Thomas A. Jackson

Plasma-Assisted Ignition in Scramjets

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.

Acoustic Characterization of an Ethylene-Fueled Scramjet Combustor with a Cavity Flameholder

*† ‡ § ** †† ‡‡ 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.

Oh planar laser-induced fluoroescence imaging in a hydrocarbon-fueled scramjet combustor

Planar laser-induced fluorescence (PLIF) imaging of OH has been completed tthrough the piloting section of a hydrocarbon-fueled scramjet. This pilot consists of flush-wall fuel injection followed by a recess, or cavity, in one wall. Images were obtained for both gaseous (ethylen) and liquid (JP-7) fuel combustion. For the gaseous-fuel tests, ethylene was introduced through four flush-wall, low-angle injectors placed upstream of the cavity. For the liquid-fuel tests, injection was normal to the crossflow through seven injectors (four in the bottom, wall, three in the top wall). Introducing a small amount of gas into the liquid in the bottom wall injectors enhanced atomization of the liquid column. Flight conditions between March 4 and 5 and dynamic pressures between 23.9 and 71.7 kPa are simulated. Instantaneous images show the dynamis of the combustion process, suggest the process is premixed in nature, and reveal the presence of large-scale structures. Average images at different axial locations show the effects of total temperature and dynamic pressure on the combustion process. Increasing temperature broadens the time-averaged flame zone, while increasing dynamic pressure tends to force the flame against the combustor sidewall. At a given axial location, the time-averaged reaction zone for ethylene is larger than that for JP-7.

Thermoacoustic Flow Instability in a Scramjet Combustor

*† ‡ § ¶ 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.

STRUCTURES OF WATER JETS IN A MACH 1.94 SUPERSONIC CROSSFLOW

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

SPRAY STRUCTURES OF AERATED LIQUID FUEL JETS IN SUPERSONIC CROSSFLOWS

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.

Investigation of Boundary Layer Bleed for Improving Scramjet Isolator Performance

Controlling shock-wave/turbulent-boundary-layer interactions is one of the most important roles of a scramjet isolator, particularly in the effort to prevent inlet unstart. Three-dimensional, steady-state numerical simulations were performed to improve the performance of a rectangular isolator using a bleeding technique. Ten different bleed slot locations and orientations were tested to determine their effectiveness in holding the shock train in the isolator, or pushing the shock system further downstream. The numerical results were based on a back-pressure condition imposed at the isolator exit. In addition, two exit-pressure conditions were set for the bleed slots to determine their impact on the isolator performance. The results show that bleeding out the low-momentum flows near the isolator corners is more effective than removing the boundary layer at the centerplane of the isolator. Numerical simulations were performed to complement experimental work to be done at the AFRL/PRA Test Cell 19 facility. Based on the CFD results and experimental resources, three different bleed slots on the isolator side wall will be tested in the wind tunnel. Eventually, the numerical and experimental results will be compared to determine the effectiveness of the bleed slots in improving the scramjet isolator performance.

A Comprehensive Study of Combustion Oscillations in a Hydrocarbon-Fueled Scramjet Engine

*† ‡ § ¶ The occurrence of combustion oscillations has recently raised a serious concern in the development of scramjet engines. This phenomenon results from the mutual coupling between the unsteady heat release and local flow fluctuations in the flame zone, and has been commonly observed in other types of airbreathing systems such as ramjet and gas-turbine engines. In a scramjet engine, acoustic waves may arise in an unsteady combustion process and propagate upstream through various subsonic flow regions (such as boundary layers, recirculation zones in flame-holding areas, and regions behind precombustion shock waves). These waves then modify the local flowfield and create vortical and entropy disturbances convected downstream into the flame zone. The ensuing pressure and velocity fluctuations perturb the heat-release process and generate acoustic waves traveling upstream. A feedback loop is thus established causing large-amplitude flow oscillations in the engine. Recent experiments have demonstrated the existence of flow oscillations in a hydrocarbonfueled scramjet engine facility with frequencies of 100-160 Hz for liquid JP-7 fuel and 300360 Hz for gaseous ethylene fuel. The present work attempts to establish an integrated theoretical/numerical framework to investigate the combustion oscillations in a scramjet combustor equipped with aerodynamic ramp fuel injectors and a cavity flameholder. Various underlying mechanisms responsible for driving instabilities in a combustor are explored systematically.

Expansion of Supercritical Methane/Ethylene Jets in a Quiescent Subcritical Environment

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.

Effect of droplet-induced breakdown on CARS temperature measurements.

This research examines the potential for coherent anti-Stokes Raman scattering (CARS) to provide reliable gas temperature measurements in the presence of liquid droplets. The droplets cause dielectric breakdown by focusing the CARS laser beams. This breakdown produces a plasma that can disrupt or obscure the CARS signal. Specifically, we examine the influence of laser induced breakdown on the CARS signal, and we determine the importance of droplet position relative to the CARS focal volume and droplet concentration on the reliability of CARS temperature measurements in droplet-laden flows. In addition, we propose a reliable data reduction procedure to minimize the disruptive influence of laser induced breakdown on CARS temperatures.