Assessment and Prediction of Air Entrainment and Geyser Formation in a Bottom Outlet: Field Observations and CFD Simulation

Abstract

Air entrainment at the intake of a bottom outlet often gives rise to air pockets in its conduit and formation of geysers. The outlet in question comprises a bulkhead gate, gate shaft, horizontal conduit, and exit. Operations show that it suffers from appreciable flow fluctuations and blowouts in the tailwater, which leads to gate operation restrictions. For the purpose of understanding the hydraulic phenomenon, both prototype discharge tests and three-dimensional computational fluid dynamics (CFD) modeling of two-phase flows are performed. The operational focus of the facility are small and large gate openings. The CFD results reveal that, with air entrained in the gate shaft, continual breakup and coalescence of air bubbles in the conduit typify the flow. At small openings below 1 meter, the air–water flow is characterized by either distinct blowouts of regular frequency or continuous air release. In terms of geyser behaviors inclusive of frequency, the agreement is good between field and numerical studies. At large openings, the gate becomes fully submerged, and the flow is discharged without air entrainment and blowouts. The paper showcases the air–water flow features in a typical bottom outlet layout in Sweden, which is intended to serve as an illustration of the study procedure for other similar outlets.

Mitigation Strategies to Reduce Detrimental Effects of Air Bubbles in Shaft Spillways: A Literature Review

“…Several studies offer beneficial contributions to explain the mechanism of occurrence of blowback incidents in hydraulic structures due to air-water flow [11,14,27], but there is a lack of predictive or analytical solutions to evaluate the critical pressure required to cause damage as a result of blowback. There is also a need to address the parameters that have positive or negative effects on blowback phenomena.…”

“…The air entrainment ratio is determined by various factors including the length, velocity, nozzle design, diameter, slope, and physical qualities of the liquid being used. Yang et al [14] hypothesized that the creation of air bubbles occurs when a combination of water and air enters a horizontal pipeline, causing the trapped air to rise and adhere to the upper part of the pipeline.…”

“…The air bubbles have the ability to accumulate and form air pockets of considerable size. Yang et al [14] indicated that the formation of air bubbles leading to the creation of air pockets can cause either air blowouts or blowbacks. This phenomenon occurs in pipes that are exposed to dynamic hydraulic conditions.…”

“…Figure 6. Air blowouts in the downstream [14] Additionally, Lauchlan et al [11] compiled and analyzed the body of information and experience on air quality issues in pipelines. The following are the findings of this literature review: The movement of air pockets or bubbles in pipelines is not well-understood, and the many prediction algorithms differ greatly from one another.…”

“…Although this study provided an explanation of the air blowback mechanism, further investigation is necessary to identify the factors that either promote or decrease air blowback. Yang et al [14] used a three-dimensional CFD model to simulate two-phase fluid flow through bottom outlets beneath a Swedish embankment dam. The outlet has a bulkhead gate, a gate shaft, a horizontal conduit, and an exit.…”