Dongjun Kim

Effect of geometric parameters on the liquid film thickness and air core formation in a swirl injector

Many theoretical and experimental studies have been conducted to investigate elements of swirl injector hydrodynamics, such as variations in liquid film thickness or air core diameter. From these studies, some theoretical relationships have been established through an approximate analytical solution of flow hydrodynamics in a swirl nozzle. However, experimental studies on elements such as the measurement of liquid film thickness have not produced conclusive results. In a swirl injector, the atomization process is significantly influenced by the liquid film thickness. Thus, it is possible to investigate the effects of various geometric parameters on spray characteristics through the measurement of liquid film thickness. We used a specially designed injector based on the electrical conductance method to measure the liquid film thickness accurately. The liquid film thickness was measured through precise calibration, and the accuracy of this measurement was demonstrated in comparison with previous theories and experiments. From these results, we present an empirical relation for the liquid film thickness by adding orifice length to an existing analytical equation. The variations and stability of the air core were also examined by visualizing the formation of the air core in the swirl chamber with a high-speed camera system. This study confirms that air core shape and liquid film thickness are directly related. Thus, study of the fluctuations of liquid film thickness under various geometric conditions can be applied to the analysis of internal flow.

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.

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.

Improved Spray Model for Viscous Annular Sheets in a Swirl Injector

A modified sheet breakup model was applied to a thin, viscous liquid film generated by a swirl injector similar to that installed in a liquid propellant rocket engine combustor. The sheet breakup model consists of three steps: determination of the swirl injector characteristics for the prediction of initial sheet conditions at the injector exit as input, linear stability analysis for primary sheet breakup, and the Taylor analogy breakup model for final drop formation. Under atmospheric pressure, the liquid sheet breakup occurs under a long-wave regime, sometimes according to simple theoretical analysis. But in high ambient pressure conditions, like a liquid propellant rocket engine combustor, the sheet breakup regime changes from a long wave to a short-wave regime due to a high gas Weber number (We 2 > 27/16), although the same injector was used. The sheet breakup model was, therefore, modified to be applicable to both long- and short-wave regimes and validated by the comparison of breakup length and Sauter mean diameter to experimental results. In both experimental and computational results, the spray cone angle and breakup length decreased as the ambient pressure increased, even though the pressure difference of the injector was constant. Local Sauter mean diameters, predicted by computation, were smaller at high ambient pressures. The comparative results show that the computational model is able to accurately predict sheet breakup length, spray cone angle, local Sauter mean diameter, and overall spray shape. Therefore, the model can be used as a design tool, ahead of analyzing spray characteristics of an injector in both atmospheric and high ambient pressure conditions.

Spray characterization in high pressure environment using optical line patternator

For the quantitative measurement in an optically dense spray, the intensity of the attenuated signal should be corrected. Therefore, the optical line patternator was applied to get the original distribution of the dense spray injected from a swirl injector at high ambient pressure up to 4.0 MPa. The optical line patternator is a combined technique of laser extinction measurement and image processing for the spray characterization. The spray was scanned with the laser beam and the line image of Mie scattering was captured simultaneously in the path of each laser beam by using a CCD camera. A photo-diode was used to obtain the transmission data that was the amount of the incident laser beam passing through the spray region. The distribution of the attenuation coefficients in the spray was obtained by processing the transmission data and Mie-scattering distribution data by an algebraic reconstruction technique. From the distribution of attenuation coefficients, we can obtain the accurate surface distribution from the Mie-scattering signal. Because the optical line patternator uses a laser beam instead of a laser sheet to scan the spray, the effect of multiple scattering, due to the increased number density of droplets in a high pressure environment is reduced significantly. The optical line patternator is suitable for investigating the characteristics of a relatively large spray under high pressure environments such as liquid rocket engines.

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.

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.

ANALYSIS OF SWIRL COAXIAL INJECTOR WITH BACKHOLE AS AN ACOUSTIC DAMPER IN LIQUID ROCKET ENGINES

Swirl coaxial injector with backhole (Bakchole injector) was analyzed to suppress high-frequency combustion instability in Liquid rocket engines. In order to analyze the effect of backhole injector as an acoustic absorber, backhole injector was regarded as a quarter-wave resonator. Also the volume and shape of the air core of the swirl injector was visualized. As a result of theoretical approaches and a.coustic tests, backhole injectors were tuned accurately to the unstable modes of combustion chamber. And the damping efficiency was estimated by measuring damping rates experimentally.