Examining Liquid Hydrogen Wettability Using Neutron Imaging

Abstract

The control of propellant boiloff is essential in longterm space missions. However, a clear understanding of propellant cryogenic condensation/evaporation in microgravity is lacking. One of the key factors in designing such systems is the location of liquid surfaces and the relation to wettability. The BT2 Neutron Imaging Facility located at the National Institute of Standards and Technology (NIST), Gaithersburg, MD, is used to image evaporation and condensation of hydrogenated propellants inside of an aluminum 6061 container. Liquid hydrogen has larger neutron crosssection area than the aluminum, allowing the visualization of the liquidvapor interface. The test cell has a conical section that enables determination of a contact angle with enhanced accuracy. If the contact angle is equal to the angle of the cone, a flat liquidvapor interface is expected. The test cell has the cone angle of 10o and a flat interface was not observed. Using the LaplaceYoung equation to fit the interface, the contact angle for hydrogen and aluminum was between 0آ° and 4آ°. The theoretical Laplace curves with contact angles of 2o and 10o are plotted on the liquidvapor interface. The of 2o curve is a closer fit as compared to the 10o curve. The uncertainty arises from resolution limits of the neutron imaging setup and edge detection. More details on the neutron imaging mechanism and relevant physics can be found from the authors' other publication of Cryogenics, 74, pp131137, 2016: doi:10.1016/j.cryogenics.2015.10.016.