Effect of blockage ratio on critical velocity in tunnel model fire tests

Experiments are conducted in a model tunnel of (1:20 of a tunnel section) to study the effect of blockage ratio on critical velocity. The vehicle blockage of the model tunnel is varied at 11 different blockage ratios, 9.4%, 15.6%, 21.9%, 25.0%, 31.3%, 34.4%, 40.7%, 43.8%, 50.1%, 56.3%, 59.5%. At the same time, the influence of heat loss in the experimental model on critical velocity is considered. The test results show that the critical velocity decreases with the increase of the blockage ratio, and the reduction ratio is less than the blockage ratio. The blockage-ratio modified coefficient obtained by experiment result is 0.545, and thus the formula of the critical velocity containing blockage ratio is proposed. The test results showed excellent agreement with from the formula of the critical velocity. The relationship between the critical velocity reduction ratio and the blockage ratio of this test and Oka and Atkinson (1995), Lee and Tsai (2012) and Kang (2010) were compared and analyzed. It is found that the main factors which lead to the different relationships about the critical velocity decrease rate and the blockage ratio are brought up, which include the height of fire burning surface, the location of fire source (the relative position about the fire source and blockage vehicles and whether the fire source is inside or outside the vehicle) and the lateral wall open area of vehicle.