Study on fire combustion characteristics and smoke spread laws in spiral tunnels under longitudinal ventilation

To reveal the combustion characteristics and smoke spread law of fires in spiral tunnels under longitudinal ventilation, and to address the inapplicability of traditional tunnel fire theories to the composite geometric features of spiral tunnels with continuous curvature and large longitudinal slope, full-scale field hot smoke tests were carried out on a separated spiral tunnel project. The influences of oil pool area, longitudinal ventilation velocity, and distance between fire source center and tunnel sidewall on fire burning rate and tunnel ceiling temperature distribution were systematically investigated, and high-precision quantitative prediction models were established. The results show that the burning rate of pool fires in spiral tunnels increases asymptotically with the growth of oil pool area, decreases rapidly at first and then tends to be stable with the increase of longitudinal ventilation velocity, and rises significantly when the fire source is close to the sidewall due to wall thermal feedback. The ceiling temperature reaches its peak near the fire source and decays asymmetrically along the tunnel longitudinal direction, with a longer high-temperature zone and slower temperature attenuation in the uphill direction; both the peak temperature and high-temperature coverage range increase remarkably with the growth of fire source scale. The prediction models for burning rate, maximum ceiling temperature rise, and longitudinal temperature attenuation established based on experimental data present high fitting accuracy with a prediction error below 10%, which can be well applied to the fire safety design of spiral tunnel engineering. This study clarifies the coupling mechanism of slope chimney effect, curve centrifugal effect and longitudinal ventilation shear effect, and reveals the unique combustion and smoke transport laws of spiral tunnel fires which are obviously different from those of conventional straight tunnels. It can provide a theoretical basis and technical support for ventilation and smoke exhaust design, disaster prevention and mitigation, as well as emergency rescue of spiral tunnels.