Cryogenic propellant handling and combustion: I. Neutron-imaging phase change experiments with LH2 and LCH4; II. Combustion of liquid oxygen droplets in hydrogen in microgravity
Time: Thu 2022-06-02 10.30 - 11.30
Location: Faxén, Teknikringen 8
Participating: James Hermanson (University of Washington)
Abstract. Predicting the evaporation and storage behavior of cryogenic liquids poses a challenge for both terrestrial energy infrastructure and long term space missions. The current understanding of cryogenic phase change and subsequent boil-off is limited, in part, because the values of accommodation coefficients (inputs to phase change models) are still lacking and the experimental data are severely limited. In order to determine these accommodation coefficients, a new cryogenic experiment was developed. Tests were conducted at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland by introducing propellant vapor into cylindrical test cells placed inside a cryostat. Phase change was induced through precise control of pressure and/or temperature and neutron imaging was used to visualize the liquid and evaporation/condensation rates. Numerical modeling, combined with temperature measurements, was used to determine the heat flux. The accommodation coefficients appear to be most strongly dependent on the saturation pressure. Analytical modeling indicates significant variations in the evaporation rate along the liquid interface.
To investigate the basic combustion processes in a cryogenic rocket engine, the combustion of individual liquid oxygen droplets in a hydrogen atmosphere under microgravity conditions is experimentally and numerically investigated. An experimental setup is used in the ZARM (Center of Applied Space Technology and Microgravity) drop tower in Bremen, Germany. In a combustion chamber cooled with liquid nitrogen, a single oxygen droplet is produced on a quartz suspender and ignited with a laser-induced plasma spark. The subsequent combustion is investigated with shadowgraph imaging and OH-chemiluminescence. As the pressure increases, the burning rate increases, but the droplet-to-flame diameter ratio decreases slightly. The condensation or freezing of water vapor outside the reaction zone and near the droplet surface is investigated for different pressures. Numerical simulations predict the emergence of three, distinct flame regions during the ignition transient; two premixed flames and a central, diffusion flame. For the quasi-steady diffusion flame, the numerical results point to condensation/freezing outside the reaction zone, qualitatively confirming the experimental results.