Numerical study on the smoke extraction efficiency and the improvement through a smoke reservoir in the naturally ventilated tunnel with vertical shaft

Plug-holing of the smoke layer is an unfavorable phenomenon for the smoke extraction efficiency in the naturally ventilated tunnel with vertical shaft. In this work, theoretical analysis and large eddy simulation (LES) are carried out to investigate how to prevent the occurrence of the plug-holing phenomenon and improve smoke extraction efficiency. The results show that the occurrence of plug-holing is influenced by the height, cross-sectional area and total pressure loss coefficient of the vertical shaft, as well as the thickness of the smoke layer. A modified dimensionless number Fr* is proposed to represent the proportion of smoke in the exhausted mass flow. Fr* varies as the -1/4 power of the cross-sectional area of the shaft, -1/2 power of the height of the shaft, and increases linearly with the thickness of the smoke layer. The critical value of Fr* for predicting whether the plug-holing phenomenon occurs is determined to be 0.34 by combining the results of the previous small-scale experiment. In addition, a smoke reservoir is designed under the shaft to increase the thickness of the smoke layer, which is proven to be effective in preventing the occurrence of the plug-holing phenomenon and improving the smoke extraction efficiency of the vertical shaft. The findings of this paper contribute to the understandings of smoke extraction for naturally ventilated tunnel with vertical shaft, and the results are also useful to the fire protection engineering in tunnels.