Jaye Koo

Analysis of signal attenuation for quantification of a planar imaging technique

An optical patternator can measure the fuel mass distribution of a spray using laser induced fluorescence (LIF) images obtained from a planar imaging technique. In the dense spray region, however, attenuation by scattering from particles in the path of the detector is so significant that the true spray pattern might be altered into a false distribution. Therefore, the method to find the geometric mean value of the intensities obtained from two cameras was evaluated and verified in a solid-cone spray under the assumption that the optical depth is constant throughout the spray. The variation of the averaged optical depth detected by the two cameras was negligible in the present study, so that the deviation of signal attenuation was within ±5% of the average value. The measurement of the mass distribution at Z = 25 mm using a single camera was estimated to have a large range of signal attenuation from 0 to 17%, while the method using two cameras has a small range of signal attenuation from 6.5% to 12.5%. Therefore, the effect of signal attenuation can be dealt with by the two-camera method. This method was also applied to measure the size distribution by taking the ratio of fluorescence to Mie-scattering signals. It was found that Mie-scattering and LIF signals were attenuated at a similar rate within the range of the signal wavelength used in this measurement. Hence, the effect of signal attenuation on the size measurement was cancelled in the single image detection and agreed well with other techniques, such as PDPA.

INJECTION PRESSURE EFFECTS UPON DROPLET BEHAVIOR IN TRANSIENT DIESEL SPRAYS

Abstract : This paper reports on the investigation of injection pressure upon the droplet behavior in transient diesel sprays. Phase/Doppler results for a Diesel spray with a maximum fuel injection line pressure of 105 MPa are compared with previously acquired droplet size and velocity measurements for a Diesel spray with an injection pressure of 21 MPa. All measurements reported here were made in atmospheric conditions at a position near the nozzle. It is shown in these results that the droplet velocity and size profiles do maintain similarity despite the substantial change in injection pressure. Specific characteristics, for example, the appearance of subtle waves in the time dependent spray data, are present in both data sets. Comparison of the measured droplet velocities and diameters with Weber number based stability criteria shows that increased injection pressure produces a higher percentage of droplets that are likely to breakup. This is mostly the result of increases in droplet velocities with higher injection pressure. The interior region of the higher pressure spray is an area extremely difficult to probe, despite the application of temporal optimization of the phase/Doppler anemometer. Inherent characteristics of the injection that affect the ability to acquire data are described, as well as some of the operational difficulties experienced in using a phase/Doppler for diesel spray droplet measurements.

Comparison of Mixing Characteristics of Unlike Triplet Injectors Using Optical Patternator

Mixing characteristics of an unlike split triplet fuel-oxidizer-oxidizer-fuel (F-O-O-F) injector, which separates the single oxidizer hole of an unlike triplet fuel-oxidizer-fuel (F-O-F) injector for liquid-rocket engines, were investigated under cold-flow conditions. The spray mass distributions of the fuel and oxidizer were measured using an optical patternator based on the planar liquid laser-induced fluorescence technique, and the mixing mechanism and the mixing efficiency of the F-O-O-F injector were compared with those of the F-O-F injector. Results showed that the mixing efficiency of the F-O-O-F injector is less sensitive to the changes of the momentum ratio or impingement angle than that of the F-O-F injector because the mixing of the F-O-O-F injector is determined by the coalescing process of two liquid sheets, whose impinging momentum is weak. Therefore, the F-O-O-F injector has an advantage in starting injection wherein the momentum ratio is usually different from the operating condition.

Combustion Characteristics of a Coaxial Porous Injector

A coaxial porous injector was proposed for gas–liquid combustion for improved mixing performance compared to a conventional shear coaxial injector. The coaxial porous injector discharged a gas jet in a direction normal to the center liquid core, and it was surmised that the momentum transfer from the radial gas jet to the axial liquid jet occurred more efficiently than with a traditional shear coaxial injector. To validate the effectiveness of this idea, the combustion performances of a coaxial porous injector and a shear coaxial injector were compared by conducting a hot-firing test using nitrous oxide-ethanol as propellant. The momentum flux ratio at the injector tip was selected as a spraying condition index, and the characteristic velocity efficiency was considered as a combustion performance parameter. The coaxial porous injector demonstrated higher combustion efficiency than the conventional shear coaxial injector, especially at lower-momentum flux ratio conditions. In addition, the comparison of sp...

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.

Injection Condition Effects of a Pintle Injector for Liquid Rocket Engines on Atomization Performances

Effects of injection conditions on a pintle injector which is proper to recent liquid rocket engines requiring low cost, low weight, high efficiency and reusability were studied. The pintle injector with a typical moving pintle was used for atmospheric experiment using water and air. Injection pressures of water were considered 0.5 and 1.0 bar, 0.1 to 1.0 bar for injection pressures of air and 0.2 to 1.0 mm for pintle opening distance. Sauter mean diameters (SMD) of spray was measured at 50 mm distance from a pintle tip and SMD was treated as a representative parameter in this study. As a result, because of shape characteristics of the pintle injector, there was a transient region between the pintle opening distances of 0.6 and 0.7 mm and this region affected to mass flow rates and SMDs. Also, Reynolds numbers for gas, Weber numbers and momentum ratios were adopted as major non-dimensional paramters and the momentum ratio has strong correlation with SMD.

Design Procedure of a Movable Pintle Injector for Liquid Rocket Engines

Variable-area injectors are suitable choices for developing throttleable rocket engines because it is difficult to efficiently control thrust when fixed-area injectors are used. A pintle injector is a variable-area injector that can control the mass flow rate of propellants, thereby replacing a large injector plate having several injector elements with a single injector unit. To achieve proper performance under all throttling conditions, a design procedure was established for a pintle injector based on spray characteristics. The spray angle and Sauter mean diameter of the droplets were critical performance parameters. In spray experiments using water and air as simulants under atmospheric conditions, backlight imaging and laser-diffraction methods were used to measure the spray angle and Sauter mean diameter. The velocity ratio, nondimensionalized opening distance, and Weber number were considered as major nondimensional numbers. Empirical correlations were formulated between performance parameters and no...

Effect of Weber Number and Momentum Flux Ratio on Macroscopic Characteristics of Spray from a Coaxial Porous Injector

The gas jet from a coaxial porous injector for two-phase flows is discharged from the porous surface, which encloses the center liquid jet, and the gas and liquid jet interact with each other physically. The wall injected gas jet transfers the radial momentum effectively while the radial gas jet develops to axial jet, and the performance of atomizing and mixing can be improved. In this study, the Weber number and the ratio of momentum flux were controlled by changing the gas injection area and the mass flow rate of the gas jet, and a study on the spray characteristics at the cold-flow test using water and air simulant was performed. It is concluded that the radial momentum transfer concept of a coaxial porous injector gives a positive effect on the atomization and mixing of the two-phase spray.

Effects of Wall-Injection Length on Spray and Combustion in a Coaxial Porous Injector

The mixing performance of an injector determines the combustion performance of a bipropellant rocket engine. In the previous study, a novel coaxial porous injector concept was developed to improve the mixing performance of a shear coaxial injector. It demonstrated a higher characteristic velocity efficiency than that of a typical shear coaxial injector. To clarify the efficiency-improving mechanism of a coaxial porous injector, the combustion flowfields of both injectors were visualized using a high-speed shadowgraph technique. The effect of geometry factors on the spray core length and the combustion performance as well as the differences in the reactive spray behavior between the coaxial porous injector and the shear coaxial injector were observed. The coaxial porous injector with a different wall-injection length showed higher characteristic velocity efficiencies, shorter spray core lengths, and thicker phase-change patterns at the spray core boundary. Changes in wall-injection length had a nonmonotoni...

Effects of Propellant Phases on Atmospheric Spray Characteristics of a Pintle Injector for Throttleable Rocket Engines

Atmospheric spray characteristics were experimentally compared between liquid-gas and liquid-liquid sprays of a pintle injector. In order to study spray characteristics, water and air were used as the simulants and the visualization technic was adopted. Spray images were acquired by using a backlight method by a high-resolution CMOS camera. As a result, when the pintle opening distance increased, liquid sheets became unstabled and fluttering droplets increased. In the liquid-gas case, the breakup performance increased as the pressure of gas injected from the annular orifice increased. In the liquid-liquid case, atomization efficiency decreased as the pressure of liquid injected from the annular orifice increased. Spray angles presented a similar trend between two cases. At the same momentum ratio, the spray angle of liquid-liquid case was lower than the angle of liquid-gas case.