Theoretical analysis of heat loss from ceiling jet in tunnel fires

Abstract

In tunnel fires, a hot gas propagates beneath a ceiling called the ceiling jet. The characteristics of the ceiling jet must be understood for safety and evacuation assessment. Small-scale experiments are useful to simulate the ceiling jet flow. The tunnel models used in previous studies have different dimensions (height and width) and wall thicknesses, and the walls were made of various materials. The heat loss through the tunnel walls (ceiling and side walls) significantly affects the property of the ceiling jet flow. However, few studies have investigated the influence of the properties of the tunnel walls on heat loss. In this study, the scaling parameters of the heat loss through the tunnel walls were introduced by conducting a theoretical analysis. Furthermore, the influence of the thermal properties of the tunnel walls, i.e., the material type and thickness, on the heat loss were quantitatively evaluated before and after thermal penetration. The results showed that the heat loss through the tunnel walls was expressed as the function of the Biot number and the Fourier number. Furthermore, the combustion time that should be satisfied in the model experiments was introduced.