Search

Researchers at JPL have developed, built, and tested an innovative heat exchanger that offers reduced thermal expansion, increased structural strength, low pressure drop, and improved thermal performance while lowering the weight associated with typical heat exchangers. This innovation would benefit the commercial thermoelectric generator, aircraft, and industrial processing (i.e., glass, steel, petrochemical, cement, aluminum) industries by improving energy management/efficiency, reducing carbon dioxide emissions, and increasing system durability due to the reduced stress from thermal expansion. <b>The Problem</b> Thermoelectric generator systems require high-performance hot-side and cold-side heat exchangers to provide the temperature differential needed to transfer thermal energy while withstanding temperatures up to 650 &#176;C. Because the hot-side heat exchangers must have a high heat flux, they are often made of metals such as stainless steel or Inconel^&#174; alloys. Although these materials can operate at high temperatures, resist corrosion, and are chemically stable, they also have several drawbacks: (1) Their lower thermal conductivity negatively affects their thermal performance. (2) Their higher thermal expansion leads to stresses that compromise system structural integrity. (3) Their high mass/volume reduces the power density of generator systems into which they are integrated. As a result, they are difficult to integrate into viable energy recovery systems. They also make the systems unreliable, non-durable, and susceptible to failures caused by thermal-structural expansion. <b>The Solution</b> JPL researchers chose to replace the metal in traditional heat exchangers with graphite, which offers an improved conductivity-to-density ratio in thermal applications as well as a low coefficient of thermal expansion. In addition, they used a mini-channel design to further increase thermal performance. Combining more advanced materials with the innovative thermal design has yielded significant improvements in performance. For example, a 200-cm³, 128-g version of JPL's exchanger successfully transported 1,100 W from exhaust at nearly 550 &#176;C with approximately 20 W/cm² thermal flux and a pressure drop of only 0.066 psi. JPL's technology combines lightweight, high-strength graphite material with a mini-channel design that offers high thermal performance. Further development and testing are underway. <em>Inconel is a registered trademark of Special Materials Corporation.</em>

The advent of additive manufacturing makes available new and innovative integrated thermal management systems, including integrating an oscillating Heat Pipe (OHP) into the leading edge of a hypersonic vehicle for rapid dissipation of large quantities of heat. OHPs have interconnected capillary channels filled with a working fluid that forms a train of liquid plugs and vapor bubbles to facilitate rapid heat transfer. Multiple additive manufacturing techniques may be used, including powder bed fusion, binder jetting, metal material extrusion, directed energy deposit, sheet lamination, ultrasonic, and electrochemical techniques. These high performance OHPs can be made with materials such as Refractory High Entropy Alloys (RHEAs) that can withstand high temperature applications. The structure of the OHP can be integrated into the constructed leading edge. The benefits include a heat transport capacity of 10 to 100 times greater than before. Integrated OHPs avoid the bends or welds in traditional heat pipes, especially at the locations where the highest thermal stresses might cause thermal-structural failure of a leading edge. Alternating the diameters of the OHP channels alleviate start-up issues typically found in liquid metal oscillating heat pipe designs in high temperature applications by aiding in the instigation of a circulating flow due to multiple forces acting upon the working fluid.