Calculation and analysis of optimal layer density for variable density multilayer insulation based on layer by layer model and Lockheed equation

Abstract

The extremely high energy density and carbon-free characteristics of liquid hydrogen (LH2) make it suitable for replacing liquefied natural gas (LNG) as a future energy commodity. However, the extremely low evaporation temperature of LH2 results in high heat leakage. Multilayer insulation (MLI), which is the most thermally efficient insulation system known, is widely used in the fields of hydrogen energy, aerospace, and other cryogenic fields. Variable density multilayer insulation (VDMLI), which has emerged in the last two decades, has improved the performance of MLI by adding spacers to change the density of MLI. However, there has been no significant progress in the optimization theory for VDMLI. In this paper, based on the layer by layer model and the Lockheed equation, a generalized optimal layer density calculation model for VDMLI is established by considering the influences of spacers, compressive pressure, gas pressure, etc. The results indicate that compared to general VDMLI and MLI on liquid hydrogen tanks, the optimal layer density VDMLI (OVDMLI) can reduce heat leakage by 21.1% and 25.3%, respectively. After comparison and analysis, the characteristics and reasons for temperature and heat transfer conductivity profiles of MLI, VDMLI, and OVDMLI are pointed out. The effect of the spacer number on heat flux and spacer profile is analyzed. Additionally, gas pressure profile and heat transfer are investigated with consideration given to the vacuum degree at both boundaries.