Experimental studies on the gas temperature and smoke back-layering length of fires in a shallow urban road tunnel with large cross-sectional vertical shafts

Abstract

A reduced-scale (1:10) tunnel with four vertical shafts is constructed to investigate the fire smoke propagation in a natural ventilation tunnel equipped with large cross-sectional ceiling openings. A series of fire tests are performed to explore the impact of fire HRR (Heat Release Rate), shaft height and shaft interval on the gas temperature and smoke back-layering length. It is observed that the smoke cannot fill up the entire cross-section of the shaft, but just propagates adhering to the shaft sidewalls and then spills out of the tunnel. This flow mode of hot smoke in the large cross-sectional shaft is significantly different from the general plug-holing phenomenon, especially for the fresh air entrainment. In such case the fresh air is entrained just along the shaft sidewalls rather than exhausted directly from the lower part of the tunnel. Moreover, experimental results indicate that the dimensionless ceiling gas temperature on the upstream side of the shaft appears to be independent on the shaft height, while the temperature decay rate on the downstream side increases with the shaft height. The variation of the smoke back-layering length under different conditions implies that the smoke back-layering length initially decreases with the shaft height, then keeps constant with the HRRs when the HRRs are greater than 25.8 kW. Additionally, the smoke back-layering length generally increases with the fire-shaft interval when the shaft height is lower than 0.65 m, while it levels off initially and then increases again when the shaft height is higher than 0.65 m. Experimental testing in this paper contributes to understanding of the smoke movement in the naturally ventilated tunnel with large cross-sectional vertical shafts.