Experimental and numerical study on the flow field of longitudinally ventilated tunnels with water spray system

Abstract

Experiments and numerical simulations have been conducted to study the effect of water sprays on the airflow in longitudinally ventilated tunnels. A physical model tunnel containing single and double column spray systems was built at 1:11 scale and tested in a wind tunnel. The experiments captured the velocity redistribution in the upstream area of the sprays, with the most notable feature being flow acceleration close to the zone of the ceiling. The numerical reproduction of the experiments using Fluent 2021R1 was achieved with good agreement, enabling a detailed study of the flow field. The investigation revealed that the water spray induces a water-air jet, causing blockage to the incoming air and deflecting the air stream towards the ceiling. The CFD method was also implemented to study a ceiling spray system and a side-wall spray system, whose spray properties were scaled from a practical design of water a spray system. Dimensional and theoretical analyses showed that the maximum increase in the mean velocity in the near-ceiling acceleration zone is determined by the momentum ratio of the water spray to the ventilation air and the proportions of areas occupied by the water spray in the ceiling zone and the whole tunnel cross section. A correlation model was proposed using the governing parameters, which accurately predicted the velocity increases. The developing trends of the mean ceiling velocities within and downstream of the spray zone were also discussed in detail. The findings are helpful for the coupling design of the tunnel fire fighting system and can provide guidance for the water curtain strategy, which uses water sprays to prevent smoke from spreading.