Experimental study on the fire spread caused by a large fire in tunnels under natural ventilation

Small-scale tunnel fire tests were carried out to investigate the characteristics of the fire spread from initial burning source to the neighboring combustible targets under natural ventilation. HRR, combustible target height, target orientation and target type were taken into consideration. The fire spread distance, incident radiative heat flux and ceiling gas temperature rise at ignition moment were analyzed. The results indicate that the target height has a significant impact on the fire spread distance and critical condition for their auto-ignition. As the target approaches to the tunnel ceiling, the critical fire spread distance increases and the critical ceiling gas temperature decreases, indicating that in addition to the incident radiative heat flux, the surrounding temperature around the combustible target also plays an important role in the ignition of the pyrolysis gases produced by the combustible under heat exposure. Besides, it is also found that in comparison with the pinewood target, corrugated cardboard can be ignited with flame under a relatively lower ceiling temperature rise within a relatively shorter heat exposure period. However, when the radiative heat flux is low, a longer heat exposure time may result in the flaming ignition for the pinewood target while it may just lead to the smoldering ignition of the corrugated cardboard due to their material difference. Based on the test result, the critical ceiling temperature rises are obtained. Finally, a method by combining the critical ceiling temperature rise and the maximum longitudinal ceiling gas temperature is used to predict the fire spread distance. The results of the critical ceiling temperature and fire spread distance can be used as the input information for the fire protection, such as activating the fixed fire-fighting system in these ranges to cool the combustible, keeping enough space between the burning vehicle and the remaining one during the fire rescue.