Experimental study on temperature profile with fire sources both inside and outside a naturally ventilated tunnel

Considering the randomness of the fire location and the high probability of tunnel fires occurring near the tunnel portal, a series of experiments were conducted using a 1:15 tunnel model. In the event of double fires with an extra external fire source except for the internal fire source, the characteristics of smoke spread and smoke stratification are influenced by asymmetric flow fields. The distribution and decay of the smoke temperature beneath the tunnel ceiling, the smoke layer thickness, and the smoke layer stratification were measured and analyzed. The findings indicate that despite a considerable dimensionless fire spacing, the maximum temperature remains longitudinally positioned between the two fire sources. The characteristics of temperature attenuation showed the variation with the dimensionless fire spacing, particularly when the fire spacing was minimal. Furthermore, a model of the pre-exponential factor was built. There was a stable area of smoke layer thickness upstream of the internal fire, showing no sensitivity to variations in dimensionless fire spacing or pool size. Additionally, an obvious smoke interface drop was found between two fire sources.