Observation of bubbles inside cryogenic liquids using capacitive multi-electrode sensors

The investigation of fluid phenomena is of high interest in the management of cryogenic liquid propellants used in heavy-lift launchers. In order to efficiently design orbital cryogenic propellant storage facilities or cryogenic propulsion systems experiments concerning fill-levels, bubble formations and boiling have to be conducted in microgravity and on earth. Thus measurement devices are needed that can give insight into the spatial distribution and the behavior of the fluids inside the cryogenic system. One possibility to observe liquids inside a given volume without intruding into the system and thereby disturbing the flow characteristics is to use a capacitive system with electrodes embedded into the walls of the vessel. In this paper experiments with such a system at cryogenic temperatures are presented. To simulate the behavior of rocket fuel in space a cryostat was filled with liquid nitrogen and by way of controlling the pressure inside or locally heating the liquid bubbles were created. Two capacitive measurement systems have been submerged into the cryogenic liquid to monitor the location and size of bubbles inside the liquid and to observe the state of the liquid/gas interface. One system consisted of a cylindrical polycarbonate ring with four embedded electrodes and a polycarbonate lid at the top carrying one ring-shaped electrode. Many small bubbles thus could be trapped and coalesced in this half closed cylinder (bubble trap) forming a big single gas volume. The other system was a polycarbonate ring with 16 electrodes arranged along its circumference. The task of these systems was to measure the filling level inside the cylindrical bubble trap and to detect and quantify rising bubbles inside the cryostat by measuring the mutual capacitances of selected electrode pairs. To interpret the measurement results, FEM simulation results were used to generate a characteristic curve for the relationship between filling level and measured capacitances. To validate the results, two cameras were mounted inside the cryostat which allowed the simultaneous recording of the filling level and the existence of bubbles and bubble streams. We observed a good agreement between measurement and simulation. Overall the suitability of the system for cryogenic applications has been successfully demonstrated