For missions to dark, dusty, or distant destinations, solar power is not practical. General-purpose heat sources used in conjunction with a heat engine represent a viable solution. The critical challenge is minimizing the size and mass of the system components without compromising reliability. Regardless of the cycle used for the dynamic power converter, the heat exchangers are frequently one of the larger/heavier components, particularly when one of the fluids is a gas rather than a liquid. In this project, Ultramet will design, fabricate, and test compact heat exchangers based on open-cell graphite foams with extremely low pressure drops. With foam-based heat exchangers, the ligaments comprising the foam are the extended surface used for heat transfer. Because of their high volumetric surface area, foam-based exchangers can be far more efficient than a fin-based heat exchanger. The foams to be used in this project, which are fabricated by chemical vapor deposition, have very low pressure drops and have demonstrated heat transfer coefficients that are 3.4 times greater than those of wavy aluminum fins. The technology is cycle-agnostic and can be used in Stirling, Brayton, and/or Rankine cycles. The high efficiency of these heat exchangers will allow them to be much smaller and weigh considerably less than a conventional heat exchanger, which will significantly reduce the overall mission cost.
NASA applications for compact foam-based heat exchangers include not only Brayton, Stirling, and Rankine cycle heat engines, but also environmental control on crewed spacecraft, heat sinks for electronics, evaporators in heat pipes, cryogenic fluid management systems, regeneratively cooled rocket engines, heaters for nuclear-thermal propulsion systems, and many more. Essentially, whenever heat needs to be moved from one location to another via a working fluid, a foam-based heat exchanger can be used.
Commercial applications include heat sinks for electronics and for high-power systems such as radars and lasers, regeneratively cooled rocket engines, systems for cooling cryogenic superconductors, rocket engine igniters, condensers in Rankine cycle power plants, and coolers used in supercritical CO2 power plants. Ultramet has developed foam-based heat exchangers for each of these applications.