Downstream stratification of the buoyant contaminants produced by a highly lazy plume in a longitudinally ventilated tunnel

Abstract

In an underground tunnel with longitudinal ventilation, the stratification of the buoyant contaminants is crucial for the safety of the workers and passengers. A series of experiments are conducted in a water tank to investigate the density stratification in the downstream of the buoyancy source. Highly lazy plumes are produced by releasing coloured brine solution into a longitudinally ventilated tunnel which is placed in a large reservoir filled with fresh water. The transient distribution of the reduced gravity is measured using a light attenuation technique. The results suggest that the vertical distribution of the reduced gravity in the buoyant layer is approximately linear. The gradient Richardson number demonstrates that the downstream stratification can be very unstable under the effect of forced longitudinal ventilation. The difference between the reduced gravity at the ceiling and that at the floor Δg′, the average reduced gravity g′‾ and the thickness of the buoyant layer h are chosen to quantify the stratification. With the increase in the longitudinal ventilation velocity, both Δg′ and g′‾ decrease but h tends to increase. As the source buoyancy flux increases, all the three parameters become larger. Based on the experimental data and dimensional analyses, quantitative models are proposed to estimate the above three parameters. It is hoped that the models may provide useful references for the downstream distribution of the buoyant contaminants produced by highly lazy plumes in a longitudinally ventilated tunnel.