초음파 가진이 탄화수소계 연료의 연소특성에 미치는 영향

Abstract

This study was performed to analyze the combustion characteristics of storable hydrocarbon-fuels with ultrasonic wave energy. The demand for launching satellites is rapidly increasing due to the decreasing cost of space launch vehicles. In particular, the demand for small satellites is steadily rising because of the reduced development and launch costs, as well as the short lead time. Small satellites are primarily operated in Low Earth Orbit (LEO), which has the disadvantage of short mission duration due to perturbations such as gravity and atmospheric drag. To make up for these disadvantage, propulsion system is required for orbit transition and attitude control. Among them, chemical propulsion systems that use storable fuels are suitable as fuels for small satellites due to their high thrust performance and high energy density per unit volume. But, the characteristic of storability requires sufficient phase change energy for combustion. This can cause combustion instability, such as ignition delay, which can degrade the performance of thruster. However, ultrasonic atomization can be used to create droplets that can be burned immediately. In addition, when ultrasonic energy is excited to the fuel, the combustion efficiency can be increased due to the cavitation effect, such as the collapse of the hydrocarbon chain structure. To take advantages of this effect, an experiment was conducted to explicate the behavior of an ultrasonically-atomized kerosene lifted-flame in accord with position of ultrasonic standing wave (USW) and the carrier gas flow-rate. The combustion region was visualized by using DSLR, ICCD camera and Schlieren photography with the high-speed camera. As a result, when the flame was exposed to ultrasonic standing wave, it was confined within the boundary region of the standing-wave field, and its OH radical (OH*) emission field was intensified. Flame chopping by USW could be also observed in detail through the distribution of OH*. And in order to determine the effect of ultrasonic-wave excitation time on the combustion characteristics of kerosene, an experiment was conducted. The fuel, kerosene and oxygen were injected through a coaxial shear injector and burned in a model combustion chamber. The ultrasonic-wave excitation time(te) was set to 0, 30, and 60 seconds, and the composition of the fuel was analyzed by gas chromatography mass spectrometry (GC/MS). The OH* intensity was then acquired by ICCD camera. As a result, the temperature of the fuel was higher with the increase of te, and the hydrocarbon chain structure of high molecular composition was decomposed into low carbon numbers composition and free radicals by the cavitation effect due to the ultrasonic-wave excitation energy. As a consequence, OH* emission intensity increased with te.