By John McHale
WEST LAFAYETTE, Ind.-Mechanical engineers at Purdue University have developed techniques for modifying household refrigeration technology with small devices to cool future weapons systems and computer chips.
The devices, called “microchannel heat sinks,” circulate coolant through numerous channels about three times the width of a human hair.
Such devices might be attached directly to electronic components in military lasers, microwave radar, and weapons systems, as well as in future computers that will generate more heat than present computers, says Issam Mudawar, a professor of mechanical engineering who is leading the research.
The researchers are adapting refrigeration systems by using the microchannel heat sinks to replace conventional “evaporators”-components in household refrigerators that contain a labyrinth of tubing. As coolant circulates through the tubing, heat is removed from the refrigerator to cool the food inside.
“We are substituting these conventional evaporators-which might be well over a meter long in the typical refrigerator-with a heat sink that’s only about 1 inch square,” Mudawar says. “The challenge is how to unplug this large evaporator and put in its place this tiny heat sink and make the whole system work.”
Recent findings were detailed in two research papers that appeared in the February issue of the International Journal of Heat and Mass Transfer. The papers were written by mechanical engineering doctoral student Jaeseon Lee and Mudawar.
Electronics for new weapons systems, as well as chips in future computers, will generate five to 10 times more heat than chips in conventional electronic products, requiring better cooling systems, Purdue researchers say.
Computers and other electronic equipment are typically cooled with bulky assemblies that use metal fins to dissipate heat and fans to circulate the hot air away from components. But electronic components in new weapons systems, such as advanced lasers and chips in future computers, will generate too much heat to be cooled with conventional systems that use fans, Mudawar says.
One possible solution is a “two-phase” cooling system-the same basic technology used in a conventional refrigerator -in which a liquid coolant absorbs heat, turns into a vapor, and is then pressurized by a compressor and condensed back into a liquid to begin the cycle over again.
In work funded by the U.S. Office of Naval Research, Mudawar’s team has successfully integrated the microchannel heat sink into an ordinary refrigerator. The device, which was attached to a heating element that simulates a hot electronic component, has been tested with a refrigerant called R134a, which is used in household air conditioners and refrigerators.
“This system successfully combines the cooling attributes of a two-phase microchannel heat sink with the low-temperature capability of a fairly standard refrigeration system,” Lee says. “The result is a high-performance cooling system capable of removing large amounts of heat while maintaining low chip temperatures unattainable by any competing cooling technology.”
“The rapidly increasing use of electronics in military hardware is resulting in unprecedented thermal management needs,” says Mark S.
Spector, thermal management program officer for the Office of Naval Research. “Future all-electric warships and combat vehicles are envisioned to have high-power electrical systems for propulsion, pulsed-power weapons and sensor arrays. Such systems are expected to generate waste heat densities approaching 1,000 watts per square centimeter. Innovative methods such as those being developed at Purdue are necessary to acquire, transport and dissipate these heat loads.”
The research papers provide information for other engineers interested in designing similar systems, said.
“The work is quickly maturing for deployment within the next three years,” Mudawar says.
The researchers also discovered that so-called “throttling valves,” which are already present in standard refrigeration systems, help to alleviate a problem with microchannel devices: pressure tends to increase and decrease dramatically along the parallel channels, hindering performance. But the throttling valves in conventional refrigeration units have been shown in the experiments to “virtually eliminate” those pressure oscillations, Mudawar says.
Such a miniature refrigeration device would keep electronic components cooler than conventional cooling technologies, enabling them to operate faster and perform better, Mudawar says.
The microchannel heat sink is a copper plate containing numerous grooves 231 microns wide-or about three times as wide as a human hair-and 713 microns deep.
“This is really pushing the envelope in how small you can go with these channels and still have a working device,” Mudawar says. “But there is another issue. In conventional systems, the evaporator is actually a very long tube that is wound around many times. So the tube might be a meter in length or more.
One advantage of modifying conventional refrigeration systems is that most of the technology is already available.
“We are not trying to reinvent the wheel with respect to the refrigeration cycle,” Mudawar says. “We want to use existing technical know-how for the refrigeration loop and then implant our new technical know-how for this micro-channel device.”
Microchannel heat sinks could help solve future heating problems because conventional refrigeration is a proven and effective technology that could remain viable for decades, Mudawar says. “One of the big dilemmas I see is that industry and government spend a fortune trying to develop a particular technology just to meet the needs for today, and when the heat dissipation goes up, there goes all of your investment because you have to invent something entirely new,” he says. “So why not begin to develop cooling schemes that are so capable they will stay with you for 20 years?”
