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Technical Paper

Nanofluids as Working Media for Loop Heat Pipes

Nanofluids have been recently investigated as new working media for two-phase thermal control systems. However, conflicting reports have emerged, in which contradictory effects of the nanoparticles on the working performance of heat pipes have been described. Some studies have shown that gold or silver nanoparticles significantly improve heat transfer performances of heat pipes. Other studies have provided evidence that metal nanoparticles have no particular effect. This study is aimed at determining whether nanofluids are good candidates as heat carriers in a Loop Heat Pipe (LHP) system. Here, a nanofluid consisting of well-characterized citrate-stabilized gold nanoparticles in water is examined. The metallic nanoparticles are functionalized with citrate ligands in order to be soluble and stable in water at room temperature. An LHP hardware set-up was developed for this investigation.
Technical Paper

A Laboratory Setup for Observation of Loop Heat Pipe Characteristics

Heat pipes, loop heat pipes and capillary pumped loops are heat transfer devices driven by capillary forces with high-effectiveness & performance, offering high-reliability & flexibility in varying g-environments. They are suitable for spacecraft thermal control where the mass, volume, and power budgets are very limited. The Canadian Space Agency is developing loop heat pipe hardware aimed at understanding the thermal performance of two-phase heat transfer devices and in developing numerical simulation techniques using thermo-hydraulic mathematical models, to enable development of novel thermal control technologies. This loop heat pipe consists of a cylindrical evaporator, compensation chamber, condenser along with vapor and liquid lines, which can be easily assembled/disassembled for test purposes. This laboratory setup is especially designed to enable the visualization of fluid flow and phase change phenomena.
Technical Paper

A High Performance Miniature Loop Heat Pipe

This work presents the results of an experimental High Performance Miniature Loop Heat Pipe. The evaporator utilizes a wick structure with the non-inverted meniscus evaporation concept, which allows using high thermal conductivity materials for the evaporator case and capillary wick structure, and hence will further reduce the thermal resistance between the evaporator elements. The heat fluxes at the evaporator can therefore be significantly higher than that of a LHP using inverted meniscus evaporation approach. Tests were conducted in the Material and Thermal Laboratory at the Canadian Space Agency. The evaporator heat input cross-section area was 2.4 cm2. When water is used as the working liquid the heat transfer rate has reached values as high as 215W, corresponding to a heat flux density of 90W/cm2 (temperature drop between heat source and LHP evaporator was ∼7°C). Working temperature oscillations (with amplitude ∼2-10°C) were observed for steady state regimes of LHP operation.
Technical Paper

Passive Dynamically-Variable Thin-film Smart Radiator Device

This paper describes a new approach to spacecraft thermal control based on a passive thin-film smart radiator device (SRD) that employs a variable heat-transfer/emitter structure. The SRD employs an integrated thin-film structure based on V1-x-yMxNyOn that can be applied to existing Al thermal radiators. The SRD operates passively in response to changes in the temperature of the space structure. The V1-x-yMxNyOn exhibits a metal/insulator transition with temperature, varying from an IR transmissive insulating state at lower temperatures, to a semiconducting state at higher temperatures. Dopants, M and N, are employed to tailor the thermo-optic characteristics and the transition temperature of the passive SRD. The transition temperature can be preset over a wide range from below -30°C to above 68°C using suitable dopants. A proprietary SRD structure has been developed that facilitates emissivities below 0.2 to dark space at lower temperatures to reduce heater requirements.
Technical Paper

Vapour Compression Heat Pump for a Lunar Lander/Rover Thermal Control

The thermal control of lunar landers/rovers necessitates the use of a system to allow heat rejection to the high temperature lunar environment. In this context a vapour compression heat pump which is a proven technology in terrestrial and aeronautical applications has been studied; its suitability in providing 2 kW cooling capability with adequate temperature lift for final heat rejection by space radiators is assessed. The stringent requirements of space-based hardware in terms of temperature lift, compactness, mass, performance and reliability necessitates optimization studies. Mass optimization of the heat pump components has been carried out, as well as selection of refrigerants and thermodynamic cycles most suited for the application.
Technical Paper

Space-Based Heat Pumps for a Lunar Lander/Rover Thermal Control

The paper addresses the thermal control of a lunar lander/rover by use of heat pumps enabling payload heat to be rejected at a higher temperature to the lunar day environment. The heat pump technologies considered include absorption, vapor compression, adsorption, hybrid and chemical heat pumps technologies. A trade-off of the various heat pump technologies for a 2kW payload cooling capability is presented based on the needs of space-based hardware in terms of low mass and power, high performance, reliability and compactness of the systems. Finally the selection of a novel variant of the chemical heat pump concept is presented as a promising technology to be further investigated through breadboard development.