| description abstract | Geysers are explosive eruptions of air-water mixture from manholes in drainage systems. When the design capacity of storm water drainage systems is exceeded during heavy rainfall, rapid inflows can lead to air-water interactions that give rise to geysers, causing damages and threatening human lives. Over the past two decades, the dynamics of air pockets in vertical shafts and horizontal pipes has been extensively studied. Although the role of entrapped air in causing large pressure transients is revealed, the mechanism of geyser formation remains elusive primarily because of the lack of detailed observations. A comprehensive series of experiments has been performed on a physical model of a simplified drainage system, which consists of a vertical riser and a horizontal pipe with diameter D connected to a constant head tank. The system is filled with water; an air pocket is then released into the horizontal pipe and the trajectories of the air pockets in horizontal pipe and vertical riser are measured by videos and a high speed camera. Pressures are measured using pressure transducers near the pipe end and at the bottom of the riser. Parameters considered include riser diameter (Dr), upstream head (H0), and initial air pocket volume (Vair). Without an external pressure head, the air pocket migration in a water-filled vertical riser is found to be similar to a slug flow. The rising velocity of the air pocket relative to the free surface (vnet) is nearly constant and close to the speed of a Taylor bubble; no geyser is observed. When an external pressure head is applied to the horizontal pipe, two types of flow are observed. For large riser diameter and small air volume, the rise of the air pocket in the vertical riser resembles a Taylor bubble, with an air pressure equal to the hydrostatic pressure of the water column above. The air breaks within the riser and no geyser is observed. For small riser diameter and large air volume, the horizontal air pocket shoots past the riser junction and only partially enters the shaft; the vertical air pocket migration is very different from that of a Taylor bubble. The air pocket pressure is found to be significantly higher than the hydrostatic value, resulting in rapid acceleration of air and water, and jetting out of air-water mixture from the riser top. Experimental results show that geysers are more likely to occur for small risers (Dr/D≤0.62) and large air volumes {Vair/[(πDr2/4)H0]≥3.42}. | |
