Stability of smoke stratification under the impact of water sprays with various droplet size ranges in longitudinally ventilated road tunnels

This paper presents a simplified model of tunnel smoke stratification, which can be used to study the effect of sprinkler water spray on stratified stability. The stratified flow downstream of a tunnel fire is represented by a two-layer thermal flow, with the hot upper layer representing the smoke and the cold lower layer representing the ventilation air. The dragging effects of sprays from sprinkler nozzles with various K-factors were numerically studied using FDS code (version 6.7.3). Experiment data reported in the literature successfully validated the accuracy of the CFD method. Spray-induced smoke movements, including the smoke logging distance and the horizontal movement, were determined by releasing a series of massless tracers from the smoke layer inlet. The findings of the work indicate that the dragging effect of a water spray is droplet-size-dependent. The smaller the droplets, the larger the dragging force for a given volumetric spray rate. By applying dimensional analysis and introducing the volume mean diameter into the governing parameters, some correlation models that account for the droplet size were proposed to predict the spray-induced smoke movements and smoke layer heat loss. The new correlation models help complete previous work by the authors and are of practical use for tunnel fire safety design where water sprays are included alongside a longitudinal ventilation system.