Study on the smoke back-layering length in a tilted tunnel with vertical shaft under natural ventilation

Abstract

By varying tunnel slope, shaft height and fire source location, a series of numerical simulations were conducted to investigate the smoke back-layering length in a tilted tunnel with a vertical shaft. The simulation results were verified by corresponding reduced-scale experiments. Five possible smoke exhaust modes in tilted tunnels with single shaft were concluded through the flow field analysis. The dimensionless relationships between tunnel slope, shaft height, fire source distance and back-layering length were analyzed. When the fire source is located upstream of the shaft, smoke back-layering occurs when the tunnel slope is less than 1.5%. When the fire source is located downstream of the shaft, smoke back-layering disappears until the tunnel slope is bigger than 2.5%. The dimensionless analysis shows that the back-layering length is related to the fire source location and the tunnel slope, but has no significant dependency on the shaft height. Using the Froude number, the location of hot smoke is identified, when Frc < 1, part of the high-temperature smoke below the shaft cannot enter the shaft but spread to the tunnel exit. Dimensionless correlations between smoke back-laying length and Richardson number (Ri) were proposed. Moreover, the critical conditions that changes the smoke exhaust mode of the tunnel were determined only by the tunnel slope and the shaft height.