A New Mexico State University assistant professor is investigating methods of increasing heat exchanger efficiency that could enable space missions to remain in orbit for longer periods of time and lead to better automotive, defense, data center and power generation thermal management systems.
“The problem we are addressing is how to extend NASA space missions,” Krishna Kota, assistant professor of mechanical engineering, said.
“The goal is to enable prolonged space missions to a few months as opposed to a few weeks.”
Currently, the length of space missions is severely limited because of the gradual loss of cryogenic propellants due to boil-off caused by solar heating. Cryogenic propellants are used to power machinery in space in place of ordinary fuel, which cannot be used because of the absence of an environment that supports combustion. These propellants, such as liquid oxygen and liquid hydrogen, are stored in liquid form in on-board insulated tanks at extremely low temperatures to prevent vaporization.
Since liquid occupies less space than gas, the systems required for handling the liquid forms of these propellants are smaller and lighter in comparison to those needed for handling the fuel in gas form, thus reducing the weight of the spacecraft, Linda Fresques of the NMSU College of Engineering said in a feature written for the El Paso Times. However, when solar radiation causes the liquid to heat up and change phase to a gas, pressure inside the storage tank increases. To prevent the need for a thicker tank material that will add to the spacecraft’s weight, relief valves are usually attached to the tanks to release extra gaseous propellant to maintain the design pressure, resulting in a gradual loss of the propellant overall.
According to Kota cryocoolers are often employed to prevent a phase-change from happening by keeping the liquid propellant cool.
“One of the primary components of the cryocoolers is the heat exchanger, which plays a very crucial role in determining how well the cryocooler can perform to keep the contents of the storage tanks cool,” he said. “The size of the heat exchanger is the biggest problem. The heat exchanger could be sometimes tens of times larger than the cryocooler itself.” A reasonably-sized heat exchanger with high effectiveness is an extremely crucial component for achieving high cooling performance; however, smaller heat exchangers often suffer from low effectiveness, while more effective heat exchangers are often large and bulky.
Kota and research partner Anthony Hyde, of the NMSU College of Engineering’s Manufacturing Technology and Engineering Center, are working to alleviate this problem by investigating both the flow and thermal interaction of the liquid propellants with the surface of the tubing connected to the heat exchanger.
“We’re doing some really exciting research to increase the performance of heat exchanger without increasing the size, actually maybe even lowering the size of the heat exchanger,” Kota said. The team is currently focusing on modifying surface topology of the tubing to alter the way the fluids interact with it.
Thus far, the team has found that under certain operating conditions, cryogenic heat exchangers could be reduced to at least one-third the size of current models, with more than 10-15 percent improvement in thermal performance. In cooperation with Brian Motil, chief of the Fluid Physics and Transport Branch of NASA GRC, they are working to integrate their findings into cryocooler systems.
“This is really, really exciting, because we have fluid flowing through pipes in innumerable applications—from drug delivery in medicine, thermal management of defense and automotive electronics, to cooling supercomputing data centers in which we have thousands of computing servers generating large amounts,” Kota said. “[We also] have power generation systems involving heat exchangers and a lot of piping and tubing for transfer of fluids.”
“The driving force is energy. Basically, what we’re trying to do is improve energy efficiency by lowering the electrical costs of pumping and improving thermal transfer using engineered surfaces and bio-inspired designs.”