Shear Coaxial Injector Research Articles

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.

Numerical Simulation of Liquid Fuel Injection in Combustion Chamber

Combustion of cryogenic propellants is of great technological interest for researchers and scientists nowadays. Due to high specific impulse, the use of liquid oxygen and hydrogen as cryogenic propellants will have preferences in the years to come. In the present study, RCM-3 configuration of MASCOTTE test facility is used for the investigation of supercritical cryogenic fuel combustion. The injector of this test case is a shear-coaxial injector consisting of a core of liquid oxygen surrounded by a high-speed flow of gaseous hydrogen. Pressure-based steady-state 2D axisymmetric scheme with non-premixed combustion model and compressibility effect under non-adiabatic conditions is used in the present study. Rich fuel stream flammability limit of 0.2 is used to accommodate non-equilibrium effects. Simulations are carried out for different turbulence models with ideal and real gas assumptions. Pressure-implicit with splitting of operators is used for pressure–velocity coupling while standard k– $$\varepsilon $$ , standard k– $$\omega $$ , and SST k– $$\omega $$ turbulence models are used for the parametric study. Effects of ideal gas and real gas assumptions are also studied. A very low value of under-relaxation factor for density (0.01) is used to encounter stability issues. Computed results show that SST k– $$\omega $$ turbulence model with real gas assumptions provide qualitatively as well as quantitatively reasonable and encouraging results. Although peak temperature value is under-predicted, its location and temperature profile along the axis are accurately predicted through real gas assumption, whereas results with ideal gas are far away from experimental values. This provides conclusive evidence that the ideal gas assumption is not appropriate for fluids in the cryogenic state as liquid oxygen in the present study. Improved modeling and inclusion of detailed chemical kinetic mechanism will provide much improved results. Present results are very promising and encouraging to use CFD for the simulation and modeling of cryogenic fuel combustion in the supercritical state.

Full-length visualisation of liquid oxygen disintegration in a single injector sub-scale rocket combustor

This work presents results of an effort to create an extended experimental database for the validation of numerical tools for high pressure oxygen-hydrogen rocket combustion. A sub-scale thrust chamber has been operated at nine load points covering both sub- and supercritical chamber pressures with respect to the thermodynamic critical pressure of oxygen. Liquid oxygen and gaseous hydrogen were injected through a single, shear coaxial injector element at temperatures of around 120 K and 130 K, respectively. High-speed optical diagnostics were implemented to visualise the flow field along the full length of the combustion chamber. This work presents the analysis of shadowgraph imaging for characterising the disintegration of the liquid oxygen jet. The large imaging data sets are reduced to polynomial profiles of shadowgraph intensity which are intended to provide a more direct means of comparison with similarly reduced numerical results. Comparing half-lengths of these profiles across operating conditions show clear groupings of load points by combustion chamber pressure and mixture ratio. All load points appear to collapse to an inverse dependence of length on impulse flux ratio.

Effects of nitrogen film cooling on ignition transition of gaseous oxygen/kerosene spray combustor

ABSTRACTThe effects of nitrogen film cooling on ignition transition under different conditions of film cooling injection pressure were observed. Gaseous oxygen and liquid kerosene were used as propellants, injected using a shear coaxial injector. Gaseous nitrogen as the film coolant was injected along the gap between the combustion chamber and outer injector walls. A dynamic pressure transducer and a high-speed camera with a band-pass filter were respectively used. The over-peak pressure and pressure rise time were detected in cases with film cooling. With the increase in the differential pressure, the over-peak pressure decreased, and pressure rise time increased. These phenomena are explained by the interruption of normal propellant injection in the mixing zone of propellant. The combustion instability intensity decreased with the increase in the differential pressure between the combustion chamber and injected film coolant. Flame structures were visualized to confirm the unstable combustion flame during the ignition transition.

Experimental and numerical investigation of combustion characteristics on GO2/GH2 shear coaxial injector

This study presents the results of a visualization investigation of the gas–gas injection combustion characteristics using a shear coaxial injector of non-premixed gaseous oxygen and gaseous hydrogen (GO2/GH2) under elevated ambient pressures. Advanced optical diagnostic approaches, including hydroxyl (OH) planar laser-induced fluorescence (PLIF) and high-speed imaging, are employed to capture the spatial distribution of the OH radical and the instantaneous flame dynamics near the faceplate. Experimental results indicate that an instantaneous reaction zone represents an apparent local extinction and re-ignition phenomenon, but does not affect the overall time-averaged reaction distribution. In addition, this study adopts the same ignition device and sequence used in a previous study; however, a different ignition process occurs. The reasons for this difference may be attributed to the fact that the exorbitant pressure peak disturbs the originally stable, downstream, cold-flow fields near the axis, which radically change the initial ignition and flame propagation processes. In order to demonstrate the feasibility and effectiveness of the applicability of computational fluid dynamics (CFD), several different associations of Reynolds averaged Navier–Stokes (RANS) approaches are compared with each other and with the experimental results. This proves that the turbulent model plays a dominant role in affecting the flow field structure. Based on the comparison with optical diagnostic results, a numerically-aided approach using RANS simulation is proposed to quantify the experimental results. This study is a first attempt to establish a standard corresponding relation between the simulation results and the optical diagnosis results.

Laser ignition of a multi-injector LOX/methane combustor

This paper reports the results of a test campaign of a laser-ignited combustion chamber with 15 shear coaxial injectors for the propellant combination LOX/methane. 259 ignition tests were performed for sea-level conditions. The igniter based on a monolithic ceramic laser system was directly attached to the combustion chamber and delivered 20 pulses with individual pulse energies of $${33.2 \pm 0.8 \,\text{ mJ }}$$ at 1064 nm wavelength and 2.3 ns FWHM pulse length. The applicability, reliability, and reusability of this ignition technology are demonstrated and the associated challenges during the start-up process induced by the oxygen two-phase flow are formulated. The ignition quality and pressure dynamics are evaluated using 14 dynamic pressure sensors distributed both azimuthally and axially along the combustion chamber wall. The influence of test sequencing on the ignition process is briefly discussed and the relevance of the injection timing of the propellants for the ignition process is described. The flame anchoring and stabilization process, as monitored using an optical probe system close to the injector faceplate connected to photomultiplier elements, is presented. For some of the ignition tests, non-uniform anchoring was detected with no influence onto the anchoring at steady-state conditions. The non-uniform anchoring can be explained by the inhomogeneous, transient injection of the two-phase flow of oxygen across the faceplate. This characteristic is verified by liquid nitrogen cold flow tests that were recorded by high-speed imaging. We conclude that by adapting the ignition sequence, laser ignition by optical breakdown of the propellants within the shear layer of a coaxial shear injector is a reliable ignition technology for LOX/methane combustors without significant over-pressure levels.

Numerical and experimental investigations of single-element and double-element injectors using gaseous oxygen/gaseous methane

The objective of this study is to investigate the effects of fuel/oxidizer (F/O) momentum ratio on the mechanism of mixing and combustion of single-element and double-element shear coaxial injectors numerically and experimentally using gaseous oxygen/gaseous methane (GO2/GCH4) and non-intrusive optical diagnostics technique based on planar laser induced fluorescence (PLIF). Instantaneous OH-PLIF images demonstrate that the flame with higher F/O momentum ratio exhibits appearance characteristics with more obvious wrinkled shear layers, extinction and kernels events. Simulation and time-averaged results show that with the increase of F/O momentum ratio at the condition of fixed total mass flow rate, the initial combustion position becomes closer to the front of the chamber. The large mass flow rate of the double-element combustor is beneficial to the thermal protection of the chamber wall. Based on the high-speed images and velocity distribution obtained from numerical data, it is detrimental for the propagation of the flame front at the injection condition of high F/O momentum ratio. In addition, according to the infrared thermal pictures, the flame outside the chamber becomes broader with the increase of F/O momentum ratio, and the plume of double-element combustor will challenge the thermal protection around the equipment. Moreover, new methods are applied in this paper to extract the flame boundary of the OH-PLIF diagrams and to calculate the number of the pixels of the plume from infrared thermal imager results.

전단 동축형 인젝터를 통해 분사된 기체메탄-기체산소 확산화염의 연소특성

전단 동축형 인젝터를 통해 분사된 기체메탄-기체산소 확산화염의 연소특성

Application of the Time-Domain Impedance Boundary Condition to Large-Eddy Simulation of Combustion Instability in a Shear-Coaxial High Pressure Combustor

The time-domain impedance boundary condition (TDIBC) is used within a large-eddy simulation (LES) framework to investigate self-sustained longitudinal combustion instability in the Continuously Variable Resonance Combustor (CVRC) rig, a high-pressure, shear-coaxial injector combustor studied experimentally at Purdue University. A modified version of the constant mass characteristic boundary condition is used to account for the impedance at the oxidizer inlet, which is truncated and therefore becomes an approximation of the full inlet. Both linear and non-linear models for the inlet reflection coefficients are developed and compared against the full injector LES. The CVRC rig exhibits different amplitude levels of the pressure oscillation depending on the length of the oxidizer injector, and thus, offers a range of conditions to evaluate the use of the TDIBC. The combustor with a truncated oxidizer injector length of 2.05 cm is used to simulate conditions equivalent of three different oxidizer injector lengths of 9 cm, 12 cm and 14 cm, which are known to exhibit semi-stable, unstable and highly unstable behavior, respectively. The results are compared with LES that explicitly resolve the full oxidizer post without impedance. It is shown that the prediction by the non-linear reflection coefficient model is much better than the linear model. Further analysis is carried out to highlight the strengths and the limitations of the model.

Large-Eddy Simulation of Supercritical Combustion: Model Validation Against Gaseous Injector

Supercritical combustion has attracted significant interest due to its applications in high-pressure combustion devices. Validation of supercritical combustion modeling, however, has not been well reported because of the lack of experimental data with sufficient spatial and temporal resolution as well as the complexity associated with modeling and simulations. The present work begins to bridge this gap with a systematic examination of gaseous combustion of a shear coaxial injector at supercritical conditions, using both large-eddy simulation and detached-eddy simulation approaches. The formulation accommodates the full conservation laws and real-fluid thermodynamics and transport theories. Turbulence/chemistry interactions are treated by means of the flamelet and flamelet/progress-variable approaches. A nonpremixed jet flame (Sandia flame D) was first considered for code validation at ideal-gas conditions. The gaseous combustion at supercritical conditions was then studied systematically using different combinations of turbulence closure and combustion models. Special attention was given to comparison with measured wall heat flux. The large-eddy simulation/flamelet, detached-eddy simulation/flamelet, and large-eddy simulation/flamelet/progress-variable approaches produced qualitatively similar results in terms of flow and flame structures as well as wall heat flux. The present work was also compared with studies conducted by other research groups.

Comparative Study of Mixing Characteristics of Particle-Laden Shear and Swirl Coaxial Jets

The effect of velocity ratio and recess on the mixing characteristics of gas-particle swirl and shear coaxial injectors was investigated. The concentration and mixing efficiencies of the particles and gas were measured by planar nephelometry based on Mie scattering and acetone planar laser-induced fluorescence methods, respectively. The mixing mechanism and mixing efficiency of the shear and the swirl coaxial injectors were compared with and without recess. The results showed that the mixing efficiency of the former was higher than that of the latter in the case without a recess because the swirling momentum of the coaxial gas did not change the particle concentration half-width. The mixing efficiencies of both injectors in the case without a recess were sensitive to changes in the velocity ratio. However, in the case of injectors with a recess, the mixing efficiency of the swirl coaxial injector was higher than that of the shear coaxial injector because the internal interaction of two propellants led to an increase in the particle concentration half-width, and the mixing efficiency of the swirl coaxial injector was less sensitive to changes in the velocity ratio than that of the shear coaxial injector.

Numerical Simulations of the Flame of a Single Coaxial Injector

The processes of mixing and combustion in the jet of a shear-coaxial injector are investigated. Two test cases (nonreacting and reacting) are simulated using the commercial computational fluid dynamics code ANSYS CFX. The first test case is an experiment on the mixing in a nonreacting coaxial jet carried out with the use of planar laser induced fluorescence (PLIF). The second test case is an experiment on the visualization of hydrogen-oxygen flame using PLIF of OH in a single injector combustion chamber at pressure of 53 bar. In the first test case, the two-dimensional axisymmetric simulations are performed using the shear-stress turbulence (SST) model. Due to the dominant flow unsteadiness in the second test case, the turbulence is modeled using transient SAS (Scale-Adaptive Simulation) model. The combustion is modeled using the burning velocity model (BVM) while both two- and three-dimensional simulations are carried out. The numerical model agrees with the experimental data very well in the first test case and adequately in the second test case.

Coupled heat transfer analysis of thrust chambers with recessed shear coaxial injectors

To investigate the effects of recessed lengths on combustion performance and heat loads in LOX/methane thrust chambers with shear coaxial injectors, a coupled numerical methodology is developed to solve the combustion and heat transfer in thrust chambers with regenerative cooling. In this methodology, the transcritical turbulent combustion is modeled by a validated non-adiabatic flamelet model considering real-fluid properties; turbulent flows within the thrust chamber and cooling channels are computed by a pressure-based coupled algorithm. The validation indicates that the prediction with detailed chemistry mechanism and the Chung method confirms quantitatively to literature experimental data. The results reveal that the recess causes an increase of wall heat flux in the whole thrust chamber and makes the heat flux peak in the combustion chamber moves downstream. Furthermore, both the heat flux peaks in the combustion chamber and nozzle increase first and then decrease as recessed lengths increase. Meanwhile, chamber pressure, hot-gas temperature, and the averaging heat flux of the combustion chamber wall are positively correlated with recessed lengths. However, the heat loads are more sensitive to the recessed lengths than chamber pressure and hot-gas temperature. Much attention should be paid to the protection of chamber wall.

Experimental investigations of coaxial injectors in a laboratory-scale rocket combustor

An investigation is performed through experiments of a laboratory-scale optically accessible combustor equipped with a single-element shear coaxial injector. Various optical diagnostics approaches, such as hydroxyl (OH) planar laser-induced fluorescence (OH-PLIF), high-speed imaging and infrared imaging, are simultaneously used to determine the OH radical spatial distribution, flame instantaneous fluctuations, and temperature fields outside the combustor nozzle. This study emphasizes the combustion characteristics and flame stabilizations of two propellant combinations under the same thrust level, including gaseous hydrogen/gaseous oxygen (GH2/GO2) and gaseous methane/gaseous oxygen (GCH4/GO2). Moreover, to validate a pressure-scaling criterion, different pressure conditions are also considered. Based on the results of the high-speed and OH-PLIF images, combustion similarities are evident under different chamber pressures for each propellant couple. The two kinds of flame are seen to be stabilized in the vicinity of the oxidizer injector lip but the anchor region in the GCH4/GH2 cases is found to change dynamically near the lip. The lift-off GCH4/GO2 flame exhibits evident unsteady combustion phenomenon in the combustor. Experimental data indicates that GCH4/GO2 flame stabilization is hard to achieve compared to the GH2/GO2 flame. Furthermore, the GCH4/GO2 flame has a greater heat releasing intensity and gentler reaction zone distribution. This indicates that relative to the GH2/GO2 flame, the GCH4/GO2 flame causes major issues with regard to heat protection of the inner wall in the combustor. Meanwhile, according to the infrared images, the GCH4/GO2 plume has a lower outlet core temperature and larger temperature gradient, which is an advantage for employment in future space engines. This study, identifying preliminary behavior differences between the GH2/GO2 and GCH4/GO2 flames, will be applicable for future engine design.

Disintegration process and performance of a coaxial porous injector

In order to understand the breakup performance of coaxial porous injectors, the sprays of coaxial porous injectors with two different porous material cylinder lengths were compared with those of conventional shear coaxial injectors. To allow comparison, the wall injection lengths were designed to be equivalent to the value of the recess depth. Cold flow sprays were visualized using back-lit photography methods and analyzed quantitatively with a laser diffraction apparatus, in order to study the effects of the momentum flux ratio and Weber number on the breakup for each type of injector. In case of the shear coaxial injector, the large liquid core was observed in low air mass flow rate condition. However, the destabilization of the liquid jet from the coaxial porous injector is almost complete within the inner region, near the injector face plate. Additionally, better breakup performance in low gas flow rate condition was obtained when the porous cylinder length decreased, while the shear coaxial injectors showed better breakup efficiency when the recess length increased. In conclusion, the different breakup process caused by the radial momentum in the inner region of the porous injector disintegrated the liquid core.

Combustion properties of gaseous CH4/O2 coaxial jet flames in a single-element combustor

Fundamental combustion properties of gaseous methane (CH4)/oxygen (O2) coaxial jet flames in a single-element combustor are experimentally evaluated as a preliminary step for subsequent studies of injection at very low temperature or using liquid O2 for CH4/O2 bipropellants, recently appearing to be the oncoming liquid bipropellant. A combustion chamber with quartz windows, a single shear coaxial injector and an exhaust nozzle on the downstream of the chamber is considered for the present study. Focusing on the measurements of the ignition and combustion stability limits of the coaxial jet flames in the chamber, flame visualization is also conducted by OH∗ chemiluminescence, schlieren imaging and direct imaging. Results show the ignition limits restricted than the combustion stability limits. Flame behaviors are largely classified into two, the stably attached flame and the oscillating, liftoff (near-blowout) flame. Due to cooling effects on the wall of the chamber, stably liftoff flame is not observed. The stability of the flame is greatest at fuel-rich condition (based on the injected amounts of CH4 and O2). From the flame visualization flame thickness is found to be smaller than the injector lip thickness and insensitive to injection conditions. The laminar-flow behavior near the injector exit due to the strong burning of pure O2 is observed even for high Reynolds numbers (Re). The flame visualization also exhibits the recirculating O2 that enhances burning in the combustor through the reaction with the outer fuel jet. The results of ignition limits, combustion stability limits and flame visualization can be used as a database for researches of modeling the gaseous CH4/O2 jet flames in the combustion chamber.

Combustion behaviors of GO2/GH2 swirl-coaxial injector using non-intrusive optical diagnostics

This research evaluates the combustion behaviors of a single-element, swirl-coaxial injector in an atmospheric combustion chamber with gaseous oxygen and gaseous hydrogen (GO2/GH2) as the propellants. A brief simulated flow field schematic comparison between a shear-coaxial injector and the swirl-coaxial injector reveals the distribution characteristics of the temperature field and streamline patterns. Advanced optical diagnostics, i.e., OH planar laser-induced fluorescence and high-speed imaging, are simultaneously employed to determine the OH radical spatial distribution and flame fluctuations, respectively. The present study focuses on the flame structures under varying O/F mixing ratios and center oxygen swirl intensities. The combined use of several image-processing methods aimed at OH instantaneous images, including time-averaged, root-mean-square, and gradient transformation, provides detailed information regarding the distribution of the flow field. The results indicate that the shear layers anchored on the oxygen injector lip are the main zones of chemical heat release and that the O/F mixing ratio significantly affects the flame shape. Furthermore, with high-speed imaging, an intuitionistic ignition process and several consecutive steady-state images reveal that lean conditions make it easy to drive the combustion instabilities and that the center swirl intensity has a moderate influence on the flame oscillation strength. The results of this study provide a visualized analysis for future optimal swirl-coaxial injector designs.

동축형 다공성재 분사기의 분열 메커니즘

동축형 다공성재 분사기에서는 기체가 분사기 중심을 지나는 액체 제트 주위를 둘러싼 다공성재를 통해 액체 제트를 향해 분사된다. 분사 방법의 차이로 인한 전단동축 분사기와 동축형 다공성재 분사기의 분열 메커니즘 차이를 살펴보기 위해, 전단동축 분사기와 동축형 다공성재 분사기를 사용하여 수류시험과 2-D 축대칭 수치해석을 진행하였다. 같은 유량조건에서의 가시화 이미지와 분무 평균입경을 비교하였으며, 수치해석을 통해 분사기 내부에서의 속도 분포가 액체 제트에 어떤 영향을 끼치는지 고찰하였다. 결과적으로, 기체의 유량이 늘어날수록 분사기 내부 속도가 낮음에도 동축형 다공성재 분사기의 미립화 및 혼합 성능이 전단동축 분사기에 비해 유리함을 확인할 수 있었다. In a coaxial porous injector, a gas propellant is injected through the porous cylinder surface to the liquid jet which is encircled by a porous cylinder. In this study, to observe the differences in disintegration mechanisms between a shear coaxial injector and a coaxial porous injector, cold-flow tests and 2-D axisymmetric numerical analysis have been carried out. The shadowgraph images and Sauter mean diameters were compared in similar experimental conditions, and the effects of velocity distributions at the inner injector region on the disintegration of liquid jet were investigated through the numerical calculations. As a result, in high air mass flow rate condition, the disintegration performance of coaxial porous injector is better than shear coaxial injector, in spite of a lower velocity at the inner injector region.

Dynamic characteristics of the recessed chamber within a gas–liquid coaxial injector

In this article, a linear dynamic characteristic model of the recessed chamber in a gas–liquid shear coaxial injector is developed. The gaseous injector flow before the recessed chamber is considered to be steady, and we just investigate the response of pressure drop across the recessed chamber to the mass flow rate fluctuation of the liquid injector. The transfer function of the recessed chamber is obtained, considering the distribute characteristics of liquid droplets within the recessed chamber. The results show that the amplitude of pressure drop pulsation would decrease as the liquid flow pulsation frequency increases, when the variation of liquid velocity is not taken into account. While considering the variation of liquid velocity, the result is opposite. The amplitude of pressure drop oscillation on the boundary of the recessed chamber increases with the increase in liquid velocity oscillation frequency.

Effects of kerosene heating on dynamic characteristics of GOx/kerosene combustor

The objective of this study was to observe low-frequency instabilities caused by heating of kerosene under supercritical operating conditions. Gaseous oxygen and liquid kerosene were injected using a shear-coaxial injector. Under specific heating conditions, the fuel heating system induced an extremely low frequency pressure fluctuation ranging from 9.9 to 11.4Hz. When pressure oscillation occurred in the heating system, low-frequency combustion instability was subsequently induced in the range of 30–200Hz. To understand the effects of the fuel heating temperature on the combustion instability, the dynamic pressure and OH* chemiluminescence intensity were measured in a combustion chamber at high speed. Further, the reacting spray of the combustion was visualized by a shadowgraph technique. In this experiment, an approximate fuel pressure of 3.0MPa was employed in order to attain a supercritical condition of kerosene. The measured dynamic pressure and chemiluminescence intensity in the time domain were converted to frequency-domain spectra by fast Fourier transform. Analysis of the dynamic pressure and chemiluminescence intensity measurements confirmed that the low-frequency pressure oscillation in the heating system had an influence on the combustion instability. From the visualization data, it was also revealed that there existed varying amplitude levels of flow rate fluctuation. This fluctuation in turn caused a periodic injection of kerosene at a frequency similar to both the combustion instability frequency and the OH* chemiluminescence intensity frequency.