Experimental investigation of stratification with liquid nitrogen in a large scale cryogenic tank demonstrator

Abstract

Increasingly complex modern space missions demand for flexible launchers particular considering future cryogenic upper stages. Extension of cryogenic storage times, reignition, propellant movement and evolving heat fluxes induce challenging scenarios of thermal stratification. Thus, identifying propellant thermal residua is complex. Moreover natural convection is strongly dependent on geometric tank scales. The need of experimental data with a large scale test tank is evident in order to predict upper stage stratification regimes precisely during accelerated mission phases. Subcontracted by Ariane Group in the framework of an ESA FLPP3 project three test campaigns with liquid nitrogen were performed. Thermal stratification within a chilled tank structure was examined in the second campaign using autogenous pressurization with nitrogen. Initial tank pressure and liquid fill level are altered as well as the type of pressurization (self-pressurization/external). As a test vessel the cryogenic tank demonstrator (CTD) with a volume of 717 l is equipped densely with sensor elements for temperature, pressure, fill level and mass flows. In addition, two movable sensor rods gauge the temperature evolution in both fluid phases. Steady boil-off data is used for extensive calibration of liquid wall and sensor rod temperature sensors. Stratification test cases are examined. The character of liquid free convection is fully turbulent with distinct effects on the s-shaped vertical temperature distribution in the thermally stratified region. The sidewall heat flux is identified to change intensively with vertical tank position due to double-walled or flange dominated tank wall sections. The tank structure in the lower bulkhead additionally also damps temperature rising in the liquid. As a result, thermal boundary conditions are significantly different comparing static boil-off with stratification. The resulting free convection flow pattern is of large complexity.