Volume 17 | Number 2 | Summer 2005 | Environmental Issue

Researchers Use High-speed Cameras to Better Understand Fluid Flow in Taylor Vortex-based UV Disinfection System

Georgia Tech researchers John Pierson (right) and Aklilu Giorges adjust a high-speed camera to conduct flow visualization studies for the Taylor vortex-based ultraviolet disinfection system. Photo by Gary Meek

Over the past year, Georgia Tech’s innovative Taylor vortex-based advanced UV (ultraviolet) disinfection system has proven its effectiveness at pathogen control in turbid liquid streams. Using aerobic plate counts (APC), researchers have shown a 5-log removal of pathogens in sample streams. The high level of inactivation occurs even with juices and beverages that are opaque to germicidal UV light as well as other liquids that contain suspended solids and turbidity (cloudiness). Even though this is a significant accomplishment, the research team is now focused on further improving the system to gain higher flow throughput and disinfection for liquid streams containing significant turbidity. To do this, however, researchers realized they needed a better analytical tool to more fully understand fluid flow in the stream. Because APC does not tell the researchers exactly how each pathogen in the fluid is moving, they turned to a different technology altogether, high-speed cameras.

“Understanding the flow field of the reactor’s disinfection column is crucial to grasping how disinfection is taking place,” says Aklilu Giorges, a research engineer spearheading the flow visualization study supported by the high-speed cameras.

The term flow field describes the location of a particle in fluid. To ensure the Taylor vortex design maximizes inactivation, researchers would like to know the location of pathogens as they move through the reactor, especially their proximity to the UV lamps. As a particle (or pathogen) moves about in a fluid, its speed changes from point to point. Rather than trying to follow the history of each individual particle, it is more convenient to describe the flow field in terms of velocity, pressure, density, etc., at every point of space relative to time. Predictions can then be made relative to how effectively the lamps will be in inactivating pathogens in a particular part of the flow field. High-speed cameras are helpful in that thousands of images can be captured each second and then analyzed with regard to particle movement.

The flow visualization study used Photron© computer-based high-speed imaging technology. To enhance the visibility of the flow field, rheoscopic fluid was used. Rheoscopic fluid, a water-based suspension of microscopic crystals, is commonly used in flow visualization experiments. The microscopic crystals reflect light so the high-speed camera can track crystal movement throughout the fluid.

The flow field was recorded with a camera frame rate of 1,000 frames per second, explains Giorges. These images were then replayed at slower speeds that allowed a qualitative assessment of the flow field.

“When the images were played at the slower speed, it was obvious that several pairs of counter-rotating vortexes were formed within the Taylor column. An entrance effect was observed that was limited to the bottom of the column. In addition, the entire flow field was observed pulsating because of the pump feed,” explains Giorges. There was, however, no indication that the counter-rotating flow pattern was disrupted because of these pulsations, adds Giorges.

When the rotor speed was increased, the number of counter-rotating vortexes decreased and the flow became chaotic. Giorges says the optimal operating conditions observed in previous APC experiments correlated well with the higher number of vortex rings and laminar (ordered) flow generated during the imaging study.

According to Giorges, the flow path visualization process clearly shows that the vortex rings are effectively moving the fluid into and out of the region where effective UV penetration is achieved even with turbid fluids.

Such visualization techniques provide researchers with improved tools not only for assessing current performance but also for gaining a better understanding of the impact design changes can have on providing even greater performance enhancements.