Showing papers in "Journal of Propulsion and Power in 1999"

Design of Three-Dimensional Hypersonic Inlets with Rectangular-to-Elliptical Shape Transition

Abstract: A methodology has been devised for the design of three-dimensional hypersonic Inlets, This methodology makes extensive use of inviscid stream-tracing techniques to generate an inlet with smooth shape transition from a rectangular-like capture to an elliptical throat. Highly swept leading edges and a significantly notched cowl enable use of these inlets in fixed geometry configurations. The design procedure includes a three-dimensional viscous correction and uses established correlations to check for boundary-layer separation caused by shock wave interactions. Complete details of the design procedure are presented and the characteristics of a modular inlet with a design point of Mach 6.0 are examined. Comparison with a classical two-dimensional inlet optimized for maximum total pressure recovery indicates that these three-dimensional inlets demonstrate good inviscid performance even when operating well below the design point. An estimate of the on-design viscous performance corresponds with that of an efficient inlet for scramjet applications.

Mechanism of Combustion Instability in a Lean Premixed Dump Combustor

Abstract: Results from an experimental study of the mechanism of unstable combustion in a coaxial, optically accessible, bluff-body-stabilizeddumpcombustorwithnaturalgasasthefuelarereported.Aparametricstudywasperformed to investigate the effects of equivalence ratio, inletvelocity, inlet fuel distribution, inlet swirl, and centerbody recess oncombustionstability. It wasfoundthatall of theseparametershadan effectonthestability characteristicsofthis combustor.Atselectedunstableoperatingconditions,phase-resolvedCHchemiluminescenceimageswerecaptured to study the heat-release structure during one period of pressure oscillation. The e ame‐ e owe eld interaction that is depicted in these images indicates that e ame‐ vortex interactions, and the resultant e ame area changes, play a signie cant role in the instabilities that occur when there is no swirl. A simple analysis of these images, however, showedthate uctuatinge ameareaandequivalenceratioe uctuationsbothcontributetotheheatreleasee uctuations that drive the instability. Unstable combustion with swirl appears to be fundamentally different from unstable combustion without swirl in that instabilities with swirl occur near lean blowout and appear to be associated with repeated detaching and reattaching of the e ame from the centerbody.

Performance Increases for Gas-Turbine Engines Through Combustion Inside the Turbine

Abstract: In a conventional gas-turbine engine, fuel is burned in separate combustors before the heated high-pressure gas expands through the turbine to provide shaft power for the compressor, fan, propellers, helicoptor rotors, or an electric generator in a ground-based powerplant application. It is proposed in this paper that combustion be continued purposely inside the turbine to increase the efficiency and specific thrust/power of the engine. We term such a turbine with combustion a turbine-burner. A thermodynamic cycle analysis is performed to demonstrate the performance gains of turbojet engines with the turbine burner over the conventional turbojets. Ground-based gas-turbine engines for power generation are also analyzed, with the results showing even better performance gains compared without conventional engines. A mixing-layer analysis with combustion in an accelarated flow similar to the conditions in the turbine burner shows that there is also potential reduction of NOr by using the turbineburner compared with conventional combustors, where the burning is at a constant pressure. Challenges and related research issues that must be addressed to use the turbine-burner technology are identified in this paper.

Burning Rate of Solid Propellant Ingredients, Part 1: Pressure and Initial Temperature Effects

Abstract: Burning rate data are presented with respect to initial temperature and pressure for three compounds that are currently used in propellant and explosive formulations. The dependence of burning rate on initial temperature and pressureispresented forammonium perchlorate(AP), HMX, and RDX.Data foringredients being considered for advanced propellants, such as CL-20, ammonium dinitramide (ADN), and hydrazinium nitroformate (HNF) are also presented and compared to the more traditional compounds. These ingredients are all capable of selfdee agration,andtheirbehavioroftencontrolsthebehaviorofthepropellantsinwhichtheyarethemainingredients.

Reduced Kinetics Mechanisms for Ram Accelerator Combustion

Abstract: Two skeletal kinetics mechanisms for reactive CH 4/O2 and H2/O2 ram accelerator e owe elds are presented. Both models were derived from a 190-reaction, 38-species kinetics mechanism (RAMEC or RAM accelerator MEChanism) that successfully reproduces the high-pressure (>50 atm), low-dilution (<70%), fuel-rich chemistry of ram accelerator mixtures. The reduction procedure for the CH 4/O2 mechanism utilized a detailed-reduction technique with ignition delay time and heat release as the selection criteria. The methane-based mechanism (REDRAM or REDuced RAM accelerator mechanism ) contains 34 reactions and 22 species and predicts ignition times to better than 5% and postcombustion temperatures to within 10 K of the full mechanism for a representative range of ram accelerator mixtures and conditions. This CH 4/O2 mechanism is an improvement over existing reduced methane-oxidation mechanisms that arebased on lower-pressure, higher-temperature chemistry. An 18-step, 9-species mechanism is presented for hydrogen-based ram accelerator combustion that is based on the H2/O2 submechanism of the RAMEC/Gas Research Institute GRI-Mech 1.2 methane-oxidation mechanism. The H2/O2 kinetics model includes HO 2 and H2O2 chemistry near the second and third explosion limits, necessary for ignition at ram accelerator pressures but lacking in certain e nite rate chemistry models currently in use.

Ignition Delay Times of Ram Accelerator CH/O/Diluent Mixtures

Abstract: An experimental study was performed to determine ignition delay times for CH4/O2/diluent mixtures and conditions relevant to forebody combustion on ram accelerator projectiles. All measurements were performed in the reflected-shock region of a high-pressure shock tube. Temperatures from 1040 to 1600 K and pressures between 35 and 260 atm were studied, and the CH4/O2/diluent mixtures had an equivalence ratio of 0.4, 3.0, or 6.0 with either N2, Ar, or He as the bath gas. Reaction progress was monitored primarily via piezoelectric pressure transducer and visible emission. For each mixture and condition, the ignition developed as a strong ignition front beginning at the endwall with little or no preignition deflagration. Ignition delay time (rigll) correlations were generated for each mixture and the entire data set; the latter correlation indicates that ignition delay is dependent only on the fuel and oxidizer concentrations and, therefore, not on the diluent species or concentration. At temperatures below approximately 1300 K for the fuel-rich mixtures, the Arrhenius temperature dependence of rign changes from an average activation energy of 32.7 kcal/mol, at higher temperatures, to approximately 19.0 kcal/mol, at lower temperatures. The transition occurs at higher temperatures as the pressure is increased, and is indicative of a shift in chain-branching kinetics between the highand intermediate-temperature regimes.

Numerical Study of Cavitation in the Wake of a Hypervelocity Underwater Projectile

Abstract: The focus of this study is on cavitation in the wake of a high-velocity underwater projectile. A physical model based on the Euler equations is presented in terms of two-phase mixture properties. Mathematical closure is achieved by providing equations of state for the possible thermodynamic states: compressible liquid, compressible two-phasemixture,andcompressiblepurevapor.Fortheoperatingconditionsstudiedhere,allstatesaresubcritical. Theproposedmodelissolvedusingahybridcomputationalschemedevelopedtoaccuratelyresolvepropertyproe les across discontinuities. The model is validated with several one-dimensional test cases that have known analytic solutions. For modeling the hypervelocity underwater projectile, the model is shown to compute unsteady shockwave development as well as the projectile-wake cavitation zone. The model is then used to conduct a parametric study on the affect of e ow and projectile properties on cavitation.

Condensed Combustion Products at the Burning Surface of Aluminized Solid Propellant

Abstract: Experimental results are presented on the formation of condensed combustion products (CCPs) at the burning surface of aluminized solid propellants. A number of experimental techniques were developed for describing the characteristics of the CCPs as a function of the oxidizer particle size and pressure. The results of this investigation provide qualitative descriptions of the CCPs formed at the propellant burning surface as functions of oxidizer particle size and pressure. Nomenclature ak = oxidizing potential of gas mixture (mole share of oxidizing components in the mixture) D = diameter of agglomerate D43 = mass-medium diameter of agglomerates D AP = mass-medium diameter of ammonium perchlorate particles d = diameter of e ne oxide particle d HDO 43 = mass-medium diameter of e ne oxide particles Fm.D/ = mass function of agglomerate size distribution fm.D/ = mass function of agglomerate size distribution density fm.d/ = mass function of e ne oxide particle size distribution density P = pressure r = propellant burn rate T = temperature Zm = share of unburned metal in the agglomerates relative to the initial aluminum in the propellant Z a = share of initial metal in the propellant used to form the agglomerates as a whole Z HDO = share of initial metal in the propellant used to form the e ne oxide particles Z ox = share of initial metal in the propellant used to form oxide in the agglomerates ° = edge angle of wetting ´ = mass share of oxide in agglomerates

Pyrolysis Behavior of Hybrid-Rocket Solid Fuels Under Rapid Heating Conditions

Abstract: An experimental investigation of the thermal pyrolysis behavior of several hybrid-rocket solid fuels under rapid heating conditions was conducted to determine pyrolysis laws and to identify and quantify the products of fuel pyrolysis. The study focused on four fuel formulations: pure hydroxyl-terminated polybutadienes (HTPB), 80% HTPB/20% Alex, 80% HTPB/20% Al, and the Joint Industrial Research and Development fuel formulation. A rapid conductive-heating technique was developed and employed to determine Arrhenius-type pyrolysis laws. All four fuels displayed two sets of Arrhenius parameters, depending on the range of surface temperature. For pure HTPB,Ea = 4:91 kcal/mol andA = 11.04mm/s above722K, whileEa = 13:35 kcal/mol andA = 3965mm/s below 722 K. These results agree well with those obtained previously using a lab-scale hybridmotor operating under realistic conditions. The gas chromatograph/mass spectrometer tests of the nonmetalized fuels, using a  ash-heating oven, indicated that the relative concentrations of the pyrolyzed species depended strongly on temperature. For pure HTPB seven major products were identiŽ ed, with 1,3-butadiene representing the dominantproduct at all temperatures tested, up to 1073 K. The measured mole fractions of the pyrolysis products and deduced pyrolysis laws of the fuels studied can be utilized in a comprehensive model simulation for combustion performance predictions.

Critical Assessment of Dual-Bell Nozzles

Abstract: A critical assessment of dual-bell nozzles is given in this paper. The principal flow field development in dual-bell nozzles, as well as design aspects for the contour of the base nozzle, the wall inflection, and the nozzle extension are discussed. Special regard is focused on the transition behavior from sea level to altitude operation and its dependence on the contour type used for the nozzle extension. Parametric numerical simulations of the flowfield development were performed to quantify the different loss effects. It is shown that the additional performance losses caused by the dual-bell nozzle contour are surprisingly low. An analytical derivation of the flow transients from the separated to the fully attached flow is presented. The necessity of further experimental investigations on dual-bell nozzles is emphasized, which will lead to a better understanding of the flow transition in dual-bell nozzles. Finally, new ideas are presented to minimize the duration of the critical flow transition by varying the thrust chamber pressure on system level, to ensure a sudden and controlled jump of the separation point from the wall inflection (sea-level operation) to the exit plane (altitude operation).

Surface Pressure Measurements in Supersonic Transverse Injection Flowfields

Abstract: Pressure-sensitive paint (PSP) was used to examine the surface pressures in flowfields generated by transverse injection of air through circular and elliptical nozzles into a supersonic freestream. Four jet-to-freestream momentum flux ratios (J) were investigated for each jet. Two different pressure paints (pyrene-based and PtOEP-based) were examined. Results of the paint comparison show that the PtOEP-based paint is significantly more temperature-sensitive than the pyrene-based paint. This results from its longer phosphorescence lifetime as compared to pyrene's short fluorescence lifetime. A comparison of the PSP results with conventional pressure measurements indicates good agreement between the two with no special fitting of the paint data. Results further show that jet operating conditions and injector geometry significantly affect the surface pressure field around a transverse jet. Increases in J dramatically alter the wall pressure field upstream and downstream of the injector. Injector geometry strongly affects the upstream extent of the separation region and the bow shock, and the character of the wake region downstream of the jet. Also, the effective back pressures computed from the PSP data for the elliptical injector cases are significantly higher than for the circular injector cases presenting a possible explanation for recently observed differences in transverse penetration.

Analyses and Application of Gas Sampling to Scramjet Engine Testing

Abstract: Gas sampling has been used in combustor studies and in scramjet engine testing. Because the gas sampling is based on the assumption that the gas composition is frozen in the sampling process, the critical Damkohler numbers necessary to quench reactions in the gas-sampling probes were evaluated using a reduced kinetic model. The phase plane analysis showed that reactions in probes can be extinguished if the probe Damkohler number is less than about 10. The analytical results were cone rmed by numerical calculations using full kinetics. The shock swallowing into sampling probes was examined using numerical simulations for the low-Reynolds-number e ow. These theoretical results were verie ed by experiments using four kinds of probes with various cone gurations in a Mach 2.5 supersonic combustor. Based on the results, e ne sampling probes with a tip diameter less than 0.3 mm are recommended for scramjet testing. Based on these calibration studies, gas sampling was successfully applied to scramjet engine testing under a e ight Mach number up to 8, to reveal interesting features in the internal e ow in swept-back engines.

Hybrid Monte Carlo -Particle-in-Cell Simulation of an Ion Thruster Plume

Abstract: A numerical code is described that simulates the plumes of ion thrusters and Hall current thrusters. In the present study, computed e owe eld results are compared with existing experimental measurements for the UK-10 ion thruster. The experimental measurements consist of ion e ux, ion density, and e oating potential data. The numerical code combines the direct simulation Monte Carlo method for modeling collisions with the particle-incell method for modeling plasma dynamics. Xenon neutrals and ions are modeled directly. Electrons are described by the Boltzmann relation. The effect of a e nite back pressure experienced in laboratory experiments is included. Agreement between simulation and experiment issatisfactory. The simulation results are found to bevery sensitive to input conditions assumed at the thruster exit plane. In particular, there is uncertainty in specifying the effects of the curvature of the dished grids of the UK-10 thruster on the ion exit velocity proe le. The sensitivity of the simulations to certain model parameters is also examined. These include the cross section for charge-exchange reactions, the mechanics of charge-exchange reactions, and the electron temperature. The beam ions are found to be only moderately dependent on these variations, whereas the charge-exchange ions are sensitive to them.

Modeling of Stationary Plasma Thruster-100 Thruster Plumes and Implications for Satellite Design

Abstract: A computational model of a stationary plasma thruster (SPT) has been developed using a quasineutral particle-in-cell/direct simulation Monte Carlo (PIC-DSMC) model. This model is based on theoretical work showing that the plume consists of a quasineutral plasma with collisionless electrons in which the magnetic field can be neglected. Details of the PIC DSMC method are presented as well as axisymmetric and three-dimensional results. Comparisons are made to new and previously reported experimental data. The model is shown to produce results similar to laboratory measurements of the ion current density and plume-induced sputter erosion rates. The model does not compare as well with retarding potential analyzer measurements of the ion energy distribution. The results confirm previous observations that measurements made in some ground facilities may substantially overpredict the amount of backflow current that will be experienced under operational conditions. A surface-sputtering model is used to predict the impact the plume has on solar array interconnects and to show the impact an SPT thruster could have on a communications satellite. The results show that the thruster should be canted with respect to the solar array, lowering its effective thrust and specific impulse.

Expansion and Mixing Processes of Underexpanded Supercritical Fuel Jets Injected into Superheated Conditions

Abstract: Theexpansionandmixingprocessesofunderexpandedsupercriticalfueljetsinjectedintosuperheatedconditions were experimentally studied. Ethylene was used as the fuel, and nitrogen was the ambient gas. The near-e eld jet plume structure was characterized by the location and size of the Mach disk and the expansion angle. The Mach disk location of the supercritical ethylene jet matches that of an ideal-gas jet. The size of the Mach disk and the expansion angle, however, increase as the injection temperatureapproaches the critical value. Thefar-e eld mixing processes were characterized by measuring fuel mole fraction and temperature distributions using spontaneous Raman scattering. Fuel mole fraction distributions follow a Gaussian function, whereas temperature distributions exhibit a dee cit inside the jet plume because of the expansion and acceleration of the fuel jet. As the injection condition approached the critical point, the following observations were made: 1 ) the ethylene centerline mole fraction increased, 2 ) the jet width at the stoichiometric level increased, 3 ) the jet width at half the maximum concentration remained the same, and 4 ) the temperature dee cit became more signie cant. These results were attributed to the larger injected fuel mass e ow and fuel condensation when the jet injection conditions approach the critical point.

Burning Rate of Solid Propellant Ingredients, Part 2: Determination of Burning Rate Temperature Sensitivity

Abstract: aep D [± rb=±T0]p Care must be taken in how one evaluates aep when making comparisons of various materials. The collection of burning rate data, number of points measured at a given condition, initial test conditions, and inherent data scatter will have an effect on the value obtained for the burning rate temperature sensitivity. ‡‡;§§ The largestvariationsinburning ratemeasurementsgenerally occur at low pressures, where one approaches the dee agration limit of thematerial,andatregionswhereabreakintheburningratepressure exponentoccurs.Coincidentally,theseareoftentheareasofgreatest interest when determining the burning rate temperature sensitivity. The changes in aep that occur due to data smoothing vs averaging, and with the selection of the initial pressure and temperature conditions, can be signie cant. The effect of initial temperature selection on aep will be demonstrated using the HMX data, and e tting techniques will be illustrated with the ADN data, where considerable data scatter is present. Samples Thedee agrationdataforthesixmonopropellantslisted inTable1 of Ref. 2 will be used to demonstrate the evaluation of burning rate temperature sensitivity. The tabular burning rate data may be found in Refs. 3 and 4. The sample type and preparation was described in Ref.4.Alloftheburningratemeasurementsusedinthispaperforthe calculations were made using the cinephotomicroscopy technique also described in Refs. 3, 4, and elsewhere.

Closed-Loop Active Control of Combustion Instabilities Using Subharmonic Secondary Fuel Injection

Abstract: The results of a study of closed-loop active control of combustion instabilities using subharmonic secondary fuel injection are presented. The tests were conducted in a laboratory-scale dump combustor at atmospheric pressure withaninlettemperatureof 415 ± C,aninletbulkvelocity of 9.3 m/s,andovera rangeoffuel-lean equivalenceratios from lean blowout to the stoichiometric condition. The observed instability was a longitudinal mode instability with a frequency of » 350 Hz. Control was achieved using modulated secondary fuel injection on every fourth cycle of the unstable e ame. The injection of secondary fuel was phase-locked with the pressure oscillation in the combustor, where the relative phase was adjusted to achieve maximum control. The most effective control was achieved at an equivalence ratio of 0.85, where the combustion-generated noise level was reduced by 22 dB, with a secondary fuel e ow rate that was only 2% of the primary fuel e ow rate.

Single-Point Thermometry in High-Pressure, Sooting, Premixed Combustion Environments

Abstract: We have performed nonintrusive thermometry in the burnt gases of rich, pressurized ethylene/air flames using a frequency measurement based on laser-induced gratings. Light from a continuous-wav e probe beam is coherently scattered from a thermal or electrostrictive grating induced by a pair of crossed, pulsed pump beams. The measured Doppler shift of the signal beam is a function of the local speed of sound from which a temperature can be extracted. At equivalence ratios of 1.6, the transient grating temperature agreed with a corrected thermocouple temperature. At higher soot loading, it is necessary to account for the change in local gas composition caused by soot particle vaporization. Soot particles, acting as blackbody absorbers, were observed to generate thermal gratings of diagnostic value.

Control of Flame Oscillations with Equivalence Ratio Modulation

Abstract: A novel technique for controlling oscillating combustion is proposed and demonstrated. For oscillations that exist over a limited range of equivalence ratios, we suggest that periodic modulation around the unstable condition can effectively avoid the oscillating condition, but otherwise produces the desired timeaverage equivalence ratio. Tests of this concept were carried out in an atmospheric-pressure, swirl-stabilized combustor with a nominal heat input of 30 kW. The fuel was natural gas. We show that it was possible to control a 300-Hz oscillation by modulating the fuel flow at frequencies less than 20 Hz, reducing the observed rms pressure from 2.7 to 0.8 kPa. Limitations on when this technique may produce successful control are also discussed.

Probabilistic Analysis of Aircraft Gas Turbine Disk Life and Reliability

Abstract: Two series of low cycle fatigue (LCF) test data for two groups of different aircraft gas turbine engine compressor disk geometries were reanalyzed and compared using Weibull statistics. Both groups of disks were manufactured from titanium (Ti-6Al-4V) alloy. A NASA Glenn Research Center developed probabilistic computer code Probable Cause was used to predict disk life and reliability. A material-life factor A was determined for titanium (Ti-6Al-4V) alloy based upon fatigue disk data and successfully applied to predict the life of the disks as a function of speed. A comparison was made with the currently used life prediction method based upon crack growth rate. Applying an endurance limit to the computer code did not significantly affect the predicted lives under engine operating conditions. Failure location prediction correlates with those experimentally observed in the LCF tests. A reasonable correlation was obtained between the predicted disk lives using the Probable Cause code and a modified crack growth method for life prediction. Both methods slightly overpredict life for one disk group and significantly under predict it for the other.

Internal Ballistics Considerations of Nozzleless Rocket Motors

Abstract: Thefundamentalsoftheinternalballisticsofsolid-propellantnozzlelessrocketmotorshavebeenstudiedtheoreticallyandexperimentally.Aimedatthemajorbasicaspects,thetheoreticalanalysisconsistedofaquasi-steady-state one-dimensionale ow ofa perfect gas in a constant-area port, assuming a uniform burning rate along the grain asa resultofthecompensating effectsoferosiveand pressure-dependentburning contributions.Thesimplie edanalysis has beenfound to be an excellent tool to exhibit fundamental characteristics. Itrevealed that thetheoreticalachievable specie c impulse in a nozzleless rocket cannot exceed approximately 86% of its value in an adapted-nozzle rocket operating at the same average chamber pressure. An experimental parametric study using cylindrical propellant grains has demonstrated good agreement with the major parameters addressed in the theoretical analysis, indicating that the assumption of a uniform burning rate is a realistic approximation.

Dynamic Modeling of Hybrid Rocket Combustion

Abstract: The combustion dynamics in hybrid rockets is studied to provide a basic input into transient motor processes. The model treats the time-dependent heat e ow into the ablating fuel surface. A variable surface temperature is considered with an effective activation energy to describe the surface-temperature variation during the transient. Two time scales are observed for throttling: a short lag near the surface related to the activation energy, and the larger well-known thermal lag as a result of conductivity. The model is also applied to an oscillating surface heate ux input. Weobserved an amplie cation oftheregression-rateoscillations for low frequencies.Although thiseffect is not the cause of instability, it can aggravate existing oscillations at these low frequencies. We next formulated a quasisteady combustion model, which is then coupled with thethermal lag system with boundary-layer delaysthat account for the adjustment of the boundary layer to the changes in the freestream conditions and blowing from the surface. A linearized treatment of this coupled system evidences some low-frequency instabilities. The scaling of the oscillation frequencies and the erratic character of the experimentally observed instabilities are successfully explained.

Effect of Biodiesel Addition to Diesel Fuel on Engine Performance and Emissions

Abstract: In this paper, exhaust emission measurements are presented, using three conventional diesel-engine vehicles and one stationary Fetter diesel engine. The engines were operated with typical automotive diesel fuel, containing 0.2 or 0.05 wt% sulphur, and blends of the fuels and 10 vol% biodiesel. The starting material for biodiesel preparation was sunflower oil, which is an abundant plant oil in many Mediterranean areas. The addition of biodiesel in the traditional diesel fuel resulted in significant reduction of black smoke emissions, and the combination of low-sulphur diesel fuel and biodiesel comprises the best fuel blend tested. The combination of low-sulphur diesel fuel and biodiesel resulted in reducing the participate matter emissions at higher loads. With regard to fuel consumption, the fuels that contained biodiesel resulted in slightly increased fuel consumption. Wear metal measurements indicated that while the detected metals Ag, Cu, Pb, and Cr were not significantly affected by biodiesel addition, the fuel blend that contained biodiesel led to a slight increase of Fe into the used lubricant.

Reconstruction Technique for Reducing Hybrid-Rocket Combustion Test Data

Abstract: A new integral technique has been developed for the purpose of analyzing combustion test data in hybrid rocket firings. The method, which utilizes a complete reconstruction of the regression history, is shown to have several advantages when compared with the current endpoint technique, in which average regression rate and port mass flux are determined based on fuel grain initial and final dimensions. One significant advantage is that the integral method permits one to ascertain the sensitivity of regression rate to changes in mass flux in a single test. Action (burning) times greater than 5 s have been found to be preferred over shorter values more typically used to date, because this minimizes errors in action time determination and permits the fuel to reach a steady-state thermal profile.

Experimental Investigation of the Role of Downstream Ramps on a Supersonic Injection Plume

Abstract: An experimental study was conducted to investigate penetration and plume expansion enhancement of a discrete low-angled (25 deg) supersonic (M =1.9) injection into a supersonic (M =2.9) crosse ow. The enhancement was achieved by injecting the low-angled jet parallel to a compression ramp. Seven compression ramp cone gurations were studied. The jet-ramp interaction enhancement mechanisms included baroclinic torque vorticity, ramp spillage vorticity, bulk compression, and the Magnus force. Shadowgraph photography was used to identify shock structures. Measurements of mean e ow properties quantie ed the e owe eld total pressure losses. Mie scattering images were used to qualitatively assess the e owe eld and to quantify the plume size, trajectory, and concentration decay rate. The results indicated that up to a 22% increase in penetration, a 39% plume expansion (» mixing), and a 27% increase in the concentration decay rate, with a corresponding 17% increase in total pressure loss, can be achieved by injection over a compression ramp as compared with low-angled injection alone.

Transcritical Vaporization of Liquid Fuels and Propellants

Abstract: An overviewis provided of somechallenges associated with predicting spray combustion processes in propulsion systems operating at pressures and temperatures that are above the critical values of the pure fuel or propellant injected in the liquid phase (e.g., diesel engines and cryogenic liquid rocket engines). The issues determining high-pressure phase equilibria are outlined e rst. Then, the case of the gasie cation of a liquid fuel (propellant) droplet in a quiescent environment is considered. The reviewed literature shows that the more advanced models now provide consistent predictions regarding, for instance, the variation of droplet lifetime with pressure. The droplet gasie cation process at these conditions is essentially unsteady. Recent studies using molecular dynamics simulations to investigate transcritical droplet vaporization are briee y discussed. Next, the effects of convection, secondary atomization, and the proximity of neighbors on supercritical droplet combustion are considered. Published results indicate that the latter tends to preclude droplets in clouds from reaching the critical mixing state. Forced convective effects on the behavior of a droplet at supercritical conditions are considerable because they couplewith a signie cantly reduced surfacetension coefe cient to produce secondary atomization and a one order of magnitude reduction in the droplet lifetime. Finally, a specie c example is given of how supercriticality ine uences the overall performance of propulsion systems.

Assessment of Parameters Influencing the Prediction of Shear-Layer Mixing

Abstract: A multi-site team has completed an investigation of modeling high speed mixing layers using the computational methods currently being applied to predict HSCT nozzle flow fields. The objectives of this investigation were to: (1) calibrate the codes used by the various team members against benchmark experimental data, and (2) assess the accuracy of the Navier-Stokes codes in calculating turbulent flows having flow characteristics similar to those of HSCT engine nozzles. Two flow geometries were investigated using the five codes of NASTAR, PAB3D, GIF3D, NASTD, and NPARC. The first was the heated supersonic round jet of Seiner. For this configuration, with a jet exit Mach number similar to that of the primary flow from mixer chutes, three nozzle flow temperatures were investigated with the five codes. Using the same grid, boundary conditions, and k-e turbulence model (in the mixing region), very similar results were obtained for all codes, but the solutions did not agree well with the experimental velocity and temperature profiles. Further calculations using different turbulence models, compressibility corrections, and axisymmetric dissipation corrections improved the agreement with experimental data, but the corrections are not universally applicable. The second configuration was the two-dimensional supersonic mixing layer of Goebel & Dutton. For the flow case examined, with two supersonic streams, the five codes again produced very similar results using the same grid, boundary conditions, and turbulence model. The agreement with experimental data was better than for the Seiner round nozzle.

Evaluation of Stored Energy in Ultrafine Aluminum Powder Produced by Plasma Explosion

Abstract: Ultrae ne aluminum powder produced by plasma explosion (ALEX) exhibits burn behavior unlike that of ordinary aluminum powders. Others have previously suggested that the source of this unique behavior might be stored internal energy. The objective of this study is to evaluate theoretically the feasibility of energy being stored in ALEX as a result of work performed by means of the compression of the liquid portion of an ALEX particle by its shrinking solid shell during rapid solidie cation of the particle. A theoretical model of this process of energy storage was developed. This model was then formulated and numerically solved on a computer for a variety of conditions. Results show that the postulated mechanism results in measurable stored energy for only unrealistically high cooling rates. It is concluded that, although the postulated mechanism could store energy, the amount of energy stored is realistically negligible. This theoretical e nding is in agreement with recent experiments that show no observable stored energy in ALEX particles. Nomenclature A = surface area, m 2 B = bulk modulus, N/m 2 Cp = specie c heat, J/kg ¢K D = diameter, m Est = thermal energy stored during solidie cation, J H f = latent heat of fusion, J/kg h = heat transfer coefe cient, W/m 2 ¢K k = thermal conductivity, W/m ¢K m = mass, kg N = surface node P = pressure, Pa r = radius,m T = temperature, K Ti = initial temperature, K V = volume, m 3 W = work, J ¯ = thermal coefe cient of expansion, 1/K 1t = time-step increment, s Ω = density, kg/m 3

Pyrolysis of paper and cardboard in inert and oxidative environments

Abstract: Data on the thermal destruction behavior of paper and cardboard under controlled conditions are presented. The decomposition behavior was determined using a thermogravimetric analyzer and differential scanning calorimetry. Tests were carried out on paper, cardboard, and cellulose at two different heating rates of 10 and 50°C/min, and surrounding gas environments of argon (for pyrolysis), air, or oxygen (for oxidative pyrolysis). The temperature range for the thermal decomposition behavior was varied from 25 to 1000°C to investigate the entire decomposition spectra. Global decomposition data show that the maximum decomposition shifts to higher temperatures at higher heating rates as a result of the competing effects of heat and mass transfer, product diffusion, and reaction kinetics. The Arrhenius parameters for thermal decomposition were determined using a first-order decomposition reaction of the form: dm = —k x m x df. Results showed that the activation energy, heat of pyrolysis, and char yield are strongly dependent on the heating rate. An increase in heating rate results in a decrease in activation energy and an increased char yield. The heating rate dependence of the kinetic parameters is discussed. The decomposition behavior of the materials examined is endothermic, whereas the overall process is exothermic because of the presence of oxygen in the material. In general, parameters such as heat transfer, mass diffusion, product evolution, heating rate, temperature, and the surrounding environment control the decomposition process. The results show significant variation in the thermal decomposition behavior of the samples. Furthermore, marked variation have been found from sample-to-sample. These variations suggest challenges associated with the exact determination of thermal decomposition characteristics of real wastes. Despite these variations, data presented in this paper are useful for design guidelines for solid waste thermal destruction systems.

Adaptive control of combustion instability with on-line system identification

Abstract: In this study, an adaptive control system is developed for suppressing pressure oscillations in a generic combustor, where the unsteady e owe eld associated with the combustion instability is described by a nonlinear inhomogeneous wave equation. Control action is achieved by injecting auxiliary liquid fuel, and is modeled as an array of time-delayed combustion sources. The adaptive controller employs on-line system identie cation (SID) for robustness with respect to transientoperating states, modeling uncertainties,externaldisturbances, andadditional modesof instability that may arisein thecourseof control. Comparisonsare made between the adaptive controller and a previously developed proportional-plus-integral (PI) controller, in terms of performance (degree of oscillation suppression, and speed with which oscillations are damped ), robustness against plant parameter changes (particularly unknown factors of the auxiliary characteristics ), and control-fuel mass expenditure. The adaptive controllerexhibitsgoodperformancewith aclearadvantageoverthePIcontrollerinrobustness. Also, theadaptive controller with on-line SID stabilizes the pressure oscillations without exciting additional mode (s).