Fluid and Heat Transfer Characteristics of a Downward-facing Condensing Film Exposed to a Cyclic Environment in Regular and Micro-Gravity Conditions
- Paper number
IAC-04-J.2.08
- Author
Mr. Sanjoy Som, United States
- Year
2004
- Abstract
Spacecrafts exploring inner or exterior planets encounter a very different thermal environment from near-Earth orbit. A wide variety of thermal control materials or devices have been used to achieve the appropriate temperature of the spacecraft for all such mission phases. However, to solve the problem of complex and variegated thermal designs of the spacecraft, it is desired to develop new functional thermal control materials. La1-xSrxMnO3 is one of the most attractive materials as such new thermal control materials. It is a manganese oxide with a perovskite-type chemical structure, that can be obtained from LaMnO3 by substitution of La3+ ions with Sr2+ ones. By choosing an appropriate molar ratio of Sr2+, they show notable changes of the radiative and optical properties according to the metal-insulator transition near room temperature. Namely, the infrared reflectance remains low (high emissivity) above the transition temperature due to insulator-like behavior; on the other hand, it becomes high (low emissivity) below the temperature because of metallic behavior. However, such changes and spectral characteristics are determined by its chemical structure. In other words, the optical properties are inherently fixed unless the molar ratio of Sr2+ changes. In recent years, the surface microstructured gratings are strenuously studied to control the optical properties of optoelectronic materials, photonic materials, selective emitters for thermophotovoltaics and so on. Their optical properties are numerically calculated based on the rigorous coupled-wave analysis (RCWA). RCWA method deal with general 3D gratings diffractions and is a powerful technique to investigate surface structures with desired spectral features. The purpose of this study has been focused on the investigation of the surface structure for improving their radiative properties. Numerical calculations have been performed using RCWA method and measured optical constants of La1-xSrxMnO3 including temperature dependence. The investigated surface gratings are required the spectral selectivity which decrease the reflectance at around 10 µm leading to effective increase of the emisivity and maintain the large changes of the infrared spectral reflectance due to metal-insulator transition. As a first step, we carried out numerical calculations changing the following parameters of periodic gratings: periodicity Λ, aperture a, and depth d. The surface structure calculated was restricted to rectangular cavities. Some calculated spectra with different structural parameters were compared and evaluated in this study. We presents the optical properties of La1-xSrxMnO3 and calculation results of the spectral selectivity with the surface microstructured gratings.
- Abstract document
- Manuscript document
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