Airflow characteristics of the stepped dropshaft in the deep tunnel storage system

ABSTRACT The deep tunnel storage system, mainly consisting of underground tunnels and dropshafts, is effective for dealing with urban waterlogging. A stepped dropshaft is suitable for the system to safely transport water to underground tunnels and efficiently release air carried by water. In this study, the air inflow discharge, air vent discharge, and air discharge into underground tunnels are investigated experimentally. As the dimensionless water flow discharge increases to 0.56, the water flow successively forms the nappe flow, transition flow, and skimming flow regimes, the relative air inflow discharge decreases from 0.97 to 0.32, the relative air vent discharge decreases from 0.85 to 0.22, and the relative air discharge into underground tunnels fluctuates below 0.20. The exhaust capacity (the ratio of air vent discharge to air inflow discharge) reaches a maximum of 0.87 at the threshold between the nappe flow and the transition flow. Influences of flow discharge, step height, step rotation angle, outlet diameter of central exhaust pipe, and underground tunnel blockage on airflow characteristics are revealed. The prediction formulas for air inflow discharge, air vent discharge, and exhaust capacity are obtained with accuracies over 80%, providing a guide to the practical design and operation of stepped dropshafts.