Effect of Extraction Vent Length on Critical Exhaust Volumetric Flow Rate in Long-distance Subway Tunnel Fires with Two-point Extraction Ventilation

Abstract

In recent years, point extraction ventilation mode is considered as a competitive alternative to control the smoke in the tunnel fire. The principle of point extraction ventilation is that sufficient exhaust volumetric flow rate is supplied to confine the smoke to an acceptable zone. However, the issue of an appropriate exhaust volumetric flow rate to confine the smoke to a safe zone has not been studied sufficiently. In this study, the critical exhaust volumetric flow rate to control the smoke to the zone between two extraction vents was proposed and a series of small-scale experiments were carried out to investigate the effect of extraction vent length on the critical exhaust volumetric flow rate. The heat release rate (HRR), extraction vent length and the distance between two vents varied during the tests. Smoke temperature below the ceiling and smoke configurations were measured and analyzed. Experimental results show that it is not the case that the longer the extraction vent, the smaller the critical exhaust volumetric flow rate. There exists a critical extraction vent length that is 0.1 m in the current study. Finally, an empirical equation was developed to predict the critical exhaust volumetric flow rate based on theoretical analysis. The coefficients in the equation were determined by experimental data. Compared with experimental data, it is obvious that the derived equation can well predict the critical exhaust volumetric flow rate. Research outcomes can provide some helpful references for the design of point extraction ventilation in the tunnel fire.