CuPGS Laminate Core for a Matrix Microchannel Heat Exchanger

University essay from Luleå tekniska universitet/Institutionen för system- och rymdteknik

Abstract: Cryocooling is a continuously developing field of engineering, applied in the fieldsof aerospace, military, and medical sciences among others. There is a demand forsmaller and more efficient cryocoolers for spaceborne low-light observation missions,with many custom cooling systems having completed successful missions. The Stir-ling cycle is the most prevalent refrigeration technique used for space applications,with the pulse-tube, Joule-Thomson or reverse Brayton cycles being used in somespecial cases.A matrix heat exchanger is designed with 3D-printed 17-4 PH stainless steel end capsstreamlined for computer numerical control (CNC) production. The heat exchanger (HX) core consists of 1mm thick stainless steel spacers and 250μm thick copperchips that are tolerance-matched for photo etching, as well as pyrolytic graphitesheets (PGS) of 25μm, the thickest commercially available PGS without addedadhesive film material.The experiments of joining PGS and copper chips with Epo-Tek 301-2 epoxy tocreate a solid core structure for the heat exchanger did not result in a pressure-resistant laminate material. The graphite surface proved difficult to adhere to usingthis epoxy, creating voids, and easily delaminated into separate layers of PGS. Bond-ing the stack together using indium, testing epoxy with a higher ability to permeatethe PGS or diffusion-bonding through other means are presented as options forfurthering the HX development.Pressure testing of a copper-only laminated heat exchanger core showed that theend cap recess adhesion capability is a potential point of failure, as the designedstructure makes it impossible to inspect the results of the bond without curingthe epoxy and pressurising the system. The difficulty in establishing a tight seambetween the main counter-flow channels of the HX is also demonstrated here, asleakage between the channels occurred at pressures in the vicinity of 2

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