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In applications such as bulk solids conveying, vacuum drying and cleaning, and liquid aeration, which may be conducted only periodically during maintenance and turnaround operations, the permanent installation of equipment such as pumps, or large auger systems is often cost-prohibitive. In these situations, eductors, which are simple, inexpensive devices devoid of moving parts can be used instead.

Eductors rely on Bernoulli’s principle to transfer energy from one fluid stream to another: liquid to gas, liquid to liquid, or gas to powder. Bernoulli’s principle states that increasing a fluid’s velocity will simultaneously result in a decrease in that fluid’s pressure. Eductors leverage this principle by accelerating the motive fluid stream through a nozzle, resulting in a low-pressure region inside the nozzle and near its discharge where velocities remain high. This low-pressure region is used to aspirate another fluid or bulk solids stream, typically through a side branch connection on the eductor body.

In a recent study, we used eductors to examine the effects of geometry, operating conditions, and scale on liquid aeration performance. In our tests, the motive fluid was water and glycerin, and the aspirated gas was ambient air. Performance was quantified by measuring air entrainment rates, and by characterizing the two-phase flow generated downstream.

Air bubble sizes and densities were assessed by photographing a transparent pipe section with a high-speed camera, and by visualizing inside the pipe with a borescope coupled to another high-speed camera. The borescope shows dispersed bubbles generally in the 100-5000 µm range, however, due to its central location in the pipe, and a relatively limited field of view, it doesn’t tell the whole story.

When visualizing the entire pipe cross-section, we can see a significant bubble size and density gradient in the vertical direction. These gradients are caused by buoyancy forces, which cause bubbles to rise and coalesce near the top of the pipe. Our previous post on flow regime maps covered such two phase flows in more detail.