Design of an innovative compact heat exchanger manufactured with additive manufacturing processes

In the framework of human space flight, the active control of temperature inside crewed modules assumes a paramount importance in protecting the habitable environment from the harsh thermal conditions of the outer space. This study, carried out in collaboration with Thales Alenia Space Italia, concerns the conceptual design of a Compact Heat Exchanger (CHX), for space applications, to be used with two streams of 3M Novec HFE 7200 DL in the temperature range from -50 to 6.5 °C. The proposed design considers a Triply Periodic Minimal Surfaces (TPMS) Heat Exchanger in stainless steel based on prismatic gyroids, obtained filling homogenously all the available volume allocated for the component. According to literature, this design should grant superior thermohydraulic performance, as well as mechanical resistance to withstand stresses experienced during the launch and the mission, while being peculiarly suited for AM production. A very detailed analysis of a single gyroid cell is performed using a 3D conjugate heat transfer model and the commercial Computational Fluid Dynamics (CFD) software STAR-CCM+, assuming negligible thickness of the wall separating the two fluids and smooth walls surfaces. In the thesis, it is demonstrated that the results of the single cell model are fully representative of the entire CHX, by developing a CFD model of the CHX. The fluids on both sides of the CHX are driven in the laminar regime in view of the very large flow area that the TPMS allows. The heat transfer coefficient resulting from the CFD analysis is compared to the target value, identified starting from the requested performance of the CHX. A second analysis is performed introducing as thickness of the wall separating the two fluid streams the minimum value achievable by state-of-the-art manufacturing techniques. With respect to the target values, a lack of performance, together with a severe overweight of the device, is identified, suggesting to move into the turbulent regime to reduce the HX mass and increase the heat transfer coefficient, and to assess the possible beneficial effect of the surface roughness. Also, possible design improvement still considering TPMS, is addressed, changing the TPMS structure from simple prismatic gyroid to cylindrical gyroid. Two mock-ups of the gyroid CHX, with different cell size, have been then designed and are currently under test, to allow the numerical model calibration and validation, accounting for turbulence and roughness effects on the thermal-hydraulic performance. In this thesis, the results of predictive simulations of the mock-ups thermal-hydraulic behavior are presented and discussed.