Effects of ambient pressure on transport characteristics of thermal-driven smoke flow in a tunnel

Thermal-driven smoke flow is the greatest threat to the trapped people in tunnels, and the basic understanding of the smoke transport characteristics is necessary for the engineering applications of smoke control and exhausting system. However, most of the previous studies only focused on the smoke transport at normal pressure. As a matter of fact, the ambient pressure decreases with the increasing of the altitude, and the influence of different ambient pressures on the transport characteristics of thermal-driven smoke has been rarely considered before. This paper presents an investigation on the effect of ambient pressure on the smoke transport characteristics in a tunnel. A series of fire simulations were conducted in a full scale road tunnel with ambient pressure ranged from 1001cPa to 50 kPa and the heat release rate (HRR) varied from 3 MW to 15 MW. The smoke mass transport along the tunnel for different conditions is analyzed, and results show that for a certain HRR, the smoke mass flow rate decreases with reduced ambient pressure due to the decreased air density and the weakening of air entrainment strength. Based on the dimensional analysis, a quantitative model to predict the average smoke mass flow rate in the tunnel is proposed considering both ambient pressure and HRR. Moreover, the credibility of the predictive model is validated by comparing with a wide range of experimental results from both full scale and reduced scale experiments at different pressures.