Krishna Kota, New Mexico State University assistant professor of mechanical engineering, is conducting research that may lead to longer duration of space missions - a high priority of NASA. It may also lead to energy efficiencies in many other applications, ultimately reducing consumption of fossil fuels and the carbon footprint.
"The problem we are addressing is how to extend NASA space missions. Right now most space missions are limited to a few weeks. The goal is to enable prolonged space missions to a few months as opposed to a few weeks," said Kota, who directs the Surface-Fluid Interaction Research Laboratory. His research is funded by NASA through the New Mexico Space Grant Consortium.
This project focuses on one of the critical issues in realizing long duration space missions: the gradual loss of cryogenic propellants due to their boil-off as a result of radiation exposure in space.
Cryogenic propellants, for example, liquid oxygen and liquid hydrogen, require extremely low temperatures to remain in a liquid state. Oxygen needs to be stored at below -183 degrees centigrade; hydrogen at below -255 degrees centigrade, the temperatures at which they vaporize.
The propellants are stored in on-board insulated tanks and the problem lies in the weight of these propellant containment systems. Because liquid occupies less space than gas, the systems required for handling liquid forms of propellants are much smaller in comparison to those needed when they are in a gaseous phase, thus reducing the weight they add to the spacecraft. However, when the liquid propellants vaporize, they increase the pressure of the storage tanks and will require thicker tank material that adds to the weight. Hence, relief valves are usually attached to the tanks to release gaseous propellant to maintain the design pressure but it results in gradual loss of the propellant.
"It's very advantageous for the propellants to be in a liquid form and prevent their boil-off," explained Kota. Cryocoolers are employed to keep them at very low "cryotemperatures" in liquid form. "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. 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 heat exchanger with high effectiveness is extremely important for achieving high performance numbers for cryocoolers. Current state-of-the-art heat exchangers are either compact and suffer from low effectiveness or have a high effectiveness but are large-sized and bulky.
Development of compact and portable mesoscale cryocoolers is crucial to enable extended storage of cryogenic propellants for orbital missions and has been deemed a high-priority future technology area by NASA.
"We're doing some really exciting research to increase the performance of the heat exchanger without increasing the size, actually maybe even lowering the size of the heat exchanger," Kota said.
Kota is examining two fundamental interactions: the flow and the thermal interaction of the liquid propellants with the surface of the tubing through which they flow in the heat exchanger.
To improve performance, Kota and his team are tailoring engineered surfaces, such as dimpled surfaces, like that on a golf ball only much smaller, and innovatively textured wavy surfaces, that will optimize the flow.
"This is the first time that anyone has looked at such wavy channels for this purpose," Kota said.
"We have multiple ways to modify the surface topology," Kota said. "One way is through conventional machining, like milling or drilling."
Anthony Hyde of the College of Engineering's Manufacturing Technology and Engineering Center is working with Kota to develop cost-efficient manufacturing methods to produce these textured surfaces.
The other method is to use chemical modification of surfaces.
"On the microscale we can modify the surfaces using chemicals or micro-fabrication techniques performed in a clean room, and can fundamentally alter the way these surfaces interact with different fluids," Kota said.
For example, chemical treatments produce thousands of nanostructures on the surface of copper, in this case, making the surface that is water-phobic or hydrophobic: water slides on the surface. The surface structure is inspired by the lotus leaf, a naturally occurring hydrophobic surface.
"This reduces the drag of the fluid or the friction of the fluid as it flows through the pipes. When the drag goes down it saves a lot in the pumping power, which means less electricity consumption," Kota said.
"It is significant that we are applying this technology to the surface of copper, which is good for heat transfer. We could do this on Teflon or some polymer surfaces that are naturally water repellant, but these materials are not good for heat transfer. The challenge is realizing these surfaces with adequate robustness on materials that have good heat transfer properties. Copper is one of the best thermal conductors."
Based on preliminary analysis, Kota's research has found that, under certain operating conditions, cryogenic heat exchangers being pursued could be at least one-third of the size of the current state-of-the-art, with more than 10-15 percent improvement in thermal performance.
Kota and Hyde are working with students at NMSU to optimize flow and heat transfer of cryogenic fluids through the proposed textured channels in addition to identifying a cost-effective manufacturing solution. Along with Brian Motil, chief of the Fluid Physics and Transport Branch of NASA GRC, they are pursuing a goal of integrating their findings into actual 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 of heat, and we have power generation systems involving heat exchangers and a lot of piping and tubing for transfer of fluids," Kota said.
"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. If we can improve energy efficiency from the thermal-fluid perspective, we can actually see a lot of reduction of the carbon footprint as a result of power generation by burning of fossil fuels. It could also make renewable power generation economical."