Gas-centered Swirl Coaxial Injectors Research Articles

Effects of injector spacing and momentum flux ratio on combustion instability in a model chamber with gas-centered swirl coaxial injectors

This study investigates the effects of injector spacing and momentum flux ratio on combustion instability in a model chamber with single and two injectors, both experimentally and numerically. For the experiments, injectors similar to those used in actual rocket engines were installed in a laboratory-constructed model chamber. Numerical simulations were conducted using ANSYS Fluent, employing a detailed combustion model to complement the experimental data. Flow conditions were categorized as fuel-lean and fuel-rich based on the fuel mass flow rate. The injector spacing varied from 15 to 45 mm, and the momentum flux ratio was adjusted from 6.4 to 139.8 by manipulating the fuel and oxidizer flow rates. Both experimental and numerical results consistently revealed distinct trends in combustion instability for lean and rich conditions, influenced by injector spacing and momentum flux ratio. Combustion stabilization was consistently more effective under fuel-rich conditions. Increasing the momentum flux ratio intensified combustion instability regardless of the fuel condition. For fuel-lean conditions, a rapid transition in combustion stability was observed, with the specific transition point varying depending on the injector spacing. Considering the trade-off between instability and injector number density, a spacing of 30 mm between injectors is recommended. Additionally, for both single and two injectors with various spacings under both lean and rich conditions, combustion stability was generally better when the momentum flux ratio was less than 48.8. These findings provide valuable insights into the control and management of combustion instability, aiding in the design of rocket engine combustors.

Experimental Study of Spray and Combustion Characteristics in Gas-Centered Swirl Coaxial Injectors: Influence of Recess Ratio and Gas Swirl

The spray and combustion characteristics of a gas-centered swirl coaxial (GCSC) injector used in oxidizer-rich staged combustion cycle engines were analyzed. The study focused on varying the recess ratio, presence of gas swirl, and swirl direction to improve injector performance. The impact of the recess ratio was assessed by increasing it for gas jet-type injectors with varying momentum ratios. Gas-swirl effects were studied by comparing injectors with and without swirl against a baseline of a low recess ratio gas injection. In atmospheric pressure-spray experiments, injector performance was assessed using backlight photography, cross-sectional imaging with a structured laser illumination planar imaging technique (SLIPI), and droplet analysis using ParticleMaster. Increasing the recess ratio led to reduced spray angle and droplet size, and trends of gas swirl-type injectors were similar to those of high recess ratio gas jet-type injectors. Combustion tests involved fabricating combustion chamber heads equipped with identical injectors, varying only the injector type. Oxidizer-rich combustion gas, produced by a pre-burner, and kerosene served as propellants. Combustion characteristics, including characteristic velocity, combustion efficiency, and heat flux, were evaluated. Elevated recess ratios correlated with increased characteristic velocity and reduced differences in the momentum–flux ratios of injectors. However, increasing the recess ratio yielded diminishing returns on combustion efficiency enhancement beyond a certain threshold. Gas swirling did not augment characteristic velocity but notably influenced heat flux distribution. The trends observed in spray tests were related to combustion characteristics regarding heat flux and combustion efficiency. Additionally, it was possible to estimate changes in the location and shape of the flame according to the characteristics of the injector.

Characteristics of spray and combustion in gas-centered swirl coaxial injector with varying gas nozzle diameter

Gas-centered swirl coaxial (GCSC) injectors, a type used in the combustion chamber of liquid rocket engines, have been widely investigated because their characteristics are key parameters for the performance of the engine. This study investigated the spray and combustion characteristics of GCSC injectors used in the combustion chamber of oxidizer-rich staged-combustion cycle engines. When the propellant mass flow rate and oxidizer-to-fuel mass flow ratio (O/F ratio) were identical, the diameter of the gas nozzle was changed such that the ratio of the gas-to-liquid momentum could be changed; thus, the swirl strength of the liquid also changed because of the increase in the injector diameter. In the spray experiments, the spray characteristics were identified in accordance with changes in the diameter of the gas nozzle through the spray angle, droplet mean velocity, and droplet size by using backlight photography and the ParticleMaster imaging system. For combustion tests, four types of reduced combustion chambers were designed to spray propellants through the same two injectors used in the spray experiments. To generate an oxidizer-rich combustion gas, combustion tests were performed using a pre-burner. The changes in combustion characteristics according to the change in the injector shape under the same combustion chamber shape, propellant flow rate, and O/F ratio were identified through the characteristic velocity and heat flux of the combustion chamber. As the gas–liquid momentum flux ratio increased in accordance with the change in diameter of the gas nozzle, the characteristic velocity increased from at least 1700 m/s to a maximum of 1721 m/s, which was related to the decrease in the Sauter mean diameter observed in the spray experiments. Few studies have investigated GCSC shape variables by simultaneously conducting atmospheric spray experiments and high-pressure combustion tests on GCSC injectors using kerosene and oxidizer-rich combustion gases as propellants. This study observed the changes in spray characteristics according to the change in the diameter of the injector gas nozzle through atmospheric spray experiments of the GCSC injector and compared and analyzed the combustion characteristics through combustion tests using the same injector. The results of this study will potentially aid the design optimization of GCSC injectors and facilitate future developments.

Effect of ambient pressure on the performance of a gas-centered swirl coaxial injector

In this study, water and air were used as simulated media to investigate the spray characteristics of a gas-centered swirl coaxial (GCSC) injector under different ambient pressures through experimental, simulation, and theoretical approaches. The results showed that for the same liquid mass flow rate, the breakup length of the liquid film decreased with increasing ambient pressure, with the extent of the decrease diminishing as the ambient pressure increased. In addition, it was found that under the same ambient pressure, the larger the liquid mass flow rate, the shorter the liquid film breakup length. Moreover, for any gas–liquid mass flow ratio (GLR), the spray angle increased significantly with ambient pressure, and the liquid film suddenly expanded compared to the no-ambient pressure condition. At the same time, the breakup length of the liquid film increased. As ambient pressure increased, the spray angle increased while the breakup length decreased. When the GLR was small, self-pulsation occurred, which gradually disappeared with increasing ambient pressure. The frequency of self-pulsation decreased with increasing ambient pressure. The periodic self-pulsation of the spray was the result of the combined effects of the centrifugal force of the rotating liquid film, surface tension of the liquid film, aerodynamic force, and pressure difference inside and outside the liquid film. Overall, these findings provide insight into the spray characteristics of GCSC injectors under different ambient pressures, which can be of importance to the design and optimization of liquid rocket engines.

Analyzing Combustion Efficiency According to Spray Characteristics of Gas-Centered Swirl-Coaxial Injector

The momentum flux ratio (MFR) significantly affects the mixing characteristics and combustion efficiency of propellants in rocket engine injectors. The spray characteristics of three gas-centered swirl-coaxial injectors used in a full-scale combustion test were investigated according to the change in the momentum flux ratio. The difference in combustion efficiency was analyzed through the comparison with combustion test results using spray visualization and quantification. The spray cross-sectional shape and droplet distribution were measured using a structured laser illumination planar imaging technique. As the swirl effect was more apparent at a low MFR, the flow rate of the liquid that was sprayed outside was high. The flow rate of the liquid sprayed around the gas injection increased with the MFR. The Sauter mean diameter (SMD) of each injector liquid spray was obtained using the laser shadow imaging method. The SMD decreased as the MFR of all injector types increased, and the injector with a high liquid flow rate and small SMD injected towards the gas center exhibited higher combustion efficiency than the injector with a dominant liquid spray and the large SMD at a large injection angle. The outcomes of the study could help contribute to the increase in the combustion efficiency of the full-scale staged combustion cycle engine combustor.

Numerical investigation on the performance of internal flow and atomization in the recessed gas-centered swirl coaxial injectors

Recessed gas-centered swirl coaxial (GCSC) injector has been widely applied in the liquid rocket engine (LRE), and its internal flow and atomization have a significant impact on the combustion downstream. The oxygen (gas)-kerosene (liquid) two-phase flow in the recessed chamber of GCSC injectors under the conditions of back pressure of 17 MPa is numerically simulated by the volume-of-fluid (VOF) method. The liquid film, atomization and gas-liquid two-phase flow fields are obtained for liquid flow with or without swirling. The atomization of liquid kerosene does not have typical atomization characteristic of a swirling injector but accomplishes in the recessed chamber by action of pneumatic force. The swirling flow in the outer injector is helpful to achieve higher gas-to-liquid axial momentum ratio for good atomization performance and smaller injector size as well as lower gas-to-liquid momentum ratio for longer liquid-film length and more steady mass flowrate. The droplet sizes of the spray, the length of liquid film and the sizes of GCSC injectors decrease with increase of the gas-to-liquid (axial) momentum ratio, while the liquid mass flowrate in the outer injector becomes much unstable due to a recirculation flow adjacent to the exit of outer injector. Therefore, it is found that increasing the gas-to-liquid (axial) momentum ratio has two contradictory effects on improving atomization performance and promoting combustion instability. To balance these contradiction influences, the corresponding limits of the gas-to-liquid (axial) momentum ratio for well-working GCSC injectors are defined in this paper. The lower and upper limits of the gas-to-liquid momentum ratio are 7.6 and 14, and the gas-to-liquid axial momentum ratio corresponds to 17.6 and 31. In actual engineering project, the gas-to-liquid (axial) momentum ratios of the GCSC injector investigated in this paper is 9 (20). It indicates that the limits proposed in this paper are reasonable.

Influence of Self-Pulsation on Atomization Characteristics of Gas-Centered Swirl Coaxial Injector

There is a lack of understanding of the spray characteristics of gas-centered swirl coaxial (GCSC) injectors during self-pulsation occurs. Therefore, the self-pulsation of a GCSC injector was investigated experimentally in this study. Experiments were conducted at atmospheric pressure with filtered water and dried air supplied through a propellant feed system. A back-lighting high-speed photography technique was used to capture unsteady spray features. A laser-based particle size analyzer (LPSA) was used to measure the size of the droplets in the spray. The effects of recess and gas-liquid ratio on spray self-pulsation were analyzed. It was found that the recess of the injector strongly determines the spray pattern. When spray self-pulsation occurs without recess, both the center and periphery of the spray oscillate. With an increase in the mass flow rate of the gas, the boundary between the center and the periphery of the spray becomes more noticeable. Meanwhile, small droplets in the spray center oscillate, with the periphery of the spray being characterized by a periodic “shoulder.” Under the same operating conditions but with a small recess (2 mm), the spray adheres to the injector faceplate. With a larger recess (7 mm), when spray self-pulsation occurs, the spray periodically forms “shoulder” and “neck,” similar to the behavior of self-pulsation in a liquid-centered coaxial injector. Therefore, it can be concluded that spray self-pulsation enhances atomization at the center of the spray to a certain extent. However, atomization becomes worse in the periphery with an oscillating spray.

Dynamic behavior of jet-swirl spray under resonance condition with external excitation

Gas-centered swirl coaxial (GCSC) injectors are used in many liquid propellant rocket engines to improve combustion efficiency. In this study, fluctuation of the mass flow rate is analyzed to investigate influence of external excitation in gas feed line on the mass flow rate distribution of sprays. Water and air are used as simulants for the liquid kerosene and gas oxygen, respectively. With external excitation (354 and 1062 Hz), atomization processes changes and periodic breakup phenomenon of a spray occurs. Liquid film thickness at the exit of the injector shows that the width of liquid mass flow rate wave is reserved when the instability mode changes. However, behavior of the liquid droplets captured with a structured laser illumination planar imaging (SLIPI) system for laser diagnostics indicates that time domain concentration of liquid mass becomes more severe with the second instability mode outside the injector. This study can provide a reference for liquid mass distribution with external excitation on gas and imply that resonance in injector feeding systems is one of the important factors in combustion instability.

Experimental Study of the Combustion Efficiency in Multi-Element Gas-Centered Swirl Coaxial Injectors

The effects of the momentum-flux ratio of propellant upon the combustion efficiency of a gas-centered-swirl-coaxial (GCSC) injector used in the combustion chamber of a full-scale 9-tonf staged-combustion-cycle engine were studied experimentally. In the combustion experiment, liquid oxygen was used as an oxidizer, and kerosene was used as fuel. The liquid oxygen and kerosene burned in the preburner drive the turbine of the turbopump under the oxidizer-rich hot-gas condition before flowing into the GCSC injector of the combustion chamber. The oxidizer-rich hot gas is mixed with liquid kerosene passed through combustion chamber’s cooling channel at the injector outlet. This mixture has a dimensionless momentum-flux ratio that depends upon the dispensing speed of the two fluids. Combustion tests were performed under varying mixture ratios and combustion pressures for different injector shapes and numbers of injectors, and the characteristic velocities and performance efficiencies of the combustion were compared. It was found that, for 61 gas-centered swirl-coaxial injectors, as the moment flux ratio increased from 9 to 23, the combustion-characteristic velocity increased linearly and the performance efficiency increased from 0.904 to 0.938. In addition, excellent combustion efficiency was observed when the combustion chamber had a large number of injectors at the same momentum-flux ratio.

동축형 분사기의 연소 특성에 영향을 주는 인자에 관한 연구

Factors affecting the combustion characteristics of gas-centered swirl coaxial (GCSC) injectors are investigated numerically and experimentally. The factors are fuel volume flow rate (Qf), momentum flux ratio (MFR) and oxidizer/fuel (O/F) ratio. Firstly, the flame pattern becomes asymmetric as the fuel volume flow rate increases with a fixed momentum flux ratio. Then, the suitable fuel volume flow rate is selected according to the results. Next, the momentum flux ratio is increased by increasing the oxidizer volume flow rate (QSUBo/SUB) with a fixed fuel volume flow rate. Results show that the spreading angle decreases as momentum flux ratio increases, which agrees with our previous results. Finally, in order to check effects of oxidizer/fuel ratio, the QSUBf/SUB, QSUBo/SUB, and MFR are kept the same but the O₂ mole fraction of oxidizer is changed. From numerical and experimental results, it is found that flame pattern depends on oxidizer/fuel ratio and spreading angle increases as the O₂ mole fraction decreases.

Characteristics of Gas-Centered Swirl-Coaxial Injector with Liquid Flow Excitation

Gas-centered swirl-coaxial injectors are typically adopted for staged combustion-cycle engines. The dynamic characteristics of gas-centered swirl-coaxial injectors were investigated in this study by inducing excitation on liquid flow through a mechanical pulsator. The effect of excitation was expressed as the injector transfer function, which was defined by the ratio of manifold pressure to film thickness. The geometry effect was investigated by changing the tangential inlet diameter and gap thickness. As the tangential inlet diameter increased, the excitation effect was significant in the film thickness and spray pattern. When the gap thickness decreased, the gain increased and the frequency at the gain decrease became higher. In addition, the momentum flux ratio was used to confirm the gas flow effect. The gain increased with the momentum flux ratio; however, shear instability resonance did not occur.

Spray Patterns and Injection Characteristics of Gas-Centered Swirl Coaxial Injectors

AbstractSpray characteristics of gas-centered swirl coaxial injectors, which are commonly used in staged combustion cycle engines, are investigated numerically and experimentally. The spray angle a...

Characteristics of Gas-Centered Swirl Coaxial Injector with Acoustic Excitation of Gas Flow

Various research activities on combustion instability that significantly influences engine efficiency have been conducted. The instability of liquid fuel flow by perturbation of gas oxidizer flow in gas-centered swirl coaxial injectors is investigated using air and water. Gas perturbation is generated by a speaker, and variation in the film thickness at the end of the injector is measured. The spray patterns have different shapes, depending on the excitation frequency. The excitation effect is enhanced by the increment of the momentum flux ratio. Different gain values are achieved according to the input frequency, and a high value is realized in both low and high frequencies. The high- and low-frequency peaks are due to the resonant frequency of the gas feeding line and gas–liquid interface instability, respectively. The high-gain frequency can possibly cause combustion instability.

Spray and atomization characteristics of gas-centered swirl coaxial injectors

To achieve uniform and efficient combustion in a rocket engine, a fine uniform spray is needed. The same is achieved by designing an injector with good atomization characteristics. Gas-centered swirl coaxial (GCSC) injector elements have been preferred recently in liquid rocket engines because of an inherent capability to dampen the pressure oscillations in the thrust chamber. The gas-centered swirl coaxial injector chosen for this study is proposed to be used in a semi-cryogenic rocket engine operating with oxidizer rich hot exhaust gases from the pre-burner and liquid kerosene as fuel. In this paper, nine different configurations of gas-centered swirl coaxial injector, sorted out by studying the spray angle and coefficient of discharge with swirl number varying from 9 to 20 and recess ratio of 0.5, 1, and 1.5 are investigated for their atomization characteristics. Spray uniformity, spray cone angle, and droplet size in terms of Sauter mean diameter and mass median diameter are studied at various momentum flux ratios for all configurations. Sauter mean diameter is almost independent of recess ratio, whereas cone angle was inversely proportional to the recess ratio. A finer atomization was observed for injectors of high swirl number but the pressure drop also increased to achieve the same flow rate. An injector of medium swirl number and recess ratio of 1.5 is deemed most fit for above-mentioned application.

Transverse Instability Studies in a Subscale Chamber

High-frequency combustion instabilities result from a coupling between modes of heat release and chamber acoustics. Because the geometric dimensions of the chamber define the relevant acoustic modes, it is particularly difficult to study transverse instabilities in anything other than a full-scale chamber. In this paper, results from experimental studies of a two-dimensional subscale chamber that can nearly replicate full-scale acoustic modes are described. The acoustic modes are primarily driven by injector elements at the endwalls of the rectangular chamber. They were designed to be unstable, based on a vortex-shedding mechanism postulated on earlier results from longitudinal studies. The 27-cm-wide by 19-cm-long chamber produced transverse instabilities with a first width-mode frequency of about 2000 Hz, with no longitudinal mode content in the pressure signal. The chamber comprises a linear array of seven gas-centered swirl coaxial liquid injectors, designed to flow either bipropellants, only oxidizer, or nothing. Eight different flow configurations were tested, yielding peak-to-peak pressure oscillation amplitudes ranging from about 5% to nearly 100% of chamber pressure. The strongest instabilities were observed when endwall injectors flowed both fuel and oxidizer, and amplitudes were reduced as the driving injectors were moved inward. An asymmetric setup demonstrated that the unsteady pressures in warm regions and hot regions have differing pressure amplitudes. To demonstrate the potential use of the chamber for measuring the combustion response to velocity and pressure oscillations, the synchronized measurement and analysis of pressure and flame emission are presented.

Study on spray patterns of gas-centered swirl coaxial (GCSC) injectors in high pressure conditions

Recess ratio and momentum flux ratio are the key parameters in determining spray characteristics and mixing efficiency of the propellants in rocket injectors. In this study, effects of the recess ratio and momentum flux ratio on the spray patterns of gas-centered swirl coaxial (GCSC) injectors have been experimentally investigated through cold flow tests in a high pressure chamber, in which the liquid fuel and gaseous oxidizer are simulated by water and gaseous nitrogen, respectively. By elevating the chamber pressure, the cold flow test conditions are adjusted to match the ratios of momentum flux, density, and velocity between the gas and liquid flows to those of the actual hot-firing conditions. The results showed that the spray patterns change drastically from a wide hollow cone to a narrow solid cone as the gas to liquid momentum flux ratio increases. Also, the injector with the shorter recess needs a higher momentum flux ratio to have a similar spray pattern to that with the longer recess. Based on the measured spray contraction parameters, the critical momentum flux ratio, which determines the near-injector spray pattern as internal or external mixing, correlates with the recess ratio.

Experimental Study on Spray Characteristics of Gas-Centered Swirl Coaxial Injectors

Gas-centered swirl coaxial injectors have become an important subject of study for staged combustion rocket engines with hydrocarbon fuels. While these injectors are employed successfully in rocket engines, it is very rare to find the related research results as applicable to design data. An experimental study on spray characteristics of gas-centered swirl coaxial injectors has been performed. The effects of momentum flux ratio and recess length on the spray characteristics have been investigated by cold flow tests with a photographic technique. The liquid intact length L, which profoundly affects the global spray characteristics, decreases as the momentum flux ratio M increases. The critical momentum flux ratio Mc is introduced to identify the flow patterns as internal or external mixing in the injectors. Concerning the effect of the recess length lR, it is shown that the spray cone angle and the drop size decrease as lR increases.