Study of critical velocity and back-layering length with fire sources both inside and outside a tunnel

The smoke back-layering length and the critical ventilation velocity in double fire scenarios where the fire sources were both inside and outside the tunnel were studied by small-scale tests and theoretical analysis. Results showed that the smoke back-layering length increased with an increase of the fire separating distance when the dimensionless fire separating distance was less than 3, and the values were larger than the results of a single internal fire at each fire separating distance. The dimensionless critical ventilation velocity segmentally increased with the increase of the dimensionless fire separating distance when the dimensionless fire separating distance was less than 10, and then remained constant. Moreover, the constant value was found to be close to that in the case of a single tunnel fire. In addition, an empirical model of dimensionless critical ventilation velocity was established for the double tunnel fire scenario both inside and outside a tunnel. •Tunnel fire scenarios occurring both inside and outside a tunnel were studied.•Critical velocity increased first and then remained stable with increasing spacing.•An empirical model of dimensionless critical ventilation velocity was established.