Water Flow and Turnover Rates in Bubble Plume Aeration Systems
Tech Talk 122
Bubble plumes, unconfined airlifts or non-ducted aeration systems have been studied for many years in a variety of applications. These systems have been used for lake and reservoir destratification, gas transfer and reaeration, bubble curtains for prevention of ice formation, containment of water quality contaminants (phytoplankton and suspended solids) and ocean sequestration of carbon dioxide.
Common to all of these applications is the difficulty in predicting water flowrates and lake or pond turnover rates. This is because of the complex interaction of limnological and climatological conditions. Weakly stratified conditions will affect water flow differently than strongly stratified conditions. The temperature, solar radiation and wind speed also affect the water flow patterns in bubble plumes.
The flowrate of unconfined systems can be quantified, either by measuring the time required to destratify a large lake-like impoundment or by taking direct measurements of their flowrate. Researchers have shown that water flowrates in bubble plumes will vary depending on gas flowrate, lake stratification and weather conditions. There are several types of flow patterns generated, which can be described as Types 1, 2 and 3. The figure below shows the three flow patterns.
In unconfined bubble columns, air is forced through diffusers at the lake bottom. As the bubbles rise to the water surface, water is entrained (carried with the bubbles) in a mixture of bubbles and water. This rising bubble plume carries deeper water that is colder and heavier than the water above. The plume will rise until the negatively buoyant water is no longer entrained. At this level, water within the bubble plume is detrained (ejected) or peels off. The bubbles continue to rise and entrain more water. The detrained water will sink to a level of neutral density and then spread out horizontally away from the bubble plume.
In the case of a stratified lake, the buoyant plume will rise to different levels before water is detrained and moves out horizontally. This will vary depending on the degree of stratification as shown in the figure.
Because of these complications, it is difficult to describe water flowrates from bubble plumes without complex limnological and climatological input data. Several researchers have used sophisticated water quality models to describe bubble plume behavior in different applications.
Some manufacturers have placed their diffusers in a draft tube to determine the water flowrate. However, using a confining tube will affect the velocity and flowrate (causing a "chimney effect"), making the diffuser appear to perform better than it does. Others have completed flowrate testing in tanks or containers where walls and boundaries affect the results. These methods of testing are so unlike the actual conditions of unconfined bubble plumes that the results should not be used to predict flowrates in an open water body and should not be used in making comparisons between diffuser systems.
At present, there is no way to accurately compare different unconfined bubble systems because there is no standardized method of testing to evaluate water flowrates. Turnover rates are especially difficult to calculate and, until a standard measurement is established, should never be used as a basis for system sizing. Without a standard method, there is no way of comparing water flow and turnover rates for competing systems. What is clear is using confined flow or flow in a ducted system is not an accurate method of describing flowrates in unconfined bubble plume systems.
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