Liquid-centered Swirl Coaxial Injector Research Articles

Numerical Study on the Self-Pulsation Characteristics of LOX/GH2 Swirl Coaxial Injector

To investigate the self-pulsation characteristics of a liquid-centered swirl coaxial injector with liquid oxygen (LOX) and gas hydrogen (GH2) as the working mediums under supercritical condition, a numerical simulation was employed. The transient simulation of the flow and injection process of cryogenic propellant was carried out using the RNG k−ε turbulence model, VOF model, and Peng-Robinson equation of state. The frequency spectrum of calculated pressure oscillation agreed with the experimental data. The amplitude-frequency characteristics of recess region, LOX, and GH2 paths when self-pulsation occurs were analyzed. The effects of operating parameters, such as the flow rate of LOX or GH2 and the initial GH2 temperature, on the self-pulsation, were evaluated specifically. Results reveal that the self-pulsation results from the periodic variation of pressure and velocity caused by the periodic blocking of annular gas by the liquid sheet. The dominant frequencies of pressure oscillation in the recess region, upstream of LOX, or GH2 path are diverse. But for the points in each region, the dominant frequency is about the same. When the LOX/GH2 mixing ratio increases, the liquid sheet thickness and the number of liquid filaments increase. The position where filaments are massively broken into droplets moves further downstream. For the same mixing ratio, the flow rate of LOX has a greater impact on the atomization features. The pressures corresponding to low or high frequency increase when the initial GH2 temperature raises. The higher temperature would shift the dominant oscillation between the low and high regimes.

Experimental Study on the Unsteady Spray Combustion Process of a Liquid Oxygen/Methane Swirl Coaxial Injector.

The present study experimentally investigated the dynamic spray combustion process of a liquid-centered swirl coaxial injector using liquid oxygen/methane in an optically accessible liquid rocket engine. Data were obtained at combustor pressures from 0.4 to 1.8 MPa and the ratio of the oxidizer mass flow rate to the fuel rate between 1.32 and 1.55. Liquid oxygen was injected at 120 K, and the injection temperature of gaseous methane was about 285 K. Based on the obtained spatial distribution and oscillation characteristics of liquid oxygen/methane flame, the combustion process was described by four subprocesses: ignition, low-frequency oscillation combustion, quasi-steady state combustion, and shutdown. In the quasi-steady state combustion subprocess, both the flame length and the normalized flame area are the largest, and the flame expansion angle is the smallest. At the initial stage of combustion, the instability of the liquid oxygen phase state leads to flame instability, which generates low-frequency unstable combustion with a dominant frequency of 93.74 Hz. In addition, the high-frequency (2500–3000 Hz) oscillation of the flame appeared in the whole combustion process. It has been confirmed to be caused by the self-pulsation of spray. Furthermore, with the increase in liquid oxygen manifold pressure, the liquid oxygen phase state changes from a two-phase mixture of liquid and gaseous oxygen to a liquid phase, which increases the mass flow rate of liquid oxygen entering into the combustor, thus generating the increase in the high oscillation frequency of the flame through the whole combustion process.

The break phenomenon of self-pulsation for liquid-centered swirl coaxial injectors

Self-pulsation boundaries that spray switches between stable and self-pulsated behaviors are experimentally obtained under water and air flow conditions for a liquid-centered swirl coaxial injector configured with different recess lengths. The underlying mechanisms responsible for self-pulsation are investigated comprehensively. Results show that for the non-recessed injector, self-pulsation only occurs in a relatively narrower region, associated with weaker spray oscillation with the amplitude less than 2 mm2/Hz correspondingly. In particular, for the injector with larger recess length, a break region is generated with increasing the momentum flux ratio. It is indicated that spray transforms from the self-pulsated behavior to the stable behavior, and develops into self-pulsated behavior once again. The break phenomenon of self-pulsation reveals a destabilizing/stabilizing effect created by the violent gas-liquid flow dynamics. Self-pulsation occurring on either side of the break region is found to be caused by different mechanisms. As such, self-pulsation of the liquid-centered swirl coaxial injector is classified into two types. The first type occurs in the injectors configured with relatively small recess length, or with large recess length before the occurrence of the break phenomenon. This self-pulsation is caused by the periodic blockage of the conical liquid sheet. Whereas, the second type is attributed to the periodic squeezing influences of annular gas on the conical liquid sheet, corresponding to the self-pulsation occurring in the injectors with larger recess length and after the disappearance of the break phenomenon.

Numerical investigation on spray self-pulsation characteristics of a liquid-centered swirl coaxial injector

Self-pulsation characteristics of a liquid-centered swirl coaxial (LCSC) injector are investigated numerically based on the coupled volume of fluid VOF and Level Set (CLSVOF) model. Good agreements of simulation results and experiment data, in aspect for self-pulsation frequency and spray angle, are generally achieved. On the basis of the flow dynamics of the injector, mechanism of spay self-pulsation in recessed LCSC injector is proposed. The self-pulsation arises from the periodic blocking of annular gas by the liquid sheet. Self-pulsated “Christmas tree” spray patterns result from the periodic vortex rings generating and shedding around the liquid sheet. Theoretical analysis demonstrates that the self-pulsation frequency is almost the same as the frequency of the liquid sheet impacting the recess chamber wall, and the model may be able to predict the self-pulsation frequency. Pressure oscillation inside the air-core is closely related to the primary breakup of the liquid sheet, and the frequency is consistent with the self-pulsation frequency. Effects of the gas-liquid mass flow rate ratio (GLR) on the self-pulsation frequency and pressure oscillation frequency inside the air-core are negligible. The self-pulsation intensity peaks under conditions with the critical mixing flow pattern, while, the pressure oscillation intensity inside the air-core increases with the elevating of GLR. It seems that the spray self-pulsation could improve the spray angle.

Numerical investigation of spray self-pulsation characteristics of liquid-centered swirl coaxial injector with different recess lengths

The self-pulsation characteristics of a liquid-centered swirl coaxial (LCSC) injector are investigated numerically based on the coupled level set and volume of fluid (CLSVOF) model. Effects of recess length on self-pulsation characteristics are evaluated and discussed. The results demonstrate the crucial role of recess length in exciting self-pulsation. As the recess length increases, spray patterns vary from stable hollow cone to self-pulsated “Christmas tree” shape, and the characteristic frequency increases marginally. Compared with the stable spray, when self-pulsation occurs, the droplets are distributed more widely in the radial direction. Self-pulsation is easily excited under conditions with the critical mixing flow. Several methods to suppress self-pulsation have been proposed. The breakup of the liquid sheet is successively dominated by surface waves and spray self-pulsation as the recess length increases. The waves in both the azimuthal and the axial directions are captured and investigated. The unstable waves are composed of sinuous mode and varicose mode. The varicose waves are responsible for the primary breakup of liquid sheet. The results for the self-pulsated spray suggest that the primary breakup of the liquid sheet is coupled with pressure oscillation inside the confined region. Self-pulsation of recessed LCSC injector is suggested to be caused by the periodical blocking of the annular gap by the liquid sheet.

Numerical investigation of the effects of gas-liquid ratio on the spray characteristics of liquid-centered swirl coaxial injectors

We investigate the effects of gas-liquid ratio (GLR) on the spray and atomization process of a liquid-centered swirl coaxial injector using the coupled level-set and volume of fluid (CLSVOF) method. The flow field, gas-liquid interaction, and breakup of liquid film during adaptive mesh refinement are evaluated and analyzed. The results show that, with the increase of GLR, the atomization process becomes dominated successively by the instability of the liquid film, strong gas-liquid interaction and spray self-pulsation. Moreover, more droplets are found near the center of the spray cone while the spray angle decreases. The inclined surface waves on the liquid film may be formed owing to Rayleigh-Taylor instability. When GLR is relatively large, holes are produced and break up on the liquid film due to the high velocity of the gas. Then, the gas-liquid interaction pushes small droplets towards the center of the spray cone, while large droplets and ligaments distribute on the periphery of the spray. With even larger GLRs, self-pulsation occurs, which is found to determine the flow structures inside the spray cone and to generate, shed and dissipate vortices periodically. Furthermore, when self-pulsation occurs, the near-field pressure and mass flow rate fluctuations increase rapidly.

Effects of annulus width and post thickness on self-pulsation characteristics for Liquid-Centered Swirl Coaxial Injectors

Based on the experimental and numerical methods, parametric studies on annulus width and post thickness of a liquid-centered swirl coaxial (LCSC) Injector were performed under atmospheric conditions to evaluate their effects on the internal flow dynamics, self-pulsation characteristics and atomization qualities. Filtered water and dried air were employed as simulant mediums. 2-D unsteady numerical simulations based on the swirl axi-symmetric model were conducted. The instantaneous self-pulsated spray images were captured with a back-lighting photography technique. By means of the Dantec Phase Doppler Anemometry (PDA) system, the atomization qualities including Sauter mean diameter (SMD), velocity of droplets and mass flow rate have been characterized. Results showed that annulus width and post thickness of inner injector together with the recess length have significant influences on self-pulsation. Violent liquid sheet pulsations are caused by the tremendous dynamic pressures imposed from the blocked annular gas stream. Whether self-pulsation occurs or not and self-pulsation intensity essentially depend on the relative magnitude of liquid film angle and recess angle. Increasing the annulus width and post thickness leads to an accompanying decay in self-pulsation. Furthermore, it was found that the mass flow rate distribution of self-pulsated spray is double-peaked while that of stable spray is single-peaked. SMD and velocity distributions of all droplets are more susceptible to the injection conditions rather than the geometrical parameters.

Effects of backpressure on self-pulsation characteristics of liquid-centered swirl coaxial injectors

A combined experimental and theoretical investigation is conducted to explore the self-pulsation characteristics generated by a liquid-centered swirl coaxial injector (LCSC) under various chamber backpressures. Filtered water and dried air were employed as simulant mediums. Data were obtained at pressures from 0.1 to 3.01 MPa and the momentum flux ratios from 0.004 to 15.99. The back-lighting photography technique was employed to capture the instantaneous self-pulsated spray and stable spray images with a high speed camera. A new empirical equation was presented to calculate the liquid film thickness at the pressure swirl injector exit without trumpet, and the flow characteristics in the trumpet of the inner injector have been calculated. Based on these researches, a theoretical analysis model has been proposed to analyze the flow pattern by calculating the liquid film angle in recess chamber. With the increase of backpressure, the flow in the LCSC injector changes from the outer mixing flow to the critical mixing flow, and develops into the inner mixing flow ultimately. Meanwhile, self-pulsation is intensified initially and then suppressed, while self-pulsation frequency fluctuates within a certain range. Self-pulsation is the strongest when liquid film angle is around the recess angle. Both variations of backpressure and injection conditions can change the flow pattern in recess chamber of the LCSC injector. When the flow pattern deviates from the critical mixing flow, self-pulsation is suppressed and the spray angle decreases accordingly. Backpressure affects self-pulsation by transforming the flow pattern in recess chamber and then strengthening or weakening the blocking actions of the conical liquid sheet.

Investigation of self-pulsation characteristics for a liquid-centered swirl coaxial injector with recess

A combined experimental and numerical investigation was conducted to explore the self-pulsation characteristics generated by a liquid-centered swirl coaxial injector with an inner post recess of 5 mm. A back-lighting photography technique was employed to capture the instantaneous flow patterns in recess chamber with a high speed camera. Pressure oscillations in recess chamber were also measured by the miniature pressure transducer. 2-D unsteady numerical simulations basing on swirl axi-symmetric model were performed. Good agreements were generally achieved between numerical simulation and experiment in aspect for the mechanism and characteristic frequency of self-pulsation. It was found that the blocking actions of the conical liquid sheet play a crucial role in self-pulsation for the coaxial injector with recess. Self-pulsation occurs coinciding with strong pressure oscillation in recess chamber. The frequencies of spray oscillation and gas pressure in recess chamber correspond well with each other. Self-pulsation frequencies are approximately linearly proportional to the liquid Reynolds number under certain conditions. In addition, the spray oscillation transforms from high frequency pulsation (at about 3000 Hz) to ultra-high frequency pulsation (ranging from 8000 Hz to 9000 Hz) with an increase in the gas Reynolds number when the mass flow rate of liquid is large enough.

Fluid Oscillations of a Swirl Coaxial Injector Under High-Frequency Self-Pulsation

This study examines data from experiments conducted on a liquid-centered swirl coaxial injector under self-pulsation. The element was operated over ranges of water [] and air [] flow. Inner post recess was varied from 0 to 2 mm. Strong self-pulsations in the range of 1000–4000 Hz were observed with and without inner post recess. An analytical survey of potential fluid-dynamic/resonant oscillators is carried out, which includes consideration of swirl post pressure eigenmodes and liquid film hydrodynamics. Measured self-pulsation frequencies are found to correspond well to a family of computed injector responses attributed to liquid surface wave patterns within the swirl injector vortex chamber. This result provides support for the conclusion that the principal fluid oscillator in defining the frequency of self-pulsation for the test article and conditions investigated here is natural surface wave motion within the injector, and it guides the broader notion that self-pulsation is a state that can develop when internal swirl injector hydrodynamics become sensitive to downstream forcing and feedback.

Details on the Mechanism of High-Frequency Swirl Coaxial Self-Pulsation

In this paper, lower boundaries at which a liquid-centered swirl coaxial injector switches from nonpulsatile behavior to self-pulsation are experimentally resolved under water and air flow conditions for an element configured both with and without inner post recess. Data-based modal decomposition analyses are used to extract distinct near-field spray dynamics from high-speed schlieren imagery. Self-pulsation is found to be instability-induced by gas–liquid interactions that strip liquid from the contiguous spray cone. These interactions take place in a regime where coaxial flows are known to experience fiber-type breakup due to hydraulic and aerodynamic instabilities. Periodic, nonpulsatile spray breakup patterns are detected as a precursor to self-pulsation. The frequency of these nonpulsatile patterns ranges from and compares well with the frequency of self-pulsation at its onset, which directly supports conventional ideas related to the mechanism of self-pulsation. The transition of spray dynamics from nonpulsatile to self-sustained oscillations shows a destabilizing/stabilizing effect as gas velocity increases, establishes temporally unstable spray oscillation at onset, and suggests that self-pulsation may be excited when stripping behavior engages one or more fluid oscillators of the injector element.

Planar Sauter Mean Diameter measurements in liquid centered swirl coaxial injector using Laser Induced Fluorescence, Mie scattering and laser diffraction techniques

An experimental technique is carried out to demonstrate the measurement of planar Sauter Mean Diameter (SMD or D32) distribution in a liquid centered swirl coaxial injector (LCSC) using simultaneous measurements of Mie scattering, Planar Laser-Induced Fluorescence (PLIF) and Laser diffraction technique (LDT). Here water is used as the test fluid with addition of optimized quantities of the organic dye (Rhodamine 6g) for PLIF measurements. Experiments are carried out at three experimental conditions with momentum flux ratios of 6.25, 12.14, and 19.95 respectively. Experiments are carried out to study the effect of dye concentration in LDT. LDT (line of sight) is corrected for multiple scattering effects. The SMD distribution obtained from Liquid Sheet Drop Sizing (LSDS) technique is calibrated using LDT (Malvern particle analyzer) that utilizes the principle of diffraction; the results obtained from both the methods are compared and analyzed using the respective histograms. The variations in the distribution of droplet diameter along the axial and radial locations in the spray field are also studied in detail.

Effects of Recess on the Self-Pulsation Characteristics of Liquid-Centered Swirl Coaxial Injectors

The self-pulsation characteristics of liquid-centered swirl coaxial injectors have been investigated using a Photron Fastcam SA-1.1 camera and a PULSE system (B&K Corp., 3650 type). The effects of both recess length and injection condition were analyzed and discussed. The flow pattern in the recess chamber of a liquid-centered swirl coaxial injector can be classified into three types, namely, outer, critical, and inner mixing flow. Once the critical flow approaches, self-pulsation is about to occur. When self-pulsation occurs, the spray oscillates with a Christmas tree shape. The Christmas-tree-like spray periodically produces droplet clusters, making horizontal spray width and mass flux oscillate periodically. Self-pulsation is the strongest when the flow is around the critical mixing flow. Both variations of recess length and injection condition can change the flow pattern in the recess chamber. When the flow pattern moves away from the critical mixing flow, self-pulsation is suppressed. All the frequency behaviors of self-pulsation indicate that self-pulsation is strongly related to Kelvin–Helmholtz instability. A possible mechanism of self-pulsation is also presented. This mechanism interprets the experiments of the present study and the results of literature well.

Experimental study on the spray characteristics of a dual-manifold liquid-centered swirl coaxial injector

A throttleable rocket engine enables operational possibilities such as the docking of spacecraft, maneuvering in a certain orbit and landing on a planet's surface, altitude control, and entrance to atmosphere-less planets. Thus, throttling methods have long been researched. However, dual-manifold injectors, which represent one throttling method, have been investigated less than others. In this study, dual-manifold and single-manifold injectors were compared to determine the characteristics of dual-manifold injectors. Also, the effects of gas injection were investigated with various F/O ratios. To investigate the characteristics, mass flow rate, spray pattern, spray angle, and droplet size were measured. The spray angle and droplet size were captured by indirect photography. About 30 images were taken to assess the spray patterns and spray angle. Also, 700 images were analyzed to understand the droplet distribution and targeting area, moving to the right from the centerline with 1.11-cm intervals. The droplet size was obtained from an image processing procedure. From the results, the spray angle showed two transition regions, due to swirl momentum in the swirl chamber regardless of the F/O ratio. The droplet size showed similar trends in both dual-manifold and single-manifold injectors except in the low mass flow rate region. In the case of the dual- manifold injector, the spray cone was not fully developed in the low mass flow rate region due to low angular momentum in the swirl chamber.

Comparative Study of Spray Characteristics of Gas-Centered and Liquid-Centered Swirl Coaxial Injectors

This study investigated the spray and Sauter mean diameter characteristics of gas-liquid swirl coaxial injectors by measuring and analyzing spray angles and mean drop sizes. Two different types of gas-liquid swirl coaxial injectors were designed and tested for this experiment: 1) a gas-centered swirl coaxial injector and 2) a liquid-centered swirl coaxial injector. The spray patterns were obtained for both types of injectors. The spray angles of these liquidcentered swirl coaxial injectors showed a decreasing trend as a function of the momentum flux ratio over the conditions studied. Good agreement was generally achieved between the predicted and measured spray angles. On the other hand, with increasing momentum flux ratio, the spray angle of the gas-centered swirl coaxial injector initially decreased when the momentum flux ratios were relatively low, but the trend reversed over the high momentum flux ratio range studied. The Sauter mean diameter data of these two types of injectors were also obtained using an image-processing method. Results showed that the Sauter mean diameter distribution of the liquid-centered swirl coaxial injector had a solid cone shape, whereas the gas-centered swirl coaxial injector exhibited two distinct Sauter mean diameter distribution regimes. The gas-centered swirl coaxial injector had a solid cone shape along the centerline, and it had a hollow cone shape at the periphery.