Aug. 6, 2004 PORTLAND - A successful new "desorption" technology using branching microchannels, which takes its inspiration from the human circulatory system, was described this week at a professional conference and cited as a significant step towards the creation of man-portable cooling systems that may find important uses in the military, fire fighting and elsewhere.
The latest advance was explained by engineers from Oregon State University at the Micro Nano Breakthrough Conference in Portland, an event that is highlighting some of the most exciting new work being done in micro and nanotechnology. It is sponsored by OSU and the Pacific Northwest National Laboratory.
One of the major projects already under way in cooperative research between OSU and PNNL is the creation of revolutionary new cooling systems that could be extremely small, lightweight, and at first might find applications for soldiers in hot weather combat or other professionals working in very hot or dangerous conditions.
The newest advance is an important technical hurdle to overcome, researchers say, and in some ways takes its inspiration from the blood pumping system of the human body, which uses a comparatively small pump to move blood - according to some estimates, through at least 60,000 miles of veins, arteries and capillaries.
"If you think of what our heart and circulatory system does, we have a rather modest pump, the heart, that moves blood through great distances," said Deborah Pence, an OSU associate professor of mechanical engineering. "To accomplish this each artery branches into two smaller arteries, yet the overall flow area increases, thus requiring less effort to move the blood. Conceptually, that's what we are doing with this engineering approach."
For traditional cooling, Pence said, a large, bulky compressor pressurizes a vapor for flow through the refrigeration cycle. One way to make a cooling system smaller is to change the type of cycle, so that the large compressor can be replaced with a small pump. This can be done, researchers say, by eliminating the compression cycle and substituting an absorption cycle, a major component of which is a desorber.
"It's already been shown that part of this cycle of evaporation and condensation can be done with microchannel heat exchangers, about the width of a hair," Pence said. "But there are high pressures involved, problems with flow maldistribution and vapor lock. What we've discovered is that branching channels work much better."
Using this approach to microchannels, which create mechanical systems that loosely resemble what a circulatory system uses in pumping blood, gives the system enhanced movement of the liquid at the microscale without large pressure requirements, the researchers found.
"We still have a lot of work to do," Pence said. "This addresses part of the problem with the desorption process. However, PNNL is working on the absorption process, which is somewhat more complicated, and eventually we must make all these components between OSU and PNNL work together. But we're much closer now to really miniaturizing the pump that's needed for the cycle using this branching channel array."
The scientists are also planning to create some transparent silicon disks that perform this function so they can better see inside the process and determine exactly what is happening as the fluid moves through - the velocities of liquids, vapors, flow instabilities and other issues.
According to Pence, the project is an interesting example of a new approach being used in some of this microtechnology research, of first trying to create something that engineers could build from a preliminary design and then see if it would actually work for the desired purpose. It's somewhat different than the carefully plotted, one-step-at-a-time procedures more typical of science. "As a colleague of mine, Kevin Drost, has said, instead of ready, aim, fire, this engineering approach is more like ready, fire, aim," Pence said. "But it's one way that we're going to create working technologies much more rapidly, instead of just something that's interesting in a laboratory but difficult to build or produce in the real world."
This research is part of a three-year, $5.2 million project being supported by the U.S. Department of Defense.
The initial goal is a portable cooling unit that could be carried by a soldier to pump coolant through a specially designed suit, preventing heat exhaustion when working in extremely hot weather conditions. But later consumer applications could be enormous, researchers say, such as automotive cooling or individualized heat pumps for each room of a house.
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