Micro Pumps Take the Heat

When you think of space exploration, images of sophisticated spacecraft probably come to mind. But future NASA missions will rely on down-to-earth technology—miniature pumps that help remove heat from electronic components. While working as graduate students at the University of Washington, Adrián Gamboa, Jone Chung, and Chris Morris developed a new type of micro pump that will circulate a cooling fluid in space-based electronics. The design of those tiny pumps, which require no moving valves, has earned Gamboa, Chung, and Morris the Design News/ANSYS College Student Design Engineering Award.

The heat-removal project started at MicroEnergy Technologies (Vancouver, WA), an R&D company that develops heat transfer systems. After it received an exploratory contract from NASA, the company approached the Mechanical Engineering Department at the University of Washington. "They have a pump technology that fits well with the cooling systems we develop," says Dan Faulkner, a senior design engineer at MicroET. "We asked them to design a pump that would transfer heat with high efficiency." A prototype system now removes 35W of heat, and the system designers aim to reach 100W.

The micro pumps rely on an actuator made of lead zirconate titanate (PZT), a piezoelectric material. Application of an electrical signal to a PZT disk produces pressure pulses in the pump chamber. For the valve mechanism, the team refined an invention developed by Nikola Tesla (1856-1943), which led to a "flapless" design.

Because the valve involves no moving parts, it offers advantages over other designs. First, the pump requires neither repairs nor replacement parts. Second, the pump works with any fluid, even air, which may make it useful as a sampling device in security systems. Third, the pump will not clog. Particles suspended in a drug mixture, for example, won't block the valve. A typical valve channel measures 300 microns wide and 750 microns deep.

"Other people built and tested valve configurations to try to optimize the general design," Gamboa says. "I don't think, though, that anyone tried to use computational fluid dynamics [CFD] with an optimizer to determine the best design. Automation of the CFD tasks revealed a lot about what makes this valve work."

After he simulated the valve design, Gamboa built a mock-up to determine if the pressure drop across the valve matched that predicted by the CFD data. To measure small pressure differences in tiny channels, Gamboa built a water manometer. He explained that this instrument will measure small pressure change, which amounted to a few centimeters of water for the pumps. (Although water served well for testing, in all likelihood, space applications will use a different heat-exchange fluid.)

At first glance, the valve design looks like it shouldn't work: Fluid can flow freely in both directions. But, the return path through the loop causes greater turbulence and restricts the backflow. In effect, more pressure loss (resistance) occurs in one direction than in the other, and as a result, the pump pumps.

While Gamboa concentrated on the valve design, Jone Chung developed the pump mechanism. Although the group tried silicon-based MEMS technologies, to produce prototypes Chung chose a micro CNC milling machine that receives the patterns for PZT actuators and valves from a PC. The width of the polycarbonate and acrylic valves varies from 100 to 300 microns. "Not many people work with plastic at this small a scale," Chung notes.

The team designed the plastic pump base first so it could optimize the valves. Next, it used ANSYS software to determine the characteristics for a covering membrane and for the PZT actuator that would produce the maximum pump volume.

After milling a pump base from plastic, the team had to apply a membrane to the open side of the pump body to seal it. At the same time, it had to keep sealant out of the valve channels. Chung explained the team used glue to bond the membrane to the pump body, but small amounts of glue got into the valves. Even though the pump produced a reasonable flow, it didn't reach the predicted value.

Current experiments involve applying a thin layer of grease to the pump body to seal it to the membrane. A rigid piece of plastic clamped to the pump body and against the membrane completes the assembly. "When we use the clamp method, the pump doesn't leak, so it looks promising," Chung says. "But sealing the pumps and keeping the channels clear remains a challenge."

As far as the design group can tell, they are the first to "stack" micro pumps in parallel to increase a flow rate. In a stacked arrangement, the pressure remains constant while the flow rate increases, a benefit when space is at a premium. Each pump has its own PZT actuator and chamber; however, they share the inlet and outlet tubes.

According to the team's advisor, Dr. Fred K. Forster, Associate Professor in Mechanical Engineering at the University of Washington, the group did a good job. Forster sees another facet of the students' work. "Many students who undertake a project in a traditional area of engineering don't confront issues of the 'newness.' Gamboa, Chung, and Morris had to ask themselves: What does micro electromechanical systems [MEMS] technology mean to my career and should I go in this direction?" According to Forster, the research experience let each student learn what MEMS is all about and where it can take them in the future.

Meet the Design Team

Like many engineers, Jone Chung and Adrián Gamboa developed an early affinity for mechanical gadgets. "As a kid, I tinkered a lot in my Dad's garage," Chung says. "I was always curious about how things worked and I once took apart my bike and put it together again just to see how the brakes worked." When Chung got to college, he decided on a mechanical engineering major because as an engineer he would get to design "cool" things such as cars and medical equipment. Prior to starting his master's degree program, Chung worked on Boeing's 777 aircraft and he contributed to a hydraulic boat-lift design as part of a design team at Sunstream.

"Unlike Jone, I couldn't always get things back together," Gamboa quips. "As early as I can remember, I had a fascination with mechanical things and enjoyed helping my Dad fix the car." Although Gamboa built model cars, most of the fun came in figuring out how to reconstruct them after they broke. "I took a lot of math and physics in college. While studying physics, it dawned on me I had learned the principles behind how things work." As a result, Gamboa pursued the practical side of mechanics and completed his BS and MS in mechanical engineering at the University of Washington. He now works as an engineer with Lockheed Martin.

Team member Chris Morris graduated from the University of Washington with an MS in mechanical engineering and has enrolled in the Ph.D. program in electrical engineering.