Influence of Noncondensable Gases on Thermodynamic Control On-Ground Experiments Using a Substitute Fluid

A cryogenic propellant submitted to heat load during long duration space missions tends to vaporize to such an extent that the resulting pressure rise must be controlled to prevent storage failure. The thermodynamic vent system (TVS), one of the possible control strategies, has been investigated using on-ground experiments with NOVEC₁₂₃₀ as substitution fluid. Results obtained for self-pressurization (SP) and TVS control phases have been reported in a previous work. The unexpected inverse thermal stratification observed during these experiments is analyzed in the present work and related to the influence of noncondensable gases. Noncondensable gases, present inside the tank in the form of nitrogen—ten times lighter than the substitution fluid vapor—generate a concentration stratification in the ullage. Assuming the NOVEC₁₂₃₀ remains at saturation in the whole ullage, the density stratification which results from this concentration stratification can explain the observed inverse thermal stratification.