Measurement of Apparent Thermal Conductivity of Multilayer Insulation

Abstract

There are many system in which producing and maintaining low temperature is the primary function. These system require cryogenic temperature for the operations. The National Bureau of Standards have chosen to consider the field of cryogenics as the involving temperature below -150°C (126K). Heat leakage is the common problem involving cryogenics applications. Sometimes it is undesirable and should be maintained at minimum level. At low temperature it becomes more difficult to remove given amount of heat and discharge it at ambient temperature. These unwanted heat leakage accounts into economical burden and acts as barrier in cryogenics applications. Therefore the cryogenics vessels and transfer lines are insulated with different types of Multi Layer Insulations (MLI) which are very effective.

MLI consists of alternate layers of high reflective shields or foils and separated by low thermal conductivity spacers. One of the most effective cryogenics insulations (MLI) involves high vacuum. It is known that in high vacuum radiation plays a major role because gas conduction and convection are negligible. In order to improve mechanical strength and ease of application, Plastic materials like Mylar and Fibre glass are coated with Aluminium foil. The spacers for MLI are made of high resistive material. For the estimation of heat transfer, Apparent Thermal Conductivity must be known. Due to unpredictable changes in parameters such as uniform contact pressure and interstitial pressure, accurate theoretical performance of MLI is very difficult. Thus an experimental investigation has been carried out on a few indigenous MLI materials like Fibre glass cloth, R P Tissue, Nylon net etc. For that, A cylindrical boil o calorimeter has been developed and standardized for testing of thermal performance of insulation. It's measurement principle for determining heat flux (Q) and Apparent Thermal Conductivity (KA) of a test specimen at fixed conditions. The heat transferred to the test vessel results in the formation of nitrogen vapor. From the amount of liquid nitrogen boil o rate, the heat transfer rate and subsequently The Apparent Thermal Conductivity of MLI is estimated. The present work is to develop optimum combination of shield and spacer from available materials ( Aluminum foil-Fibre glass cloth, Aluminium foil-R P Tissue, Aluminum foil-Nylon net) by experimental investigation of apparent thermal conductivity. The present work also includes the experimental investigation for apparent thermal composite materials from these different combinations of MLI.