For spacecraft thermal management systems, it is crucial to diminish the overall mass of on-board thermal storage system and minimize the temperature fluctuations when the environmental temperature changes drastically. Since there is no atmosphere in outer space, heat can only be rejected to space using radiation (e.g. radiators). The heat sink conditions, and the heating power subjected to be rejected vary continuously at the orbiting stage of the spacecraft. Without thermal storage capability, the radiator is required to be large enough to release the highest power at the hottest of the heat sink. By engaging and integrating Phase-Change Materials (PCM) within a passive two-phase heat exchanger, the radiator can be designed and sized for the average rather than the maximum power.To meet the NASA needs, the prototype developed and fabricated at the University of the District of Columbia is a novel two-phase heat exchanger that integrates PCM within a vapor chamber. The entire solid structure of the prototype is manufactured additively using Maraging Steel with the Direct Laser Metal Sintering (DLMS) technique. The heat exchanger’s casing consists of multiple ultra-thin drawers loaded with bulk PCM at melting temperature of 25 ℃, functioning as thermal storage medium. The exterior surface of each drawer and the internal surfaces of the casing are also wrapped with screen mesh to serve as the wick structure to transport the chosen working fluid (i.e. Methanol) back to the heated surface at the evaporator using the capillary pumping pressure.Depending on the heat sink temperature and PCM melting temperature, the developed heat exchanger can either function as a thermal capacitor or a two-phase heat exchanger with thermal storage capability, that is, two modes of operation. During the spacecraft launching and landing stages where the heat sink temperature is higher than the PCM melting temperature and no heat can be dissipated at the condenser surface, the saturated vapor condenses on the PCM drawer’s surface and the excessive heat is stored within the bulk PCM in the form of latent heat of fusion until the heat sink temperature drops below the PCM melting point (e.g. orbiting stage). The heat stored within the PCM is then released back to the two-phase medium of the heat exchanger (i.e. vapor gaps between drawers) and the device turns to the normal heat exchanger mode.In the present study, the geometry and sizing of the PCM-loaded drawers enclosed inside the heat exchanger’s casing are optimized based on the mass ratio (the ratio of PCM mass to the total heat exchanger mass), additive manufacturing (AM) constraints, printing orientation, and total thermal resistances imposed on the system for either of the operation modes, heat exchanger mode or heat storage mode (dry mode). The AM printed prototype with its all components will also be represented along with the technical drawings and 3D CAD model developed in Creo. Furthermore, the wick structure design procedure will be discussed and its main performance characteristics (e.g. effective pore size, effective thermal conductivity, porosity, and permeability) will be reported to analytically examine the capillary pumping limit for the proposed heat exchanger. Lastly, the support structure model generated in Materialise Magics for metal printing of the heat exchanger will be exposed and discussed.

Additively Manufactured Two-Phase Heat Exchanger Integrating PCMs for Spacecraft Thermal Management

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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