3D Numerical Modeling of Geyser Formation by Release of Entrapped Air from Horizontal Pipe into Vertical Shaft

Abstract

Geysers are explosive eruptions of air-water mixture from manholes in drainage systems. When the design capacity of urban storm water drainage systems is exceeded during extreme rainfall, rapid inflows into the drainage network can lead to air-water interactions that give rise to geysers—causing damage to the water infrastructure and threatening human lives. Although extensive research has revealed the role of entrapped air in causing large pressure transients in drainage tunnels, the mechanism of geyser formation remains elusive. In this study, an unsteady three-dimensional (3D) computational fluid dynamics (CFD) model is developed to simulate the pressure transients and air-water interactions during geyser events using the volume-of-fluid (VOF) technique. Extensive numerical simulations are conducted to study the air-pocket dynamics caused by release of trapped air from a horizontal tunnel into a vertical riser. Model predictions of the air-water interface in the vertical shaft are in good agreement with laboratory measurements by a high-speed camera; the mechanism for the formation of geysers is elucidated. It is found that during a geyser event, compression of the air pocket in the riser can lead to rapid acceleration of the overlying water column and its expulsion from the riser; the air-pocket pressure is significantly higher than the hydrostatic pressure of the overlying water column, and the velocity is greater than that of a Taylor bubble. Comprehensive numerical modeling has been conducted to study the effect of the vertical shaft diameter, the upstream pressure head and the air-pocket volume on the air-pocket dynamics; the results show that geyser formation is primarily controlled by the riser to tunnel diameter ratio, Dr/D. 3D CFD simulations have also been carried out for an idealized prototype drainage system; it is shown that the geyser behavior can be characterized by extremely large vertical air velocities that result in dispersed air-water mixtures often observed in the field.