Abstract: Ventilation is an effective method for controlling smoke during a tunnel fire. The "critical ventilation velocity", ucr is defined as the minimum velocity at which smoke is prevented from spreading under longitudinal ventilation flow in tunnel fire situations. However, all the previous studies conducted experiments or numerical simulations are using fullcrosssection ventilation to investigate the ucr. In real tunnels, the ventilation pattern is not full cross section. This study used 1:20 smallscale tunnel to study the influence. The fires were gasoline fires poured into square pans with side length of 6 and 9 cm. Ventilation pattern investigated included full, upper half cross section and jet fan. The fires located at 150, 200, 250 cm from the ventilation fan. The data show that the ucr increased as the area of ventilation exit decreased and the ucr decreased because of the vehicle existed in the tunnel. Additionally, the ucr changed with the distance between the fire source and ventilation fan. Therefore, using the ucr based on fullcrosssection ventilation may not control the smoke during a fire.

Key words: tunnel fire, ventilation, critical velocity