Characteristic parameters and patterns of buoyancy-driven currents in inclined tunnels with a closed end

Inclined narrow spaces with a closed end, such as inclined shafts and tunnels under construction, are common in modern society. This study presents a series of brine-water experiments examining buoyancy-driven flow in such inclined corridors or tunnels. In the experiments, once buoyant fluid is released into the tunnel, the buoyancy-driven outflow advances forward at an approximately constant velocity, which is mainly determined by the buoyancy flux. Dye-tracing observations reveal that, in addition to the outflow layer, buoyancy drives an inflow layer and creates an interior recirculating zone between these two layers. Velocity and reduced gravity exhibit an approximately linear variation across the tunnel, with slightly steeper gradients in the outflow and inflow currents. The thickness of the recirculating zone is minimally affected by source parameters but increases with the tunnel inclination angle. This recirculating flow results in contaminant accumulation in the center of the tunnel, with gradual dilution occurring due to ongoing turbulent exchange with inflow and outflow layers. Notably, contaminant dilution is enhanced as the tunnel inclination angle increases. The findings provide important insights into smoke movement and the timescale required to remove pollutants, such as gaseous contaminants and airborne aerosols, in such scenarios.