Effects of ambient pressure on characteristics of smoke movement in tunnel fires

Abstract

This paper systematically studies the effect of ambient pressure on smoke movement in tunnel fires by numerical investigations. Because of the correlation between ambient pressure and air density, the influence of pressure on smoke movement can be considered as the effect of ambient air density in this study. The results of numerical simulation show that under a certain fire HRR, the average smoke layer thickness downstream of fire increases with the increase of both ambient pressure and longitudinal ventilation velocity. Compared with ambient pressure, the ventilation velocity has a greater impact on smoke layer thickness. The concept of turning velocity VT was put forward for the first time when analyzing the effect of ambient pressure on smoke backlayering length. When V<VT, the smoke backlayering length will decrease with the decreasing pressure. When VT<V<VC, the smoke backlaying length will increase with the decreasing pressure. Such results can be attributed to the two-sides effect of ambient pressure on the thermal buoyancy of smoke plume at the smoke backlayering front. Furthermore, the critical velocity has a negative correlation with the ambient pressure. According to the results of dimensional analysis, the prediction models of the smoke backlayering length and critical velocity considering the effect of ambient pressure were established. These findings will provide some basic references for the design of ventilation system in high-altitude tunnel.