A perforated MLI blanket system is targeted for a number of large-scale cryogenic facilities. Space applications and particle accelerators are two fields concerned with the thermal shielding of devices used at cryogenic temperatures. Because radiation heat transfer varies with T⁴ insulation, systems operating between the boundary temperatures of 300 and 77 kelvin (K) are of prime importance. The heat transfer in this range is the dominant portion of the total heat transfer, even for devices operating with a cold boundary temperature as low as 2 K. Systems operating under conditions of degraded vacuum levels are also a key consideration because of heat transfer by residual gas conduction.

An experimental study of a perforated MLI blanket was performed in 2001. The blanket, manufactured by Jehier (Chemille, France), is a cryogenic thermal insulation system composed of perforated double-aluminized Mylar separated by polyester net spacers. Two blankets of 15 layers each were installed on the cylindrical cold mass of the research cryostat as shown in figure 1. The total thickness of the 30 layers was 7 millimeters (mm). Complete characterization of the blanket thermal performance between 300 and 77 K using a steady-state liquid nitrogen evaporation method was produced using Cryostat-1.

A summary graph of the variation of apparent thermal conductivity (k-value) with cold vacuum pressure (CVP) is given in figure 2. The k-value at the high vacuum condition of 0.01 millitorr was measured to be 0.029 milliwatt per meter-kelvin (mW/m-K). (The equivalent R-value per inch is 5,000.) Comparison of experimental data for other types of MLI systems is also presented. Layer temperature profiles for the different vacuum levels are presented in figure 3.

The testing was performed at the Cryogenics Test Laboratory of NASA Kennedy Space Center as part of a comparative study of cryogenic vacuum insulation systems. Basic heat transfer questions and practical methods of application regarding the performance of high-performance MLI blankets continue to be addressed. A target is to develop thermal insulation systems that can operate effectively for large-scale, energy-efficient industrial application on Earth and in space.

Contact: J.E. Fesmire (James.Fesmire-1@ksc.nasa.gov), YA-C2, (321) 867-7557
Participating Organizations: Dynacs Inc. (Dr. S.D. Augustynowicz) and Fermi National Accelerator Laboratory (C. Darve)