Optimal range of ventilation velocity in tunnel fires under subcritical longitudinal ventilation

In tunnel fires under longitudinal ventilation, a large ventilation velocity, such as the critical velocity, may disrupt smoke stratification downstream of the fire source. During the early stages of a fire, when evacuation is in progress, a subcritical ventilation velocity can strike a balance between maintaining smoke stratification and restricting backlayering length. To determine the optimal range of subcritical ventilation velocity, both brinewater and fire smoke experiments are conducted, considering the combined effects of smoke backlayering and stratification. Due to boundary buoyancy loss, the backlayering length observed in fire smoke experiments is slightly shorter than that in brine-water experiments. Smoke stratification is characterized using the thickness of the smoke layer and a dimensionless temperature rise. The thickness of the smoke layer is primarily influenced by ventilation velocity and source buoyancy flux, with minimal impact from boundary heat transfer during the early stages of a tunnel fire. Furthermore, the dimensionless temperature rise tends to increase with higher source buoyancy flux and lower ventilation velocity. Based on human safety criteria, i.e., backlayering length is manageable and smoke does not descend into human activity zone, the optimal range of subcritical ventilation velocity is proposed. These findings provide valuable insights for optimizing ventilation strategies in tunnel fires under subcritical longitudinal ventilation.