Low-pressure air micro-plasmajets with a length of 10mm were generated through a nozzle with a diameter of 0.7mm at the pressures of 31.3 and 5.3kPa. Spectroscopic measurements were conducted at the point of 0.5mm from the nozzle exit on the central axis. The N2+ 1- and N2 2+ bands were predominant in the wavelength region of 280 to 420nm and the band shape was almost independent of the pressure. From temperature determination by a spectral matching method, it was found that the plasmajets were in a thermal nonequilibrium state and that vibrational temperature was much higher than rotational one. The experimental intensity distribution was reconstructed by the equilibrium radiation theory, in good agreement except for the N2 2+ bands with v′ ≥ 2 as in case of atmospheric air micro-plasmajet. For these bands, much better agreement was obtained by taking the effects of predissociation and non-Boltzmann rotational population distribution for N2C3Πu state into consideration in the theory. Discussions were made about potential cause of high vibrational temperature and slight difference between experimental and theoretical band shapes of the N2 2+ bands at the low-pressures.
To validate our proposed atomization theory, a series of microgravity experiments were conducted for a water jet issued into an otherwise quiescent atmosphere. The present paper reports findings from the case of liquid Weber number being nearly equal to unity. The water contained in a syringe is pushed by a piston which moves at a constant speed. The initial overshoot of liquid issue speed produces a long water column, whose length is reduced at every instant of subsequent disintegration interacting with the nozzle exit. The balance between tip contraction speed and liquid issue speed, which is attained by the selection of the Weber number, enables us to observe the disintegration process in detail. It is revealed that the short-wave breakup mechanism is characterized as the local destabilization which takes place at the short neck part bridging the tip bulb and the upstream high pressure crest part and that the neck maintains the nature of propagative capillary wave until a final stage of disintegration.
Strain and temperature measurement, especially in cryogenic environments, was studied using fiber Bragg grating (FBG) sensors for the purpose of the aerospace structural health monitoring. Although the relationship between the applied strain and the Bragg wavelength shift was the same as that at room temperature, the temperature-wavelength relationship became non-linear under cryogenic environment. In order to show the applicability of the sensor in aerospace applications, FBG strain and temperature sensors were embedded in a composite liquid hydrogen tank and measured in the cryogenic and pressurized environment. Encapsulated and small-size temperature sensors were used in this article and the temperature drift of the strain sensor was compensated by using the output of the temperature sensor. It was revealed throughout the experiment that the optical power loss could be critical in the case of existing large temperature difference. The practical solution for this issue was also discussed in this article.
Discharge current oscillation at the frequency of 10--100kHz causes serious problems in using an anode layer type Hall thruster in space. As a novel approach to the oscillation reduction, azimuthally nonuniform propellant flows were created in an acceleration channel. As a result, the small-amplitude oscillation region became significantly wide. Although the thrust efficiency decreased in exchange due to increase in electron current from the channel exit, the tradeoff curve between oscillation amplitude and thrust efficiency was improved by introduction of a new parameter ``differential flow rate.'' In this study, the small-amplitude oscillation region with 42--64mT width and the thrust efficiency 39%, which is roughly equivalent to the magnetic layer type thrusters with the same channel diameters, was achieved.