Numerical and experimental study on the effects of a ceiling beam on the critical velocity of a tunnel fire based on virtual fire source

Abstract

Ceiling beams, such as smoke screens, supporting beams, and ceiling pipelines, exist in several types of tunnels. The ceiling structure of a tunnel has a significant influence on smoke propagation. This study determines the critical velocity required to control fire smoke in a tunnel with a ceiling beam. We conducted model experiments and numerical simulations to analyze the effects of a ceiling beam on air flow structure and smoke propagation. An air re-circulation length model was fitted to determine whether a fire source lies within the vortex swirling zone. Using a virtual fire source, we obtained the critical velocity prediction model for a tunnel with a ceiling beam. The results indicate that the re-circulation length of the vortex swirling zone is determined by the beam height and tunnel height, but is independent of the longitudinal airflow speed. The form of the critical velocity model for a tunnel with a ceiling beam is consistent with that of conventional models, wherein the dimensionless critical velocity is proportional to the dimensionless heat release rate raised to the power of one-third. This implies that a virtual fire source can be used to determine the critical velocity. The proposed prediction model of the critical velocity for a tunnel with a ceiling beam is an improvement on previous models and can be used in the design of smoke control systems in tunnels with beams.