Study on the ceiling gas temperature distribution, fire merging, and flame length induced by twin tunnel fires under reduced pressures

A comprehensive understanding of the development characteristics of multiple fires in tunnels holds significant importance in estimating the thermal safe distance required for both people and facilities. In this paper, a series of numerical and experimental works are performed to examine the ceiling gas temperature, fire merging, and flame length of twin fires in a tunnel. Varied thermal hazard scenarios were simulated by altering the ambient pressure, heat release rate, and pool spacing. The findings indicate that as the ambient pressure reduces, the air entrainment coefficient decreases, resulting in a higher ceiling gas temperature. Large pool spacings demonstrate two peak impact points in ceiling gas temperature. However, as the pool spacings decrease further, only one peak impact point appears above the center of two fire sources. As pressure mounts, the low-oxygen zone at the tunnel ceiling contracts progressively, and it primarily appears in the additional region between two fire sources. The temperature processing method is adopted to determine the fire merging and flame length. The fire merging probability is predicted by introducing a piecewise model. Furthermore, a physical model is proposed based on the air entrainment theory to establish the relationship between flame length and the effects of pool spacing, ambient pressure, and heat release rate, which can be applied to both open spaces and tunnels.