Experimental study on backlayering length of thermal smoke flow in a longitudinally ventilated tunnel with blockage at upstream of fire source

Experiments were conducted in a model tunnel to investigate backlayering length of thermal smoke flow in a longitudinally ventilated tunnel with vehicular blockage at upstream of the fire source. Two kinds of blockage ratios were considered and the horizontal distance between the fire and the blockage is in the range of 1-5 m. It is found that with no blockage, the experimental data can be well collapsed by Li model (without blockage). With blockage, the backlayering length is found to be lower than that in an empty tunnel. Both Tang model and Li model (with blockage) fail to predict the backlayering length in our present work. The backlayering length decreases or increases with increasing blockage-fire distance at blockage ratio of 0.26 and given heat release rates, however, at blockage ratio of 0.51 and given heat release rate, the backlayering length increases with increasing blockage-fire distance at large ventilation velocity. At certain heat release rate and ventilation velocity, the backlayering length is larger for cases with smaller blockage ratio, and this discrepancy gradually decreases with increasing blockage-fire distance at larger ventilation velocity, and finally longer backlayering length can be obtained for cases with larger blockage ratio. Larger backlayering length could be obtained for cases with higher heat release rates irrespective of the blockage-fire distance at blockage ratio of 0.26 and given ventilation velocities.