*† ‡ § ** †† ‡‡ The occurrence of combustion oscillations has recently raised serious concerns about the development of scramjet engines. Previous studies on supersonic combustion for high-speed airbreathing propulsion applications indicated that combustion may take place in subsonic regions, such as boundary layers and recirculation zones in flame-holding cavities. During this process, a longitudinal mode of thermoacoustic instability may develop in a spatial domain reaching from the shock train to the flame zone. The present work experimentally and analytically investigates such thermoacoustic instabilities inside an ethylene-fueled scramjet combustor with a recessed cavity flameholder. High-speed pressure transducers are utilized to record acoustic signals. The effects of fuel/air equivalence ratio, fueling scheme, and simulated flight conditions on the stability characteristics of the combustor are examined systematically. A companion analytical analysis is also established to help explore the underlying mechanisms responsible for driving and sustaining thermoacoustic flow instabilities. In particular, the interactions between the unsteady heat release, fuel injection and mixing, and shock response are examined. The measured oscillation frequencies agree well with the characteristic frequencies related to the acoustic feedback loop between the shock and flame and the acoustic-convective feedback loop between the fuel injection and flame.

This study assesses the prospect of main-fuel ignition with plasma-generating devices in a supersonic flow. Progress from this study has established baseline conditions for operation, such as the required operational time of a device to initiate a combustion shock train as predicted by computational fluid dynamics computations. Two plasma torches were investigated: a direct current constricted-arc design and an alternating current unconstricted-arc design based on a modified spark plug. Both plasma torches are realistic in size and operate within the same current and voltage constraints, although differing substantially in orifice geometry. to compare the potential of each concept, the flow physics of each part of the igniter/fuel-injector/combustor system was studied. To understand the constraints involved with the ignition process of a hydrocarbon fuel jet, an experimental effort to study gaseous and liquid hydrocarbons was conducted, involving the testing of ethylene and JP-7 fuels with nitrogen and air plasmas. Results from individual igniter studies have shown plasma igniters to produce hot pockets of highly excited gas with peak temperatures up to 5000 K at only 2 kW total input power. In addition, ethylene and JP-7 flames with a significant level of the hydroxyl radical, as determined by planar laser-induced fluorescence, were also produced in a Mach 2 supersonic flow with a total temperature and pressure of 590 K and 5.4 atm. Information from these experiments is being applied to the generation of constraints and the development of a configuration with perceived high ignition potential in full scramjet combustor testing.

Ignition and flame stability have long been a serious concern in the development of scramjet engines due to the difficulties of achieving efficient ignition and steady combustion in a high-speed environment. The situation becomes more challenging during the engine start-up stage during which the low chamber pressure and unsettled fuel/air mixing tend to blow off the flame, even when a flame holding device such as a cavity is employed. To circumvent this difficulty, one of the ignition aids is to modulate the flow structures in the isolator and combustor in order to reduce the local flow velocity and increase the pressure by means of air throttling downstream of the flame holder. The purpose was to establish a proper shock train in the isolator to facilitate ignition and flame stabilization. In experiments, compressed air was 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-walls, in which the fuel/air mixing process is considerably more efficient due to the shock-induced flow distortion and larger 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 then terminated immediately after the flame is stabilized, in order to minimize the amount of throttling gas. A proper shock system in the isolator for sustaining combustion will be maintained if the subsequent heat release in the combustor is sufficiently high. Insufficient heat release often leads to an unstable shock train, and a premature removal of air throttling may result in flame blowout. It should also be noted that the shock train interacts with the inlet flowfield. Significant flow spillage or even inlet unstart may occur if the combustor is over pressurized. A dynamic optimization of the shock train is needed.

An experimental investigation of the primary breakup properties of round aerated-liquid jets in the annular flow regime exposed to a supersonic crossflow is described. Single- and double-pulse shadowgraphy and holography were used to study the properties of the conical liquid sheet that extends from the jet exit for finite degrees of aeration as well as the outcomes of primary breakup in the dense-spray region near the liquid jet itself The results show that the gas jet along the axis of the annular flow leaving the injector passage is underexpanded so that the excess pressure of the flow in this region forces the annular liquid sheet into a conical shape that extends from the injector exit. Primary breakup occurs in a similar manner along both the upstream and downstream sides of the liquid jet (relative to the crossflow), which suggests that there are relatively weak aerodynamic effects due to the crossflow near the jet exit. Surface velocities of the liquid sheet were measured and were used to develop correlations for the liquid sheet thickness. The sizes of ligaments and drops were measured along the liquid surface and were found to have constant diameters of 29 and 43 mu m, respectively, independent of position along the liquid sheet for the wide ranges of aeration levels, liquid/gas momentum flux ratios, injector exit passage diameters, and liquid properties considered during the present investigation. Finally, drop size distributions satisfied Simmons's universal root-normal drop size distribution function with mass median drop diameter (MMD)/Sauter mean diameter (SMD) = 1.07, which implies more nearly monodisperse drop size properties after aerated-liquid jet primary breakup than is encountered for other primary breakup processes.

A procedure for simulating the injection of supercritical ethylene into nitrogen is used to investigate aspects of the injection of supercritical fuels, considered to be. an enabling technology in the design of hydrocarbons-fueled scramjet engines. The method solves the compressible Navier-Stokes equations for an ethylene/nitrogen mixture, with the thermodynamic behavior of ethylene described using the Peng-Robinson equation of state. Homogeneous equilibrium and finite-rate phase-transition models are used to describe the growth of a condensed ethylene phase in several axisymmetric and three-dimensional injector nozzles. Predictions are compared with shadowgraph and direct-lighting imaging data, mass flow rate measurements, mole-fraction and temperature measurements in the jet mixing zone, and wall pressure distributions. Qualitative trends relating to jet structure, the appearance of a condensed phase, and the effects of back, pressure and injectant temperature are in good agreement with experimental results but indicate the need for improved characterization of the nozzle flow before injection and the inclusion of a better turbulence model for the jet mixing zone. For conditions where both are applicable, a nucleation/ growth phase transition model provides a similar bulk fluid response as a homogeneous equilibrium model but also yields predictions of number density and average droplet size.

Simulations of the injection of ethylene and ethylene / methane mixtures at supercritical conditions are performed using a numerical model that accounts for finite-rate nucleation and growth of a condensed droplet phase. The Peng-Robinson equation of state, extended to binary mixtures, is used to model the thermodynamics of both the liquid and vapor phases. Predictions are compared with experimental shadowgraph and direct-lighting imaging data and with pressure measurements obtained along the centerline of the injector. Selected predictions are also compared with results obtained using a homogeneous equilibrium model. The effects of interfacial tension and species diffusion coefficient are examined, as are the effects of modifying the binary interaction parameter in the Peng Robinson equation. Results obtained for pure ethylene injection are in good agreement with available experimental data, but results obtained for the injection of methane/ethylene mixtures consistently underpredict the location of condensation onset in the injector nozzle.

*† ‡ § ¶ This paper deals with the thermoacoustic instability and ensuing flow oscillation in a scramjet engine, a phenomenon commonly known as combustion instability. The analysis is based on a quasi-one-dimensional treatment of unsteady flow motion, which simulates the main features of the oscillatory flowfields in both the isolator and combustor. The model also accommodates the response of local heat release to acoustic excitation. The calculated oscillation frequency agrees well with the measured values of around 350 Hz. A companion analytical analysis is also established to help explore the underlying mechanisms responsible for driving and sustaining thermoacoustic flow instabilities. In particular, the interactions between the unsteady heat release, fuel injection and mixing, and shock response are examined. Their influence on the acoustic oscillation characteristics is identified.

support requirements and volumetric energy density considerations. The engine cycle and fuel choice pose unique challenges related to the jet-in-crossflow problem, and the design of a successful scramjet combustor remains one of the most challenging problems in supersonic aerodynamics. Among the many formidable tasks facing scramjet designers are the injection of fuel without introducing unacceptably large losses, the efficient mixing of the fuel with the freestream (where efficiency must be measured in terms of both time or distance and energy loss), the stabilization of a combustion region, and the management of heat transfer to walls and injector surfaces. Gaseous injection studies have investigated the effects of discharge orifice shapes (circular vs. elliptic), injector configurations (normal vs. angled, cascade, aeroramp), and different injectants (air vs. helium). The injection of pure and aerated liquids into high-speed crossflows has been studied. Liquid injection studies have investigated the fundamental breakup processes of liquid column, spray penetration height, and the effects of injection angle, liquid aeration, and injection conditions on spray structures.

The structures of water jets injected into an M=1.94 crossflow were studied experimentally. Two plain orifice nozzles with L/d0 of 20 and orifice diameters of 0.5 and 1.0 mm were tested. Liquid injectors were flush mounted on the bottom plate of the wind tunnel to provide normal injection. Wide ranges of test conditions for jet-to-air momentum flux ratios, aeration levels, and freestream air velocity were tested. A two-component phase Doppler particle analyzer (PDPA) was utilized for the measurement of droplet and spray plume properties along the centerline and across the half plane of spray plumes at various freestream locations. Based on the PDPA measurements, correlations for the penetration heights of pure- and aerated-liquid jets were developed. It was found that once the jet is aerated, the penetration height and cross-sectional area of the spray plume increase dramatically to create a more uniformly distributed spray plume for injected liquid. The atomization processes of pure- and aerated-liquid jets are completed at x/d0<100 at the M=1.94 crossflow, due to the strong action of the supersonic freestream air. The flux- averaged SMD is fairly constant for x/d0≥100 and is on the order of 10 µm for both pure- and aerated-liquid jets. Centerline distribution profiles of droplet and spray plume properties in the freestream direction can be normalized by the penetration height of each spray to obtain universal curves for both pure- and aerated- liquid jets in regions where the liquid atomization process is complete. The normalized distribution profiles for droplet size and x-component droplet velocity exhibit S and mirrored-S shapes, respectively. These S-type distribution profiles are caused by the presence of the bottom floor. These universal curves can potentially be used for the modeling of the far-field structure of liquid jets in supersonic crossflows. NOMENCLATURE

Aerated-liquid atomization has been shown to produce well-atomized sprays in a quiescent environment with only a small amount of aerating gas at relatively low injection pressures. This technology is one of those being considered to facilitate rapid vaporization, mixing, and combustion of hydrocarbon fuels in scramjet engine concepts. The present work simulates the internal flow structure of an aerated liquid injector using a “mixture” model of two-phase flow. Details of this model and its numerical formulation using low-diffusion upwinding methods are presented in this work. Simulation results for aerated-liquid injector flowfields at gas-to-liquid (GLR) mass ratios of 0.08% and 2.45% are presented. The effects of the choice of reference velocity, the type of inflow boundary conditions applied, and the use of “unsteady” preconditioning are discussed. In accord with experimental visualization data, two-dimensional results for GLR = 0.08% indicate a combination of slugging and core-annular two-phase flow in the injector. Twoand three-dimensional results for GLR = 2.45% indicate that a core-annular flow mode dominates, again in agreement with experimental trends.

k = Boltzmann constant The structure of supercritical methane/ethylene jets injected into a quiescent environment was investigated experimentally and numerically. Round injectors with orifice diameters of 0.5, 1.0, and 1.5 mm and a passage length/diameter ratio of 4 and a transparent injector with a 1.0-mm square exit cross section and two internal geometric configurations were tested inside a high-pressure chamber to provide vertical downward injection. Methane/ethylene mixtures with methane mole fraction of 0.0, 0.1, and 0.9 were used as the test fluids. Visualization of the near-field jets and measurement of shock/jet structures were performed using shadowgraph images. Qualitative predictions of condensation were made using models of various fluid properties, including a generalized equation of state and pressure and temperature from an ideal-gas flow solver. It was found that supercritical methane/ethylene jets undergo ideal-gas-like expansion with visible barrel shock and Mach disk inside the jets when injected at high reduced temperatures. Opaque jet images with condensation occurring at the injection plane or even inside the injector were observed when the supercritical methane/ethylene jet is injected at a temperature close to critical temperature. Both experimental observation and simulation are consistent with the condensation phenomena of homogeneous nucleation, which can generate a large quantity of small droplets spontaneously at a high liquid temperature.

d0 = injector orifice diameter GLR = aerating gas-to-liquid mass ratio The structures of aerated-liquid jets injected into subsonic crossflows were studied experimentally. An aerated-liquid injector with a diameter of 0.5 mm was flush mounted on the bottom plate of a subsonic wind tunnel to provide normal injection. Freestream Mach numbers, M, of 0.2 and 0.3 were tested. Water at room temperature was used as the test injectant. Wide ranges of test conditions for jet-to-air momentum flux ratios, q0, and aeration levels, GLR, were tested. A phase Doppler particle analyzer (PDPA) was utilized to quantitatively measure droplet and spray plume properties. The obtained data was used to develop correlations for the properties of droplet and spray plume for aerated-liquid jets, using the least squares method. It was found that the atomization processes of a typical aerated-liquid jet are completed at a relatively short x/d0. The droplet size decreases as the GLR and M increase. The droplet velocity increases with GLR, x/d0, and M and has no significant dependence on q0. The cross-sectional area of the spray plume increases with GLR, q0, and x/d0 and decreases with M. As GLR increases, the increase in the cross-sectional area of spray plume, Aj, mainly comes from the increase in spray penetration height. The spray width, however, is fairly independent of GLR. The aspect ratio of the spray plume increases with GLR. It was also found that A70%/Aj and A30%/Aj of aerated-liquid jets increase with GLR and x/d0 and have values of 32.5 and 10.6, respectively. For the present study, the values of A70%/Aj and A30%/Aj for the pure-liquid jets are 29.3 and 8.5, respectively. h = spray penetration height L = nozzle passage length M = freestream Mach number Oh = Ohnesorge number, μL/(ρL(SMD) σ) Q = volumetric flow rate q0 = jet-to-freestream momentum flux ratio at GLR=0, ρLw0/ρ∞u∞ SMD = Sauter mean diameter, Σ di/Σ di, i for all droplets T = temperature u = velocity component in the x direction w = velocity component in the z direction; also spray width We = Weber number, ρL(SMD)(u∞-up)/σ x = distance in the freestream direction y = distance in the cross stream direction z = distance in the direction of liquid injection θ = jet injection angle

The spray structures and the spray penetration heights of aerated liquid jets in a supersonic crossflow were studied experimentally and theoretically. Experiments were carried out inside a 25-mm square supersonic wind tunnel with performance Mach number of 1.85. An effervescent injector was flush mounted on the bottom plate of the supersonic wind tunnel to provide normal injection into the supersonic crossflow. Several plain orifice nozzles with orifice diameters of 380 to 890 pm were tested. Test liquids included water, ethyl alcohol, and a 33% alcohol/water solution. Laser sheet illumination photography and shadowgraph were used for spray visualization and penetration height measurement. Theoretical modeling was based on the assumptions of co-annular spray structure and equal liquid and barbotaged gas velocities at the nozzle exit plane. It was found that the spray transits from the dual mode spray to the pure barbotage mode spray as the amount of barbotaged gas increases. The spray penetration height in the barbotage mode increases with the amount of barbotage gas, due to the increased jet-to-air momentum flux ratios. The theoretical prediction is in good agreement with the experimental data.

Recent results from combustion experiments in a direct-connect supersonic combustor are presented. Successful ignition and sustained combustion of gaseous ethylene have been achieved using an injector/flameholder concept with low-angle, flush-wall fuel injection upstream of a wall cavity. Two interchangeable facility nozzles (Mach 1.8 and 2.2) were used to obtain combustor inlet flow properties that simulate flight conditions between Mach 4 and 6 at a dynamic pressure of 47.9 kPa. Mainstream combustion was achieved at equivalence ratios between 0.25 and 0.75 using only a spark plug and no other external ignition aids. Delta-force levels between 667 and 1779 N were measured, with corresponding combustor pressure ratios between 3.1 and 4.0. Video records of the flame zone show an intensely active combustion zone with rapid flame spreading. One-dimensional performance analysis of the test data indicates a combustion efficiency around 80% with an average combustor skin friction coefficient of 0.0028.