Dependence of multi-layer insulation thermal performance on interstitial gas pressure

Abstract

Four examples of multi-layer insulation (MLI) blankets, differing in layer density, thickness, and spacer type, were evaluated using the Cryostat-100 fixture, a cylindrical calorimeter, in the Cryogenic Test Laboratory at Kennedy Space Center. The steady state thermal performance of each was measured at pressures ranging from high vacuum (down to 10−6 Torr) up to 1 atmosphere. The four heat flux versus pressure data sets were reduced to a single "universal curve", demonstrating the essential parameters that determine how the performance of a generic blanket depends on interstitial gas pressure. A simple phenomenological model based on molecular collision probabilities is followed by a systematic curve fitting procedure encompassing the entire pressure range. The final result is a closed-form expression for the pressure-dependent heat flux that can be readily generalized to arbitrary thermal boundary temperatures, gas species, and MLI blanket thickness and layer density.