Showing papers in "Journal of Propulsion and Power in 2016"

Experimental Study of the Performance of a Rotating Detonation Engine with Nozzle

Abstract: A rotating detonation engine is experimentally tested with various nozzle configurations for the purpose of measuring the propulsive performance of these devices in terms of thrust and specific impulse. Particular attention is given to comparing different internal nozzle configurations, which include bluff body, aerospike, and choked aerospike arrangements. The nozzle throat exit choke present in the rotating detonation engine exhaust is analyzed to provide insight into the stagnation pressure gain nature of the device.

Injector-Driven Combustion Instabilities in a Hydrogen/Oxygen Rocket Combustor

Abstract: Self-excited combustion instabilities of the first tangential mode have been found in a research combustor operated with the cryogenic propellant combination of hydrogen/oxygen. In a series of consecutive test campaigns, the influence of operating conditions on these self-excited combustion instabilities was examined. This included a variation of the combustion chamber pressure, the mixture ratio, and the propellant temperatures. It has been shown how these operating parameters influence the resonance frequencies of the combustion chamber. The analysis of the influence of operating conditions on the oscillation amplitude of the first tangential mode indicated that the instability occurred when the frequency of the first tangential mode of the combustion chamber was shifted into the frequency of the second longitudinal mode of the liquid oxygen injector. With a variation of the injector length, and therefore its longitudinal resonance frequencies, this hypothesis has been tested. Based on the experimental ...

Comparing Direct and Indirect Thrust Measurements from Passively Fed Ionic Electrospray Thrusters

Abstract: Highly ionic beams of several hundred microampere per squared centimeter have been measured from porous glass ionic liquid electrospray sources fabricated using a conventional mill. The thrust output from three prototype devices, two emitting the ionic liquid 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide and one emitting 1-ethyl-3-methylimidazolium-tetrafluoroborate, was measured directly using a precise balance. Thrusts up to 50 μN were measured when emitting 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide in a bipolar, alternating potential configuration at less than 0.8 W input power and with propellant supplied from an internal reservoir. Measurements of mass spectra via time-of-flight spectrometry, angle resolved current distributions, ion fragmentation, and energy deficits have been applied to accurately calculate thrust and mass flow rates indirectly from the same devices. For two of the three cases, calculated and directly measured thrusts were in agreement to within...

Thermal Behavior Inside Scramjet Cooling Channels at Different Channel Aspect Ratios

Abstract: To study the thermal behavior inside scramjet cooling channels at different aspect ratios, a three-dimensional model of fuel flow in terms of the fuel real properties is built and validated through experiments. The whole cooling channel is divided into noncracking and cracking zones, and only the noncracking zone is studied in this paper. The simulation results indicate that heat transfer deterioration occurs very easily in scramjet engine cooling channels due to its small fuel mass flux, relatively high wall heat flux, and near-critical operating pressure. Increasing the channel aspect ratio is not always beneficial for reducing the wall temperature because of thermal stratification, and there exists an optimum value below 8 to make the wall temperature lowest. In addition, increasing the channel aspect ratio will reduce the thermal diffusion coefficient, resulting in a stronger thermal stratification and making the utilization of chemical heat sink and heat transfer design in the cracking zone of the co...

Quasi-One-Dimensional Model of Scramjet Combustor Coupled with Regenerative Cooling

Abstract: A quasi-one-dimensional model for scramjet combustor coupled with regenerative cooling has been developed for comprehensive and rapid prediction of engine performance in system design and further research of advanced scramjet engine cycles with regenerative cooling. The model consists of two sets of ordinary differential equations for the reacting flow in the combustor and the cooling flow in the cooling channels separately. Additional models for wall heat transfer, sonic fuel injection, mixing efficiency, and finite-rate chemistry are also included. The SUNDIALS code is used to solve the stiff ordinary differential equations for combustion flow and the nonstiff ones for coolant flow individually, which are thermally coupled and iteratively solved to obtain the variables of the flowfields and the performance of the combustor with the consideration of the effect of regenerative cooling. Examples of hydrogen cooled scramjets were simulated under Mach 7.7 with strut injectors. Compared to the combustor witho...

Mode Transitions in Hall-Effect Thrusters Induced by Variable Magnetic Field Strength

Abstract: Mode transitions in a 6 kW laboratory Hall-effect thruster were induced by varying the magnetic field intensity while holding all other operating parameters constant. Ultrafast imaging, discharge current, and thrust measurements were used to characterize the change in discharge channel current density and thruster performance through mode transitions. The modes are described here as global oscillation mode and local oscillation mode. In global mode, the entire discharge channel is oscillating in unison and spokes are either absent or negligible with discharge current oscillation amplitude (root mean square) greater than 10% of the mean value and can even be as high as 100%. In local oscillation mode, perturbations in the discharge current density are seen to propagate in the E×B direction. Spokes are localized oscillations that are typically 10–20% of the mean discharge current density value. The discharge current oscillation amplitude and mean values are significantly lower than global mode. The mode tra...

Electrical Facility Effects on Hall Thruster Cathode Coupling: Performance and Plume Properties

Abstract: The impact of facility conductivity on Hall effect thruster cathode coupling is experimentally investigated. The 3.4 kW Aerojet Rocketdyne T-140 Hall effect thruster operating at a discharge voltage of 300 V, a discharge current of 10.3 A, and an anode flow rate of 11.6 mg/s serves as a representative Hall effect thruster test bed. The nominal facility operating pressure during thruster operation is 7.3×10−6 Torr corrected for xenon. Two 0.91×0.91 m square aluminum plates are placed adjacent to, but electrically isolated from, the walls of the conductive vacuum chamber at two locations with respect to the center of the thruster exit plane: 4.3 m axially downstream along the thruster centerline, and 2.3 m radially outward centered on the exit plane. The plates and body of the Hall effect thruster are configured in three distinct electrical configurations with corresponding measurements: 1) electrically grounded with measurements of currents to ground, 2) electrically isolated with measurements of floati...

Ignition experiments of hydrocarbons in a mach 8 shape-transitioning scramjet engine

Abstract: The design and testing of a cavity flameholder in a Mach 8 rectangular-to-elliptical shape-transitioning scramjet is presented. The experimental results presented are from the shock tunnel testing of the engine, fitted with the cavity combustor and fueled by gaseous ethylene, methane, and hydrogen. Axisymmetric simulations of a cavity in a diverging combustor were used to confirm that the cavity flow would fully establish in the limited test time available in our impulse facility. The cavity flameholder was designed such that autoignition and flameholding of gaseous ethylene–air mixtures should occur. Static pressure measurements were taken along the entire flowpath of engine. The engine was fueled at equivalence ratio ranging from 0.58<φ<0.71. Successful combustion and flameholding of ethylene and hydrogen was observed at a flight Mach number of 7.3 and a dynamic pressure of 53.5 kPa at an altitude of 28.7 km. Methane did not ignite at any of the conditions tested.

Hybrid Rockets with Axial Injector: Port Diameter Effect on Fuel Regression Rate

Abstract: A numerical rebuilding of a series of test data obtained with static firings of a lab-scale hybrid rocket is carried out with a Reynolds-averaged Navier–Stokes solver including detailed gas–surface interaction modeling based on surface mass and energy balances. Two experimental campaigns are considered in which gaseous oxygen is fed into axisymmetric hydroxyl-terminated polybutadiene grains through an axial conical subsonic nozzle. A validation rebuilding of all of the firing tests has been performed first to highlight numerical prediction capabilities and modeling limits. Despite the several geometrical simplifications, which allows using a reduced number of cells in the computational domain and thus performing parametric analyses efficiently, the present computational fluid dynamics approach is able to capture the main features of the motor internal ballistics, fairly reproducing the average chamber pressure values and the fuel regression rate trends with oxidizer mass flux and port diameter. Computed f...

Large-Eddy Simulation of Coaxial LN2/GH2 Injection at Trans- and Supercritical Conditions

Abstract: Large-eddy simulations are carried out for the coaxial injection of liquid nitrogen and preheated hydrogen at supercritical pressures. The conditions are similar to that in typical liquid-propellant rocket combustors. By using nitrogen as a model gas, the mixing process is studied without the interference with chemical reactions. An analysis of the thermodynamic conditions that arise in the shear layer reveals that local phase separation may occur if the injection condition is transcritical. A novel volume-translation method on the basis of the cubic Peng–Robinson equation of state is introduced for the use in multispecies large-eddy simulations and is tested for both trans- and supercritical injection conditions. The new thermodynamic model corrects the deficiencies of the Peng–Robinson equation of state in the transcritical regime at minimal extra computational cost. Two independently developed large-eddy simulation codes are used for the simulations and the results are compared. The outcome indicates t...

Investigation on Ignition Enhancement Mechanism in a Scramjet Combustor with Dual Cavity

Abstract: The spark ignition and flame stabilization of kerosene in a dual-cavity scramjet combustor is investigated with the isolator entrance Mach number of 2.52, total pressure of 1.6 MPa, and stagnation temperature of 1486 K, which corresponds to Ma=5.5 flight condition. The effects of dual-cavity configuration on ignition and flameholding were analyzed. Two distinct cavity-organized flame regimes were found: cavity flame and cavity shear-layer flame. The driving force of the transition of flame stabilization mode from cavity flame to cavity shear-layer flame is the high pressure resulted from combustion, which occurs in the cavity. The dual-cavity flameholding scheme can promote the ignition ability. First, the downstream cavity could stabilize the flame convected from the upstream cavity. Second, compared with the upstream cavity, the downstream cavity is a more suitable ignition location because the fuel has a longer distance for evaporation and mixing. The boundary-layer separation and upstream moving of pr...

Graphene Oxide/Ammonium Perchlorate Composite Material for Use in Solid Propellants

Abstract: An energetic composite material based on graphene oxide and ammonium perchlorate was prepared using a recrystallization method involving acetone and ethyl acetate as the solvent and antisolvent, respectively. This method resulted in the production of small crystals, where the graphene oxide wraps around the ammonium perchlorate particle to form a composite particle. The resulting composite material is found to enhance the thermal decomposition of ammonium perchlorate over just physically mixing. Furthermore, using this material, bimodal ammonium perchlorate/hydroxyl-terminated polybutadiene solid propellants were prepared. The combustion behavior of the propellants were investigated, and the graphene-oxide-based solid propellant resulted in a 15% increase in the burning rate at a pressure of 80 atm over the baseline material.

Effects of Bleed Position on the Stability of a Supersonic Inlet

Abstract: Effects of various boundary-layer bleed locations on the stability of an axisymmetric supersonic inlet has been investigated experimentally. The bleeds were located on the inlet compression cone and were designed to improve the stability margin of the inlet. The main objective of the study was to identify the boundary-layer bleed location and its effects on the improvement of the stability margin of the inlet. In addition, a buzz precursor detection methodology based on the rms level has been introduced. Experiments have been carried out on a mixed-compression inlet with a design Mach number of 2.0 and three different bleed locations at three freestream Mach numbers of 1.8, 2.0, and 2.2. All tests were conducted at an angle of attack of 0 deg. The results show that, based on the individual design of an inlet, a location can be found that the spilled normal shock stands at this location and consequently separation starts at the buzz onset. If the bleed slot is located in this place, the subcritical perform...

Detecting the Onset of an Impending Thermoacoustic Instability Using Complex Networks

Abstract: Recently, M. Murugesan and R. I. Sujith (“Combustion Noise is Scale-Free: Transition from Scale-Free to Order at the Onset of Thermoacoustic Instability,” Journal of Fluid Mechanics, Vol. 32, June 2015, pp. 225–245) showed that the transition from combustion noise to thermoacoustic instability can be represented as a change from a scale-free to a regular structure in the topology of complex networks. These topological changes of the complex networks during this transition can be quantified by calculating the network properties. In this paper, the variation of network properties, namely, clustering coefficient, characteristic path length, network diameter, and global efficiency is presented as the system dynamics undergoes transition from combustion noise to thermoacoustic instability. These network properties capture the change in system dynamics well before the rise in pressure amplitude levels in the combustors. These network properties can be used as early warning signals to detect the onset of impendi...

Survey of Selected Additively Manufactured Propellants for Arc Ignition of Hybrid Rockets

Abstract: Results of a testing campaign to assess multiple commercially available three-dimensional printer materials for effectiveness in an arc-ignition system for hybrid rockets are presented. Previously, a form of additive manufacturing known as fused deposition modeling was used to fabricate high-density acrylonitrile butadiene styrene (ABS) fuel grains so that, when properly layered, they possess unique electrical breakdown properties. When subjected to an inductive charge, an electrical arc flows along the layered material surface and seeds combustion when the arc occurs simultaneously with the introduction of an oxidizing flow. This study investigates commercially available three-dimensional printable materials to search for equivalent or possibly superior fuel alternatives to ABS. Test specimens include photopolymers processed using polyjet (stereolithography) and fused-deposition printing. Comparison metrics include general arc ability, pyrolysis rate, dissipated power, characteristic velocity, and abilit...

Solid amine-boranes as high performance hypergolic hybrid rocket fuels

Abstract: An experimental study is conducted to evaluate amine–borane materials in solid fuels that create hypergolic combustion with nitric acid. Ignition delays as short as 3.3 ms have been measured for fuels containing amine–boranes in an epoxy binder: one of the fastest ignition delays ever recorded for a hypergolic hybrid fuel. The heats of formation of the amine–boranes were measured and theoretical (ideal) performance calculated. Some amine–boranes exhibit a double peak performance allowing for a range of oxidizer/fuel combinations resulting in high performance. Amine–borane/fuel binder combinations can produce a 3.8 and 8.2% increase in Isp and ρIsp, respectively, over monomethylhydrazine and a 4–11% increase in Isp and a 17% decrease to 5% increase in ρIsp over other hypergolic hybrid rockets that use similar oxidizers. Amine–boranes also exhibit trends of being less toxic and more air stable than their base amines. In addition to the hypergolic properties, performance alone motivates the use of these mate...

Nanosecond Pulsed Plasma Activated C2H4/O2/Ar Mixtures in a Flow Reactor

Abstract: The present work combines numerical and experimental efforts to investigate the effect of nanosecond pulsed plasma discharges on the low-temperature oxidation of C2H4/O2/Ar mixtures under reduced pressure conditions. The nonequilibrium plasma discharge is modeled using a one-dimensional framework, employing separate electron and neutral gas temperatures, and using a detailed plasma and combustion chemical kinetic mechanism. Good agreement is seen between the numerical and experimental results, and both results show that plasma enables low-temperature C2H4 oxidation. Compared to zero-dimensional modeling, the one-dimensional modeling significantly improves predictions, probably because it produces a more complete physical description (including sheath formation and accurate reduced electric field). Furthermore, the one- and zero-dimensional models show very different reaction pathways, using the same chemical kinetic mechanism and thus suggest different interpretations of the experimental results. Two kine...

Effects of Dynamic Backpressure on Pseudoshock Oscillations in Scramjet Inlet-Isolator

Abstract: To estimate the effects of combustion-induced backpressure on the pseudoshock oscillation within a scramjet inlet isolator, a computational-fluid-dynamics code was used to study the motions of pseudoshock under different sinusoidal dynamic backpressures. The relations of the pseudoshock oscillation with the pressure histories, standard deviation, and spectral density distributions were explored. Second, the impact of frequency and amplitude of dynamic backpressure on the pseudoshock oscillation was investigated. The results show that the leading edge of the oscillating pseudoshock is not at the place where maximum values of standard deviation and spectral density occur because sinusoidal dynamic backpressure is applied at the exit of the isolator. Moreover, the frequencies of dynamic backpressure have minor influence on the maximum upstream position of the pseudoshock leading edge with the increase of frequency. In comparison, the amplitudes of the dynamic backpressures have great impact on the maximum up...

Experimental Visualization of Cryogenic Backflow Vortex Cavitation with Thermodynamic Effects

Abstract: The world’s first test facility that allows the visualization of cavitation on a rotating inducer in both cryogen and water was used for comparing the cavitation features in liquid nitrogen at 77.9 K and water at 292.5 and 333.5 K. The test inducer was a triple-threaded helical one with a diameter of 65.3 mm and a rotational speed range of 3500–6000 rpm. The backflow vortex cavitation on the rotating inducer was quantitatively measured in the considered fluids. From the results, it was inferred that the backflow vortex cavitation orbital rate and its orbital diameter around the inducer axis depend on the head coefficient but are almost independent of the cavitation number and kind of fluid. Furthermore, the diameter of each backflow vortex cavitation column depends on the head coefficient, cavitation number, and kind of fluid. At the same head coefficient and cavitation number, the diameter in liquid nitrogen is 2.4 times smaller than that in water because of the thermodynamic effects. Moreover, the appea...

Fuel–Air Mixing Experiments in a Directly Fueled Supersonic Cavity Flameholder

Abstract: In the present study, planar laser-induced acetone fluorescence was used to quantify the streamwise concentration distribution of an ethylene fuel surrogate (acetone-seeded nitrogen) injected directly into the recirculation region of a cavity flameholder embedded in a nonreacting supersonic airflow. The effect of injection configuration and fueling rate were studied. Parallel injection from the cavity front wall resulted in an ultra-fuel-rich recirculation region (beyond the upper flammability limit of an ethylene–air mixture) with a mixing efficiency that degraded with increased fueling rate. Downstream angled injection from the cavity floor yielded a substantial improvement in terms of the spatial uniformity and flammability of the local mixture. Its mixing efficiency was observed to be independent of fueling rate. The superior mixing ability of the latter injection configuration has the potential to provide increased cavity flameholder operability through a broader range of ignition and blowout limits ...

Testing and Modeling of a Porous Axial-Injection, End-Burning Hybrid Motor

Abstract: Testing was conducted using polyethylene as the porous fuel and gaseous oxygen as the oxidizer. Nominal test articles were tested using 100, 50, and 15 μm pore sizes. Pressures tested ranged from atmospheric to 1194 kPa, and oxidizer injection velocities ranged from 35 to 80 m/s. Regression rates were determined using pretest and posttest length measurements of the solid fuel. Experimental results demonstrated that the regression rate of the porous axial-injection, end-burning hybrid was a function of the chamber pressure, as opposed to the oxidizer mass flux typical in conventional hybrids. Regression rates ranged from approximately 0.65 mm/s at atmospheric pressure to 7.74 mm/s at 1194 kPa. The analytical model was developed based on a standard ablative model modified to include oxidizer flow through the grain. The heat transfer from the flame was primarily modeled using an empirically determined flame coefficient that included all heat transfer mechanisms in one term. An exploratory flame model bas...

Transverse Instability Studies in a Subscale Chamber

Abstract: High-frequency combustion instabilities result from a coupling between modes of heat release and chamber acoustics. Because the geometric dimensions of the chamber define the relevant acoustic modes, it is particularly difficult to study transverse instabilities in anything other than a full-scale chamber. In this paper, results from experimental studies of a two-dimensional subscale chamber that can nearly replicate full-scale acoustic modes are described. The acoustic modes are primarily driven by injector elements at the endwalls of the rectangular chamber. They were designed to be unstable, based on a vortex-shedding mechanism postulated on earlier results from longitudinal studies. The 27-cm-wide by 19-cm-long chamber produced transverse instabilities with a first width-mode frequency of about 2000 Hz, with no longitudinal mode content in the pressure signal. The chamber comprises a linear array of seven gas-centered swirl coaxial liquid injectors, designed to flow either bipropellants, only oxidizer...

Experimental and Numerical Investigation of a Solid-Fuel Rocket Scramjet Combustor

Abstract: A new scramjet configuration using solid fuel as propellant is proposed, namely, the solid-fuel rocket scramjet. Experimental and numerical investigation of the solid-fuel rocket scramjet combustor was conducted to evaluate its performance. The experiment simulated a flight environment of Mach 4 at a 17 km altitude. Magnesium-based solid fuel was used as propellant in this study. The results show that secondary combustion occurs in supersonic combustor. The combustion efficiency of the propellant is about 65%, and the total pressure recovery is about 0.5. These imply that the scramjet configuration using solid fuel as a propellant is feasible. The results of the numerical analysis show that a normal shock wave is generated in the core flow of the combustor, and this is due to all of the fuel-rich gas being injected in the same cross section.

Electrical Facility Effects on Hall-Effect-Thruster Cathode Coupling: Discharge Oscillations and Facility Coupling

Abstract: The physical mechanisms that govern the electrical interaction between the Hall-effect-thruster electrical circuit and the conductive vacuum-facility walls are not fully understood. As a representative test bed, an Aerojet Rocketdyne T-140 Hall-effect thruster is operated at 3.05 kW and a xenon mass flow rate of 11.6 mg/s with a vacuum facility operating neutral pressure of 7.3×10−6 torr, corrected for xenon. Two electrical witness plates, representative of the facility chamber walls, are placed 2.3 m radially outward from thruster centerline and 4.3 m axially downstream from the thruster exit plane. The cathode is radially translated from 18.1 to 77.8 cm away from the thruster centerline. At each cathode position, the discharge current and the electrical waveform of the radial and axial plates are simultaneously measured. As the cathode radial position changes from 18.1 to 77.8 cm from the thruster centerline, the discharge-current oscillation frequency decreases between 17 and 35% for the electrically...

Transverse Combustion Instability in a Rectangular Rocket Motor

Abstract: A computational analysis of transverse acoustic instability is presented for an experimental combustion chamber with rectangular cross section. The analysis is shown to be efficient and accurate. The governing equations are solved on multiple, coupled grids, which are two-dimensional in the combustion chamber and nozzle and one-dimensional in the injector port. Thus, they allow for a fast simulation, even in a serial run. Because of the lengthscale difference, the jet flame behavior at the injectors (including effects of turbulence) can be decoupled from the acoustic effects and solved on a local grid for each jet flame emerging from an injector. Wave propagation through the injector feed ports is evaluated on additional, one-dimensional grids for each injector port. The overall algorithm is used to simulate the Purdue seven-injector rocket engine; good quantitative agreement between simulations and experiment is achieved. All simulations that are predicted to be unconditionally unstable are confirmed by ...

Characterization of Flashing Phenomena with Cryogenic Fluid Under Vacuum Conditions

Abstract: Flashing occurs when liquid undergoes a sudden depressurization and triggers a violent atomization and vaporization. This phenomenon may happen at the transient startup of the upper-stage rocket engine. In this paper, experimental investigation of cryogenic fluid injection into a low-pressure environment is performed using liquid nitrogen instead of the real cryogenic propellants (liquid oxygen and liquid methane). With the help of a high-speed shadowgraph technique and with the temperature measurement, the characteristics of flashing are explored, including its evolution process, the spray angle, the spray thermal behaviors along the injection centerline, and the spray phase solidification. The results show that bubble nucleation is a reasonable controlling factor for the flashing evolution process. The nondimensional energy barrier χ is conditionally indicative of the flashing transition process, and it works well under low injection temperature conditions. The drastic temperature drop due to flashing p...

Reusable Rocket Engine: Firing Tests and Lifetime Analysis of Combustion Chamber

Abstract: The main engine of the reusable sounding rocket has been developed at the Kakuda Space Center of the Japan Aerospace Exploration Agency. A total of 54 engine firing experiments were conducted between June 2014 and February 2015. Throughout these experiments, the advanced capabilities and functions of this engine, such as wide-range throttling, accurate controllability, and health monitoring, were proved to be feasible. This rocket was also required to be reusable for over 100 flights. To confirm the long-life durability of this engine for over 100 flights in a limited experiment period, sequential multiple-firing tests were planned and conducted. Numerical analysis to evaluate damage of the main chamber due to firings was also conducted. The results of the durability tests and numerical analysis showed reuse of this engine for over 100 flights to be feasible.

Visualization of Simulated Fuel–Air Mixing in a Dual-Mode Scramjet

Abstract: Nitric oxide planar laser-induced fluorescence measurements have been performed in a small-scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 flight. A mixture of nitric oxide and nitrogen was injected at the upstream end of the inlet isolator as a surrogate for ethylene fuel, and the mixing of this fuel simulant was studied with and without a shock train. The shock train was produced by an air throttle, which simulated the blockage effects of combustion downstream of the cavity flameholder. Nitric oxide planar laser-induced fluorescence signal was imaged in a plane orthogonal to the freestream at the leading edge of the cavity. Instantaneous planar images were recorded and analyzed to identify the most uniform cases, which were achieved by varying the location of the fuel injection and shock train. This method was used to screen different possible fueling configurations to provide optimized test conditions for follow-on combustion measurements...

Large-Eddy Simulation of High-Frequency Combustion Instability in a Single-Element Atmospheric Combustor

Abstract: Large-eddy simulation is performed to simulate high-frequency combustion instability in a single-element atmospheric combustor. Simulations are conducted for corresponding combustion-instability experiments, and the self-excited combustion instability is successfully captured. The first tangential mode of the combustion chamber is excited in the large-eddy simulation, and the amplitude and frequency of the pressure fluctuations are consistent with the experimental observations. The first tangential mode in the large-eddy simulation was observed at 1 kHz, and the peak-to-peak amplitude was approximately 4% of time-averaged pressure. The higher-order modes were also observed in the large-eddy simulation at frequencies ranging from 2 to 4 kHz, although those amplitudes were approximately one-fourth of the first tangential mode. The coupling mechanism between the flame and acoustic mode is explored based on the large-eddy-simulation results. The periodic ignition of the unburnt H2/O2 mixture exhibits lifted c...

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

Abstract: 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 frequen...