plugging ratio increases from 10 % to 50 %. With an increase in plugging ratio for two-ends plugging, the flame front position at the separation time decreases from 906 mm at β = 2 % to 542 mm at β = 50 %, and the flame separation position decreases from 348 mm at β = 2 % to 65 mm at β = 50 %. The flame separation position moves to the ignition end, indicating that the two-ends plugging can effectively inhibit oxygen entrainment, thereby reducing the burning area in the cross-sectional bending tunnel. In flame shuttling stage, the instantaneous flame spread rate is larger than 10 cm/s, so it belongs to the gas-phase controlling regime. This is because the liquid temperature during flame shuttling is larger than the flashpoint of n-butanol (38 °C). In stable spreading stage, the flame spread belongs to liquid-phase controlling regime and the flame spread rate is slightly increased by plugging ratio for ignition-end plugging, but decreased considerably for non-ignition-end plugging and two-ends plugging. This indicates that in a real fire scene in a circular cross section tunnel, it is recommended to increase the plugging ratio and use the two-ends plugging method to put out the liquid fire. This is consistent with the findings of the flame pulsation frequencies under varied plugging ratios and plugging methods.