Thermal Performance Of Various Multilayer Insulation Systems Below 80k

Author : Wesley Louis Johnson
Publisher :
ISBN 13 :
Total Pages : 85 pages
Book Rating : 4.:/5 (74 download)

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Download or read book Thermal Performance of Cryogenic Multilayer Insulation at Various Layer Spacings written by Wesley Louis Johnson and published by . This book was released on 2010 with total page 85 pages. Available in PDF, EPUB and Kindle. Book excerpt: Multilayer insulation (MLI) has been shown to be the best performing cryogenic insulation system at high vacuum (less than 10−3 torr), and is widely used on spaceflight vehicles. Over the past 50 years, many numerous investigations of MLI have yielded a general understanding of the many variables associated with MLI. MLI has been shown to be a function of variables such as warm boundary temperature, the number of reflector layers, and the spacer material in between reflectors, the interstitial gas pressure and the interstitial gas. Because conduction between reflectors increases with the thickness of the spacer material, and yet the radiation heat transfer is inversely proportional to the number of layers, it stands to reason that the thermal performance of MLI is a function of the number of layers per thickness, or layer density. Empirical equations that were derived based on some of the early tests showed that the conduction term was proportional to the layer density to a power. This power depended on the material combination and was determined by empirical test data. Many authors have graphically shown such optimal layer density, but none have provided any data at such low densities, or any method of determining this density. Keller, Cunnington, and Glassford showed MLI thermal performance as a function of layer density of high layer densities, but they didn't show a minimal layer density or any data below the supposed optimal layer density. However, it was recently discovered by the author that by manipulating the derived empirical equations and taking a derivative with respect to layer density, a solution for on optimal layer density may be obtained. Several manufacturers have begun manufacturing MLI at densities below the analytical optimal density. This trend is apparently based on the theory that increased distance between layers lowers the conductive heat transfer and that there are no limitations on volume. By modifying the circumference of these blankets, the layer density can easily be varied. The most direct method of determining the thermal performance of MLI at cryogenic temperature is by evaporation (or "boil-off") calorimetry. Several blankets were procured and tested at various layer densities by the Cryogenics Test Laboratory at NASA Kennedy Space Center. The blankets were tested over a wide range of layer densities including the analytical minimum. Several of the blankets were tested at the same insulation thickness while changing the layer density (thus a different number of reflector layers). Heat transfer optimization of the layer density of multilayer insulation systems would remove the variable of layer density from the complex method of designing such insulation systems. Since the layer density is one of the variables that in those complex equations that require more experience to understand fully grasp, this significantly simplifies the blanket design process. Additional testing was performed at various warm boundary temperatures and pressures. The testing and analysis was performed to determine thermal performance data and to simplify the analysis of cryogenic thermal insulation systems.