액체금속을 이용한 우주용 가변 열전도 라디에이터에 관한 연구

Abstract

In spacecraft design, the function of the thermal control subsystem(TCS) is to keep all the spacecraft’s component systems within acceptable temperature ranges during all mission phases. It must cope with the external environment, which can vary in a wide range as the spacecraft is exposed to deep space or direct sunlight, sunlight reflected off Earth (albedo), and infra-red(IR) energy emitted from Earth, and the internal heat generated by the operation of the spacecraft itself. During launch or in exceptionally low orbits, there is also a free molecular heating effect caused by friction in the rarefied upper atmosphere. In this environment, The thermal control subsystem protects the equipment from overheating, either by thermal insulation from external heat fluxes, or by proper heat removal from internal sources and the equipment from temperatures that are too cold, by thermal insulation from external sinks, by enhanced heat absorption from external sources, or heat release from internal sources. Today’s satellites have been getting high function and performance. This leads to an ever-larger heat produced by the payloads in the spacecraft, which will result in a shortened life, malfunction or even failure of satellite and these developments demands many requirements in order to realize maximum performance. For example, TCS is necessary to keep specific components (such as optical sensors, batteries, atomic clocks, etc.) within a specified temperature stability requirement, to ensure that they perform as efficiently as possible. Furthermore, the recently satellites are getting smaller that led to increase of extremely high heat flux density and serous heat loads. It request on cooling beyond what can be offered by the common thermal control method and alternative ways are being required and implemented gradually at this time. Therefore, satellites need more effective thermal control technologies for both survive in harsh space thermal environment and guarantee high performance that shows within a temperature stability. Recently, it is realized that using a liquid metal or its alloys with a low melting point as coolant could significantly lower the electric component temperature. The thermal conductivity of a metal is much higher than that of general liquid such as water, oil or more organic fluids. Thus, if a certain liquid metal or its alloys with a low melting point is adopted as the cooling fluid, a much higher cooling capacity than that of traditional fluid can be obtained. The liquid metal cooling is expected to open a new world for spacecraft cooling because of its evident merits over traditional coolant. Also, because of its high conductivity, a liquid metal can be driven by an MHD(Magneto-hydro-dynamic) pump which exploits the Ampere force arising from the application of direct current normal to the magnetic filed in the region of the fluid between two permanent magnets. Advantages of adopting an MHD pump lies in the ability toe pump these liquid efficiently with a silent, vibration-free, low energy consumption rate of the non-moving and compact MHD pump. In this study, we investigate the possibility of using a liquid metal as coolant to enhance a efficiency of space thermal control. For this, we proposed a variable conductance radiator(VCR). VCR provide a variable thermal resistance between the heat source and radiator and can thereby maintain the heat sources at a relatively constant temperature while the radiator temperature varies. Conventional radiator usually provide a single thermal emissivity and conductance. Unfortunately, though, variations in environment and component heat-generation rates, along with the degradation of surface finishes over time, can drive temperature variations in a passive design to ranges larger than some components can withstand. Heaters therefore are sometimes required in a thermal design to protect components under cold case environmental conditions or to make up for heat that is not dissipated when an electronic box is turned off. When VCR is applied the spacecraft, VCR makes it possible the effective thermal control by optimizing the conductance according to the temperature condition. VCR is possible to change conductance that exploits the highly thermo-physical properties of liquid metals with very high heat transfer coefficients to adopted in a wide range of temperature. In this case of hot case, panel of VCR is filled with liquid metal inside, it is transferring the heat from the payload to radiator surface. In this case of cold case, panel of VCR is vacant with evacuate, it is insulating from the cold case environmental condition. We can use two type of thermal management in hot and cold condition by moving the liquid metal. This new method is expected to be flexible thermal management of a space radiator, where both efficient thermal control and extremely low energy consumption are of major concern. In this paper, we have introduced the VCR that proposed a novel concept of radiator for spacecrafts and have fabricated a laboratory-scale test section of the VCR to demonstrate effectiveness of performance that change for heat conductivity and verify validation of thermal control. Additionally, we have developed a practical development model of VCR and verified efficiency by comparing to a conventional radiator from thermal analysis.