Taeock Khil

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.

Effects of Orifice Internal Flow on Breakup Characteristics of like-Doublet Injectors

Under cold-flow and atmospheric ambient pressure conditions, the breakup characteristics of liquid sheets formed by a like-doublet injector were investigated. The sheet breakup wavelength, which induces the sheet to be broken into ligaments, and the sheet breakup length, which is important for finding the flame location, were measured using stroboscopic light. After sheet breakup, liquid ligaments are formed intermittently, and their wavelengths are believed to be related to the combustion instability of liquid-rocket engine. Therefore, the wavelength and the breakup length of ligaments broken into fine drops were also measured. Because these spray characteristics are affected by the flow characteristics of the two liquid jets before they impinge on each other, the focus was on the effects of orifice internal flow, such as the cavitation phenomenon that occurs inside a sharp-edged orifice. From the experimental results, it was found that liquid jet turbulence delays sheet breakup and shortens the wavelengths of both sheets and ligaments. Because the turbulence strength of a sharp-edged orifice is stronger than that of a round-edged orifice, the shape of orifice entrance gives large differences in the spray characteristics. With these results, empirical models of the spray characteristics of a like-doublet injector were proposed, and these models can provide useful and practical data for use in designing liquid-rocket combustors.

Dynamic Characteristics of Simplex Swirl Injector in Low Frequency Range

Combustion instability is generally generated by mutual coupling between heat release and acoustic pressure in the combustor. Acoustic pressure causes the mass flow rate of propellant injected from atomizers to oscillate, which affects the combustion process. Since the early 1970s, great efforts have been made in Russia to elucidate the underlying mechanisms of the injector’s dynamic characteristics to better manage combustion instability. In the current work, to characterize the injector dynamics, a mechanical pulsator was developed to produce forced pressure oscillation. An analytical method was also developed to quantify the mass flow rate of the propellant oscillating at the injector exit. The pulsating values of the mass flow rate, pressure, liquid film thickness, and axial velocity generated at the simplex swirl injector exit were all measured and quantified in real time in a low frequency range of the fluctuating pressure. Furthermore, the dynamic characteristics of each parameter were analyzed using these pulsating values. Because of themeasured time and position differences between themanifold and the orifice of the swirl injector, phase and amplitude differences were identified and further characterized between the input and output values.

Experimental Study on Simplex Swirl Injector Dynamics with Varying Geometry

The effects of swirl chamber’s diameter and length on injector’s dynamic characteristics were investigated through an experimental study. A mechanical pulsator was installed in front of the manifold of a swirl injector which produces pressure oscillations in the feed line. Pressure in the manifold, liquid film thickness in the orifice and the pressure in the orifice were measured in order to understand the dynamic characteristic of the simplex swirl injector with varying geometry. A direct pressure measuring method (DPMM) was used to calculate the axial velocity of the propellant in the orifice and the mass flow rate through the orifice. These measured and calculated values were analyzed to observe the amplitude and phase differences between the input value in the manifold and the output values in the orifice. As a result, a phase-amplitude diagram was obtained which exhibits the injector’s response to certain pressure fluctuation inputs. The mass flow rate was calculated by the DPMM and measured directly through the actual injection. The effect of mean manifold pressure change was insignificant with the frequency range of manifold pressure oscillation used in this experiment. Mass flow rate was measured with the variation of injector’s geometries and amplitude of the mass flow rate was observed with geometry and pulsation frequency variation. It was confirmed that the swirl chamber diameter and length affect an injector’s dynamic characteristics. Furthermore, the direction of geometry change for achieving dynamic stability in the injector was suggested.

Quantification of the transient mass flow rate in a simplex swirl injector

When a heat release and acoustic pressure fluctuations are generated in a combustor by irregular and local combustions, these fluctuations affect the mass flow rate of the propellants injected through the injectors. In addition, variations of the mass flow rate caused by these fluctuations bring about irregular combustion, which is associated with combustion instability, so it is very important to identify a mass variation through the pressure fluctuation on the injector and to investigate its transfer function. Therefore, quantification of the variation of the mass flow rate generated in a simplex swirl injector via the injection pressure fluctuation was the subject of an initial study. To acquire the transient mass flow rate in the orifice with time, the axial velocity of flows and the liquid film thickness in the orifice were measured. The axial velocity was acquired through a theoretical approach after measuring the pressure in the orifice. In an effort to understand the flow area in the orifice, the liquid film thickness was measured by an electric conductance method. In the results, the mass flow rate calculated from the axial velocity and the liquid film thickness measured by the electric conductance method in the orifice was in good agreement with the mass flow rate acquired by the direct measuring method in a small error range within 1% in the steady state and within 4% for the average mass flow rate in a pulsated state. Also, the amplitude (gain) of the mass flow rate acquired by the proposed direct measuring method was confirmed using the PLLIF technique in the low pressure fluctuation frequency ranges with an error under 6%. This study shows that our proposed method can be used to measure the mass flow rate not only in the steady state but also in the unsteady state (or the pulsated state). Moreover, this method shows very high accuracy based on the experimental results.

Quantifying the Variation of the Mass Flow Rate generated in a Simplex Swirl Injector by Pressure Fluctuation

When a heat release and acoustic pressure fluctuations are generated in a combustor by irregular and local combustions, these fluctuations affect the mass flow rate of the propellants injected through the injectors. In addition, variations of the mass flow rate by these fluctuations bring about irregular combustion, which is associated with combustion instability. Therefore, it is very important to identify a mass variation through the pressure fluctuation on the injector and to investigate its transfer function. Therefore, quantification of the variation of the mass flow rate generated in a simplex swirl injector via the injection pressure fluctuation was the subject of an initial study. To acquire the transient mass flow rate in the orifice with time, the flow axial velocity and the liquid film thickness in the orifice were measured. The axial velocity was acquired through a theoretical approach after measuring the pressure in the orifice. In an effort to understand the flow area in the orifice, the liquid film thickness was measured by an electric conductance method. In the results, the mass flow rate calculated by the axial velocity and the liquid film thickness measured by the electric conductance method in the orifice were in good agreement with the mass flow rate acquired by the direct measuring method in a small error range within 1 percent in the steady state and within 4 percent for the average mass flow rate in a pulsated state. In addition, the amplitude(gain) of the mass flow rate acquired using the proposed direct measuring method was confirmed using the PLLIF technique for low pressure fluctuation frequency ranges. This study shows that the proposed method can be used to measure the mass flow rate not only in the steady state but also in the unsteady state(or the pulsated state), as the mass flow rate in the orifice can be acquired with time. Moreover, this method shows very high accuracy based on the experimental results.

BREAKUP CHARACTERISTICS OF LAMINAR AND TURBULENT LIQUID SHEETS FORMED BY IMPINGING JETS IN HIGH PRESSURE ENVIRONMENTS

Breakup characteristics of liquid sheets formed by the impingement of two water jets were investigated as increasing the injection velocity up to 30m/s and the ambient gas pressure up to 4.0MPa. We compared the breakup characteristics between laminar and turbulent sheets, which are formed by round and sharp edged orifices, respectively. The results showed that the aerodynamic force significantly affects the breakup of laminar sheet when the gas based Weber number is higher than unity. It was also found that the turbulent sheets have three breakup regimes, i.e. expansion regime, wave breakup regime and catastrophic breakup regime according to the gas based Weber number. Using the experimental results, we could suggest empirical models on breakup lengths for laminar and turbulent sheets.

The Breakup Characteristics of Liquid Sheet Formed by Like-Doublet Injectors

The atomization process of uni-element like-doublet injector includes formation of liquid sheet, fragment into ligaments, and breakup into fine drops. During the processes, breakup characteristics of the liquid sheet such as breakup length and breakup frequency affect the combustion instability as well as the efficiency of liquid rocket combustor. In this paper we proposed spray models on the breakup characteristics of the liquid sheet based on empirical data. We focused on the effects of the orifice inner flow characteristics such as cavitation phenomenon inside the sharp-edged orifice. From our experimental results, we found that the effects of turbulence inside the sharp-edged orifice are significant for the breakup of liquid sheet, and the cavitation tends to increase the turbulence strength. Since the turbulence strength of jet at the orifice exit increases as the orifice length becomes short, we also considered the effects of orifice length. Our empirical formula for the beakup length and the breakup frequency of liquid sheets formed by like-doublet injectors are believed to give some useful and actual data for designing liquid rocket combustors.

Effect of Orifice Length on Particle Distribution in Particle-laden Jet

As a propellant of a high speed underwater vehicle, the hydro-reactive solid metal particles using seawater as a oxidizer maximizes its specific impulse when the solid metal particles and the seawater are uniformly mixed in the combustion chamber. The purpose of this study is to investigate the effects of injector geometry on the particle distribution of similarity point of view. For the purpose of this similarity of the mean velocity and particle number density along the radial direction was measured by Particle Image Velocimetry(PIV).

Effect of Injector Geometry on Cryogenic Jet Flow

Characteristics of cryogenic single jet flow were investigated. Liquid nitrogen was injected into a high-pressure chamber and formed single jet. Ambient condition around jet was changed from subcritical to superctirical condition of nitrogen. Injector geometries also were changed. A shape of the jet and core diameter were measured by flow visualization technique, and core spreading angle was calculated. Flow instability was found at atmospheric pressure condition. As ambient pressure increased, core spreading angle was increased and maintained after certain pressure.