Study on the variation law of smoke layer thickness with ceiling structure height in long-narrow space with one end opening

Abstract

The ceiling structure in a long-narrow space has a significant impact on the thickness of the fire smoke layer. This paper mainly studies the influence of the height of the ceiling structure on the thickness of the smoke layer before the ceiling structure. Firstly, a theoretical analysis is conducted on the relationship between the critical thickness that affects the smoke flow state and the smoke layer thickness directly below the ceiling structure, and a formula for calculating the smoke layer thickness under gradient flow conditions is derived. Then a small-scale experimental platform was built to study the deviation of actual fire smoke relative to theoretical calculations. Experimental results show that due to the smoke cannot maintain gradient flow when crossing the ceiling structure, there is difference with theoretical calculations in the smoke layer thickness directly below the ceiling structure and some distance before the ceiling structure (which may be of greater concern for personnel evacuation). The gradient flow coefficient and thickness coefficient are defined to represent the above differences. After analysis, it was found that the gradient flow coefficient and thickness coefficient showed different linear variation patterns when the height of the ceiling structure was longer than 35 cm or shorter than 30 cm. Through numerical analysis, this study suggests that the main reason for the change in this linear pattern is whether the potential energy conversed from kinetic energy of the thin layer close to the ceiling structure can directly make the smoke to cross the ceiling structure. The research conclusions of this paper can assist in the engineering design of building smoke control and personnel evacuation.