Influence of ambient pressure on critical ventilation velocity and backlayering distance of thermal driven smoke in tunnels with longitudinal ventilation

Thermal driven smoke backlayering in tunnels with longitudinal ventilation is a huge threat to the trapped people during fires. To explore the influence of ambient pressure on smoke backlayering distance and critical ventilation velocity of thermal driven smoke flow, fire simulations with varying ambient pressures and heat release rates (HRRs) were carried out in a full scale tunnel with longitudinal ventilation. The results suggest that for a certain HRR and longitudinal ventilation velocity, the smoke backlayering distance increases with the decrease of ambient pressure due to the weaker inhibiting effect of longitudinal airflow, which is characterized by the decrease of the ratio of the horizontal inertial force of longitudinal airflow to thermal buoyancy of the smoke backlayering. Moreover, a parallel analysis for critical ventilation velocity was also made, and it has been found that the critical ventilation velocity increases with the decrease of ambient pressure for a certain HRR. Based on the dimensional analysis, a correlation of dimensionless critical ventilation velocity is proposed and is compared with previous correlations at normal pressure. Finally, the credibility of correlation are validated by comparing with experimental results from large and full scale tunnel fire tests.