Studies On Tunnel Fire Ceiling Jet Characteristic With Longitudinal Ventilation And Ceiling Smoke Extraction

In recent years, with the continuous development of road traffic, tunnel plays an increasingly important role in transportation. However, with the growth in the number of the tunnels, the tunnel fire accidents gradually increased. Though tunnel fire is a rare event, but due to its special narrow structure, and relatively confined space, evacuation for the personnel is rather difficult in the event of fire, a large number of high-temperature smoke and flame accumulate in the tunnel, easily leading to the collapse of the tunnel structure and heavy casualties, causing serious economic losses, resulting in adverse social impact.Tunnel ventilation and smoke extraction system has an important role on controlling the spread of fire smoke, reducing the damage of high-temperature smoke and flame on the tunnel structure and personnel. Currently, the tunnel fire control systems can be divided into two kinds:the longitudinal ventilation and the ceiling smoke extraction. In this paper, theoretical analysis and redeced-scale model experiment are used to study the tunnel fire ceiling jet characteristics, concentrating on the tunnel fire smoke back-layering length and temperature distribution under ceiling smoke extraction, flame extension length beneath ceiling under longitudinal ventilation, and the effect of tunnel slope on the tunnel smoke temperature characteristics, specific work include:Smoke layering length under ceiling smoke extraction is studied. Results show that, for ceiling smoke extraction with no longitudinal ventilation, smoke presents symmetrical distribution from the fire source, and smoke layering length decreases with the ceiling smoke extraction velocity increases; for ceiling smoke extraction combined with longitudinal ventilation, smoke presents asymmetrically distribution from the fire source, with longer downstream smoke layering length and shorter upstream smoke layering length. By theoretical analysis, based on the smoke back-layering length model under longitudinal ventilation system in tunnel fires of Y.Z. Li from SP Technical Research Institute of Sweden, this paper develops the tunnel smoke layering length model under ceiling smoke extraction system.Smoke temperature distribution under ceiling smoke extraction is also studied. Results show that, for ceiling smoke extraction with no longitudinal ventilation, smoke temperature distribution from both sides of fire source are roughly the same, the maximum temperature of the smoke is occurred near the top of the fire source, with the increase of the distance from the fire source, the temperature is gradually reduced as well as the descent gradient. With different ceiling smoke extraction velocities, smoke temperatures near the fire source are much more varied, but with the increase of the distance from the fire source, the smoke temperature differences are reduced. For ceiling smoke extraction combined with longitudinal ventilation, the upstream smoke temperature decay along the tunnel ceiling increases as longitudinal ventilation velocity increases; while the downstream smoke temperature decay along the tunnel ceiling decreases as longitudinal ventilation velocity increases. By theoretical analysis, based on the smoke temperature decay theoretical model in tunnel fires of Longhua Hu, this paper develops the tunnel smoke temperature decay prediction model under ceiling smoke extraction.The flame extension length beneath tunnel ceiling with longitudinal ventilation is investigated. Results show that, for the condition of flame extension beneath ceiling does not touch the tunnel side walls, the flame extension lengths beneath the tunnel ceiling increase with heat release rate, and decrease with source-ceiling height, burner size and longitudinal ventilation velocity With longitudinal venlation effect, the flame extension length is longer downstream than upstream. For the condition of flame extension beneath ceiling touches the tunnel side walls, the flame extension behaviors present the similar phenomenon. Based on the unburnt fuel theory, the flame extension behaviors beneath tunnel ceiling are analyzed by the unburnt part of the fuel after impingement and the air entrainment into the ceiling flow to consume the unburnt fuel. The predictive model for flame extension length beneath tunnel ceiling with longitudinal ventilation is proposed.Maximum smoke temperature and smoke temperature distribution under sloped tunnel ceiling are analyzed. Results show that, the maximum smoke temperature model in tunnel fires proposed by Kurioka and Y.Z. Li agrees well with experimental data when the tunnel slope is 0. However, the predictions are quite different from the experimental results when the tunnel slope is 3% and 5%. With the increase of tunnel slope, maximum smoke temperature beneath tunnel ceiling decreases. Based on the models of Kurioka and Y.Z. Li, consider the tunnel slope effect, this paper develops the maximum smoke temperature prediction model of sloped tunnel under longitudinal ventilation. The smoke temperature attenuation rate increases with increasing tunnel slope. Based on the smoke temperature decay theoretical model in tunnel fires of Longhua Hu, consider the effect of tunnel slope, this paper also develops the smoke temperature decay prediction model of sloped tunnel under longitudinal ventilation.