Experimental and numerical study of a 3D-printed aluminium cryogenic heat exchanger for compact Brayton refrigerators

Some cryogenic applications, such as for superconducting generators of wind power plants, require compact refrigerators. Heat exchangers are the most bulky components within the cold box of a cryogenic refrigerator. Thus, a reduction of their size would be advantageous for the development of a compact system. Volume and performance of a heat exchanger depend strongly on the hydraulic diameter of the channels and of the efficiency of the heat transfer surface. To improve the performance, a concept of aluminium 3D printed heat exchanger with optimised fin geometry has been developed. A fin design with hydraulic diameters of 1.48 mm and 1.07 mm for the high and low pressure sides, respectively, has been realized. Two prototype aluminium heat exchangers with improved fins have been produced by 3D printing and studied within this project. The accuracy of the geometry reproduction within the printing process has been investigated. The heat exchangers have successfully withstood pressure tests. The pressure drop has been measured at atmospheric and at higher pressure using helium and air as working fluids. Furthermore, the heat transfer with a temperature difference of 40 K along the heat exchanger has been investigated using helium. The Colburn and Darcy friction factors of designed and printed fin geometry have been calculated using Ansys CFX. A full heat exchanger simulation model with consideration of longitudinal heat conduction through inner and outer walls has been developed in Python. The results of the numerical simulations have been successfully validated against measurements. Based on the obtained data, suggestions for an improved heat exchanger geometry for cryogenic applications have been worked out.