Showing papers in "Journal of Propulsion and Power in 2008"

Survey of Aerodynamic Drag Reduction at High Speed by Energy Deposition

Abstract: A selected survey of aerodynamic drag reduction at high speed is presented. The dimensionless governing parameters are described for energy deposition in an ideal gas. The types of energy deposition are divided into two categories. First, energy deposition in a uniform supersonic flow is discussed. Second, energy deposition upstream of a simple aerodynamic body is examined. Both steady and unsteady (pulsed) energy deposition are examined for both categories, as well as the conditions for the formation of shock waves and recirculation regions. The capability of energy deposition to reduce drag is demonstrated experimentally. Areas for future research are briefly discussed.

Multiple surrogate modeling for axial compressor blade shape optimization

Abstract: A major issue in surrogate model-based design optimization is the modeling fidelity. An effective approach is to employ multiple surrogates based on the same training data to offer approximations from alternative modeling viewpoints. This approach is employed in a compressor blade shape optimization using the NASA rotor 37 as the case study. The surrogate models considered include polynomial response surface approximation, Kriging, and radial basis neural network. In addition, a weighted average model based on global error measures is constructed. Sequential quadratic programming is used to search the optimal point based on these alternative surrogates. Three design variables characterizing the blade regarding sweep, lean, and skew are selected along with the three-level full factorial approach for design of experiment. The optimization is guided by three objectives aimed at maximizing the adiabatic efficiency, as well as the total pressure and total temperature ratios. The optimized compressor blades yield lower losses by moving the separation line toward the downstream direction. The optima for total pressure and total temperature ratios are similar, but the optimum for adiabatic efficiency is located far from them. It is found that the most accurate surrogate did not always lead to the best design. This demonstrated that using multiple surrogates can improve the robustness of the optimization at a minimal computational cost.

Flame Characteristics in Supersonic Combustor with Hydrogen Injection Upstream of Cavity Flameholder

Abstract: Flame characteristics and a plausible flameholding mechanism in a supersonic combustor, with hydrogen injection upstream of a cavity flameholder, were investigated in the present study. Instantaneous OH radical distribution of the combustion flowfield was obtained using OH planar laser-induced fluorescence. According to the similarity of experimental observations with different cavities, a typical L/D = 7 cavity was chosen, and its supersonic combustion flowfield with hydrogen injection was calculated by large-eddy simulation. The results showed that the cavity shear layer plays a very important role in the flameholding process. An approximately steady flame existed in the cavity shear layer and hot combustion products were transported into the injection jet by the vortex interaction ofthejet-with-cavity shear layer. Flame then spread gradually following the counter-rotating vortex induced by the jet until the whole injection jet was ignited. Combustion products, which generated from the cavity shear layer and the jet, were convected into the cavity by the unsteady motion of the cavity shear layer and transported with the recirculation flow to the cavity front wall. These hot products and their intermittent combustion then heated up the cavity, and the fuel that entered into the cavity shear layer was preheated. Thus, the flameholding cycle was formed.

Dynamics of Laminar Premixed Flames Forced by Harmonic Velocity Disturbances

Abstract: This paper describes the dynamics of constant-burning-velocity premixed flames responding to harmonic velocity disturbances. Results are derived from analytical and computational solutions of the nonlinear G equation and compared with available experimental data. It is shown that the flame dynamics are controlled by the superposition of two waves propagating along the flame sheet: those originating at the flame-anchoring point and from flow nonuniformities along the flame. They may either constructively or destructively superpose, and so the overall linear flame response depends upon two Strouhal numbers, St 2 and Stc, related to the amount of time taken for a flow (St c ) and flame-front (St 2 ) disturbance to propagate the flame length, normalized by the acoustic period. The nonlinear flame response is controlled by flame propagation normal to itself, which smoothens out the wrinkles induced by the forcing at an amplitude-dependent rate. The flame's nonlinear response is shown to exhibit two qualitatively different behaviors. For parameter values at which these disturbances constructively interfere, the nonlinear flame response saturates. When the flame disturbances destructively interfere, the nonlinear transfer function may actually exceed its linear value before saturating. This result explains experimentally observed variations of the nonlinear flame response with frequency.

Near-Surface Electrical Discharge in Supersonic Airflow : Properties and Flow Control

Abstract: The results of experimental and numerical investigations of the interaction between the near-wall electrical discharge and supersonic airflow in an aerodynamic channel with constant and variable cross sections are presented. Peculiar properties of the surface quasi-direct-current discharge generation in the flow are described. The mode with flow separation developing outside the discharge region is revealed as a specific feature of such a configuration. An interaction model is proposed on the basis of measurements and observations. A regime of gas-dynamic screening of a mechanical obstacle installed on the channel wall is studied. Variation of the main flow parameters caused by the surface discharge is quantified. The ability of the discharge to shift an oblique shock in an inlet is demonstrated experimentally. The influence of relaxation processes in nonequilibrium excited gas on the flow structure is analyzed. Comparison of the experimental data with the results of calculations based on the analytical model and on numerical simulations is presented.

Nanosecond-Pulsed Discharges for Plasma-Assisted Combustion and Aerodynamics

Abstract: The efficiency of nanosecond discharges as an active-particle generator for plasma-assisted combustion and ignition has been shown. The kinetics of alkane oxidation have been investigated from methane to decane in stoichiometric and lean mixtures with oxygen and air at room temperature under the action of high-voltage nanosecond unform discharge. The study of nanosecond barrier discharge influence on a flame propagation and flame blowoff velocity has been carried out. A significant increase of the flame blowoff velocity has been demonstrated. A decrease of 2-3 orders of magnitude of the plasma-assisted ignition delay time in comparison with the autoignition has been registered. Detonation initiating by high-voltage gas discharge has been demonstrated. The energy deposition in the discharge ranging from 70 mJ to 12 J for propane-oxygen-nitrogen mixtures leads to the transition to detonation at a distance of less than one diameter of the detonation tube. The influence of pulsed surface dielectric discharge on the flow separation for airfoils at a high angle of attack has been investigated within the velocity range from 20 to 110 m/s for the power consumption less than 1 W/cm of the wing span. The conclusion has been made that the main mechanism of plasma impact is the boundary-layer turbulization rather than acceleration.

Chemical Erosion of Carbon-Carbon/Graphite Nozzles in Solid-Propellant Rocket Motors

Abstract: A comprehensive theoretical/numerical framework is established and validated to study the chemical erosion of carbon-carbon/graphite nozzle materials in solid-rocket motors at practical operating conditions. The formulation takes into account detailed thermofluid dynamics for a multicomponent reacting flow, heterogeneous reactions at the nozzle surface, condensed-phase energy transport, and nozzle material properties. Many restrictive assumptions and approximations made in the previous models have been relaxed. Both metallized and nonmetallized AP/HTPB composite propellants are treated. The predicted nozzle surface recession rates compare well with three different sets of experimental data. The erosion rate follows the trend exhibited by the heat-flux distribution and is most severe in the throat region. H 2 O proved to be the most detrimental oxidizing species in dictating nozzle erosion, followed by much lesser contributions from OH and CO 2 , in that order. The erosion rate increases with increasing chamber pressure, mainly due to higher convective heat transfer and enhanced heterogeneous surface reactions. For nonmetallized propellants, the recession rate is dictated by heterogeneous chemical kinetics because the nozzle surface temperature is relatively low. For metallized propellants, the process is diffusion-controlled due to the high surface temperature. The erosion rate decreases with increasing aluminum content, a phenomenon resulting from reduced concentrations of oxidizing species H 2 O, OH, and CO 2 . The transition from the kinetics-controlled to diffusion-controlled mechanism occurs at a surface temperature of around 2800 K.

Plasma-Assisted Ignition in Scramjets

Abstract: 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.

Aluminum-Rich Al-MoO3 Nanocomposite Powders Prepared by Arrested Reactive Milling

Abstract: Fuel-rich Al-MoO 3 nanocomposites were prepared using arrested reactive milling. Powder composition was varied from 4Al + MoO 3 to 16Al + MoO 3 . Powders were evaluated using electron microscopy, thermal analysis, x-ray diffraction, heated-filament-ignition experiments, and constant-volume-explosion experiments. Uniform mixing of MoO 3 nanodomains in the aluminum matrix was achieved for all prepared powders. Multiple and overlapping exothermic processes were observed to start when the nanocomposite powders were heated to only about 350 K. In heated-filament experiments, all nanocomposite powders ignited at temperatures well below the aluminum melting point. Ignition temperatures for these powders were estimated for the higher heating rates that are typical of fuel-air explosions. Constant-volume-explosion experiments indicated that flame propagation in aerosols of nanocomposite thermite powders in air is much faster than that in pure aluminum aerosols. The energy release, normalized per unit mass of aluminum, was higher for the nanocomposite materials with bulk compositions 4Al + MoO 3 and 8Al + MoO 3 and lower for pure aluminum and for the 16Al + MoO 3 nanocomposite sample. The reaction rate was the highest for the 8Al + MoO 3 nanocomposite powder. The combustion efficiency inferred from the measured pressure traces correlated well with the phase compositions of the analyzed condensed combustion products.

Testing a Helicon Double Layer Thruster Immersed in a Space-Simulation Chamber

Abstract: The Helicon Double Layer Thruster, a new magnetoplasma thruster that accelerates ions to supersonic velocities using a current-free electric double layer, has been tested successfully for the first time inside a space-simulation vacuum chamber. Using a retarding field energy analyzer, the presence of a current-free double layer and the associated ion beam in argon have been confirmed for operating conditions of 0.297 mgs -1 of argon, 53.3 mPa gas pressure, 100 W of radio-frequency forward power at 13.56 MHz, and a maximum axial magnetic field of 138 G. The inductively coupled plasma and ion beam formed have been characterized axially, and the measured beam velocity is about 8.7 kms -1 for these conditions. The effect of moving the Helicon Double Layer Thruster source tube relative to the magnetic field and radio-frequency antenna is investigated, and the pressure dependence of the double layer is measured from 20 to 275 mPa and compared with a recently developed theoretical model. Ions in the Helicon Double Layer Thruster exhaust are also shown to be nonmagnetized, suggesting that ion detachment has occurred.

Real-gas effects in Organic Rankine Cycle turbine nozzles

Abstract: Organic Rankine cycle turbogenerators are a viable option as stationary energy converters for external heat sources, in the low power range (from a few kW up to a few MW). The fluid-dynamic design of organic Rankine cycle turbines can benefit from computational fluid dynamics tools which are capable of properly taking into account real-gas effects occurring in the turbine, which typically expands in the nonideal-gas thermodynamic region. In addition, the potential efficiency increase offered by supercritical organic Rankine cycles, which entails even stronger real-gas effects, has not yet been exploited in current practice. In this paper, real-gas effects occurring in subcritical and supercritical organic Rankine cycle nozzles have been investigated. Two-dimensional Euler simulations of an existing axial organic Rankine cycle stator nozzle are carried out using a computational fluid dynamics code, which is linked to an accurate thermodynamic model for the working fluid (octamethyltrisiloxane C 8 H 28 O 2 Si 3 ). The cases analyzed include the expansions starting from actual subcritical conditions, that is, the design point and part-load operation, and three expansions starting from supercritical conditions. Results of the simulations of the existing nozzle for current operating conditions can be used to refine its design. Moreover, the simulations of the nozzle expansions starting from supercritical conditions show that a nozzle geometry with a much higher exit-to-throat area ratio is required to obtain an efficient expansion. Other peculiar characteristics of supercritical expansions such as low sound speed and velocity, high density, and mass flow rate, are discussed.

Wear Mechanisms in Electron Sources for Ion Propulsion, 2: Discharge Hollow Cathode

Abstract: The wear of the keeper electrode in discharge hollow cathodes is a major impediment to the implementation of ion propulsion onboard long-duration space science missions. The development of a predictive theoretical model for hollow cathode keeper life has long been sought, but its realization has been hindered by the complexities associated with the physics of the partially ionized gas and the associated erosion mechanisms in these devices. Thus, although several wear mechanisms have been hypothesized, a quantitative explanation of life test erosion profiles has remained incomplete. A two-dimensional model of the partially ionized gas in a discharge cathode has been developed and applied to understand the mechanisms that drove the erosion of the keeper in two long-duration life tests of a 30-cm ion thruster. An extensive set of comparisons between predictions by the numerical simulations and measurements of the plasma properties and of the erosion patterns is presented. It is found that the near-plume plasma oscillations, predicted by theory and observed by experiment, effectively enhance the resistivity of the plasma as well as the energy of ions striking the keeper.

Characteristics of Gliding Arc and Its Application in Combustion Enhancement

Abstract: A novel magnetically stabilized gliding arc reactor coupled with a counterflow burner was developed to study nonthermal plasma enhancement of ignition and extinction phenomena. The results showed that the new coupled plasma-flame system provides a well-defined platform for understanding of the basic mechanism of the plasma-flame interaction. It was shown that with a plasma discharge of the airstream, up to a 220 % increase in the extinction strain rate was possible at low power inputs for air and methane diluted with nitrogen. Measurements of temperature profiles via planar Rayleigh scattering thermometry and OH number density profiles via planar laser-induced fluorescence (calibrated with absorption) were taken to quantify various effects. Detailed numerical simulations at elevated air temperatures and radical addition were performed for comparison with experimentally obtained results. Results of the extinction experiments initially suggested that the enhancement effect was predominantly thermal for our particular setup of experiments. However, in ignition experiments specifically for hydrogen, temperature measurements conducted for hydrogen-air mixtures suggested the contribution of active species to justify the extent of the enhancement effect. Further comparison with numerical simulations also provides an insight into the participation of species other than radicals in the enhancement effect.

Experimental Study of a Dual-Mode Scramjet Isolator

Abstract: A constant-area isolator was fabricated and tested in conjunction with a Mach 2 hydrogen-air combustor operating at a simulated Mach 5 flight enthalpy. Predicted isolator performance was validated through pressure measurements obtained via low-frequency pressure taps. The maximum pressure ratio measured in the combustor approached the design limit of 4.5. Scramjet operability, the range of equivalence ratios over which combustion was sustained without shock-inlet interaction, was improved to 0.06-0.32, as opposed to 0.32-0.37 without the isolator. For a given change in fuel equivalence ratio, the location of the shock train was easier to control with the isolator modification. Shock-train location repeatability was found to vary somewhat with equivalence ratio. Small fluctuations in the time-resolved pressure history indicated that the shock train was relatively temporally steady for a given equivalence ratio. High-frequency pressure measurements were within a 95% confidence interval of low-frequency pressure measurements. High-frequency results indicated that an increase in pressure and large pressure fluctuations occurred near the leading edge of the shock train. Power spectral analyses also indicated that there is significant variation in the frequency content of the pressure signal upstream and downstream of the shock-train leading edge. These results suggest that methods of shock-train leading-edge detection may be developed using pressure-time history characteristics other than the pressure magnitude.

Turbulence Models Assessment for Large-Scale Tip Vortices in an Axial Compressor Rotor

Abstract: Six turbulence models frequently used in compressor aerodynamics were employed in the detailed numerical investigations of a low-speed large-scale axial compressor rotor, for which the tip flows were measured in detail with stereoscopic particle image velocimetry, to assess the predictive capabilities of the turbulence models for large-scale vortices in the tip region of the rotor The six turbulence models include: the mixing-length model, the Spalart-Allmaras model, the standard k - e model, the shear-stress transport k - w model, the v 2 - f model, and the Reynolds stress model Their results were carefully discussed and compared with the measurements both on velocity fields and turbulence stresses It was found that the Reynolds stress model is superior to the others in the prediction of the tip-leakage vortex at the design condition, whereas the standard k - e model shows the best results in the prediction of the corner vortex at the near-stall condition Although the simulation could predict the large-scale tip vortices well in the mean flowfield, the computed flow mechanism has large discrepancy with the reality

Repetitively Pulsed Nonequilibrium Plasmas for Magnetohydrodynamic Flow Control and Plasma-Assisted Combustion

Abstract: This paper demonstrates significant potential of the use of high-voltage, nanosecond pulse duration, high pulse repetition rate discharges for aerospace applications. The present results demonstrate key advantages of these discharges: 1) stability at high pressures, high flow Mach numbers, and high-energy loadings by the sustainer discharge, 2) high-energy fractions going to ionization and molecular dissociation, and 3) targeted energy addition capability provided by independent control of the reduced electric field of the direct current sustainer discharge. These unique capabilities make possible the generation of stable, volume-filling, low-temperature plasmas and their use for high-speed flow control, nonthermal flow ignition, and gasdynamic lasers. In particular, the crossed pulsersustainerdischargewasusedformagnetohydrodynamic flowcontrolincoldM � 3 flows,providing firstevidenceof cold supersonic flow deceleration by Lorentz force. The pulsed discharge (without sustainer) was used to produce plasma chemical fuel oxidation, ignition, and flameholding in premixed hydrocarbon–air flows, in a wide range of equivalence ratios and flow velocities and at low plasma temperatures, 150–300 � C. Finally, the pulser-sustainer discharge was used to generate singlet oxygen in an electric discharge excited oxygen–iodine laser. Laser gain and output power are measured in the M � 3 supersonic cavity.

Near Exit Plane Velocity Field of a 200-Watt Hall Thruster

Abstract: This work presents the near exit plane velocity field of a 200-W laboratory xenon Hall thruster at a single operating condition with a 250-V anode potential. The ionized propellant velocities were measured using laser-induced fluorescence of the 5d[4] 7/2 -6p[3] 5/2 excited state xenon ionic transition at 834.72 nm. Ion velocities were interrogated from the acceleration channel exit plane to a distance 107 mm from the exit plane (3.3 exit plane diameters). Both axial and radial velocities were measured. A nearly uniform axial velocity profile of approximately 13,800 ± 500 m/s (130 ± 10 eV) was measured at the thruster exit plane. The maximum axial velocity, measured 107 mm from the exit plane, was 16,800 m/s (192 eV). The ion flow exiting the thruster acceleration channel mixes downstream due to both the coaxial thruster geometry and a possible ion-acoustic shock. This behavior appears in regions where multiple, or broadly distributed, radial and axial velocity components occur. These regions also exhibit broadened fluorescence line shapes, likely indicative of collisions between the various velocity populations as well as possible ionization of background neutral, and correspond to the brighter, more visible portions of the plume. This region has been previously identified as a possible ion-acoustic shock. This hypothesis appears consistent with the low radial velocity ion populations measured in this more luminous portion of the plume. In addition, a limited study at five off-nominal conditions near a region of high insulator erosion indicates that the impinging ion energy on the protruding center pole boron nitride insulator is predictably changed by flow rates and anode potentials; however, it also appears to vary significantly with applied magnetic field strength.

Shock Train Leading-Edge Detection in a Dual-Mode Scramjet

Abstract: Time-resolved pressure measurements in a dual-mode scramjet isolator were examined to investigate the potential for using the measurements for shock train leading-edge detection. Changes in the pressure magnitude, standard deviation levels, and frequency content were observed as the shock train advanced upstream past each pressure measurement station. Three detection criteria were defined and examined: 1) 150% of the normalized pressure magnitude upstream of combustion influences, 2) 150% of the normalized pressure standard deviation level upstream of combustion influences, and 3) the maximum value of the normalized pressure standard deviation. Another method of shock train leading-edge detection involved the examination of the frequency content of the pressure signal using power spectra analysis. Results indicated that the second detection criterion provided the earliest method of shock train detection as the shock train moved upstream, followed by the first and third criteria. Also, the frequency content of the pressure signals significantly changed near the shock train leading edge. However, a comparison of this method to the three criteria first examined showed that it did not provide earlier shock train detection.

Investigation of Two-Dimensional Scramjet Inlet Flowfield at Mach 7

Abstract: A measurement campaign has been carried out to investigate the viscous flow effects in a fixed geometry two-dimensional scramjet inlet at Mach 7. The tests have been performed in a hypersonic blowdown wind-tunnel facility. The existing boundary layer separation due to shock interaction in the inlet throat can lead to inlet unstart. Therefore, an optional passive boundary layer bleed has been integrated at the throat. The passive bleed reduces the lip shock induced separation bubble in the throat significantly. The obtained experimental wall pressure distribution and pitot pressure profiles are discussed and compared to 2-D and 3-D computational fluid dynamics calculations. To investigate the compression behavior, different backpressure ratios have been applied to the isolator flow and the effects on the internal flow structure are analyzed by means of shadowgraph pictures, static pressure distribution, and pitot pressure profiles at the exit of the isolator. The heat transfer coefficient to the inner side wall of the isolator is calculated using the time-dependent surface temperature, measured with an infrared system. The resulting 2-D Stanton number distributions are presented for different configurations and different backpressure ratios. In addition, three different Reynolds numbers have been used to investigate the effect on the separation bubble located in the corner of the first and second ramp transitions and the resulting change in ramp heat flux.

Wear Mechanisms in Electron Sources for Ion Propulsion, 1: Neutralizer Hollow Cathode

Abstract: Upon the completion of two long-duration life tests of a 30-cm ion engine, the orifice channel of the neutralizer hollow cathode was eroded away to as much as twice its original diameter. Whereas the neutralizer cathode orifice opened significantly, no noticeable erosion of the discharge cathode orifice was observed. Noquantitative explanation of these erosion trends has been established since the completion of the two life tests. A two-dimensional model of the partially ionized gas inside these devices has been developed and applied to the neutralizer hollow cathode. The numerical simulations show that the main mechanism responsible for the channel erosion is sputtering by Xe+. These ions are accelerated by the sheath along the channel and bombard the surface with kinetic energy/charge of about 17 V at the beginning of cathode life. The density of the ions inside the neutralizer orifice is computed to be as high as 2.1 x 10(sup 22) m(sup -3). Because of the 3.5-times larger diameter of the discharge cathode orifice, the ion density inside the orifice is more than 40 times lower and the sheath drop 7 V lower compared with the values in the neutralizer. At these conditions, Xe+ can cause no significant sputtering of the surface.

Combustion of Boron-Titanium Nanocomposite Powders in Different Environments

Abstract: Combustion of nanocomposite powders with the bulk composition 2B+Ti was compared with combustion of blended boron and titanium powders with the same bulk composition and with combustion of aluminum in wet and dry gas environments. Nanocomposite powders were prepared by Arrested Reactive Milling. The gas environments were N2/O 2/CH 4 mixtures with oxygen concentration fixed at 22.5 % and methane concentration varied from 0 to 12 %. The experiments were conducted in a constant volume explosion vessel. The mass loads of metallic fuel were determined from thermodynamic calculations to ensure the maximum flame temperature for each metal fuel – gas mixture combination. The calculations showed that despite the higher adiabatic flame temperatures for Al than for 2B+Ti, a greater energy per unit mass of metal fuel was released to produce heated gaseous combustion products in combustion of 2B+Ti as compared to Al. Experiments with Al powders showed that the flame temperature did not change noticeably as a function of gas composition and remained close to 2560 K. The combustion temperature for the nanocomposite 2B+Ti increased from about 2180 to 2370 K as the methane concentration increased from 0 to 12 %. The bulk burn rates inferred from the rates of pressure rise were consistently higher for the nanocomposite 2B+Ti powder, followed by Al and then by the blended 2B+Ti powder. The efficiency of combustion for all the fuels was assessed by comparing the predicted and experimental portions of the combustion energy used to produce the heated gaseous products. Based on this assessment, nanocomposite boron-based fuels outperformed Al for all environments, with the difference increasing at the increased methane concentrations. Nearly complete combustion was observed for both 2B+Ti fuels (nanocomposite and blended powders) at high methane concentration, when the highest rates of combustion were also observed. Thus, the effect of kinetic trap associated with formation of HOBO could not be detected. It was concluded that nanocomposite 2B+Ti powders enable one to achieve rapid and highly efficient combustion in both dry and wet gaseous environments.

Metal-CO2 Propulsion for Mars Missions : Current Status and Opportunities

Abstract: *† Metal-CO2 propulsion is less known than other Mars in-situ resource utilization (ISRU) technologies This concept, based on using Martian carbon dioxide as an oxidizer in jet or rocket engines, offers the advantage of no chemical processing for CO2 and thus requires less power consumption than ISRU alternatives In this paper, we review various aspects of metal-CO2 propulsion, such as combustion of magnesium and aluminum in CO2, engine types and characteristics, production of liquid CO2 and metal fuel on Mars, and potential missions Lunar and terrestrial applications are also discussed I Introduction N-SITU Resource Utilization (ISRU) is recognized as an enabling technology for exploration of Mars, which can significantly reduce the mass, cost, and risk of robotic and human missions The critical element in future missions is the large mass of propellant for a Mars ascent vehicle, while power and propellant to accommodate a long stay and mobility on Mars are also important Transportation of propellant from Earth to Mars requires tremendous increase in the initial mass of hardware in low Earth orbit (and hence mission cost) as compared to prior no-return missions Fortunately, Mars possesses resources that can be used for propellant production The Martian atmosphere consisting of 95% CO2 is the obvious and most promising in-situ resource I

Radical farm ignition processes in two-dimensional supersonic combustion

Abstract: A two-dimensional numerical study was performed of the ignition processes associated with the concept of radical farming for supersonic combustion. In a preliminary parametric study, a range of freestream conditions attainable in a hypersonic shock tunnel was investigated and mapped according to whether or not the behavior known as radical farming is present: a combustion-induced pressure rise in second or subsequent hot pockets rather than the first Two such cases were analyzed in detail, both having mean conditions across the combustion-chamber entrance that would result in extremely long ignition lengths. The initiation, branching cycle, and heat release reactions in the combustion process become active in the radical farm, and H and OH radicals are produced. Their rate of production slows in the regions of flow expansion, but does not approach chemical freezing until toward the end of the localized expansion zones. Simultaneously, heat release elevates the local temperature. When the mixture flows through the shock at the second hot pocket, the elevated temperature and the presence of radicals enable the branching cycle and three-body recombination heat release reactions to accelerate, and significant pressure rise due to heat release is then able to occur. The extent to which this is completed in the second hot pocket depends on the inflow conditions.

Single-Dielectric Barrier Discharge Plasma Enhanced Aerodynamics: Concepts, Optimization, and Applications

Abstract: This paper deals with the physics and design of single dielectric barrier discharge plasma actuators for enhanced aerodynamics in a variety of applications. The actuators consist of two electrodes: one exposed to the air and the other covered by a dielectric material. The electrodes are supplied with an alternating current voltage that, at high enough levels, causes the air over the covered electrode to ionize. The ionized air, in the presence of the electric field produced by the electrode geometry, results in a body force vector that acts on the ambient air. The body force is the mechanism for active aerodynamic control. The plasma generation is a dynamic process within the alternating current cycle. The body force per unit volume of plasma has been derived from first principles and implemented in numerical flow simulations. Models for the time and space dependence of the body force on the input voltage amplitude, frequency, electrode geometry, and dielectric properties have been developed and used along with experiments to optimize actuator performance. This paper presents results that highlight the plasma actuator characteristics and modeling approach. This is followed by overviews of some of the applications that include leading-edge separation control on airfoils, dynamic-stall vortex control on oscillating airfoils, and trailing-edge separation control on simulated turbine blades.

Study of Heat Transfer Correlations for Supercritical Hydrogen in Regenerative Cooling Channels

Abstract: Understanding the cooling efficiency of supercritical hydrogen is crucial to the development of high-pressure thrust chambers forregeneratively cooledliquid-oxygen/liquid-hydrogen rocket engines. Available Nusselt number correlations are compared with an extensive data set of local heat transfer coefficients to determine the domains of validity for each correlation. The data set was compiled from previous heated straight-tube experiments with supercritical hydrogen. Results indicate that particular correlations perform better than others for certain regimes of fluid properties, with the accuracy of heat transfer coefficient predictions ranging from 23 to over 100%. Correlation uncertainty due to inherent uncertainties in the equation-of-state and transport properties of supercriticalhydrogenisalsoevaluated.Thepropertydependentuncertaintywasfoundtorangefrom2to10%,and therefore is not the main contributor to the larger errors in the correlation predictions. A number of published correlations for nonhydrogen supercritical fluids are shown to achieve comparable performance with hydrogen.

Unsteady Strong Shock Interactions in a Transonic Turbine : Experimental and Numerical Analysis

Abstract: The aerothermal performance of highly loaded high-pressure turbines is abated by the unsteady impact of the vane shocks on the rotor. This paper presents a detailed physical analysis of the stator-rotor interaction in a state-of-the-art transonic turbine stage at three pressure ratios. The experimental characterization of the steady and unsteady flowfield was performed in a compression tube test rig. The calculations were performed using ONERA's code elsA. This original comparison leads to an improved understanding of the complex unsteady flow physics of a high-pressure turbine stage. The vane shock impingement on the rotor originates a separation bubble on the rotor crown that is responsible for the generation of high losses. A model based on rothalpy conservation has been used to assess the pressure loss. The analysis of the unsteady forcing relates the shock patterns with the force fluctuations.

Limiting Contractions for Starting Simple Ramp-Type Scramjet Intakes with Overboard Spillage

Abstract: A method for obtaining the limiting contraction for supersonic intake-starting via overboard spillage is demonstrated for a simple ramp-type intake family The strong-shock design principle is proposed on the basis of comparison of the limiting contraction line with the Kantrowitz (self-starting) lines of a few particular ramp intakes Predicted starting characteristics compare favorably with two-dimensional inviscid numerical simulations

Optimal Design of Hybrid Rocket Motors for Microgravity Platform

Abstract: A nested direct/indirect method is used to find the optimal design for a microgravity platform which is based on a hybrid sounding rocket. The direct optimization of the parameters that affect the motor design is coupled with the indirect trajectory optimization to maximize a given mission performance index. A gas-pressure feed system is used, with three different propellant combinations. The feed system exploits a pressurizing gas, namely, helium, when hydrogen peroxide or liquid oxygen is used as an oxidizer. The simplest blowdown design is compared with a more complex pressurizing system, which has an additional gas tank that allows for a phase with constant propellant tank pressure. Only self-pressurization is considered with nitrous oxide; two different models are used to describe the behavior of the tank pressurization. The simplest model assumes liquid/vapor equilibrium. A two-phase model is also proposed: Saturated vapor and superheated liquid are considered and the liquid/vapor mass transfer evaluation is based on the liquid spinodal line. Results show that the different tank-pressurization models yield minimal differences of the optimal motor characteristics. Performance differs slightly due to the different mass of the residual oxidizer. The propellant comparison for the present case shows better performance for hydrogen peroxide/ polyethylene with respect to liquid oxygen/hydroxyl-terminated polybutadiene, while nitrous oxide/hydroxyl-terminated polybutadiene remains attractive for system simplicity and low costs.