Articles | Volume 6, issue 1
J. Sens. Sens. Syst., 6, 135–143, 2017

Special issue: Sensors and Measurement Systems 2016

J. Sens. Sens. Syst., 6, 135–143, 2017

Regular research article 02 Mar 2017

Regular research article | 02 Mar 2017

Capacitive gas-phase detection in liquid nitrogen

Christoph Kandlbinder1, Alice Fischerauer1, Mario Mösch1, Tobias Helling1, Gerhard Fischerauer1, and Martin Siegl2 Christoph Kandlbinder et al.
  • 1Chair of Measurement and Control Systems, Center of Energy Technology (ZET), University of Bayreuth, 95440 Bayreuth, Germany
  • 2Institute of Space Systems, German Aerospace Center (DLR), 28359 Bremen, Germany

Abstract. The main and upper stages of heavy lift launchers for space applications are often fuelled by cryogenic liquids. In order to enable the re-ignition of a cryogenic upper stage for orbital changes, it is crucial to study the behaviour of these fluids in microgravity. As gaseous bubbles entering the fuel lines of the engine can cause the destruction of the engine, these bubbles are a risk for the functionality of the re-ignition mode. To measure an evolving gaseous phase and its volume, a capacitive measurement system for two-phase mixtures was realised. Its electrodes are arranged in such a way that phase changes inside a vessel can be detected without parasitic heating under cryogenic conditions. Two cases have been investigated: a fill-level measurement involving a large gas bubble above a homogenous liquid on the one hand, and the identification of a bubble stream inside a liquid on the other hand. The system concept was tested in a cryogenic environment allowing the controlled generation of bubble streams inside liquid nitrogen and of a contiguous gaseous volume above the liquid. The characteristics of the measurable capacitances of different pairs of electrodes were experimentally determined and compared with finite-element simulations (Ansys). In addition, the electrical flux density was computed to corroborate the simulated capacitance curves with theoretical statements. The experimental findings closely agree with the simulated results if possible disturbances due to the characteristics of the capacitance measurement hardware are properly taken into account. Thus, by measuring various capacitances, it was possible to determine the level up to which a receptacle inside a liquid-nitrogen tank was filled with liquid (the space above the liquid being taken up by gaseous nitrogen), to identify the existence of a bubble stream in the liquid nitrogen and to demonstrate that the capacitance measurement results enable one to differentiate between the two cases.

Short summary
In this work we present a cost- and energy-efficient measurement system for the spatial detection of gas phases in liquid fluids with a low permittivity value. We showed that we can simulate the system and its environment and use the calculated results to interpret the results originating from measurements of the electrical capacitance between different electrodes. The proposed system can be modified for, e.g., observation of fluid behaviour in cryogenic tanks for reigniteable space propulsion.