Gas-centered Swirl Coaxial Injector Research Articles

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.

Study on the influence of the combustion chamber baffle on atomization characteristics of gas-centered swirl coaxial injector

The combustion chamber baffle is an important means to suppress the combustion instability of the combustion chamber of liquid rocket engine. In order to study the influence of spray impacting on the baffle on the atomization characteristics of injector, sufficient experimental research has been carried out in this paper. In the present study, the effects of different gas/liquid flow rates and different baffle structures on the atomization characteristics are investigated. The results show that for gas-centered swirl coaxial injector, a wall film is formed on the baffle when the spray impinges on the baffle and it is broken into liquid ligaments and droplets downstream of the baffle by centrifugal force, resulting in a slower spray velocity near the baffle, a significantly larger sauter mean diameter, an increase in flow flux downstream of the baffle and worse spray uniformity. In addition, it is found that there is a spray zoning phenomena by comparing the spray with different baffles.

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.

Three-dimensional simulation of a gas-centered swirl coaxial injector spray under pulsation conditions

Based on steady-state experiments, a three-dimensional (3D) simulation study under pulsation conditions was carried out. This was performed to study the effect of pulsation of the supply system on the atomization characteristics of a gas-centered swirl coaxial injector. The results indicated that the spray shape resembled a pagoda when the Klystron effect occurred. When the low-frequency pulsation (200 Hz) was applied to the liquid inlet, the peak of the exit pressure was close to the trough of the inlet mass flow rate. Meanwhile, the exit mass flow rate and pressure were approximately sinusoidal. There was no significant lag effect in the response of the exit mass flow rate to the inlet mass flow rate but with a small sawtooth-like oscillation. Increasing the amplitude of the liquid inlet disturbance amplified the exit mass flow rate fluctuation, which was greater than the set amplitude of the inlet pulsation. The dominant frequency of the injector exit mass flow rate and pressure variation was significantly less than the inlet pulsation frequency when the frequency of the liquid inlet pulsation was increased to 1000 Hz. This was due to the low-pass filtering effect of the annular slit. There was no significant change in the overall spray morphology for the gas inlet pulsation. By applying the same amplitude and frequency of pulsation to the gas and liquid inlets of the injector, the pulsation of the liquid inlet had a stronger effect on the injector exit spray than that of the gas inlet.

Spatio-Temporal Analysis of a Self-Pulsating Gas-Centered Swirl Coaxial Injector with Forced Gas Jet

The objective of the present work is to examine the effects of the acoustically forced gas jet on self-pulsating sprays generated from a gas-centered swirl coaxial injector. We conduct the experiments on self-pulsating spray by forcing the gas jet over a range of forcing frequencies and amplitudes. Various image processing techniques, including proper orthogonal decomposition (POD), have been employed to characterize the spray response. Our spatial mapping of dominant frequencies from time-resolved spray width reveals that although both natural and forcing frequencies prevail in the primary breakup region, a modulation frequency (corresponding to the double of the beat frequency) outcompetes the others. The spatial influence of the forcing is also evaluated by measuring liquid mass fluctuations and drop size statistics at various downstream locations. The experiments were extended to investigate the role of various geometrical parameters, such as swirl number, recess, and length of the gas line on the spray response. The influence of the forcing was observed to prevail even in the liquid mass shedding zone with the appearance of all dominant frequencies. This may have important consequences on the thermoacoustic stability of the combustor. In any case, far downstream of the nozzle, the influence of acoustic forcing is not significant to alter the droplet statistics.

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.

Experimental Study on the Dynamic Characteristics of Gas-Centered Swirl Coaxial Injector under Varying Ambient Pressure

To determine the dynamic characteristics of a gas-centered swirl coaxial injector under backpressure, an experimental system of dynamic injection in a backpressure chamber was constructed. Filtered water and nitrogen were used as simulant media for rocket propellants, which are typically used with this kind of injector. An inertial flow pulsator was manufactured to generate the pulsation of the flows that feed to the liquid injector. The electric conductance method was adopted to measure liquid film thickness. After the pulsation of incoming flow in the feedline was tested, and the operating conditions for the injector to start pulsating were validated, the effects of the chamber backpressure and the recess length of the injector on the dynamic characteristics of spray, such as liquid film thickness, breakup length, and amplitude of pulsation, have been investigated in detail. Experimental results demonstrated that the increase in chamber backpressure prompts the liquid sheet to rupture earlier with a shorter breakup length, which results from the increased density of the ambient gas. Chamber backpressure suppresses the pulsation of the outlet flow, especially for a longer recess length. Moreover, a decrease in the recess length results in a reduction in breakup length due to an intense gas–liquid shearing in a narrower recess section. For a lower backpressure, the amplitude of outlet flow generally increases when the recess length increases. However, this phenomenon is not obvious for the conditions of higher backpressure and lower pulsation frequency.

Effects of recess length on combustion instability in a model chamber with a gas-centered swirl coaxial injector

Combustion instability is investigated in a model chamber by changing the recess length of a gas-centered swirl coaxial injector ranging from 0 mm to 20 mm with a 4 mm interval. Effects of recess length on combustion instability are pursued by numerical simulations and experiments and compared with each other. The parameters for evaluation of instability, i.e., damping factor, decay rate, and damping coefficient, are quantified based on numerical and experimental data. The parameters show that acoustic oscillations in the combustor are suppressed relatively more as the recess length of the injector increases. Combustion stability improves more and more with the recess length and then, levels off. Improvement in stability is not appreciable when the recess length is increased further than a critical value. On the other hand, stability increases with equivalence ratio for a lean condition, reaches its maximum, and then, decreases finally for a rich condition. However, injectors with a longer recess length maintain good stability even if the equivalence ratio varies.

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.

Detailed Measurement of Oxidizer-Rich Staged Combustion Injector Dynamics in Model Rocket Combustors

This paper presents experimental results from a model rocket combustor that uses a gas-centered swirl-coaxial injector element injecting warm oxygen and ambient temperature RP-2 fuel with 900 psia chamber pressure, representative of an oxidizer-rich staged combustion cycle. Results from the two injector geometries are analyzed and compared. One was a flat-faced injector with the element ending at a sudden expansion into the chamber. The other used a truncated cone at the injector exit: a design feature of specific interest that appears in drawings for the RD-170 main-chamber injector. To improve spatial resolution of high-frequency pressure measurements, the model injector was scaled up from RD-170 dimensions to facilitate measurements in the injector and head end of the combustor. Measured pressure spectra of the flat-face combustor showed a well-organized first axial mode and harmonics. The truncated-cone geometry showed a more diffuse spectra with higher-frequency modes, with a reduction in amplitude further upstream in the cone. Intermittency in measured dynamics is characterized via multiscale analysis. Adopting concepts from fractal theory, the stability margin of the injector configurations was quantified using the Hurst exponent of pressure oscillations. Comparatively, a larger range of fractal dimensions with smaller fluctuations in the initial reacting zone were observed for the flat-face injector.

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.

Effect of Acoustic Excitation of Gas Flow in a Gas-Centered Swirl Coaxial Injector

Effect of Acoustic Excitation of Gas Flow in a Gas-Centered Swirl Coaxial Injector

Spray patterns of multi-element swirl coaxial injector of interacting spray under different injection conditions

Multi-element injectors have been used in liquid rocket engines to obtain high thrust, and the gas-centered swirl coaxial injector is a representative injection system because it provides high mixing performance. Investigations of the spray characteristics of injection systems have focused on the characteristics of single-element injectors, including the spray angle, breakup, and atomization mechanism of the liquid sheet formation. However, the spray characteristics of multi-element injectors need to be studied because of their usage in real rocket engine systems. In this paper, spray patterns and interacting spray under different injection conditions are analyzed using the backlight imaging technique, and the dominant flow fields are observed using the dynamic mode decomposition method. The spray angle in the case of the gaseous nitrogen with a high Reynolds number is calculated to be lower than the case with a low Reynolds number. From the averaged images, it is found that there are two dominant flow fields: one is located in the vicinity of the injector head and the other is in the interacting spray zone, which is related to a secondary breakup via an adjacent injector. As a result of the dynamic mode decomposition analysis, however, the spray zone near the injector is confirmed to be a more dominant flow field than the interacting spray zone.

Dynamics of Self-Pulsation in Gas-Centered Swirl Coaxial Injector: An Experimental Study

Experimental investigations on the self-pulsation dynamics in a gas-centered swirl coaxial atomizer are conducted using water and air. Experiments are performed in the pulsation regime by precisely varying the momentum flux ratio for different values of swirl number, recess length, and lip thickness. High-speed imaging is used to understand the underlying mechanism of the onset of pulsation. While the transition to the pulsation regime takes a route of intermittency in a nonrecessed injector, the transition in a recessed injector is seen to be spontaneous. The spray width fluctuates significantly in the pulsation regime, and hence the temporal variation in the spray width is used to characterize the pulsation frequencies. Three factors, the constrained shear layer instability between the liquid and the gas streams inside the recess region, the spread rate of the gas jet, and its interaction location with the liquid stream, are shown to dictate the presence of dominant pulsation frequencies and even their higher harmonics in many recessed configurations. It is further shown that the nondimensional pulsation frequency depends only on the lip thickness and is independent of the flow and other geometrical parameters of the injector considered in the present paper.

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.

기체 중심 스월 동축형 분사기가 장착된 모형연소기의 운동량비 변화에 따른 연소불안정성 분석

기체 중심 스월 동축형 분사기가 장착된 모형연소기의 운동량비 변화에 따른 연소불안정성 분석

Thermoacoustic Stability Boundaries in the Model Combustion Chamber with a Gas-Centered Swirl Coaxial Injector

The stability of a combustion chamber with a gas-centered swirl coaxial injector is investigated over a wide range of operating conditions in the aspect of thermoacoustic instabilities. First, flame shapes induced by the injector are analyzed for various recess lengths from experimental results. The spreading angle and flame size are reduced as the recess length increases. And, as a stability criterion, the damping factor is introduced. The operating conditions are divided into 7 sets with 9 tests, and fuel mass flow is fixed in each set. Stability boundaries are identified in terms of damping factor for the 63 tests by experiments without external perturbations. The stability map for the self-excited instability is obtained and it reveals two unstable regions. One is located at low dynamic head ratio and the other one is at high dynamic head ratio. A stable region is found at moderate dynamic head ratios. Relative stability is evaluated by various flow parameters over a wide range of operating conditions in the combustion chamber with a gas-centered swirl coaxial injector.

Study on the damping capacity of a fluidic baffle under different injection conditions

Passive control methods have been widely used to decrease the intensity of combustion instability, and a structural baffle injector has been applied in liquid rocket-engine development. However, the development cost of a structural baffle injector increases because the thrust of a liquid rocket engine should increase as the weight increases. The objective of this paper is to analyze the damping capacity of a fluidic baffle injector using a dissimilar injector arrangement. In this study, a cold-flow test was conducted to observe the effect of using a fluidic baffle injector, and a pulse gun was adopted to generate the pressure wave. A gas-centered swirl coaxial injector and a shear coaxial injector were used to inject the bipropellant. Gaseous nitrogen and liquid water were used for the bipropellant. In addition, multi injectors and a pulse gun were used to compare the effects of a gas-centered swirl coaxial injector and a shear coaxial injector, each acting as the fluidic baffle, on the spray intensity under different conditions. When the gas-centered swirl coaxial injector was used as the fluidic baffle injector, the amplitude increase rate increased as the momentum flux ratio decreased. However, the shear coaxial injector obstructed the pressure-wave propagation. Therefore, the shear coaxial injector was a better fluidic baffle injector than the gas-centered swirl coaxial injector.

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.