Effect of Isogrid-Type Obstructions on Thermal Stratification in Upper-Stage Rocket Propellant Tanks

Analytical models for propellant thermal stratification are typically based on smooth wall flow correlations. However, many propellant tank walls have a mass-saving isogrid, which alters the boundary layer. This work investigates the boundary-layer behavior over walls with obstruction elements that are representative of isogrid or internal stiffener rings. The experimental studies reveal that the thickness of the velocity boundary layer over an isogrid wall is more than 200% thicker than a smooth wall at full-scale upper-stage tank Reynolds numbers. For buoyancy-induced free convection flows, the computational-fluid-dynamics models demonstrate that the velocity boundary layer over a wall lined with obstruction elements may be thicker or thinner than the equivalent boundary layer over a smooth wall, whereas the thermal boundary layer is always thicker for the rough wall. A Rayleigh number scaling analysis is presented for a range of fluids, tank and obstruction sizes, heat loads, and acceleration levels. When the results are applied to a full-scale liquid-hydrogen tank with obstruction elements, the stratification layer is 18% thicker, and the stratum fluid is 31% warmer than the corresponding results for the smooth wall tank. The increase is attributable to the augmented heat transfer area and enhanced mixing of fluid due to the obstruction element.