Showing papers in "Journal of Propulsion and Power in 1988"

Calculation of Unsteady Wake/Rotor Interaction

Abstract: This paper presents a numerical analysis of wake/rotor interaction using a time-marching solution of the unsteady, nonlinear Euler equations. The incoming wakes are specified through the unsteady inflow boundary conditions. The lagged periodic boundary condition that arises due to unequal rotor/stator pitches is handled by a new technique that inclines the computational plane in time. Comparison of results for a flat-plate cascade with results using a linear analytic theory demonstrates the method's capability to accurately predict unsteady forces, moments, and radiated sound. Results are also presented for a low-speed turbine.

Double-multiple streamtube model for studying vertical-axis wind turbines

Abstract: This work describes the present state-of-the-art in double-multiple streamtube method for modeling the Darrieus-type vertical-axis wind turbine (VAWT) Comparisons of the analytical results with the other predictions and available experimental data show a good agreement This method, which incorporates dynamic-stall and secondary effects, can be used for generating a suitable aerodynamic-load model for structural design analysis of the Darrieus rotor 32 references

Spray characteristics of aerated-liquid pressure atomizers

Abstract: The atomizing performance of a novel type of injector is investigated. Essentially, the injector comprises a plain-orifice atomizer with means for injecting air or gas into the liquid upstream of the final discharge orifice. Measurements of mean drop size and drop-size distribution are made using a Malvern spray analyzer. The liquid employed is water, and all tests are carried out at normal atmospheric pressure. Water injection pressures are varied from 34.5 to 690 kPa (5 to 100 psid) and air/liquid ratios from 0.002 to 0.022 by mass. The results obtained with injector orifice diameters of 0.8,1.6, and 2.44 mm show that good atomization can be achieved using only small amounts of atomizing gas at injection pressures down to 34.5 kPa (5 psi). The system appears to have considerable potential for practical applications in which the available gas and liquid pressures are low and in which small holes and flow passages cannot be employed owing to the risk of plugging by contaminants in the liquid.

Laser-induced-fluorescence visualization of transverse gaseous injection in a nonreacting supersonic combustor

Abstract: A laser-induced-fluorescence (LIF) visualization study is reported of transverse gaseous injection into a Mach 2.07 freestream. The purpose of the study was to obtain an experimental data base for validating calculations of three-dimensional mixing in supersonic combustor flowfields without chemical reaction. Three injection configurations were studied: 1) injection from a single orifice into a constant-area duct, 2) injection from a single orifice behind a rearward-facing step, and 3) injection from staged orifices behind a reaward-facing step. Densitometer scans of LIF photographic negatives allowed injectant penetration and spreading profiles to be quantitatively determined for each injection configuration over a range of injection dynamic pressure ratio. These data represent the first detailed measurements of the three-dimensional flowfield in the vicinity of the injectors. Shock-structure tirple points were located visually as a second measure of injectant penetration. Parametric functions were fit to the data to establish quantitatively the effect of a rearward step, staged injection, and injection dynamic pressure ratio on injectant penetration and spreading into a supersonic flow without chemical reaction.

Pulsed instability in rocket motors - A comparison between predictions and experiments

Abstract: A series of 18 pulsed motor tests was conducted in heavy-wall solid rocket motors having an internal case diameter of 8.38 cm. The motor length was varied from 0.61 -1.22 m. Three related reduced-smoke propellants and four different grain designs were tested. All of the motors were linearly stable and all were pulsed into nonlinear instability. The data from this test series were used to evaluate the validity of previously developed pulser models and to evaluate the ability of a previously developed nonlinear instability analysis to predict the observed trends in the data. In most cases, the combined pulser and chamber stability models were found to be capable of predicting measured pulse amplitudes to within 10%. The ejecta pulser model had to be modified to account for the effect of internal wave reflection when propellant grain extended up to the nozzle entrance plane. The nonlinear instability analysis demonstrated the ability to predict many of the observed wave amplitude and shift trends in the data as a function of propellant and grain design. This combined analytical/experimental investigation also provides additional insight into the nature of nonlinear pulse triggered instability and the factors that influence its occurrence and severity.

Combustion Processes in Supersonic Flow

Abstract: Three general classes of models that describe the processes occurring in diabatic flow in ducts having supersonic entry conditions are discussed. They are: integral techniques, finite-difference methods, and exact two-dimensional planar flame models formulated on the basis of instantaneous heat release. All three methods rigorously satisfy the conservation equations. The first two methods provide a basis for predicting and analyzing supersonic combustor performance. The careful interpretation and judicious use of experimental observations are crucial for the successful application of these methods. Comparisons of analytical and experimental results are presented, and generalized parametric studies are included. The third method is based on an idealized mixing and combustion model that may not be achievable, but nonetheless serves as a valuable analytical tool for explaining complex processes involving shock waves and heat addition. Results from four types of flow structures are discussed. Nomenclature A = cross-sectional area Af = projected area of inlet Aw = wall area

Forced mixer lobes in ejector designs

Abstract: Forced mixer lobes in augmentor primary ejectors obtain a 100-percent increase in pumping over conventional design, together with nearly complete mixing in very short mixing ducts, through the generation of large scale axial vorticity in the mixing duct. The vorticity causes rapid mixing of the primary and secondary flows with low losses; since mixing length is minimized, wall friction losses are reduced, allowing more secondary flow to be pumped for a given total pressure in the primary flow. Analytical results are presented that are judged to have significant implications for future ejector test work.

Experimental evaluation of hydraulic accumulator efficiency with and without elastomeric foam

Abstract: Gas-charged hydraulic accumulators utilize the compressibility of a gas to store energy. This energy storage capability has numerous applications, many of which are sensitive to frictional and thermal losses that occur during the charge/discharge cycle. Under some conditions, the magnitude of these losses can be quite high. The thermal loss is caused by variations in the gas temperature during compression and expansion and is highly dependent upon the cycling rate. Experimental data show that the thermal loss can be minimized by inserting elastomeric foam in the gas side of the accumulator. This study reports experimental data taken with an 8.185-liter (2.162-gal) piston-type hydraulic accumulator. Measured accumulator efficiencies are presented, and the effect of holding time, precharge pressure, rate of charging and discharging, and foam mass on the thermal loss and accumulator efficiency are discussed. Frictional and thermal losses are quantified for this accumulator, and their relative magnitudes are determined. For this accumulator, foam insertion improved the efficiency by as much as 60 percent. 18 references.

Combustion of Ammonium Perchlorate and Various Inorganic Additives

Abstract: The combustion of mixtures of ammonium perchlorate and various inorganic compounds of the following general types was studied: oxides of copper and chromium, oxides of other metals, other perchlorates, other ammonium salts, and darkening agents. The additives were generally added at the 2 and 8 wt% levels. Deflagration rates as a function of pressure (100

Vortex shedding in high-speed compressor blade wakes

Abstract: The wakes of highly loaded compressor blades are generally considered to be turbulent flows. Recent work has suggested that the blade wakes are dominated by a vortex streetlike structure. The experimental evidence supporting the wake vortex structure is reviewed. This structure is shown to redistribute thermal energy within the flowfield. The effect of the wake structure on conventional aerodynamic measurements of compressor performance is noted. A two-dimensional, time-accurate, viscous numerical simulation of the flow exhibits both vortex shedding in the wake and a lower-frequency flow instability that modulates the shedding. The numerical results are shown to agree quite well with the measurement from transonic compressor rotors.

Recent Developments in Flutter Suppression Techniques for Turbomachinery Rotors

Abstract: Recent research on passive flutter control techniques for rotors and cascades is reviewed. The proposed methods include the use of periodicity-breaking inperfections, aeroelastic tailoring, dry friction damping, and mode shape control. Numerical results are presented to illustrate the effectiveness of each method in controlling various types of flutter, and to evaluate the feasibility of incorporating the techniques in current and future technology rotors. Mistuning may be the only feasible approach for existing engines, but aeroelastic tailoring offers promising advantages for future engines.

Flame stabilization using large flameholders of irregular shape

Abstract: Measurements of blowoff velocity and drag coefficient are carried out on various two-dimensional flameholders, including flat plates and singleand double-sided vee-gutters. The vee-gutters are tested with and without slots cut in their trailing edges. The flat plates are tested at various inclinations to the flowstream, with cutouts in the leading edge, or trailing edge, or both. The test program covers wide ranges of effective pressure ratio obtained using the water injection technique, and it also includes variations in the size, blockage, and geometry of the flameholders. The results show that flameholder size is the most important parameter in determining aerodynamic blockage and that base-shaped modifications play only a minor role. However, base-shaped modifications do have a significant effect on flame stability. In particular, the random removal of material from a regular bluff body always has an adverse effect on its flameholding properties.

Current distribution and plasma acceleration in MPD arcjets with applied magnetic fields

Abstract: Current distribution and plasma acceleration mechanisms in MPD arcjets with applied magnetic fields are analyzed theoretically, based on induction and quasi-one-dimensional gasdynamic equations. The finite-element method is used to obtain current distribution and magnetic field patterns. The applied magnetic fields are of diverging type, and calculations are conducted in the cases of discharge currents of 1000 and 2000 A, and mass flow rate of 100 mg/s, for a conventional electrode geometry. Whether the magnetic Reynolds number R(m) is negligibly small or not, it is shown that current distribution on the cathode is not influenced by plasma flow, and the influence of it on current distribution is bounded in the downstream region. Information is presented concerning the relative importance of acceleration mechanisms in MPD arcjets with applied magnetic fields; it is also shown that although for most cases, an externally applied magnetic field is desirable, there exists the possibility that an efficiency-reducing current vortex will occur in the downstream region for the case of a strong magnetic field. 9 references.

Aerodynamic Analysis of the Darrieus Wind Turbines Including Dynamic-Stall Effects

Abstract: Comparisons have been made between aerodynamic performance predictions using two dynamic-stall methods and experimental data for a 17-m Sandia wind turbine. These dynamic-stall models are the incidence delay methods originating from Boeing-Vertol and from the Massachusetts Institute of Technology, which are based on numerical correlations of the dynamic-stall delay with pitch rate parameter. The MIT model calculations show that aerodynamic coefficients are underestimated in comparison with those of the Gormont model, even if the shape of the curves is quite the same. The MIT model provides a good prediction of the dynamic-stall regime. This is characterized by a plateau oscillating near the experimental data of the rotor power vs wind speed at the equator

Influence of fuel temperature on atomization performance of pressure-swirl atomizers

Abstract: The influence of fuel temperature on mean drop size and drop-size distribution is examined for aviation gasoline and diesel oil using three pressure-swirl simplex nozzles. Spray characteristics are measured over wide ranges of fuel injection pressure and ambient air pressure using a Malvern spray analyzer. Fuel temperatures are varied from 20+ 50°C. Over this range of temperature, the overall effect of an increase in fuel temperature is to reduce the mean drop size and broaden the drop-size distribution in the spray. Generally, it is found that the influence of fuel temperature on mean drop size is far more pronounced for diesel oil than for gasoline. For both fuels, the beneficial effect of higher fuel temperatures on atomization quality is sensibly independent of ambient air pressure.

Effects of additives on the microexplosion of carbon slurry droplets

Abstract: Experimental investigations were conducted on the effects of volatile additives in inducing and intensifying the microexplosion of combusting carbon slurry droplets. The mixture explosion temperatures were determined, and a new droplet generator capable of producing small droplets of well-controll ed size was developed for the slurry fuel used. Agglomerate fragments produced through microexplosion were collected by using an impactor-type sampling probe and statistically analyzed. Results show that the fragment sizes follow the log-normal distribution, and that addition of 5% volatile miscible fuels to the carbon slurry can substantially reduce the mean fragment size. ARBON slurries hold potential as high-energy-d ensity fuels for application in volume-limite d propulsion sys- tems. The realization of this potential, however, has been com- plicated by the occurrence of particle agglomeration, which is usually associated with the burning of slurry fuels. That is, it has been demonstrated that the gasification mechanism for slurry droplets consists of two stages.1"4 In the first stage the liquid component is preferentially gasified, leaving behind an agglomerate of the particles. In the second stage this agglomer- ate subsequently heats up, ignites, and burns, spanning a pe- riod that can be substantially longer than that involving liquid gasification. Thus, the overall burning time of a slurry droplet is significantly longer than those corresponding to either a liquid droplet of similar initial size or the single particles by themselves. Two suggestions have been offered to reduce the severity of this agglomeration problem. The first is to coat the particles with suitable chemicals in order to prevent particle adhesion. The second is to break up either the slurry droplet before the agglomerate is formed or the agglomerate itself after it is formed. The present investigation is concerned with the second alternative, based on the phenomenon of droplet micro- explosion. Recent theoretical and experimental studies on the droplet combustion of multicomponent fuels have demonstrated that, for suitable components and composition, rapid gasification within the droplet interior can occur spontaneously, leading to intense internal pressure buildup with consequent catastrophic fragmentation of the droplet. The occurrence of microexplo- sion can be facilitated through several major factors.5 First, the volatility differentials between the more and less volatile com- ponents should be as wide as possible in that the less volatile component is needed to drive up the droplet temperature while the more volatile component is needed for the ease with which it will spontaneously gasify. Second, internal gasification is facilitated in the presence of particles, as in slurries, which provide heterogeneous nucleation sites; without such sites the

Performance Experiments with a Shock-Tunnel-Driven Argon-Cesium MHD Disk Generator

Abstract: An extensive amount of data has been collected concerning MHD disk generator performance under different operation conditions. The results are obtained from a large number of runs with the Eindhoven shock tunnel facility. The runs are carried out at different stagnation temperatures, stagnation pressures, external loads, and seed fractions. Two channels have been used, one with and one without inlet swirl. Voltage, pressure, and radiation measurements have been employed. Current to voltage characteristics have been measured for different seed ratios. The enthalpy extractions of the disk with inlet swirl are found to be comparable with similar experiments with linear channels. The enthalpy extractions of the radial disk are found to be lower. A high enthalpy extraction (18 percent at a stagnation temperature of 2100 K) is reported at a comparatively low stagnation pressure (4.2 bar). A one-dimensional-gasdynamical analysis using measured voltages as an input is discussed. The measured fluctuations of electron temperature and density indicate that the results are obtained in a nonuniform plasma. 10 references.

Thrust Modulation in Hybrid Rocket Engines

Abstract: An experimental investigation of hybrid rocket engines (HRE) is performed to study the throttling performance within a thrust range of 10:1. In order to improve the throttling properties over the entire thrust range, the method of secondary gas injection (gasification) was used. The injection gases were reactive (O2) and inert (N2). High-pressure tests were carried out up to 70 bar to increase the burning rate and the specific impulse. The standard fuel in this study was a composition of aromatic and cyclic amine with red fuming nitric acid (RFNA) as the oxidizer. Controllable, stable, and reproducible throttling properties were demonstrated down to 10% of thrust. The performance with gasification demonstrated the general feasibility of wide-range throttling with fixed-injection geometry. The paper outlines a method for calculating the ballistic behavior of hybrid propellant combinations that is based upon the regression characteristic and the effects of gasification. a A c* F G =m/A m O/F = mn/m PC

Representation of average drop sizes in sprays

Abstract: Procedures are presented for processing drop-size measurements to obtain average drop sizes that represent overall spray characteristics. These procedures are not currently in general use, but they would represent an improvement over current practice. Clear distinctions are made between processing data for spatial- and temporal-type measurements (also referred to as concentration-sensitive and flux-sensitive, respectively). The conversion between spatial and temporal measurements is discussed. The application of these procedures is demonstrated by processing measurements of the same spray by two different types of instruments, a laser-diffraction drop sizer and a phase-Doppler particle analyzer.

Onset in magnetoplasmadynamic thrusters with finite-rate ionization

Abstract: Quasi-one-dimensional self-field flow in a magnetoplasmadynamic thruster is analyzed, including the effect of nonequilibrium ionization. This theory, like the frozen flow theory, predicts the occurrence of the destructive "onset" phenomenon due to an excessive back Electromotive Force (EMF). Comparison between this theory and experiments on a straight coaxial thruster is given and is shown to be good within the limits of quasi-one-dim ensionality. The back-EMF onset mechanism is also shown here to be influenced by wall friction and heat transfer, and a choking condition incorporating these effects is derived. a a* A B B* Bt CD DH F h h* hc J J* k kb kf

Influence of design concept and liquid properties on fuel injector performance

Abstract: An experimental investigation of the influence of design concept and liquid properties on fuel injector performance of airblast atomizers is carried out. Three injectors with distinctly different fuel filming concepts were employed in this program. In the experiments, the nozzle operating conditions (namely, air/liquid ratio and air pressure drop) and liquid properties were varied. Drop sizes were measured with a laser diffraction instrument and a photographic technique was employed for cone angle measurement. The results indicate that the method of spreading the liquid into a film has a significant effect on atomization quality. Also, surface tension has the most dominant effect on mean drop size. The drop size distribution data followed closely the modified Rosin-Rammler type under almost all conditions. Equations to correlate mean drop sizes to distribution parameters were derived to facilitate the prediction of the range of droplets in the spray.

Mixing of an underexpanded rectangular jet ejector

Abstract: An experimental investigation was carried out on a rectangular ejector (constant area mixing duct) with an underexpanded rectangular jet as the primary flow. From the wall static pressure measurements, the ejector performance was found to show irregular variations with the primary jet pressure. Hot-wire measurements, together with schlieren photographs, showed that better performance was obtained when the flow was well mixed. The well-mixed flow was found when the screech tone Strouhal number was in the 0.11-0.14 range, which agrees with the most unstable Strouhal number of the free underexpanded rectangular jet. By incorporating a phase-locked technique with shlieren flow visualization, we were able to photograph standing waves excited by the screech tones inside the ejector mixing duct. The interactions between the jet flow and the screech tones were studied using this flow visualization technique.

Analysis of Rotor Tip Clearance Loss in Axial-Flow Turbines

Abstract: Tip clearance flow is a major contributor to the losses in axial flow turbines. Tip shrouding reduces the extent of this loss at the expense of more structural complexity and increased centrifugal blade stresses. Recent technological advance in the area of active clearance control promises to minimize the tip clearance loss without the adverse tip shrouding effects. Due to complexity of rotor tip flows, a comprehensive tip clearance loss model that accounts for the tip shape, relative wall motion, tip loading, and stage characteristics has not yet been developed. In the present paper, the rotor tip clearance flow is aerothermodyn amically analyzed and a loss model is presented that includes the above-mentioned effects. Tip leakage discharge coefficient and the stage loading factor are taken as modeling parameters. Finally, earlier tip clearance loss models are reviewed and comparisons are drawn with the present work.

Mixing characteristics of a ducted elliptical jet

Abstract: Mixing between elliptical ducted air jets discharged at a sudden expansion and nitrogen, which was radially injected through the larger duct walls, was experimentally studied using hot-wire anemometry and gas sampling techniques. Mixing for this flowfield, which simulates a combustor with fuel addition through the side walls, increased considerably when the air jet was produced by an elliptical, rather than a circular, cross section. Elliptical jets discharging from orifices provide better mixing than jets discharging from elliptical and circular pipes. Additional mixing enhancement was achieved when the elliptical jets were acoustically forced by excited resonant pressure waves of the duct. The mean and turbulent velocity measurements provide insight into the mechanism of the observed mixing enhancement for this simulated boundary-layer combustion process.

Highly compact inlet diffuser technology

Abstract: The potential benefits of short, highly compact inlet diffuser systems extend from efficient propulsion/airframe performance to improved survivability The cost of these benefits may be excessively high engine face distortion This highlights the importance of accurate preliminary design methods that assure low distortion engine face flow Therefore, compact/offset subsonic diffuser performance and operational characteristics have been investigated, with the general goal of developing preliminary design guidelines and providing a useful perspective for advanced aircraft designers Incorporated are important prior works that extend the database by introducing the results of recent experiments, including passive and active BLC methods for providing separation-free diffuser flow Diagnostic pressure measurements, in conjunction with CFD calculations, are used to demonstrate the flow mechanisms responsible for the engine face distortion profile Surprisingly good comparisons of measured engine face pressure characteristics with CFD results are shown for the severely separated flow of the test article

Complex modal balancing of flexible rotors including residual bow

Abstract: A balancing procedure utilizing the Complex Modal Method is presented for linear flexible rotor dynamic systems including the effect of residual shaft bow. The method does not require trial runs; however, a valid mathematical model of the system dynamics is required to obtain the system's modal parameters, which are used to relate the balance corrections to measured responses. Several balancing strategies based on the extension of previous work are suggested for single-speed balancing. Two applications are presented: 1) a gas turbine system with computergenerated response data, and 2) an operating steam turbine-generator system.

Experimental and theoretical effects of nitrogen gas flow rate on liquid-jet atomization

Abstract: Two-phase flows were investigated by using high-velocity nitrogen gas streams to atomize small-diameter liquid jets. Tests were conducted primarily in the acceleration-wave regime for liquid-jet atomization, where it was found that the loss of droplets due to vaporization had a marked effect on drop-size measurements. In addition, four identically designed two-fluid atomizers were fabricated and tested for similarity of spray profiles. A scattered-light scanning instrument was used to measure the following characteristic drop diameters: volume median A.s, Sauter mean D32, and volume-linear mean Am and it was found that each one of the three characteristic drop diameters varied inversely with nitrogen gas flow rate raised to the 1.33 power. The exponent 1.33 power is identical to that predicted by atomization theory for liquied-jet breakup in the acceleration-wave regime, and it is somewhat higher than the value of 1.2 that was previously obtained at a sampling distance of 4.4 cm downstream of the atomizer. The difference is attributed to the fact that drop-size measurements obtained at the 2.2 cm sampling distance are less affected by vaporization and dispersion of small droplets and, therefore, should have better agreement with atomization theory.

Blade Tip, Finite Aspect Ratio, and Dynamic Stall Effects on the Darrieus Rotor

Abstract: The objective of the work described in this paper was to apply the Boeing-Vertol dynamic stall model in an asymmetric manner to account for the asymmetry of the flow between the left and right sides of the rotor. This phenomenon has been observed by the flow visualization of a two-straight-bladed Darrieus rotor in the IMST water tunnel. Also introduced into the aerodynamic model are the effects of the blade tip and finite aspect ratio on the aerodynamic performance of the Darrieus wind turbine. These improvements are compatible with the double-multiple-streamtube model and have been included in the CARDAAV computer code for predicting the aerodynamic performance. Very good agreement has been observed between the test data (Sandia 17 m) and theoretical predictions; a significant improvement over the previous dynamic stall model was obtained for the rotor power at low tip speed ratios, while the inclusion of the finite aspect ratio effects enhances the prediction of the rotor power for high tip speed ratios. The tip losses and finite aspect ratio effects were also calculated for a small-scale vertical-axis wind turbine, with a two-straight-bladed (NACA 0015) rotor. 15 references.

Rotation effects on axisymmetric sudden-expansion flows

Abstract: The dump combustor of a ramjet propulsion system was simulated by a Plexiglas tube of 63.5 mm I.D. and 76.2 cm long. A convergent nozzle with an exit diameter of 43.2 mm was fitted to the inlet of the tube. Therefore, this arrangement provided a sudden expansion and gave a step height of 10.15 mm. Tube rotation effects on mixing in the dump combustor were examined by rotating the Plexiglas tube about its own axis at a constant speed. The flow downstream of the sudden expansion was studied using a one-color, one-component, laser-Doppler anemometer. Two conditions were investigated in detail, one with the tube rotating at 840 rpm and another with the tube stationary. In addition, three other experiments with different tube rotational speeds were carried out to determine the dependence of reattachment length on rotation. The average inlet velocity was set at ~ 10.6 m/s for all experiments. The results show that the primary reattachment length decreased as rotation increased because of the destabilizing effect due to the large shear created by the rotating tube wall. A detailed comparison of the flow characteristics with and without rotation present is made, and the effects of rotation on the flow are analyzed.