University of Central Florida researchers are developing a human-like way for large machines to cool off and keep from overheating: Letting the machines "breathe."
In effect, the machines "inhale" cool blasts of water and "exhale" surface heat, says Khan Rabbi, a doctoral candidate in UCF's Department of Mechanical and Aerospace Engineering.
In a study, led by Rabbi, a pulsed water-jet cooled a machine's hot titanium surface.
"The more water we pumped out of the spray jet nozzles, the greater the amount of heat that transferred between the solid titanium surface and the water droplets, thus cooling the titanium," said Rabbi .
The water is emitted from small water-jet nozzles, about 10 times the thickness of a human hair. The tiny jets douse the large electronic system's surface and the water is collected in a storage chamber, where it can be pumped out and circulated again to repeat the cooling process.
The storage chamber holds about 10 ounces of water. By using a small amount of water, the system avoids flooding.
To be able to control the flow, and customize it for a given machine, a mechanical chopper wheel with designated holes cuts the free stream jet into tiny segments or pulses. The chopper wheel is coupled to a DC motor.
"By controlling the speed of that motor, we tuned the pulses," Rabbi told Tech Briefs.
Another main pump determines the total amount of water in the free stream jet. Using high-speed, infrared thermal imaging, the team checked the accuracy of intended water-dispensing and found the optimum amount of water for maximum cooling performance.
To remove 600KW of heat from a one-square-meter titanium surface, the researchers needed about 8 milligrams of water per pulse-cycle, says Rabbi.
The findings are detailed in the journal Physical Review Fluids .
The "breathing" approach to thermal-management could someday support the cooling of large electronics, space vehicles, batteries in electric vehicles, and gas turbines.
In an edited Q&A with Tech Briefs below, Rabbi explains why he believes his team's study is a first step in creating new kinds of liquid cooling systems.
Tech Briefs: From a technology perspective, what does it mean when we say that the machines “breathe?” What controls the flow of water?
Khan Rabbi: In our proposed cooling technology, the hot surfaces are impacted by cold water jets in a pulsating manner thus exchanging the heat. Later, the hot water leaves the system by exhaust ports.
This inhaling of cold water-jets and exhaling of hot water is analogous to "breathing." A two-phase flow pump regulates the flow of water whereas a mechanical-chopper-wheel chops down free stream jets into smaller segments. That is how the pulsation is created.
Tech Briefs: How much water is required?
Khan Rabbi: Now, the amount of water necessary to cool down a hot surface depends on different factors, such as surface size, surface material, surface temperature, thermal load, and flow conditions. Our study takes all these factors into consideration to achieve maximum cooling performance.
Tech Briefs: What inspired you to try this breathing-like idea for thermal management?
Khan Rabbi: Machines consume energy. During operations, a single form of energy gets converted into numerous other forms. One of them is thermal energy or heat. If we want to keep them in good working condition, we need to cool them down as efficiently as we can. To achieve this, we take inspiration from nature. For instance, most animals keep their body cool by perspiration, respiration or both. For example, humans keep cool by both perspiration (sweating) and respiration (breathing). Animals, however, that do not have sweat glands, like birds, use only respiration to cool down.
Tech Briefs: Has this approach been done before?
Khan Rabbi: We propose this idea where the combination of perspiration (evaporative cooling) and respiration (convective transport) is perceived both experimentally and theoretically.
This technique has been experimented before by other researchers. To our knowledge, there was no study that provides fundamental understanding of such pulsating jet-impingement cooling technology. To generate an idea of the fundamental limit, we look right at the interface between the hot surface and impacting water droplets. We also propose a predictive model to estimate maximum heat transfer performance for a range of pulsation frequencies.
Tech Briefs: What are the alternatives to this kind of method, and why is a breathing approach so valuable?
Khan Rabbi: Conventional water-jet cooling technologies use free stream jets. Higher amounts of water in these free-stream jets, however, can cause flooding, thus resulting in lower cooling performance.
In contrast, lower amounts of water can cause dry-out events where there is no coolant present to take-out the heat. This, in result, increases the temperature of the surface.
For these reasons, we propose a pulsating cooling technology that solves this problem by being in the intermediate regime where there will be no flooding or dry-outs. This technique is valuable because it can be designed according to the hardware needs by tuning the thermo fluid design parameters.
Tech Briefs: In what applications do you envision this approach being used?
Khan Rabbi: This particular technology can be used in thermal management in space and ground-based systems, such as batteries of electric vehicles, electronic packages, telecommunication equipment, data centers, turbine-systems, and materials manufacturing.
Tech Briefs: What are you working on now?
Khan Rabbi: In our lab (Interfacial Transport Lab) at University of Central Florida, we will continue to design, develop, and generate fundamental understanding at solid-liquid interfaces.
Currently, we are working on hybrid cooling technologies that combine this active cooling technique with passive pulsation cooling devices suitable for machines and electronics devices at a wide range of length scales, from micro to macro.
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