JET A1-산소 동축류 확산화염에서 산화제 조성 변화가 연소특성에 미치는 영향에 관한 실험적 연구

Abstract

In the present study, JET A1/oxygen coaxial diffusion flames were investigated to provide fundamental knowledge that is crucial to understand fires and explosion characteristics for the Korea Space launch vehicle II (KSLV-II). To this end, the experimental conditions were carefully selected to mimic atmospheric conditions similar to those inside the first stage of KSLV-II considering the characteristics of the propellant (oxygen-nitrogen /oxygen-helium) supplied to a main combustor (engine). Atmospheric parameter adjustments including the molar oxygen concentration and inert dilution in the coaxial flow, fuel mass flow and coaxial flow temperature have been made to qualitatively investigate the characteristics of ignition energy, flame length and temperature, tripple flame propagation speed, and soot particles formation in JET A-1 diffusion flames. The inert gas supplied to the coaxial flow was found to influence ignitability and triple flame spread rate. For the helium-diluted flames, the measured minimum ignition energy and triple flame spread rate greatly increased compared to the nitrogen-diluted flames due to the thermal diffusion characteristics of helium gas. Experimental results also clearly indicated that the measured flame lengths were proportional to the JET A1 fuel mass flow rate as well as increases in the temperature of the oxidizer stream. Moreover, increases in the inert gas concentration in the oxidizer stream resulted in stretching the flame. In particular, the flame was further elongated in the oxidizer steam diluted with nitrogen gas. Inert dilution in the oxidizer stream that can adjust the viscous drag and flow-induced momentum was believed to be one of important mechanism to affect the length of coaxial diffusion flames. Significant increases in soot volume fraction within JET A1/oxygen coaxial diffusion flames were observed with inert dilution in the co-axial flow. The experimental measurements demonstrate that nitrogen-diluted flames produced the higher soot volume fraction than the helium-diluted flames. The coupled analysis of the flame temperature and the residence time (affected by the flame length) provides correlation with the observed variations in sooting in JET A1/oxygen coaxial diffusion flames. As the flame temperature with molar oxygen concentration increased, the measured soot volume fraction initially increased and then began to decrease after reaching a maximum value. Experimental results clearly indicate that the flame temperature can control the sooting behavior for JET A1/oxygen coaxial diffusion flames by mediating the competition among the fuel pyrolysis reactions, fragmentation of formed soot precursors, and oxidation of soot precursors and particles which eventually result in an interesting non-monotonic sooting behavior at elevated oxygen concentration.