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.