Monte Carlo simulations of residual gas pumping out of multi-layer insulation

Abstract

In multi-layer insulation (MLI) for space or terrestrial cryogenic applications, large densities of gas molecules (particles) between individual layers reduce the thermal performance of the insulation because particle collisions transfer heat. This poses an incentive to provide pathways for particles to escape the system quickly, such as perforation.This paper uses two quality metrics for an MLI-setup: the perforation escape ratio (PER), defined as the ratio of the number of particles escaping through the layers (broadside pumping) to the total number of outgassed particles, and the pressure distribution across the layers.The present stationary Monte Carlo simulations investigate the outgassing behaviour of MLI-setups at the end of pumping, where an equilibrium state is approached and molecular flow is assumed. Some of the findings of the “Lockheed report” (Keller et al., 1974) [1] are reproduced, and the two aforementioned quality metrics are analysed for different cases. We find that the residual pressure decreases and the parameter PER increases as (a) the perforated area increases, (b) the perforation size decreases, and (c) the number of layers decreases. Both the pressure and the parameter PER increase as (a) the layer size increases, (b) the distance between layers decreases, and (c) the porosity of the edges decreases.