Study on induced airflow velocity of point smoke extraction in road tunnel fires

In the point smoke extraction system, induced airflow velocity which can restrain smoke back-layering is an important parameter to evaluate smoke control efficiency of smoke extraction systems. New formula for the induced airflow velocity and smoke back-layering length is proposed for the tunnel fires with different heat release rates through small-scale tunnel model experiments. The experiment results show that when the dimensionless back-layering length is more than four, the influence of smoke outlet area on the induced airflow velocity and smoke back-layering length can be negligible. The relationship between the dimensionless back layering length and induced airflow velocity is greatly affected by the heat release rate. In point extraction system, the smoke back-layering length increases gradually with the increasing of heat release rate, then tends to be smooth.