Abstract
ABSTRACTA geyser is an explosive flow of air–water mixture shooting out of a manhole. It has been demonstrated experimentally that the releasing motion of confined air pockets in a dropshaft is the key mechanism to trigger a geyser. Other release events, unassociated with air–water mixtures, can occur, but the intensity is significantly smaller than the air–water geysers. Existing numerical models that simulate vertical air movement in mixed-phase flows typically solve a series of lumped-mass continuity, momentum and energy equations, greatly simplifying the interactions between the water and air phases. Hence, existing models are unsatisfactory in capturing the complex dynamics of a geyser because of the violent interactions between the water and air phases. In this work, a two-phase numerical model solving the Navier–Stokes Equations was applied to investigate the driving forces in an air–water geyser formation in storm sewer system. The simulated dynamics include buoyancy, air compressibility, momentum and pressure. The numerical model revealed the key factor that triggers an air–water geyser, which involves compressed air pockets that are pushed into the dropshaft by pressure surges from the main pipe. The numerical model also captured the two distinctive features of an air–water geyser, which are a violent mixture of water–air and a high-speed jet. This study also revealed how a pressure head in the main pipe, which is much lower than the ground elevation, could lift the water to the ground and push it out of the manhole.
Highlights
Geyser phenomenon has been observed in several stormwater sewer systems (Guo & Song, 1991; Hamam & McCorquodale, 1982)
The numerical model reveals the key factor that triggers a geyser feature, namely that when a compressed air pocket is pushed into a vertical dropshaft and escapes from the system, a violent water–air mixture with a high velocity forms, and could trigger a geyser
The results indicate that just having captured air in the system is not enough to produce the air–water geyser
Summary
Geyser phenomenon has been observed in several stormwater sewer systems (Guo & Song, 1991; Hamam & McCorquodale, 1982). The compressed air pocket pushes water out of a manhole, forming a geyser This numerical model did not reveal details of pressure distribution characteristics in the air pocket rising motion. Field data analysis of a geyser in a storm-water tunnel in Minneapolis by Wright, Lewis, and Vasconcelos (2011) shows that the pressure head within the main pipe is about 20 meters below the ground, not high enough to push water out of the vertical shaft when a geyser was observed These findings indicate there might be other driving forces in a geyser. Whereas FLUENT treats a trapped air pocket as incompressible in a typical twophase flow simulation, this work incorporated the Ideal Gas law to reflect air compression effects inside of an air pocket Results of this numerical study provide insight to help guide future stormwater sewer system design to alleviate possible geyser occurrences
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