Abstract
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.
Highlights
For future space vehicles like the Ariane 6 rocket of the European Space Agency (ESA), re-ignition of the upper stage driven by cryogenic liquids will play a key role
We investigated the capacitance measurement process itself and how well the system can differentiate between single-phase boundaries and multiple phase boundaries as such effects are likely to occur in zero-gravity environments
Since the measurement system that will be described later has to be tested in cryogenic liquids, a cryostat is used as a test environment (Fig. 1a and b)
Summary
For future space vehicles like the Ariane 6 rocket of the European Space Agency (ESA), re-ignition of the upper stage driven by cryogenic liquids will play a key role. Gas bubbles can come into existence by sloshing, changes in gravitational forces, or external heat input If these bubbles enter the fuel lines of the turbopumps, the pumps can be destroyed by cavitation effects. Capacitive sensors promise a high potential to determine the position of the surface of a liquid phase (i.e. the position of the liquid-to-gas interface), but to be costefficient, simple, and to show fast reaction times (Carapelle and Colette, 2005; Medeova et al, 1998; De Kerpel et al, 2013) They neither disturb the liquid movement nor do they heat the liquid. We investigated the capacitance measurement process itself and how well the system can differentiate between single-phase boundaries (large gas bubble above a sea of LN2 as it is assumed in filllevel measurement) and multiple phase boundaries (stream of many small gas bubbles rising in a sea of LN2) as such effects are likely to occur in zero-gravity environments
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