Capacitive gas-phase detection in liquid nitrogen

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.

Research on optimization method of capacitance tomography based on contribution degree analysis

“…The mathematical expression of the capacitance residual CR is shown in formula (4), where G t is the gray value matrix corresponding to the original flow pattern; C t is the measured capacitance value, and formula (2) holds when the 'soft field' effect and grid division error are ignored. C l in formula (3) is the measured capacitance value of the empty tube; C h is the measured capacitance value of the full tube [12].…”

“…C l in formula (3) is the measured capacitance value of the empty tube; C h is the measured capacitance value of the full tube [12]. Formula ( 5) is the expansion of formula (4), where c mn represents the measured capacitance value obtained with electrode m excited and electrode n grounded; s mni is the sensitivity of the ith pixel of the electrode pair m-n, whose value ranges from positive to negative value. In formula (5), s mn is the sensitivity field of the electrode pair m-n, which is the row vector of the sensitivity matrix S:…”

“…It has the advantages of no radiation, high time resolution, simple structure, and adaptation to various complex industrial environments. Therefore, it has a broad prospect for application in industrial processes such as petroleum, chemical, pharmaceutical, electric power, and metallurgy [1][2][3][4][5].…”