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Supercritical combustion of Liquid OXygen (LOX) and kerosene with a bi-swirl coaxial injector is studied using Large-Eddy Simulation (LES) associated with Flamelet model. The injector configuration is derived from main injectors of the RD-0110 engine. Swirling oxygen at 120 K and swirling kerosene at 300 K are introduced from the injector to the combustion chamber at compressed liquid states. The operating pressure is 10 MPa. A three-component surrogate consisting of n-decane/n-propylbenzene/n-propylcyclohexane (74%/15%/11% by mole) is used to represent kerosene. A detailed chemistry kinetics with 209 species and 1673 elemental reactions is employed to establish the flamelet library. The injector near-field flow and flame structures are examined and discussed in detail to enhance understanding of supercritical combustion of liquid oxygen and kerosene with a bi-swirl coaxial injector. Suggestions are also made for future studies of supercritical combustion of dual-liquid propellants associated with bi-swirl coaxial injectors.

One-dimensional simulations are performed in air to study the dynamics of energy coupling, gas heating and generation of active species by repetitively pulsed nanosecond dielectric barrier discharges (NS DBD) in plane-to-plane geometry. The plasma is modeled using a twotemperature detailed chemistry model, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. The input energy is directly proportional to number density, and remains fairly constant on a per molecule basis from pulse to pulse. At 40 kHz pulsing rate, nearly 75% of the input energy is coupled the vibrational energy mode as compared to 40% of the energy used in vibrational excitation at 1 kHz pulsing rate. As a consequence, we hypothesize that the compression waves generated through fast gas heating in surface nanosecond discharges will be weaker at higher pulsing frequencies because of reduced energy coupling at higher E/N range (above 200 Td). Repetitive pulsing results in uniform production of atomic oxygen in the discharge volume via electron impact dissociation during voltage pulses, and through quenching of excited nitrogen molecules in the afterglow. A uniform “hat shaped” temperature profile develops in the discharge volume after multiple pulses, owing to chemical heat release from quenching of excited species. This may explain recent observations of volumetric ignition of fuel-air mixtures subjected to nanosecond volume discharges. * Graduate Research Assistant, sharath@gatech.edu † William R. T. Oakes Professor and Chair, vigor.yang@aerospace.gatech.edu 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 07 10 January 2013, Grapevine (Dallas/Ft. Worth Region), Texas AIAA 2013-0569 Copyright © 2013 by Sharath Nagaraja and Vigor Yang. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. D ow nl oa de d by G E O R G IA I N ST O F T E C H N O L O G Y o n Ju ne 1 0, 2 01 3 | h ttp :// ar c. ai aa .o rg | D O I: 1 0. 25 14 /6 .2 01 356 9

High-fidelity numerical simulations have been performed to study the formation and fragmentation of liquid sheets formed by two impinging jets using an improved volume-of-fluid (VOF) method augmented with several adaptive mesh refinement (AMR) techniques. An efficient topology-oriented strategy was further established to optimize the performance and accuracy of the AMR algorithm. Two benchmark cases pertaining to low- and high-velocity impinging jets are simulated as part of a grid refinement study. Calculated jet dynamics show excellent agreement with experimental observations in terms of the rim shape, droplet size distribution and impact wave structures. Detailed flow physics associated with the temporal evolution and spatial development of the jets are explored over a wide range of Reynolds and Weber numbers. A realistic rendering post-processing using a ray-tracing technique is performed to obtain direct insight to the flow evolution. Special attention is paid to the dynamics of the impact wave which dominates the atomization of the injected liquid. The work appears to be the first systematic numerical study in which all the flow patterns formed by impingement of two liquid jets are obtained. Fine structures are captured based on their characteristic length scales. Various atomization modes, from stable to highly unstable, are resolved with high fidelity.

The interactions between acoustic waves and gas-phase flame dynamics of a double-base homogeneous propellant in a rocket motor has been studied by means of a comprehensive numerical analysis. The formulation treats the complete conservation equations of mass, momentum, energy, and species concentration, and accounts for finite rate chemical kinetics in the gas phase and subsurface reactions. The model has been implemented to examine the detailed flow structures and heat-release mechanisms in various parts of the motor, including microscale motions near the propellant surface and macroscale motions in the bulk of the chamber. Results indicate that strong interactions between exothermic reactions and acoustic waves occur in regions with steep temperature gradients due to the large activation energy of the associated chemical kinetics. The dynamic behavior of the luminous flame plays a decisive role in determining the motor stability characteristics. Distributed combustion response in the gas phase provides the energy for driving flow oscillations, and can be treated correctly as a combination of monopole and dipole sources based on acoustic theory.

Linear and non-linear analyses of acoustic waves in baffled combustion chambers have been developed by means of perturbation-expansion techniques. The formulation is based on a generalized wave equation derived from the conservation equations for a two-phase mixture, and accommodates all influences of non-linear gas dynamics and combustion responses. Several specific effects of baffles are presented as mechanisms by which baffles eliminate combustion instabilities. Included are longitudinalization of transverse waves inside baffle compartments, restriction of velocity fluctuations near the injector face, and decreased oscillation frequency. A potential destabilizing influence of baffles is found to be the concentration of acoustic pressure at the injector face. Primary aspects of limit cycles are examined by considering second order non-linear acoustics for both two- and three-dimensional cases. Conditions for the existence of limit cycles were obtained, as well as explicit formulas for the oscillation amplitudes and frequencies in terms of linear and non-linear parameters. Important features of the energy cascade among acoustic modes are elucidated, including the energy balance during limit cycles and transfer of energy by non-linear gas dynamics.

A comprehensive numerical analysis has been developed to study the key physicochemical processes involved in the self-sustained combustion of 1,3,5-trinitr ohexahydro-s-triazine (RDX) monopropellant. The model takes into account detailed chemical kinetics and transport phenomena in the gas phase, and thermal decomposition and subsequent reactions in the condensed phase. The formation of gas bubbles in the subsurface layer due to molecular degradation and evaporation is also included to provide a complete simulation. Various important aspects of RDX burning characteristics are systematically examined over a broad range of pressure, with special attention given to the effect of the subsurface two-phase flow on propellant deflagration. Good agreement between calculated and measured burning rates as well as their pressure and temperature sensitivities is achieved. Results of species concentrations reveal a multistage reaction mechanism in the gas phase. The temperature profile, however, exhibits a monotonic increase from the surface to the final flame zone. No evidence is obtained of the existence of a temperature plateau in the dark zone, consistent with some experimental observations of selfdeflagrating RDX flames using microthermocouple techniques. Further investigation into the gas-phase chemical kinetics is required to establish a unified understanding of RDX combustion under different operating conditions.

High fidelity numerical simulations are performed to study the atomization process of impinging jets over a broad range of operating conditions. An improved volume-of-fluid (VOF) method augmented with adaptive mesh refinement (AMR) is used to simulate the formation and breakup of the liquid sheet formed by two impinging jets. In addition, a thickness-based refinement method is developed and implemented to automatically and efficiently resolve the drastic changing of requested grid resolution. The behaviors in various Reynolds and Weber number regimes are studied systemically. The predicted liquid sheet topology, atomization, and droplet size distribution agree well with experimental measurements. Several different patterns of sheet and rim configurations are obtained, including liquid chain, closed rim, fish-bone, disintegrating sheet, disintegrating rim and impact wave. The instability mechanisms of sheets and rims are studied based on the concepts of absolute and convective instabilities. New knowledge is acquired about the onset of sheet and rim instabilities. This locking-on feature of Strouhal number of impact wave is found based on the previous detailed study. Finally, schematic diagrams of all kinds of instabilities happens in impinging jet atomization are obtained. I. INTRODUCTION Collision between two cylindrical jets is one of the generic configurations for the generation of liquid sheets, the dynamics and stability of which have attracted a great deal of attention due to their relevance to the spray atomization and combustion in liquid propellant engines.[1-4] The impingement of liquid jets is a very efficient method for atomization and mixing whereby the dynamic head of the propellant is used to destabilize an opposing liquid propellant stream. This in turn results in fragmenting the liquid into ligaments and droplets.[5] The oblique collision of two cylindrical laminar jets at low jet velocities leads to the liquid flowing outward from the impingement point, producing a leaf-shaped expanding sheet which lies in a plane perpendicular to the plane containing the two liquid jets,. A rich variety of flow structures, from single oscillating jet obtained at low flow rates, to the violent disintegration of flapping sheets obtained at higher flow rates, have been observed depending on the Weber and Reynolds numbers of the jets. In the specific usage of liquid propellant rocket engine, the liquid sheet shows full-developed violent breakup with quickly growing waves. The liquid sheet destabilizes, breaks and eventually disintegrates into ligaments or droplets under the influence of surface tension, viscous, inertial, and aerodynamic forces. The impinging jets can provide rapid mixing and atomization. This unstable hydrodynamic wave is usually called impact wave[6]. Impact wave dominates the breakup and atomization process in most rocket combustors using impinging jet injectors.[7] Different from another capillary wave, impact wave shows a nonlinear behavior that cannot be described linear stability analysis.

The Weis-Fogh mechansim is quite attractive because it can generate very high unsteady lift. It is indicated that the high unsteady lift is related to the attached vortex around the leading edge and trailing edge of wing/airfoil, which is caused by the relative motion. In the present work, the numerical results prove that, when the axial spacing is reduced, the interaction between rotor/stator can generate unsteady vortex around the rotor blades, and the rotor lift coefficient is increased, which improves the performance of stage. Besides reducing the axial spacing, the rotor/controllable stator system is proposed to apply vortex lift in turbomachinery. It is indicated that unsteady lift coefficient is increased greatly by the relative motion of rotor/controllable stator and the unsteady vortex is very important. Nomenclature c = chord of the aerofoil

During the duration of this sponsorship, we broadened our understanding of combustion instabilities through both analytical and experimental work. Predictive models were developed for flame response to transverse acoustic instabilities and for quantifying how a turbulent flame responds to velocity and fuel/air ratio forcing. Analysis was performed on the key instability mechanisms controlling heat release response for flames over a wide range of instability frequencies. Importantly, work was done closely with industrial partners to transition existing models into internal instability prediction codes. Experimentally, the forced response of hydrogen-enriched natural gas/air premixed and partially premixed flames were measured. The response of a lean premixed flame was investigated, subjected to velocity, equivalence ratio, and both forcing mechanisms simultaneously. In addition, important physical mechanisms controlling the response of partially premixed flames to inlet velocity and equivalence ratio oscillations were analyzed. This final technical report summarizes our findings and major publications stemming from this program.

The investigations of the collision outcome and dynamics of mass transfer process of unequal-sized binary droplets collision are preformed computationally and theoretically. All the possible collision outcomes, bouncing, coalescence, reflexive and stretching separation are obtained numerically. The present study focuses on water droplet interactions in atmospheric air over a wide range of Weber numbers and impact parameters with two droplet diameter ratio, 0.5 and 0.25. The numerical formulation is based on the complete set of conservation equations for both the liquid and the surrounding gas phases. An improved volume-of-fluid (VOF) technique, augmented by an adaptive mesh refinement (AMR) algorithm, is used to track the liquid/gas interfaces. A novel thickness-based refinement methodology is also developed to resolve the extremely thin gas film during early approaching of droplets. Numerous high-fidelity bouncing cases are simulated using proposed efficient methodology. The smallest cell size is up to O(10 -4 ) of the small droplet diameter with minimum grid size of 0.02 μm. In addition, an advanced visualization technique using the Ray-tracing methodology is implemented to gain direct insight into the detailed physics of droplet interaction and mass transfer process. A simple theory based on the geometric relation of the unequal-sized droplet collision system is established to predict mass transfer ratio of stretching separation outcome. Good agreement is achieved with simulation results.

*† The effects of pulsed nanosecond plasma discharges on H2-air and C2H4-air mixtures are studied numerically using detailed chemistry. The premixed flow is activated by a parallel-plate nanosecond pulsed plasma discharge system with pulse duration ranging between 15 to 130 ns, and pulsing frequency between 5 to 60 kHz. The plasma discharge is modeled using a two-temperature model, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. An energy equation for electrons is solved to obtain electron mean energy self-consistently. Electron transport and reaction rate coefficients are expressed as functions of mean electron energy, and stored in lookup tables. In air, a large portion of input pulse energy is used in electron impact vibrational and electronic excitation of N2, and dissociation of O2. In H2-air mixtures, an additional pathway of H2 dissociation becomes important, and atomic oxygen formation is suppressed. In C2H4-air mixtures, formation of smaller hydrocarbon chains and atomic hydrogen utilizes much of the input pulse energy.

The flow dynamics and mixing process of a sonic ethylene jet into supersonic air crossflow is numerically investigated based on the large-eddy-simulation (LES) technique, with special attention given to the near-injector and near-wall turbulence treatment. The salient turbulent, vortical, and shock structures are examined to identify the mechanisms dictating the supersonic mixing layer. Results reveal that the periodic shedding of shearlayer vortices accounts significantly for the crossflow entrainment through the stretchingtilting-tearing mechanism. The calculated mixing is qualitatively comparable to the experimental data. The possible roles of the low-speed regions, both ahead of the jet and in the wake region, are also explored to further elucidate the system characteristics. Nomenclature a = Speed of sound, m/s C = Ethylene concentration d = Jet diameter, mm J = Jet-to-crossflow momentum ratio M = Mach number

In this paper, deformation and breakup of Newtonian liquid droplets at elevated pressures have been studied. Detailed physics pertaining to four different breakup regimes, oscillatory, bag, multimode and shear breakup modes , has been investigated using an incompressible interface tracking methodology. The accuracy and efficiency of the code was enhanced by incorporating an adaptive mesh refinement (AMR) techni que. In general, the aerodynamic drag force exerted by the ambient fluid causes the droplet to deform. The deformation is resisted by viscous and surface tension forces. The breakup mechanism becomes progressively violent as the We number increases, and moves from oscillatory to shear breakup regime. Quantitatively, the droplet lifetime decreases as the inertial force is increased in comparison to the deformation resisting, surface tension force. A criterion to mark the beginning of breakup has been quantified in terms of surface and kinetic energy associated with the droplet. Critical We numbers for the three regimes have also been identified for 100 atm pressure conditions. A generalized regime diagram, for Oh < 0.1, was developed to predict the various breakup modes, taking into account the pressure effect on critical We number, using data from previous experimental investigations , and simulations conducted during the current study.

The flow dynamics and mixing a gaseous jet into a crossflow is numerically investigated under conditions with externally imposed oscillations from the crossflow upstream region. A broad range of forcing frequencies and amplitudes are considered. The analysis is based on a large-eddy-simulation (LES) technique, with special attention given to the near-injector and near-wall turbulence treatment. Detailed information about the flow structures and mechanisms that dictates the flow evolution is obtained by means of the proper orthogonal decomposition (POD) analyses. Results reveal that the dominant structures in the stationary crossflow case, with jet Strouhal numbers around 0.1 and 0.7, have been suppressed by the excitations. The flapping and detaching movements, bearing the forcing frequencies and their subharmonics, become the dominant phenomena as the forcing amplitude increases. These induced motions lead to a longer and narrower jet plume in any transverse plane and a lower “center of gravity” within the scalar field, which substantially modify the mixing efficiency.

The physical mechanism of impact wave and mixing process in the atomization of impinging jets are computationally investigated. Using an accurate adaptive solver for surface-tension-driven interfacial flows, the simulations are found to agree well with experiment data for both wavelength of impact wave and droplet size distribution. The formation of impact wave is studied by analyzing velocity profiles and vortices contours of liquid sheet formed by impinging jets. It shows that the impact wave is caused by the interfacial shear stress which forms the capillary waves on the two sides. The interaction of waves on the two sides forces the liquid sheet to resonate at natural frequency. The effect of impact wave to mixing process of both miscible and immiscible impinging jets is also addressed.

A comprehensive theoretical analysis has been developed to study vaporization of liquid oxygen (LOX) droplets in hydrogen over a wide range of pressure. The formulation accommodates complete sets of conservation equations for both gas and liquid phases, and accounts for variable properties, thermodynamic non-idealities, and vapor-liquid phase equilibria. The model is capable of treating the entire history of a vaporizing LOX droplet, including the thermodynamic phase transition through the critical mixing point. Various distinct high-pressure effects on the droplet behavior were investigated in depth. In particular, a parametric study of the droplet lifetime as a function of the ambient pressure, temperature, and initial droplet diameter has been conducted. The droplet lifetime exhibits a strong pressure dependence and can be correlated well with the square of the initial diameter.

A comprehensive numerical analysis has been conducted to study the combustion of a double-base homogeneous propellant in a rocket motor. Emphasis is placed on motor internal flow development and its influence on propellant combustion. The formulation is based on the complete conservation equations of mass, momentum, energy, and species concentration, with consideration of finite-rate chemical reactions and variable properties. Turbulence closure is achieved using a two-layer model that takes into account the wall-injection effect arising from propellant burning. The governing equations and associated boundary conditions are numerically solved by means of a fully coupled implicit scheme capable of treating chemical reacting flows over a wide range of Mach number. Various aspects of the internal flowfield and combustion wave structure in a rocket motor environment are investigated systematically. The effects of pressure and cross flow on propellant burning rate are also studied. Results indicate that, for double-base propellants, no specific interactions between turbulence and combustion are observed. The propellant combustion can be locally modeled as a well-stirred reactor, and the influence of turbulence appears mainly in the calculation of eddy diffusivities.

High-resolution numerical study is carried out to investigate the flame stability of the turbulent supersonic combustion in a Dual-Combustion Ramjet (DCR). The auto-ignition in a shear layer between hydrogen/carbon-monoxide syngas and air was studied at elevated enthalpy condition. Comparison of a constant area combustor and a combustor with a small divergence angle shows that the supersonic combustion has a characteristics of the lifted flame and its stability is influenced significantly by the compressibility.

The present work establishes an integrated theoretical/numerical framework to study the characteristics of plasma enhanced H2-air counterflow diffusion flames. The oxidizer flow is activated by a parallel-plate nanosecond pulsed plasma discharge system with pulse duration ranging between 15 to 130 ns, and pulsing frequency between 5 to 60 kHz. The plasma discharge is modeled using a two-temperature model, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. Electron transport and reaction rate coefficients are expressed as functions of mean electron energy, and stored in lookup tables. A large portion of input pulse energy is used in electron impact dissociation and excitation of N2 and O2. Plasma activation significantly increases the flame extinction strain rates, resulting in stable combustion even at lean conditions.

The ARO Nano Engineered Energetic Materials (NEEM) MURI program has been exploring new methodologies for developing energetic material formulations with control of all constituents over a wide range of length scales from 1 nm to 1 mm and larger and employing the latest techniques in molecular self-assembly and supramolecular chemistry for synthesizing and assembling NEEMs. The synthetic efforts have been guided by theoretical calculations and dynamic performance testing methodologies that also operate on all length scales. This final report provides a summary of the accomplishments. In particular, new methods to generate metal nanoclusters were developed that are stabilized against environmental degradation while preserving their high energy content. Rapid expansion supercritical solution processes were developed and applied to synthesize nanosized particulate oxidizers and energetic oxidizer shell/fuel core composites. Surface science and experimental chemistry methods were applied to study the structures and energy releasing reaction pathways of NEEMs. Unique diagnostic capabilities were developed and applied to study the fundamental mechanisms that underlie NEEM dynamic performance. Combustion mechanisms of various NEEMs, including nanothermites and nanoparticulate fuel/liquid oxidizer systems, were experimentally analyzed. The reactive and thermal characteristics of NEEMs were studied using large (billion atoms) multiscale simulations that couple quantum-mechanical calculations to molecular dynamics calculations. In particular, the stability, structure and energetics of metallic nanoparticles with a special focus on the relationships between particle size, shape and excess surface free energy were studied. A unified theory of ignition and combustion of aluminum particles for a wide range of sizes, from nano to meso scales was established.

The present study focuses on binary droplet interactions over a wide range of Weber numbers and impact parameters. The formulation is based on the complete set of conservation equations for both the liquid and surrounding gas phases. An improved volume-of-fluid (VOF) technique, augmented by an adaptive mesh refinement (AMR) algorithm, is used to track the liquid/gas interfaces. Detailed information about the droplet dynamics, including collision, deformation, coalescence, separation, and mass and energy transfer, is obtained systematically. Various underlying processes of droplet dynamics are investigated by analyzing the overall energy budget. In addition, an advanced visualization technique using the Ray-tracing methodology is implemented to gain direct insight into the detailed physics of droplet interaction. Theories are also established to predict the droplet behavior after collision. Good agreement is achieved with simulation and experimental results.

The purpose of this study is to conduct a comprehensive study on the flow characteristics in a combustor with seven swirl injectors. The analysis is based on a 3D large eddy simulation (LES) technique. The propellant properties are defined by a surrogated model of Jet-A fuel and air mixture. Two combustors of counter-swirl injectors (four-clockwise and three-counterclockwise-swirl injectors) and co-swirl injectors (seven-clockwise-swirl injectors) are simulated to identify the effect of swirl direction. The detail turbulent flow structures; a corner recirculation zone (CRZ) and a central toroidal recirculation zone (CTRZ), are observed and the flow interactions among swirl injectors in each combustor are visualized. Pressure oscillation in the combustor was numerically investigated and compared with theoretical results based on linear theory to determine the dominant acoustic modes.

Thermo-mechanical behavior of nickel coated nano-aluminum particles, in the size range of 4-16 nm, is studied using molecular dynamics simulations. The analysis is carried out in isothermal-isobaric and isochoric-isoenergetic ensembles using an embedded atom method. Emphasis is laid on analyzing the melting of Al core, diffusion of Al and Ni atoms, and intermetallic reactions for different core sizes and shell thicknesses. The melting point of the Al core is found to exceed the heterogeneous melting point of pure nAl particle and approach the homogenous melting point of Al, irrespective of the shell thickness. The diffusion of Al atoms, after melting, is accompanied by self-sustaining inter-metallic reactions between Al and Ni atoms. The advent of these reactions is, to some extent, delayed for a thicker shell and expedited for a larger core. The amount of heat release due to the reactions increases as the Al or Ni atomic fraction increases to 0.5. Adiabatic reaction temperatures close to 2300 K are predicted for near-equiatomic particles with a 1 nm thick Ni shell. The simulation results indicate the possibility of ignition of these particles in an inert environment and also help in explaining their reduced ignition temperatures.

The combustion of nano-aluminum, alane, and ice mixtures is theoretically studied. A multi-zone theoretical framework is established by solving the energy equation in each zone and matching the temperature distribution and the heat flux at the interfacial boundaries. The effect of replacing a portion of nano-aluminum particles with micron-sized aluminum and alane particles is examined in the pressure range of 1-10 MPa and for an additive mass fraction of 25%. The addition of micron-sized alane particles results in lower flame temperatures due to the dehydrogenation reaction of alane particles prior to their ignition. The lower flame temperatures and the longer burning times of micron-sized alane particles is responsible for lower flame speeds of these mixtures. For bimodal aluminum-ice mixtures, the lower mass fraction of alumina causes an increase in the flame temperatures. However, the mixtures exhibit mildly lower flame speeds, in view of the longer burning times of micron-sized aluminum particles. The flame thickness of a bimodal mixture of aluminum particle mixtures is higher than that of mono-dispersed mixtures due to the prevalence of two distinct reaction zones. The model results are well supported by experimentally measured burning rates.

A good understanding of the flow dynamics and the flame stabilization mechanism in a liquid rocket engine is essential for the development of high-performance liquid rocket engines. The current work attempts to improve the understanding of LOX/CH4 combustion dynamics at conditions similar to contemporary liquid rocket engines. Three-dimensional Large-Eddy Simulation (LES) of LOX/Methane combustion in a shear coaxial injector is performed, focusing on the identification of the flame stabilization mechanism of the non-premixed LOX/Methane flames at supercritical conditions. The flamelet and the flamelet/progress-variable approaches are considered. Extreme caution is exercised to improve the numerical accuracy of the dual-time stepping preconditioning scheme to ensure that the primary quantities in the governing equations are indeed conserved at locations where the typical density ratio is up to 100. The mixing and the flame dynamics at supercritical conditions in the vicinity of the shear coaxial injector are studied. The flame anchoring/lift-off and the corresponding mechanisms are also systematically studied.

The dynamics of a flame stabilized by a triangular bluff body in a straight channel is conducted using a large eddy simulation technique along with a level-set flamelet approach. Both acoustically non-reflecting and reflecting inlet boundary conditions are treated. The physical processes responsible for driving combustion instabilities in the chamber are identified and quantified, including the mutual coupling between acoustic wave motion, vortex shedding, flame oscillation, and heat release. The mechanism of energy transfer from chemical reactions in the flame zone to acoustic oscillation in the bulk of chamber is investigated systematically. Strong resonance occurs between the shear layer separated from the bluff body and one of the chamber acoustic modes, when the non-reflecting inlet boundary condition is enforced. As a consequence, a close loop of acoustic wave, vortex shedding, flame oscillation, and heat release is formed, which explains the observed unstable combustion with large pressure oscillation.

ligament, are studied. The circumferentially space drops were shed from the periphery of the sheet, as well as the ligaments were fragmented from the leading edge of the sheet and then broke into droplets following the Rayleigh mechanism. The periodic waves from the point of impingement were apparent on the surface of the sheet. The impact waves caused early breakdown of the sheet downstream of the impingement point, whereas waves amplied by aerodynamic stresses controlled the breakdown of the rest of the sheet and the ligaments. The impinging jets for non-Newtonian uid are also investigated, two dierent ow patterns are observed.

The present study is dedicated to the simulation of supercritical uids owing inside small cooling channels. In a context of rocket motor engine, a strong heat ux coming from the combustion chamber (locally 80 MW/m) may lead to very steep gradients close to the wall. These density gradients have to be thermodynamically and numerically caught to really understand the mechanism of heat transfer from the wall to the uid. In this study, the shock-capturing WENO numerical scheme, usually used with the perfect gas law, is extended to real gases and applied to the simulation of the EH3C con guration. The WENO scheme is compared with a fourth-order central-di erence (4CD) scheme on a temperature slot and sinus function, and a non-periodic supersonic channel ow. For the last case, a better behavior of the WENO scheme is noticed away from the inlet, in accordance with the results of the temperature slot convection. For the EH3C con guration, only the WENO scheme achieves a substantial computation.

disparity in time and length scales exist in close proximity to one another. Although advances in the field have been made, the largest and most reliable source of information to date applicable to the design of improved combustion devices is the store of experimental data from full-scale engine tests. Consequently, the motivation behind the present work was to gain further insight into the mechanisms associated with combustion instability by providing a detailed, concise account and analyses of the design attributes which led to dynamic stability in F-l developmental injectors. Objectives were 1) to preserve the experience gained through development of the F-l engine; 2) to merge full-scale test results with corresponding theories and experiments; and 3) to analyze the effect of proposed solutions. To facilitate analysis, all available full-scale component and engine test data have been combined into a single data base. This compilation provides a complete genealogy of F-l developmental injector design configurations, and contains all available measured and observed test results. Table 1 lists the injector design parameters and test results acquired. The complete data base is available as an appendix to a separate technical report prepared by the authors.1 These data have been assembled from a variety of sources.2^ Reference 2 is a chronological tabulation of full-scale injector component test results recorded at the test site. This document lists the injectors tested along with the date, chamber pressure, thrust, run time, mixture ratio, bomb size, and damp times, as well as observations made during various tests. Reference 3 contains a set of 16 reports (four volumes of four reports each) which present a somewhat chronological account of the methodology leading to a dynamically stable injector design. Full-scale engine and component test results are discussed throughout this set of reports. Reference 4 provides a broad overview of the problems and solutions encountered with combustion instability in the F-l engine. Finally, Refs. 5 and 6 are weekly

Relevant properties of propellants such as physical properties, specific impulse, specific gravity, material compatibility, and others are briefly described. Various kinds and classifications of liquid propellants are reviewed, and common ones are briefly described. The ignition properties of these propellants are divided into hypergolic and nonhypergolic classes, and methods of igniting nonhypergolic propellants are discussed. The steps and stages of the subsequent combustion process are described generally, and the problem of combustion instabilities is introduced and briefly described, along with methods for testing for them and eliminating them. Keywords: liquid rockets; propellants; combustion; injector; instability; bipropellants; monopropellants; ignition; atomization; fuel; oxidizer

A unified numerical analysis was conducted to investigate the inlet buzz and combustion oscillation in an axisymmetric ramjet engine. The inlet buzz phenomenon in the subcritical operation arises large pressure oscillation, combustion instability, engine surge, and thrust loss, etc. The physical model of concern includes the entire engine flow path, extending from the leading edge of the inlet center-body through the exhaust nozzle. The theoretical formulation is based on the Farve-averaged conservation equations of mass, momentum, energy, and species concentration, and accommodates finite-rate chemical kinetics and variable thermo-physical properties. Turbulence closure is achieved using the combined model of a low-Reynolds number k-e two-equation model and Sarkar’s compressible turbulence model. The detail flow structures such as buzz shock train, shock/boundary layer interaction, and flame fluctuation are observed. Both the driving source to the inlet buzz and buzz effects on both flow and flame evolutions are studied.

‡* † § The analysis in Part I of this study is extended to explore the ignition transient and subsequent flame development under flow conditions with and without air throttling. In the present work, gaseous ethylene is injected into the chamber upstream of the cavity flameholder. Immediately after a steady ethylene/air flow is established in the combustor, a hot-spot igniter is employed to simulate the spark ignition at the bottom of the cavity. Various processes involved in the ignition tra nsient and flame development are studied in detail under conditions with and without air throttling. The effect s of the throttling air flow on ignition enhancement and flameholding are analyzed. Air throttling generates a precombustion shock train in the isolator. The resultant decrease in the flow velocity and increases in the temperature and pressure in the combustor section improve the ignition characteristics and the flame stabilization process. The predicted combustor performance and flow distributions agree well with experimental measurements.

Difficulties in achieving efficient ignition and steady combustion in a high-speed environment have long been serious concerns in the development of scramjet engines. The situation becomes more challenging during the engine start-up stage when the low chamber pressure and unsettled fuel-air mixing tend to blow off the flame, even if a flame holding device such as a cavity is employed. One of the ignition aids is to modulate the flow structures in the isolator and combustor by means of air throttling downstream of the flame holder, in order to reduce the local flow velocity and increase the pressure. The purpose is to establish a proper shock train in the isolator to facilitate ignition and flame stabilization. In experiments, compressed air is introduced in a controlled manner into the combustor to generate a pre-combustion shock train in the isolator. The resultant increases in the temperature and pressure of the air stream in the combustor, along with the decrease in the flow velocity, lead to smooth and reliable ignition. The shock train also gives rise to lowmomentum regions and separated flows adjacent to the combustor side-wall. The fuel-air mixing process in separated flows is considerably enhanced due to shock-induced flow distortion and large residence time. In general, air throttling is activated once steady fuel injection has been achieved and an ignition source such as a spark-plug has been turned on. Air throttling is terminated immediately after the flame is stabilized, in order to minimize the amount of throttling gas. If the subsequent heat release in the combustor is sufficiently high, a proper shock system in the isolator for sustaining combustion is maintained. Insufficient heat release often leads to an unstable shock train. A premature removal of air throttling may result in flame blowout. It should also be noted that the shock train interacts with the inlet flow field. Significant flow spillage or even inlet unstart may occur if the combustor is over pressurized. A dynamic optimization of the shock train is needed to ensure smooth ignition and flame stabilization. The present work establishes an integrated theoretical/numerical framework within which the influences of all known effects (including the location and operating timing of air throttling and fuel injection) on the engine ignition transient and flame development are studied systematically.

A theoretical analysis of active control of nonlinear acoustic instabilities in combustion chambers is developed in this article. The formulation starts with a generalized wave equation that describes the dynamic behavior of second-order nonlinear oscillations with distributed feedback actions. Control inputs are provided by the burning of the injected seconday fuel in the chamber, with its instantaneous mass flow rate modulated by a proportionalplus-integral (PI) controller located between the pressure sensor and the fuel injection mechanism. Various nonlinear stability characteristics, including the existence and stability of limit cycles, are studied analytically using the method of time averaging. In addition, an optimization procedure is developed for selecting controller gains. Nomenclature Anij = parameters associated with nonlinear acoustic coupling a = speed of sound in mixture Bnij = parameters associated with nonlinear acoustic coupling b = spatial distribution of burning of control fuel, Eq. (5b), Cv - constant-volume specific heat for two-phase mixture c = amplification factor of sensor output Dni = linear parameters, Eq. (11) Eni = linear parameters, Eq. (11) e - error signal between desired response and actual measurement

The combustion of solid fuels under supersonic crossflows has been studied using a comprehensive numerical analysis. The formulation is based on the time-dependent multidimensional compressible Navier-Stokes equations and species transport equations. Features of this approach include consideration of finite-rate chemical kinetics and variable properties. Turbulence closure is achieved using the Baldwin-Lomax algebraic model. The governing equations are solved numerically using a flux-vector splitting lower-upper symmetric successive overrelaxation technique that treats source terms implicitly. The effects of various operating conditions on the combustion behavior of the hydroxyl-terminated polybutadiene-based solid-fuel samples are treated in detail. Results indicate that both inlet temperature and pressure have strong influences on the burning rate of the fuel sample. For the operating range considered, an optimum pressure is required to maximize the burning rate. The sample burns increasingly faster with pressure from 1 to 4 atm. However, at a higher pressure, the energy released by combustion is not sufficient to further raise the temperature of the crossflow. This results in a decrease in heat feedback to the fuel sample, consequently causing a slight reversal of the burning rate trend with pressure.

Active control of longitudinal pressure oscillations in combustion chambers has been studied theoretically by means of a digital state-feedback control technique. The formulation is based on a generalized wave equation that accommodates all influences of combustion, mean flow, unsteady motions, and control actions. Following a procedure equivalent to the Galerkin method, a system of ordinary differential equations governing the amplitude of each oscillatory mode is derived for the controller design. The control actions are provided by a finite number of point actuators, with instantaneous chamber conditions monitored by multiple sensors. Several important control aspects, including sampling period, locations of sensors and actuators, controllability, and observability, have been investigated systematically. As a specific example, the case involving two controlled and two residual (uncontrolled) modes is studied. 20 refs.

Gasification Technology to Produce Synthesis Gas, G.A. Richards and K.H. Casleton Syngas Chemical Kinetics and Reaction Mechanisms, M. Chaos, M.P. Burke, Y. Ju, and F.L. Dryer Laminar Flame Properties of H2/CO Mixtures, J. Natarajan and J.M. Seitzman Fundamental Combustion Characteristics of Syngas, G. Ribert, P. Thakre, Z. Wang, R.A. Yetter, and V. Yang Turbulent Combustion Properties of Premixed Syngas, R.K. Cheng Pollutant Formation and Control, K.J. Whitty, H.R. Zhang, and E.G. Eddings Syngas Utilization, G.A. Richards, K.H. Casleton, and N.T. Weiland Catalytic Combustion of Syngas, J. Mantzaras Operability Issues Associated with Steady Flowing Combustors, T. Lieuwen, V. McDonell, D. Santavicca, and T. Sattelmayer Combustion of Syngas in Internal Combustion Engines, M.K. Fox, G. Lilik, A.L. Boehman, and O. Le Corre Solid Oxide Fuel Cells Using Syngas, R.J. Kee, H. Zhu, and G.S. Jackson Index

Liquid-propellant rocket engines are capable of on-command variable thrust or thrust modulation, an operability advantage that has been studied intermittently since the late 1930s. Throttleable liquid-propellant rocket engines can be used for planetary entry and descent, space rendezvous, orbital maneuvering including orientation and stabilization in space, and hovering and hazard avoidance during planetary landing. Other applications have included control of aircraft rocket engines, limiting of vehicle acceleration or velocity using retrograde rockets, and ballistic missile defense trajectory control. Throttleable liquid-propellant rocket engines can also continuously follow the most economical thrust curve in a given situation, as opposed to making discrete throttling changes over a few select operating points. The effects of variable thrust on the mechanics and dynamics of an liquid-propellant rocket engine as well as difficulties and issues surrounding the throttling process are important aspects of throttling behavior. This review provides a detailed survey of liquid-propellant rocket engine throttling centered around engines from the United States. Several liquid-propellant rocket engine throttling methods are discussed, including high-pressure-drop systems, dual-injector manifolds, gas injection, multiple chambers, pulse modulation, throat throttling, movable injector components, and hydrodynamically dissipative injectors. Concerns and issues surrounding each method are examined, and the advantages and shortcomings compared.

A numerical analysis is performed to study the effect of surface roughness and radiation on graphite nozzle erosion in solid rocket motors loaded with non-metalized ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants. An integrated theoretical/numerical formulation established previously to predict the chemical erosion of nozzle material has been extended to investigate the effects of surface roughness and radiation. The framework takes into account propellant chemistry, detailed thermofluid dynamics for a multi-component flow, energy transport in the solid phase, heterogeneous reactions at the nozzle surface, and nozzle material properties. Typical combustion species of non-metalized composite propellant at practical motor operating conditions are considered at the nozzle inlet. The two-layer turbulence model has been modified to account for surface roughness. The contribution from radiative heat transfer is included in the overall energy balance at the gas-solid interface. Results show that the heat- transfer rate to the nozzle material is enhanced for a rough surface, leading to a higher erosion rate than that for a smooth surface. For non-metalized propellant, the erosion rate has been found to decrease slightly due to radiation, and change in the rate strongly depends on the gas-phase emissivity.