sage action, inc.
  Airflow Visualization and Measurement
 
 
   
 

SEE THE UNSEEN

by

C. W. Reniff, Dr. D. E. Ordway & B. E. Ordway

Air is invisible, but engineers in industry, government and universities often need to “see” airflow patterns for a number of reasons.

Most people are aware of aerodynamic testing of autos or aircraft in wind tunnels, but not so familiar to the general public are behind-the-scenes efforts to determine the flow patterns of the air cooling the electronics inside a computer or paper copier. Equally important is the efficiency of transporting conditioned air through duct work and the uniform distribution of that same air throughout a room for a pleasant comfort level.

One way for the development or design engineer to see the air is to implant something into the moving air that will reveal the direction and speed of the flow which are important to his or her tests. Historically, smoke has been the best known method to trace airflow. Since smoke consists of a dense concentration of submicron-sized particles that are individually too small to be seen by the human eye, it can be useful only when the airflow is smooth or “laminar”. Once the flow becomes turbulent, however, the tiny smoke particles separate appreciably, or “dissipate”, and observable flow tracing quickly deteriorates.

Other methods have utilized short pieces of yarn, or tufts, mists created by mixing certain chemicals, carbon dioxide vapor and liquid nitrogen mixed with steam to indicate the direction of moving air.

All of these methods have one or more shortcomings. A large breakthrough came, though, with the “practical” introduction of the helium-filled soap bubble technique of airflow visualization. Superior to the methods cited above, these special 1/8” diameter bubbles “lock onto” the streamlines of the moving air into which they are implanted because they have the same average weight and density as the air itself. In other words, they contain just enough helium to offset the weight of the bubble film, making them “neutrally buoyant”.

Advancement of this technique was initiated by Sage Action, Inc., Ithaca, NY, under a contract with the Office Of Naval Research in 1968. The equipment consists of a control console which meters compressed helium, a special-formula Bubble Film Solution (BFS) and compressed air at the proper flow rates to the “Head”. The Head is the component that, in turn, forms the bubbles in rapid succession at 300 to 400 bubbles per second. The Head fits into a third component, the Mini-Vortex Filter which takes out the “bad” bubbles that are too heavy or too light. The remaining bubbles exit the Filter and are ducted to the test area by a length of flexible plastic tubing and dispersed with a special wand.

The neutral buoyancy characteristic provides the bubbles with the remarkable ability to negotiate rotating fans, even at high rpm, or to survive airspeeds up to transonic velocities. This is all possible since there is no relative motion between the bubble and the surrounding air. The bubbles have a substantial lifetime of one to two minutes, or longer, because of a plasticizer ingredient in the BFS.

For maximum visibility, the bubble stream is illuminated by a specialized light source and observed perpendicular, more or less, to the axis of the light beam against a dark background. If the light source is “pulsed” at a known frequency, in addition, the bubble traces result in distinct “dashes” which can be measured to determine the velocity of the bubble and, therefore, the air. Sage Action has developed and sells such a light source.

Larger Image

The flow tracing results can be can be photographed or videotaped by means of conventional cameras. Since these streamlines are so well-defined, exceptional evaluation of the flow patterns can be made.

SAI™ Bubble Generators have been employed for countless applications. These applications have been very diverse, going far beyond any original expectations. They range from cars to high performance aircraft, from computers to copiers, from home fans to commercial compressors, from pig pens to cleanrooms, from baby diapers to photographic films, from office ceiling diffusers to marble mosque ventilators, from vacuum cleaners to lawn mowers, from paint booths to baby incubators, from hair dryers to industrial burners, from pea plants to pine trees and from parachutes to the Space Shuttle.

In summary, the versatile and many-faceted field of airflow visualization has been greatly enhanced by the arrival of the tough, little SAI™ neutrally-buoyant bubble. With this unique capability, engineers can design or improve whole areas of our life that depend on moving air, providing the quietness and the energy-smart features that are greatly needed today.

 
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