| The hazards that a tunnel fire can pose is greater than any other building fires because of its special building structure, and thus tunnel fires are now becoming increasingly a hot topic among scientists and researchers. Tunnels with their narrow inner space and crowed structure, can easily foster a fast-spreading fire, and once fire occurs, the heat release rate of the fire source will be extremely great, making it even more difficult for firefighters to rescue personnel and property. Due to all these factors, it is of great practical significance to study the tunnel fire.Previous studies on the tunnel fires has already been relatively mature, factors that will influence the development of a tunnel fire, including the fire heat release rate, gas temperature and its distribution, crosswind speed, back-layering length and its corresponding critical velocity, the influence of blockage, have become the focus of tunnel fire researchers. In this paper, based on a detailed tunnel fires literature review, we found that there is limited report on the influence of blockage to the tunnel fire development, while in a real tunnel fire scenario, blockage, especially vehicles, are not rare to notice, and their impact on tunnel fires is obvious. We thus decided the research content of this paper based on the above discussion.In this paper, large numbers of scale experiments have been conducted in a1:5model tunnel,72m×1.5m x1.3m (length x width x height). We selected a1:5car model as the tunnel blockage, dimensions of1.3m x0.4m x0.5m (length×width×height), three different cross wind velocities (0.5m/s,0.75m/s,1.0m/s), three fire source heat release rate using liquefied petroleum as fuel (50kW,75kW,100kW), six different blockage-fire distance (1m,2m,3m,4m,5m,6m), are employed in our experiments. We found, through experiments, that the experimental data of maximum gas temperature can well collapse with previous model when there is no blockage in the tunnel (Y.Z. Li, B. Lei, H. Ingason, The maximum temperature of buoyancy-driven smoke flow beneath the ceiling in tunnel fires, Fire Safety Journal46(2011)204-210.), on the other hand, when blockage exists, previous model fails to predict the experimental result, further, with different blockage-fire distances, the maximum gas temperature will show a regular change, thus we venture an influence factor to characterize the influence of blockage-fire distance on the tunnel fire maximum gas temperature, and modify previous model with the new influence factor to better predict the experimental results.In order to further study the influence of blockage on the tunnel fire, we designed another set of experiments, trying to explore the influence of blockage-fire distance on the tunnel fire back-layering length and critical velocity. The experimental conditions are similar to the maximum temperature experiments. We decide the back-layering length and thus calculating the critical velocity through analysis of the temperature data obtained by the thermocouples installed in the tunnel roof. It is found that, the experimental back-layering length can be well predicted by Li model (Y. Z. Li, B, Lei, H. Ingason, Study of critical velocity and back-layering length in longitudinally ventilated tunnel fires, Fire Safety Journal45(2010)361-370.), and the according critical velocity can be well collapsed with Wu model (Y. Wu, M. Z. A. Bakar, Control of smoke flow in tunnel fires using longitudinal ventilation systems—a study of the critical velocity, Fire Safety Journal35(2000)363-390.). On the other hand, when blockage exists in the tunnel and influence the fire development, previous models on back-layering length and critical velocity fail to predict the experimental result, thus we invent a new factor to modify the models, namely, the blockage-fire distance factor. The value of the new factor under different experimental conditions are obtained and fitted with blockage-fire distance. Finally we modify the prediction models and compare the new models with experimental data. |
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