Natural Convection Effects in Insulation Systems of Cryogenic Storage Tanks

Large scale cryogenic tanks are used for storage and transportation of liquid hydrogen (LH2) and liquefied natural gas (LNG). The typical boil-off rate (BOR) in these tanks is 0.05 to 0.3% per day based on the tank volume, and BOR control is an important factor in the design of these tanks. In order to minimize the BOR rate due to heat ingress from ambient, these tanks are covered with an insulation system that is usually porous and of low density, where void spaces are filled with a gas such as nitrogen or hydrogen. While vacuum-based insulation is usually practiced for small-scale applications, it is not economical for large-scale storage tanks. As the interstitial gas is at ambient pressure for non-vacuum applications, the presence of natural convection due to unstable stratification of gas-density in the porous insulation can enhance the heat ingress and increase the BOR rate.

This study examines the impact of natural convection in insulation systems of storage tanks. We solve the governing equations (continuity, momentum and energy balances) for porous insulation with variation in local density, viscosity and conductivity with temperature to determine the critical Rayleigh number for the onset of natural convection. It was found that depending on the Rayleigh number, there may be a single or multiple convective cells, and local Nusselt number (or the ratio of the heat flux with and without convection in the insulation material) can be an order of magnitude larger, and must be considered carefully in the design of these systems.