Shear Coaxial Injector Research Articles

Effects of Wall-Injection Length on Spray and Combustion in a Coaxial Porous Injector

The mixing performance of an injector determines the combustion performance of a bipropellant rocket engine. In the previous study, a novel coaxial porous injector concept was developed to improve the mixing performance of a shear coaxial injector. It demonstrated a higher characteristic velocity efficiency than that of a typical shear coaxial injector. To clarify the efficiency-improving mechanism of a coaxial porous injector, the combustion flowfields of both injectors were visualized using a high-speed shadowgraph technique. The effect of geometry factors on the spray core length and the combustion performance as well as the differences in the reactive spray behavior between the coaxial porous injector and the shear coaxial injector were observed. The coaxial porous injector with a different wall-injection length showed higher characteristic velocity efficiencies, shorter spray core lengths, and thicker phase-change patterns at the spray core boundary. Changes in wall-injection length had a nonmonotonic effect on both the spray core length and the characteristic velocity efficiency. As opposed to the expected results, the coaxial porous injector with the highest combustion efficiency showed the longest spray core length. The phase-change pattern width, caused by the evaporation of liquid fuel droplets, was proportional to the efficiency.

Investigation of non-premixed flame combustion characters in GO2/GH2 shear coaxial injectors using non-intrusive optical diagnostics

Single-element combustor experiments are conducted for three shear coaxial geometry configuration injectors by using gaseous oxygen and gaseous hydrogen (GO2/GH2) as propellants. During the combustion process, several spatially and timeresolved non-intrusive optical techniques, such as OH planar laser induced fluorescence (PLIF), high speed imaging, and infrared imaging, are simultaneously employed to observe the OH radical concentration distribution, flame fluctuations, and temperature fields. The results demonstrate that the turbulent flow phenomenon of non-premixed flame exhibits a remarkable periodicity, and the mixing ratio becomes a crucial factor to influence the combustion flame length. The high speed and infrared images have a consistent temperature field trend. As for the OH-PLIF images, an intuitionistic local flame structure is revealed by single-shot instantaneous images. Furthermore, the means and standard deviations of OH radical intensity are acquired to provide statistical information regarding the flame, which may be helpful for validation of numerical simulations in future. Parameters of structure configurations, such as impinging angle and oxygen post thickness, play an important role in the reaction zone distribution. Based on a successful flame contour extraction method assembled with non-linear anisotropic diffusive filtering and variational level-set, it is possible to implement a fractal analysis to describe the fractal characteristics of the non-premixed flame contour. As a result, the flame front cannot be regarded as a fractal object. However, this turbulent process presents a self-similarity characteristic.

Properties of inverse nonpremixed pure O2/CH4 coflow flames in a model combustor

In order to evaluate the potential of oxy-combustion of inverse nonpremixed coflow flames using a clean fuel to improve the combustion and emission characteristics compared with the normal coflow flames using air, the fundamental properties of inverse nonpremixed pure oxygen (O2)/methane (CH4) coflow flames in a model combustor are experimentally investigated. The combustion stability (extinction) limits, structure and nitrogen oxide (NOx) emissions of the inverse coflow flames in the quartz-windowed combustion chamber with a single shear coaxial injector and a fully opened exhaust nozzle are measured using OH∗ chemiluminescence, a schlieren imaging system and a gas analyzer. Results show four distinct stability regimes and limits: the attached flame regime, the near-blowout flame regime, blowout limits and blowoff limits. The direct flame and OH∗ chemiluminescence images confirm the cooling effects of chamber walls that cause no steady liftoff flame and only the near-blowout flame. The extinction limits of the inverse CH4/O2 coflow flames are extended compared with the normal CH4/air coflow flames and reasonable levels of NOx emissions are observed, which supports that oxy-combustion of the inverse nonpremixed coflow flames using the clean fuel is acceptable for practical application, providing a useful database for modeling the flames in the confined chamber.

Computational Fluid Dynamics Simulations of a GO2/GH2 Single Element Combustor

A single-element combustor known as the “Penn State preburner combustor” is modeled numerically using the commercial computational fluid dynamics code ANSYS CFX. The aim of computational fluid dynamics modeling is to simulate the wall heat flux, which has been measured experimentally. The simulated combustion chamber has a single shear coaxial injector and operates with gaseous oxygen and hydrogen in a staged combustion configuration. The turbulent flow in the combustion chamber is modeled using the Favre-averaged Navier–Stokes equations and the shear-stress transport turbulence model. The turbulent non-premixed flame is modeled using an extended eddy dissipation model. The developed turbulent combustion model shows good agreement with the experimental data, good convergence, and a short computational time. A mesh convergence study is performed, and a mesh-independent solution is obtained on a mesh with 1.5 million nodes. The complexity of the model is gradually increased until the model is capable of predicting the wall heat flux. The analysis of numerical results shows a significant effect of boundary conditions on wall heat flux predictions. The comparison of the Reynolds-averaged Navier–Stokes simulations with the experimental data demonstrates the capability of Reynolds-averaged Navier–Stokes simulations to predict wall heat fluxes in a rocket combustion chamber.

Ignition of nitrous oxide and ethanol spray in a subscale thruster

To observe the transient behavior of N2O/C2H5OH spray ignition in a subscale thruster, the ignition flow field was visualized by a shadowgraph technique. The windowed combustor with 22 mm inner diameter equipped with a shear coaxial injector, which injected liquid ethanol and gaseous nitrous oxide through the center hole and the annular gap, respectively. Total mass flow rate was about 16 g/s for a combustion pressure of 9 bar. The pressure traces at three different oxidizer feed pressure conditions were compared, and the consistent trend of ignition–quenching–reignition was observed. From the shadowgraph image analysis, we found the indication of combustion inside of the gas injector, which caused the abnormal increase of injector upstream pressure. To prevent this violent phenomenon, some possible solutions were proposed.

Effects of Injector Geometry on Cryogenic Shear Coaxial Jets at Supercritical Pressures

For shear coaxial injectors, which are widely used in liquid oxygen–gaseous hydrogen rocket engines, the key parameters that characterize injector geometry are the recess length, taper angle, and wall thickness of the liquid oxygen post, which is the inner tube of the coaxial injector. In the present study, the effects of the recessed length and taper angle of the liquid oxygen post on cryogenic coaxial jets at supercritical pressures were experimentally investigated. The large-scale structure and mixing enhancement of cryogenic nitrogen and gaseous helium coaxial jets were evaluated using a high-speed backlighting technique. Instantaneous backlit images revealed that the dark core, which is believed to be dense nitrogen, exhibited sinusoidal forms induced by the presence of the outer helium jets. However, the sinusoidal oscillations were less influenced by the injector geometry. The mixing enhancement of coaxial jets was evaluated with respect to the dark-core length and the jet spread angle. The recessed liquid oxygen post enhanced the mixing of cryogenic nitrogen when the recess length was sufficiently large, which suggests that a recess of a certain length is required to improve the mixing of cryogenic coaxial jets at supercritical pressures. Because the decrease in the dark-core length with the recessed length was consistent with the increase in the jet spread angle at the injector exit, the mixing enhancement caused by the recess can be regarded as a result of an improvement in the mixing between the inner and outer jets in the recessed region. The effects of the liquid oxygen post exit taper on the mixing enhancement were more significant than those of the recess. The decrease in the dark-core length with the inner-to-outer momentum flux ratio ( number) is consistent with the relevant empirical correlation at supercritical pressures. Therefore, the mixing enhancement caused by the liquid oxygen post exit taper can be considered the result of an increase in the number, rather than the result of giving azimuthal velocity to the cryogenic nitrogen jet at the liquid oxygen post exit.

Flame Dynamics During Combustion Instability in a High-Pressure, Shear-Coaxial Injector Combustor

We present a large-eddy simulation based investigation of combustion instability in the Continuous Variable Resonance Combustor (CVRC); a high-pressure, shear-coaxial injector combustor studied experimentally at Purdue University. An important characteristic of the CVRC setup is the role that the oxidizer injector length plays in the stable/unstable combustion dynamics occurring within the combustor. We perform three simulations corresponding to different stability regions; one is a semi-stable case whereas the other two cases are unstable. The acoustic pressure fluctuation exhibits a limit cycle behavior in all cases. An abrupt change in the predicted amplitude between stable and unstable cases is also observed, consistent with the experimental trends. Further investigations to characterize the mechanism driving such abrupt change in the stability pattern when the oxidizer injector length is varied, show that acoustic interactions with the flow field lead to subtle changes in the vorticity (and mixing) dynamics, which significantly alters the distribution of species and heat release. In addition, aspects related to flame stabilization such as the mean flame anchoring location and the mode of burning, premixed or non-premixed, are also affected. The analysis of flame structures shows the presence of multi-mode burning regions where premixed and non-premixed flames appear to coexist. However, the dominant burning mode switches from premixed to non-premixed within an acoustic cycle, potentially playing an important role in flame stabilization.

Combustion Characteristics of a Coaxial Porous Injector

A coaxial porous injector was proposed for gas–liquid combustion for improved mixing performance compared to a conventional shear coaxial injector. The coaxial porous injector discharged a gas jet in a direction normal to the center liquid core, and it was surmised that the momentum transfer from the radial gas jet to the axial liquid jet occurred more efficiently than with a traditional shear coaxial injector. To validate the effectiveness of this idea, the combustion performances of a coaxial porous injector and a shear coaxial injector were compared by conducting a hot-firing test using nitrous oxide-ethanol as propellant. The momentum flux ratio at the injector tip was selected as a spraying condition index, and the characteristic velocity efficiency was considered as a combustion performance parameter. The coaxial porous injector demonstrated higher combustion efficiency than the conventional shear coaxial injector, especially at lower-momentum flux ratio conditions. In addition, the comparison of spray images between the shear coaxial and coaxial porous injectors under ambient conditions supported the results of the hot-firing experiment.

Extreme fuel-rich combustion characteristics of RBCC embedded rocket engine with gas-liquid shear coaxial injectors in continuously varying mixture ratios

In order to achieve stable and effective transition between the operation modes in a rocket-based-combined-cycle engine (RBCC), the embedded rocket engine should be capable of throttling continuously in a wide range of operation conditions. A kerosene/GO2 RBCC embedded rocket engine with gas–liquid shear coaxial injectors was designed to investigate the combustion characteristics under varying operation conditions. Throttling cold test, static hot test, and throttling hot tests in varying mixture ratios (0.102–0.221) were conducted with a throttleable venturi and a throttleable sonic nozzle. In these tests, the mixture ratio was adjusted in two ways: one is to linearly decrease the fuel mass flow rate with a constant oxygen mass flow rate, and the other is to increase the oxygen mass flow rate with a constant fuel mass flow rate. Results indicate that the combustion efficiency and the characteristic velocity increase along with the mixture ratio, but show a different growth rate depending on the mixture ratio adjustment method. To specify, characteristic velocity and combustion efficiency of hot test throttled by increasing oxygen mass flow rate grow faster. And in the range of mixture ratios ( Φ<0.25), the combustion efficiency obtained by both adjustment methods is lower than 55%. The flame behaviors are influenced significantly by the oxygen mass flow rate, and a reasonable explanation is that increasing oxygen mass flow rate can significantly increase the spray angle and decrease the mean diameter. And the flame behaviors can influence the discharge coefficient of the kerosene injector. Furthermore, the discharge coefficient decreases along with the decrease of kerosene mass flow rate in both hot and cold tests.

Prediction and Analysis of Combustion Instabilities in a Model Rocket Engine

This paper is a compilation of the research efforts of five different groups (Bertin Technologies with Centre National d’Etudes Spatiales, Institut de Mécanique des Fluides de Toulouse, ONERA, Purdue University, and Technische Universität München) on the prediction of combustion instabilities in a model rocket engine. This research was initiated by the Rocket Engine Stability Initiative group for the 2nd Rocket Engine Stability Initiative Workshop on Combustion Instability Modeling, which took place in October 2010 at Astrium, GmbH, in Ottobrunn, Germany. The target experiment consists of a single shear coaxial injector using methane and decomposed hydrogen peroxide as reactants. Both the inlet of the injector and the outlet of the chamber are choked, resulting in well-defined acoustic boundary conditions. The length of the oxidizer tube could be varied continuously, and its influence on the stability is studied. Many numerical strategies are tested, addressing different physical phenomena at play during unstable combustion. Acoustic solvers, both with and without mean-flow effects are used to draw stability maps. The weak spot of these solvers is that they require the flame response to acoustic perturbations as an input. Large-eddy simulations, requiring no such a priori knowledge, are performed with the intent to elucidate flame stabilization and flame response mechanisms.

Effect of Density Gradient on Flame–Acoustic Interaction

Dynamic interactions between shear coaxial injector flames and periodic compression waves are investigated under controlled density ratio variations. Turbulent diffusion flames are established between an oxygen slot jet in the middle and two hydrogen slot jets on the sides, representing a sectional view of a shear coaxial injector flowfield. Transverse acoustic excitation is applied using a compression driver mounted on one side of the combustor to force possible flame–acoustic interactions. While maintaining a stoichiometric fuel–oxidizer ratio, the density of each stream is controlled by diluting the propellant gases with inert helium and argon by appropriate amounts. Results show strong dependence of flame–acoustic interaction on the density gradient between the propellants. The observed trend is consistent with the vorticity production mechanism created by the baroclinic source term in the flowfield. It is observed that the flames become more resilient to acoustic coupling when the fuel–oxidizer density difference is reduced. The results open up the possibility of using density gradient manipulations to extend the overall stability margin of shear coaxial injector flames.

Breakup structure of two-phase jets with various momentum flux from a porous injector

Spray structure and atomization characteristics were investigated through a comparison of a porous and a shear coaxial injector. The porous injector shows better atomization performance than the shear coaxial injector. To increase atomization performance and mixing efficiency of two-phase jets, a coaxial porous injector which can be applicable to liquid rocket combustors was designed and tested. The characteristics of atomization and spray from a porous and a shear coaxial injector were characterized by the momentum flux ratio. The breakup mechanism of the porous injector is governed by Taylor-Culick flow and axial shear forces. Momentum of injected gas flow through a porous material which is composed of sintered metal is radically transferred to the center of the liquid column, and then liquid column is effectively broken up. Although the shapes of spray from porous and shear coaxial jets were similar for various momentum ratio, spray structures such as spray angle and droplet sizes were different. As increasing the momentum flux ratio, SMD from the porous injector showed smaller value than the shear coaxial injector

LOx Jet Atomization Under Transverse Acoustic Oscillations

Testing has been conducted with the BKH rocket combustor at the European Research and Technology Test Facility P8 for cryogenic rocket engines at DLR Lampoldshausen. BKH has multiple shear coaxial injectors and an exhaust modulation system for forcing excitation of acoustic resonances in the combustion chamber. Optical access windows allow the application of parallel high-speed shadowgraph and flame emission imaging of the near-injector region. This paper reports measurements of the intact liquid oxygen core during forced excitation of the first transverse acoustic mode. High-speed shadowgraph images show that the mechanism of core breakup and atomization differs between off-resonance and first transverse excitation conditions. The core length is found to decrease with increasing amplitude of acoustic pressure, or equivalently with transverse acoustic velocity, with a core length reduction of up to 70% for conditions approaching those of naturally occurring high frequency combustion instabilities. This dependence is shown to be distinguishable for differing operating conditions of the combustor. Fluctuation in core length also decreases with increasing excitation amplitude. Knowledge of the influence of the transverse acoustic field on the LOx core is important for understanding the driving mechanisms of combustion instabilities.

Experimental study of hydrogen-rich/oxygen-rich gas–gas injectors

Five types of coaxial injectors were investigated experimentally using hot hydrogen-rich gas and oxygen-rich gas, which were respectively provided by a GH2/GO2 hydrogen-rich perburner and a GH2/GO2 oxygen-rich preburner. The injectors were the shear coaxial injector, the oxidizer post expansion coaxial injector, the fuel impinging coaxial injector, the central body coaxial injector, and the shear tri-coaxial injector. The characteristic velocity efficiency and the combustor’s wall temperatures were obtained for different design parameters through the experiments. It can be concluded that angles of the oxidizer post expansion and the fuel impinging have little influence on the combustion performance and the wall temperatures. The contact area between fuel and oxidizer and the mass flow rate have significant impacts on the combustion performance. The shear tri-coaxial injector has the best combustion performance but also the highest wall temperatures among the five types of injectors.

다공성재를 통과하는 압축성 유체의 압력강하에 관한 실험적 연구

This study proposes the characteristics of the pressure drop in a compressible fluid through porous media for application to a porous injector in a liquid rocket engine in order to improve the uniformity of the drop size distribution and the mixing performance of shear coaxial injectors. The fluid through the porous media is a Non-Darcy flow that shows a Nonlinear relation between the pressure drop and the velocity at high speed and high mass flow rate. The pressure drop of the Non-Darcy flow can be derived using the Forchheimer equation that includes the losses of viscous and inertia resistance. The permeability and Ergun coefficient represented as a function of the pressure drop and pore size can be applied to the porous injector, where the fluid through the porous media is compressible. A generalized correlation between the pressure drop in relation to the pore size was derived.

2중 유로형 전단 동축 분사기의 기체 분사율에 따른 유동 및 입도분포

To verify the effect of inner- and outer-stage gas jets, a shear coaxial injector was designed to analyze the axial velocity profile and breakup phenomenon with an increase in the measurement distance. When the measurement position was increased to Z/d=100, the axial flow showed a fully developed shape due to the momentum transfer, aerodynamic drag effect, and viscous mixing. An inner gas injection, which induces a higher momentum flux ratio near the nozzle, produces the greater shear force on atomization than an outer gas injection. Inner- and Outer-stage gas injection do not affect the mixing between the inner and outer gas flow below Z/d=5. The experiment results showed that the main effect of liquid jet breakup was governed by the gas jet of an inner stage. As the nozzle exit of the outer-stage was located far from the liquid column, shear force and turbulence breaking up of the liquid jets do not fully affect the liquid column. In the case of an inner-stage gas injection momentum flux ratio within 0.84, with the increase in the outer gas momentum flux ratio, the SMD decreases. However, at an inner-stage gas jet momentum flux ratio over 1.38, the SMD shows the similar distribution.

Numerical and experimental study on shear coaxial injectors with hot hydrogen-rich gas/oxygen-rich gas and GH2/GO2

The influences of the shear coaxial injector parameters on the combustion performance and the heat load of a combustor are studied numerically and experimentally. The injector parameters, including the ratio of the oxidizer pressure drop to the combustor pressure (DP), the velocity ratio of fuel to oxidizer (RV), the thickness (WO), and the recess (HO) of the oxidizer injector post tip, the temperature of the hydrogen-rich gas (TH) and the oxygen-rich gas (TO), are integrated by the orthogonal experimental design method to investigate the performance of the shear coaxial injector. The gaseous hydrogen/oxygen at ambient temperature (GH2/GO2), and the hot hydrogen-rich gas/oxygen-rich gas are used here. The length of the combustion (LC), the average temperatures of the combustor wall (TW), and the faceplate (TF) are selected as the indicators. The tendencies of the influences of injector parameters on the combustion performance and the heat load of the combustor for the GH2/GO2 case are similar to those in the hot propellants case. However, the combustion performance in the hot propellant case is better than that in the GH2/GO2 case, and the heat load of the combustor is also larger than that in the latter case.

1112 CHARACTERISTICS OF H2/AIR ANNULAR JET FLAMES USING MULTIPLE SHEAR COAXIAL INJECTORS

In order to understand the combustion and heat transfer characteristics of H2/air annular jet flames using multiple shear coaxial injectors, the experimental and numerical investigations have been conducted using a small cylindrical combustion chamber under normal pressure. Each flame appears as an independent flame until the flame amalgamation. Further downstream, the high temperature regions again merge to form a large flame. Then, the flame is once squeezed, and the temperature distribution spreads rapidly in the radial direction in the downstream region. The phenomena resemble the transition to turbulent mixture of jet. The flow characteristics like the turbulent kinetic energy and the recirculation vortex are important for the heat transport in the combustion chamber, and the wall heat flux is strongly influenced by them.

313 複数同軸型噴射器を用いた小型円筒燃焼室における水素感情噴流火炎の燃焼特性(OS5-3 燃焼技術の基礎と応用研究(3),OS5 燃焼技術の基礎と応用研究)

313 複数同軸型噴射器を用いた小型円筒燃焼室における水素感情噴流火炎の燃焼特性(OS5-3 燃焼技術の基礎と応用研究(3),OS5 燃焼技術の基礎と応用研究)

Stability of Confined Gas-Liquid Shear Flows in Recessed Shear Coaxial Injectors

The linear-temporal and spatiotemporal instability behavior of a confined gas–liquid shear flow was investigated theoretically. The practical situation which motivated this investigation is the recessed gas–liquid shear coaxial injector, usually used in liquid propellant rocket engines. The corresponding dispersion relation between the complex wave growth rate and the complex wave number was derived. The temporal stability analysis shows that a more strongly confined gas–liquid shear flow exhibits a larger temporal growth rate than a weakly confined gas– liquid shear flow. A larger nondimensional outer injector radius, a larger liquid-to-gas density ratio and velocity ratio, and a smaller liquid Weber number would stabilize the confined gas–liquid shear flow. In spatiotemporal mode,whentheconfinementisstrong, flowisabsolutelyunstable.Whentheconfinementbecomesweak,the flowcan transit to become convectively unstable. Under strong confinement, the confined jet is always absolutely unstable independent of the variation of liquid–gas density ratio, velocity ratio, and liquid Weber number, i.e., the confinement is the most key parameter determining the absolutely/convectively unstable characteristics of the flow. However, the absolute growth rate and wavelength can be affected by these parameters.