Interactions between water-based fixed fire-fighting systems and longitudinal ventilation in road tunnels
Notwithstanding well-developed tunnel ventilation strategies, devastating tunnel fire catastrophes have never been absent in past decades, ringing an alarm bell for the tunnel community about the inadequacy of the conventional fire-fighting systems. In such context, proposals have been made to add Water-based Fixed Fire-Fighting Systems (WFFFS) as auxiliaries of conventional ventilation systems because of their high efficiency in active fire suppression. To promote the development of the active fire-fighting systems in road tunnels, the recent two decades have seen a great effort put into full and small-scale experiments and numerical investigations. To date, the benefits of WFFFS, like reducing the fire size, cooling down the hot smoke and preventing fire spread, have been demonstrated, and its application is widely accepted. However, it is noticed most of these studies focused on specific tunnel conditions and only drew qualitative conclusions, causing the mechanisms behind the system interactions to be rarely reported and are therefore of concern when applying the findings to different cases. Moreover, there seemed to be a research preference for exploring the WFFFS advantages in the past decades, and the drawbacks they should bring about when interacting with the ventilation systems, such as smoke stratification destruction and air flow blocking, are yet to be clearly understood. This project aims to study the system interactions, including advantages and flaws, between WFFFS and the most used longitudinal ventilation system in road tunnels to achieve the most effective implementation of the coupled fire-fighting system. The system interactions are divided into on-fire and off-fire interactions, denoting categories with and without involving direct fire fighting effects, respectively. In the off-fire zone, the interactions focus on the impact of water spray on the air/smoke flow, covering the impacts of water spray on the stability of smoke stratification, the air flow field change and the smoke spread in tunnels. In the on-fire zone, the impacts of either of the two systems on the other are investigated. In terms of the impact of WFFFS on ventilation, the heat and mass transfer effects between the water spray and the buoyant smoke are involved compared to the off-fire interactions, with which the critical velocity and air-driving force reductions due to the suppression are investigated. For the impact of ventilation on the WFFFS, the objective is to investigate the activation delay of temperature-triggered automatic nozzles, which requires a good understanding of the flame geometry in longitudinally ventilated tunnels. Outcomes of the project include a better understanding of system interaction mechanisms and quantitative methods capable of predicting spray-induced horizontal and vertical smoke movements, spray-induced critical velocity and air-driving force reductions and ventilation-induced activation delay of automatic nozzles, which are of guide values for the design and development of the system synergy between WFFFS and the longitudinal ventilation in road tunnels.
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