Determination method of smoke layer height in road tunnel fire

Buoyancy frequency methodꎬ N percentage methodꎬ integral ratio methodꎬ are three common methods to determine smoke layer height in tunnel fireꎬ the calculations of three methods are often biased from visual observations. In order to study and explain deviations of calculation results from visual observationsꎬ which were found in three common determination methods of smoke layer height in tunnel fireꎬ a long tunnel model was established based on FDSꎬ and six types of fire source power were set. Thenꎬ vertical visibility distribution was used to verify deviation between existing determination method and visual results before visibility mutation method was proposed. The results show that the deviation calculated from above four methods ranks as visibility mutation method (1􀆰 63%) < buoyancy frequency method (4􀆰 88%) < N percentage method (N= 30ꎬ 7􀆰 88%) < integral ratio method (8􀆰 50%)ꎬ among which results of integral ratio and buoyancy frequency method are higher than visual ones. It is also found that visibility value in vertical direction of tunnel remains constant and then decreases abruptly as the height increasesꎬ and the greater fire power isꎬ the faster it declinesꎬ and the lower visibility value there is.