Ji-Hyuk Im

Effect of Ambient Gas Density on Spray Characteristics of Swirling Liquid Sheets

The spray and breakup characteristics of a swirling liquid sheet were investigated by measuring the spray angle and breakup length as the axial Weber number We l was increased up to 1554 and the ambient gas pressure up to 4.0 MPa. As the We l and ambient gas density p increased, the disturbances on the annular liquid sheet surface were amplified by the increase of the aerodynamic forces, and thus the liquid sheet disintegrated from the injector exit. The measured spray angles according to the ambient gas density differed before and after the sheet broke up. Before the liquid sheet broke up, the spray angle was almost constant; however, once the liquid sheet started to break up, the spray angle decreased. As the ambient gas density and We l increased, the increasing aerodynamic force caused the breakup length to decrease. Finally, the measured breakup lengths according to the ambient gas density and We l were compared with the results of the linear instability theory. Considering the attenuation of sheet thickness in the linear instability theory, the corrected breakup length relation agreed well with our experimental results.

Comparative Study of Spray Characteristics of Gas-Centered and Liquid-Centered Swirl Coaxial Injectors

This study investigated the spray and Sauter mean diameter characteristics of gas-liquid swirl coaxial injectors by measuring and analyzing spray angles and mean drop sizes. Two different types of gas-liquid swirl coaxial injectors were designed and tested for this experiment: 1) a gas-centered swirl coaxial injector and 2) a liquid-centered swirl coaxial injector. The spray patterns were obtained for both types of injectors. The spray angles of these liquidcentered swirl coaxial injectors showed a decreasing trend as a function of the momentum flux ratio over the conditions studied. Good agreement was generally achieved between the predicted and measured spray angles. On the other hand, with increasing momentum flux ratio, the spray angle of the gas-centered swirl coaxial injector initially decreased when the momentum flux ratios were relatively low, but the trend reversed over the high momentum flux ratio range studied. The Sauter mean diameter data of these two types of injectors were also obtained using an image-processing method. Results showed that the Sauter mean diameter distribution of the liquid-centered swirl coaxial injector had a solid cone shape, whereas the gas-centered swirl coaxial injector exhibited two distinct Sauter mean diameter distribution regimes. The gas-centered swirl coaxial injector had a solid cone shape along the centerline, and it had a hollow cone shape at the periphery.

Effect of Recess on the Spray Characteristics of Liquid-Liquid Swirl Coaxial Injectors

The effects of recess in a liquid-liquid swirl coaxial injector on the spray characteristics have been investigated by measuring the spray angle and breakup length in eight cases of recess length. The orifice recess, a geometric parameter, has a strong influence on the spray and mixing characteristics by varying the interaction point between two liquid sheets. The variation of the recess length resulted in three different injection regimes: external, tip, and internal mixing injection. The coaxial spray characteristics in the external mixing injection regime are found to be mainly governed by the merging phenomenon and momentum balance between two liquid sheets. In the internal mixing injection regime, an impact wave is generated by the impingement of the inner spray on the outer liquid film inside the injector. The amplitude of the impact wave is gradually attenuated due to the fluid viscosity, flowing along the outer injector wall after impingement. This attenuation of the impact wave according to the recess length could affect the stability of a liquid sheet; therefore, the breakup length is found to increase with the increase in recess number.

The Effects of the Ambient Pressure on Self-Pulsation Characteristics of a Gas/Liquid Swirl Coaxial Injector

Self-pulsation gas strong influences on the atomization and mixing process and accompanies strong noises. A strong pressure and flow rate oscillation are detected when self-pulsation occurs. A sharp and narrow increase in frequency spectrum by self-pulsation is observed at a certain frequency. In this study, the effects of the ambient pressure on selfpulsation characteristics of a gas/liquid swirl coaxial injector are studied experimentally. A gas/liquid swirl coaxial injector is mounted in the high pressure chamber and the ambient pressure increased up to 1 MPa. The spray patterns were investigated according to the ambient pressure using digital camera and stroboscope. Also, acoustic and spray characteristics of self-pulsation are investigated with various injection and recess length conditions using acoustic device and laser system. From the results, it is found that the ambient pressure has great effects on self-pulsation and the mechanism of self-pulsation is suggested. Nomenclature do = inner orifice diameter Pc = chamber pressure Reg = axial gas Reynolds number Rel = axial liquid Reynolds number RR = recess ratio vg = axial gas velocity vl = axial liquid velocity ρg = gas density ρL = liquid density

SELF-PULSATION CHARACTERISTICS OF A SWIRL COAXIAL INJECTOR WITH VARIOUS INJECTION AND GEOMETRIC CONDITIONS

The spray and acoustic characteristics of a gas/liquid swirl coaxial injector are studied experimentally. The self-pulsation is defined as a pressure and flow rate oscillations by a time-delayed feedback between liquid and gas phase. Self-pulsation has strong influences on atomization and mixing processes and accompanies painful screams. So, the spray and acoustic characteristics are investigated. Spray patterns are observed by shadow photography technique in order to determine the onset of self-pulsation. And self-pulsation boundary with injection conditions and recess length is get. To measure the frequency of the spray oscillation, oscillation of the laser intensity which passes through spray is analyzed by Fast Fourier Transform. For acoustic tests, a PULSE System was used. Acoustic characteristics of a swirl coaxial injector are investigated according to the injection conditions, such as the pressure drop of the liquid and gas phase, and injector geometries, such as recess length and gap size between the inner and outer injector. From the experimental results, the increase of recess length leads to the rapid increase of the sound pressure level. And as the pressure drop of the liquid phase increases, the frequency of the self- pulsation shifts to the higher frequency. The frequency of spray oscillation is the same as that of the acoustic fields by self-pulsation.

THE CHARACTERISTICS OF SWIRL COAXIAL INJECTOR UNDER VARYING GEOMETRIC AND ENVIRONMENTAL CONDITIONS

It is known that the recess can augment mixing efficiency and flame stabilization through the internal mixing of propellants. So, various experiments, such as stroboscopic photography, PDPA and mechanical patternator, were performed at 8 cases of recess length to grasp its effect on the spray characteristics. In the emulsion injection region, spray angle and breakup length increase with increase of recess length, and which can be explained by the formation and decay of wave inside the recess region. As the recess length increases, the mean drop size increases due to the increase of effective film thickness and the decrease of spray angle. And the mixing efficiency increases with recess length, but in the case of very deep recess length, the mixing efficiency decreases. Also, spray characteristics in high-pressure environment were studied, and notable change of spray patterns are observed. Aerodynamic force of ambient gas significantly affects the breakup of a swirl spray, however, the breakup mechanism of liquid-liquid swirl coaxial spray is much controlled by the impact force by the interaction of propellants. Lastly, our experimental results according to the ambient gas pressure and injection velocity were compared with the linear instability theory. As a result, because attenuation of sheet thickness could affect the sheet breakup mechnism, it must be considered in the linear instability theory.

Spray Distribution Measurement at High Ambient Pressure

Abstract. Distribution of spray was measured. Optical Line Patternator (OLP) was used to measure theplanar distribution of the spray from a swirl-coaxial type injector. Ambient pressure was varied and injec-tion pressure was fixed in experiment. As ambient pressure increased, spray distribution was changed fromhallow cone to solid cone shape, and spray angle was decreased. Limitation in measuring dense spraywas found at high ambient pressure condition. Key Words : Spray(분무), Imaging Technique(이미지기법 ), High Ambient Pressure(고압환경 ), Extinction(감쇠), Mie Scattering(Mie 산란) 1. 서론 액체 연료를 사용하는 연소 추진 기관에 있어서 연료 분사는 매우 중요하다. 분사기를 통하여 분사된 분무의 액적 크기 및 유량 분포에 따라 연소기 내의 화염 발생 위치,연료 혼합 효율 등이 (3)변화하고 그 결과연소 특성이 변화하게 된다. 또한 액체 로켓 엔진의 경우 연료 액적의 (4)분포 상태가 연소불안정 현상에도 영향을 줄 수 있기 때문에, 연소기 내부의 분무 특성을계측하여 정량적으로 파악할 필요가 있다. 상압 조건에서 분무 단면의 분포 형상을 파악하기 위하여 분무장 내에 수집관을 삽입하여 직접 유량을 측정하는 기계적 패터네이터(mechanical patternator)와, Malvernparticle analyzer, PDPA 등의 광학 기법, 2차원 단면에서유량 분포를 빠르게 파악할 수 있는 광학 패터네이터(optical patternator), 광학 (1)토모그래피(optical tomography)등의 방법이 개발되었다. 액체 로켓 엔진의 경우 내부 압력이 높은 연소실 내에 액체 연료가 분사되는데, 이 경우 다른 조건이 모두동일한 경우에도 상압에서의 분무 특성과 많은 차이를보인다. 고압 조건에서의 분무 특성을 파악하기 위하여 압력을 유지할 수 있는 챔버가 필요하고, 이때 상압조건에서의 분무 계측에 비하여 실험 상 제약이 많아진다. 고압 상황에서의 분무 분포를 파악하기 위하여챔버 내에 기계적 패터네이터를 사용하는 방법