On the backlayering flow of the buoyant contaminants in a tunnel with forced longitudinal ventilation

Abstract

In a longitudinally ventilated tunnel, the backlayering flow of buoyant contaminants appears when the ventilation velocity is lower than the critical velocity. In this study, we use brine-water experiments to investigate the backlayering length and the flow structure of the backlayering flow. It is found that the “seemingly stagnant” backlayering flow actually consists of two sub-layers moving towards opposite directions. The distribution of the reduced gravity in the entire model tunnel is measured using a light attenuation technique. The contaminant concentration in the backlayering flow is significantly larger than that in the downstream current. A dimensionless parameter is proposed to normalize the density distribution in different backlayering flows. In the scenario without boundary heat transfer, the mass exchange between the buoyant fluid and ambient fluid also can reduce the buoyancy force of the backlayering flow. A model is proposed to estimate the backlayering length without boundary heat loss. Because both boundary heat loss and mass exchange contribute to the buoyancy attenuation of smoke backlayering flow, the effect of the boundary heat transfer on the backlayering length is quantitatively analyzed by comparing the present model with previous models of smoke backlayering. The results are helpful for understanding the buoyant backlayering flow in longitudinally ventilated tunnels.