Numerical investigation on the stability of tunnel smoke stratification under the effect of water spray and longitudinal ventilation

Tunnel smoke stratification has been reported to exist in the vicinity of a fire source, even under moderate ventilation conditions, which is of importance for occupant evacuation. To understand the effects of a water spray on its stability, a two-layer thermal flow model was built, and a series of scenarios considering different ventilation rates, water spray rates and smoke temperatures were numerically investigated. The ability of the Fire Dynamic Simulation (FDS) tool to predict such a two-phase problem was determined by validating against experimental and zone model data found in the literature. Then the tracer method was adopted to closely observe the smoke and droplet interactions in the two-layer model used in this paper. General effects of the ventilation flows and water spray on the maximum smoke logging distance and its corresponding horizontal displacement were observed. It was found that increasing the ventilation rate promotes the horizontal displacement but weakens the maximum downward distance, while increasing the water spray rate boosts the movements in both directions. Models based on nondimensional analysis for predicting the smoke displacements and the heat loss due to water spray were obtained. The observation of droplets indicates that a water spray globally blocks the horizontal smoke movement, and the upstream part of the water spray contributes 56.7% of the total downward drag forces. This work is helpful in revealing some of the underlying mechanisms behind the interactions of water spray and smoke stratification in the presence of longitudinal ventilation.