Systematic study to reduce the effects of cracks in multilayer insulation Part 1: theoretical model

Abstract

A theoretical model, the so-called enhanced black cavity model, has been developed by the author to explain the unexpectedly large heat flux through the crack (or slot) in a multilayer insulation (MLI) blanket of a cryogenic device (the increment of the heat load due to unit crack area was above a hundred times the heat flux of a MLI blanket without cracks). The crack in a blanket was initially considered as only a black cavity and the energy emitted from the external sources directly into the crack was estimated. The effects of the multi-reflection of the radiation flux outside the crack, and of the change of temperature distribution in a blanket caused by the crack were then added. For completeness it has also been indicated that there is a geometry factor which is a function of relative dimensions of the crack, and which also reduces the effectiveness of the black cavity. The enhanced model with our recent experimental data was used to show how to calculate the view factor, F, the radiant energy, Q S,b, the total enhancement factor, η, and the total increment of heat flux, Q S,T. Finally, based on the model, we predict some heat transfer phenomena in a MLI system with the crack and also suggest several methods to reduce the effects of the crack. These methods have been experimentally investigated and are reported in the associated paper.