Parametric study on critical ventilation velocity in case of fire inside longitudinally ventilated tunnels

Ventilation is essential to provide a smoke-free path for safe passenger evacuation and effective rescue services in case of a tunnel fire. The critical ventilation velocity، VC، is defined as the minimum velocity which creates this safe passage by preventing smoke from spreading upstream in tunnels. This research discusses smoke flow control in tunnels with a focus on the important parameters affecting critical velocity. Critical velocity values are evaluated for different heat release rates and results show good compliance with model-scale experiments. The study is extended to investigate effects of fire source blockage ratio and lateral location، tunnel slope and ventilation air relative humidity on the behavior of critical velocity. Results show a drop in VC about equal to blockage ratio occurs in presence of fire source blockage. Investigation of critical velocity in sloped tunnels illustrates that for each %1 increment in negative slope، 2. 5% higher ventilation is required. Results also show that air relative humidity does not have significant effect. However، fire lateral location impacts critical ventilation velocity in such a way that about 10-20% higher airflow is necessary to suppress smoke in a near-wall fire in comparison with a middle-tunnel fire.