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At least 217 records · Page 12

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NASA NTRS1979

Experimental study of the effects of flameholder geometry on emissions and performance of lean premixed combustors

Emissions of NOx, CO and unburned hydrocarbons (UHC) are reported for a lean premixed propane-air system at inlet conditions of 800K and 1 MPa using twelve flameholder designs. The flameholders tested represent six design concepts with two values of blockage for each concept. Data were obtained at reference velocities of 35 m/s, 25 m/s and 20 m/s at combustor stations 10 cm and 30 cm downstream of the flameholders. Flameholder pressure drop was found to be a principal determinant of emissions performance. Designs producing larger pressure drops also produced less NOx, CO and UHC emissions. The lean stability limit equivalence ratio was found to be approximately 0.35 for all designs. Flashback velocities (axial components in the flameholder passages) varied between 30 m/s and 40 m/s. A perforated plate flameholder was operated with a velocity as low as 23 m/s through the perforations at equivalence ratio 0.7 without producing flashback.

Roffe, G.↗

NASA NTRS1980

Jet decay rate effects on hover jet-induced loads

The paper explains the mechanisms for observed jet decay rate effects on jet-induced loads on a flat plate for cross-flow and hover configurations. Examples are presented for the influence of the decay rate on integrated loads on a flat plate induced by the jet issuing at a right angle from the plate into still air; knowledge of these jet-induced loadings is of particular importance for VTOL aircraft design because they produce an effective loss in the lift force available in hover. The apparent inconsistency between existing lift loss vs jet decay rate trends is explained in light of the different mechanism which determine the jet mixing and trajectory, i.e., entrainment, blockage, and the vortex pair associated with a jet in crossflow.

Kuhlman, J. M.↗

NASA NTRS1981

Analysis of effect of flameholder characteristics on lean, premixed, partially vaporized fuel-air mixtures quality and nitrogen oxides emissions

An analysis was conducted of the effect of flameholding devices on the precombustion fuel-air characteristics and on oxides of nitrogen (NOx) emissions for combustion of premixed partially vaporized mixtures. The analysis includes the interrelationships of flameholder droplet collection efficiency, reatomization efficiency and blockage, and the initial droplet size distribution and accounts for the contribution of droplet combustion in partially vaporized mixtures to NOx emissions. Application of the analytical procedures is illustrated and parametric predictions of NOx emissions are presented.

Cooper, L. P.↗

NASA NTRS1992

Blockage correction in three-dimensional wind tunnel testing based on the wall signature method

An improved wall interference assessment and correction method for three-dimensional wind tunnel testing is presented. Blockage corrections on the surface of a test article are calculated based on a limited number of wall pressure measurements. These measurements are combined with a signature analysis procedure and influence functions to determine an equivalent test article and wake representation. Pressure coefficient corrections are calculated based on this equivalent body. The signature analysis procedure is modified to improve the on-line operation of the wall signature method. A new geometry of the equivalent body is introduced which can be combined with existing panel codes more effectively. The calculation of influence functions and the determination of pressure coefficient corrections are based on a panel code. Therefore it is possible to apply the present method to any closed tunnel cross section. A numerical simulation of the idealized flow field of a wing and its wake in a rectangular wind tunnel is used to verify the improved wall signature method. The present method is considered to calculate blockage corrections in the NASA/ARC 12ft Pressure Wind Tunnel.

Ulbrich, N.↗

NASA NTRS1982

Wall interference correction improvements for the ONERA main wind tunnels

This paper describes improved methods of calculating wall interference corrections for the ONERA large windtunnels. The mathematical description of the model and its sting support have become more sophisticated. An increasing number of singularities is used until an agreement between theoretical and experimental signatures of the model and sting on the walls of the closed test section is obtained. The singularity decentering effects are calculated when the model reaches large angles of attack. The porosity factor cartography on the perforated walls deduced from the measured signatures now replaces the reference tests previously carried out in larger tunnels. The porosity factors obtained from the blockage terms (signatures at zero lift) and from the lift terms are in good agreement. In each case (model + sting + test section), wall corrections are now determined, before the tests, as a function of the fundamental parameters M, CS, CZ. During the windtunnel tests, the corrections are quickly computed from these functions.

Vaucheret, X.↗

NASA NTRS1981

RSRA vertical drag test report

The Rotor Systems Research Aircraft (RSRA), because of its ability to measure rotor loads, was used to conduct an experiment to determine vertical drag, tail rotor blockage, and thrust augmentation as affected by ground clearance and flight velocity. The RSRA was flown in the helicopter configuration at speeds from 0 to 15 knots for wheel heights from 5 to 150 feet, and to 60 knots out of ground effect. The vertical drag trends in hover, predicted by theory and shown in model tests, were generally confirmed. The OGE hover vertical drag is 4.0 percent, 1.1 percent greater than predicted. The vertical drag decreases rapidly as wheel height is reduced, and is zero at a wheel height of 6 feet. The vertical drag also decreases with forward speed, approaching zero at sixty knots. The test data show the effect of wheel height and forward speed on thrust, gross weight capability, and power, and provide the relationships for power and collective pitch at constant gross weight required for the simulation of helicopter takeoffs and landings.

Flemming, R. J.↗

NASA NTRS1984

Inlet boundary layer effects in an axial compressor rotor. II - Throughflow effects

This paper presents results of an experimental aerodynamic study conducted in the rotating frame of reference downstream of an isolated compressor rotor with both thick and thin inlet endwall boundary layers. The paper focuses on those aspects of the data having particular significance to the assumptions and application of throughflow theory. These aspects include the spanwise distributions of static pressure and blockage, and the radial redistribution of fluid as it passes through the blade row. It is demonstrated that the main contributions to total pressure loss, blockage, and the distortion of the static pressure field were due to the hub corner stall and tip leakage. This is a significant departure from previous conclusions which looked to the endwall boundary layer and to secondary flow as major loss and blockage producing mechanisms.

Wagner, J. H.↗

NASA NTRS2007

Noise Benefits of Rotor Trailing Edge Blowing for a Model Turbofan

An advanced model turbofan was tested in the NASA Glenn 9- by 15-Foot Low Speed Wind Tunnel (9x15 LSWT) to explore far field acoustic effects associated with rotor Trailing-Edge-Blowing (TEB) for a modern, 1.294 stage pressure ratio turbofan model. The TEB rotor (Fan9) was designed to be aerodynamically similar to the previously tested Fan1, and used the same stator and nacelle hardware. Fan9 was designed with trailing edge blowing slots using an external air supply directed through the rotor hub. The TEB flow was heated to approximate the average fan exit temperature at each fan test speed. Rotor root blockage inserts were used to block TEB to all but the outer 40 and 20% span in addition to full-span blowing. A configuration with full-span TEB on alternate rotor blades was also tested. Far field acoustic data were taken at takeoff/approach conditions at 0.10 tunnel Mach. Far-field acoustic results showed that full-span blowing near 2.0% of the total flow could reduce the overall sound power level by about 2 dB. This noise reduction was observed in both the rotor-stator interaction tones and for the spectral broadband noise levels. Blowing only the outer span region was not very effective for lowering noise, and actually increased the far field noise level in some instances. Full-span blowing of alternate blades at 1.0% of the overall flow rate (equivalent to full-span blowing of all blades at 2.0% flow) showed a more modest noise decrease relative to full-span blowing of all blades. Detailed hot film measurements of the TEB rotor wake at 2.0% flow showed that TEB was not every effective for filling in the wake defect at approach fan speed toward the tip region, but did result in overfilling the wake toward the hub. Downstream turbulence measurements supported this finding, and support the observed reduction in spectral broadband noise.

Woodward, Richard P.↗

NASA NTRS1991

Visualization of the freeze/thaw characteristics of a copper/water heat pipe - Effects of non-condensible gas

The freeze/thaw characteristics of a copper/water heat pipe of rectangular cross section were investigated experimentally to determine the effect of variations in the amount of non-condensible gases (NCG) present. The transient internal temperature profiles in both the liquid and vapor channels are presented along with contours of the frozen fluid configuration obtained through visual observation. Several interesting phenomena were observed including total blockage of the vapor channel by a solid plug, evaporator dryout during restart, and freezing blowby. In addition, the restart characteristics are shown to be strongly dependent upon the shutdown procedure used prior to freezing, indicating that accurate prediction of the startup or restart characteristics requires a complete thermal history. Finally, the experimental results indicate that the freeze/thaw characteristics of room temperature heat pipes may be significantly different from those occurring in higher temperature, liquid metal heat pipes due to differences in the vapor pressures in the frozen condition.

Ochterbeck, J. M.↗

NASA NTRS2001

Alleviation of Facility/Engine Interactions in an Open-Jet Scramjet Test Facility

Results of a series of shakedown tests to eliminate facility/engine interactions in an open-jet scramjet test facility are presented. The tests were conducted with the NASA DFX (Dual-Fuel eXperimental scramjet) engine in the NASA Langley Combustion Heated Scramjet Test Facility (CHSTF) in support of the Hyper-X program, The majority of the tests were conducted at a total enthalpy and pressure corresponding to Mach 5 flight at a dynamic pressure of 734 psf. The DFX is the largest engine ever tested in the CHSTF. Blockage, in terms of the projected engine area relative to the nozzle exit area, is 81% with the engine forebody leading edge aligned with the upper edge of the facility nozzle such that it ingests the nozzle boundary layer. The blockage increases to 95% with the engine forebody leading edge positioned 2 in. down in the core flow. Previous engines successfully tested in the CHSTF have had blockages of no more than 51%. Oil flow studies along with facility and engine pressure measurements were used to define flow behavior. These results guided modifications to existing aeroappliances and the design of new aeroappliances. These changes allowed fueled tests to be conducted without facility interaction effects in the data with the engine forebody leading edge positioned to ingest the facility nozzle boundary layer. Interaction effects were also reduced for tests with the engine forebody leading edge positioned 2 in. into the core flow, however some interaction effects were still evident in the engine data. A new shroud and diffuser have been designed with the goal of allowing fueled tests to be conducted with the engine forebody leading edge positioned in the core without facility interaction effects in the data. Evaluation tests of the new shroud and diffuser will be conducted once ongoing fueled engine tests have been completed.

Albertson, Cindy W.↗

NASA NTRS1983

Effects of interstage diffuser flow distortion on the performance of a 15.41-centimeter tip diameter axial power turbine

The performance of a variable-area stator, axial flow power turbine was determined in a cold-air component research rig for two inlet duct configurations. The two ducts were an interstage diffuser duct and an accelerated-flow inlet duct which produced stator inlet boundary layer flow blockages of 11 percent and 3 percent, respectively. Turbine blade total efficiency at design point was measured to be 5.3 percent greater with the accelerated-flow inlet duct installed due to the reduction in inlet blockage. Blade component measurements show that of this performance improvement, 35 percent occurred in the stator and 65 percent occurred in the rotor. Analysis of inlet duct internal flow using an Axisymmetric Diffuser Duct Code (ADD Code) were in substantial agreement with the test data.

Mclallin, K. L.↗

NASA NTRS1983

Effects of interstage diffuser flow distortion on the performance of a 15.41-centimeter tip diameter axial power turbine

Mclallin, K. L.↗

NASA NTRS1975

Development of a high efficiency thin silicon solar cell

Variations in temperatures used in experimental processing and their effect on the resulting solar cell performance parameters were investigated. Diffusion temperature variation results in a fairly distinct optimum cell performance for diffusion temperatures in the immediate vicinity of 850 C. An additional effort was also devoted to redesign of the matallization gridline pattern for both minimum light blockage and minimum fill factor alteration due to series resistance. Efforts on improvement of tantalum oxide antireflection coatings were undertaken. Fifty 2 cm x 2 cm cells having a range of thicknesses have been submitted as the first sample group. These cells were processed under conditions tentatively identified during this first contractual quarter's experiments as being optimal for resulting cell performance.

Lindmayer, J.↗

NASA NTRS1986

Development of an explicit time marching procedure for laminar and turbulent flow

A finite-volume based, time-marching method for turbulent and laminar flows is described. Extensions to the computational procedure to allow the handling of shock induced separation and large regions of strong backflow are emphasized. Two test cases are considered, the United Technologies Research Center (UTRC) separated and reattached turbulent boundary layer and the strong shock case in the McDonnell Douglas Research Lab (MDRL) transonic diffuser G. The extended method worked well on the UTRC flow with a boundary layer blockage of 58% and a maximum backflow velocity of 37% of the local maximum free-stream velocity. It also worked well on the MDRL diffuser with a shock Mach number of 1.353 and a maximum backflow velocity of -71.7 m/s. A Mach number dependent interpolation formula for effective pressure was developed for use in density-update time-marching methods.

Source record↗

NASA NTRS1989

Spectral calibration analysis of the airborne oceanographic lidar

Efforts were concentrated on the spectral resolution of the Airborne Oceanographic Lidar (AOL). This year's work was targeted towards the analysis of calibration techniques to enable the AOL to measure absolute radiances of both passive and active modes of operation. Absolute spectral calibration of the AOL is necessary in order to fully understand and monitor the sensitivity and stability of the total system. Calibration is also needed to obtain valid surface truth data, with which to improve the accuracy of satellite-borne oceanic color scanners. In particular, accurate measurements of oceanic chlorophyll concentrations rests upon reliable irradiance calibrations of both laser induced and solar induced chlorophyll fluoresence. An analysis was performed on the spectral calibration methods used by the AOL. The optical path of the instrumentation was examined to study how the radiance from a calibration sphere was influenced. Ray tracing analysis was performed, including the Cassegrain-telescope optics. It was determined that the calibration radiance was significantly effected by optical-defocusing, due to close positioning of the calibration sphere with respect to the telescope. Since the multi-mode usages of the AOL require varying altitudes and trajectories, a computational algorithm was developed to compensate for image distortions of the telescope optics. Secondary mirror blockage, secondary vignetting, and beam divergence was determined, in order to account for the actual amount of calibrated flux received at the spectral sensors.

Rousey, Carlton E.↗

NASA NTRSDate not supplied

Flow Physics at a Large Rotor Tip Gap in a Multi-Stage Axial Compressor

The flow physics in a large rotor tip gap in a 1.5-stage axial compressor is investigated in the current study. The flow structure in the rotor tip region is complex with several dominant vortical structures of opposite rotation, resulting in inhomogeneous and highly anisotropic turbulence. Earlier measurements show that eddy viscosity is negative over large parts of the tip region and eddy viscosity varies among stress/strain components. The present study aims to understand how the complex nature of rotor tip leakage flow affects compressor performance when the tip gap size is greater than 4–5% of the rotor span, which is typical of advanced small core engines. Unsteady Reynolds-averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) techniques are applied to study flow physics in a large rotor tip gap (5.5% of rotor span) in a 1.5-stage axial compressor. Calculated flow fields from the two different approaches are compared with available measurements and examined in detail. LES calculates the pressure rise in the present compressor fairly well, while URANS with a standard two-equation turbulence closure underpredicts the pressure rise by 15-20% of the measured values. The current study shows that URANS with the current turbulence closure produces much higher all-positive eddy viscosity in the tip-gap region compared to measurements and LES. The distribution of eddy viscosity in the URANS simulation is also wrong. Consequently, the flow in the tip region is highly damped with significantly larger blockage generation, which results in the tip leakage vortex (TLV) staying closer to the blade suction side compared to the measurement. When the TLV stays closer to the blade, both flow turning and the pressure rise across the compressor are reduced compared to the measurements. It appears that this effect is amplified by a large rotor tip gap.

tip gap↗

NASA NTRS2009

Improving the Visible and Infrared Contrast Ratio of Microshutter Arrays

Three device improvements have been developed that dramatically enhance the contrast ratio of microshutters. The goal of a microshutter is to allow as much light through as possible when the shutters are in the open configuration, and preventing any light from passing through when they are in the closed position. The ratio of the transmitted light that is blocked is defined here as the contrast ratio. Three major components contribute to the improved performance of these microshutters: 1. The precise implementation of light shields, which protect the gap around the shutters so no light can leak through. It has been ascertained that without the light shield there would be a gap on the order of 1 percent of the shutter area, limiting the contrast to a maximum of 100. 2. The precise coating of the interior wall of each microshutter was improved with an insulator and metal using an angle deposition technique. The coating prevents any infrared light that finds an entrance on the surface of the microshutter cell from being emitted from a sidewall. Since silicon is in effect transparent to any light with a wavelength longer than .1 micrometer, these coatings are essential to blocking any stray signals when the shutters are closed. 3. A thin film of molybdenum nitride (MoN) was integrated onto the surface of the microshutter blade. This film provides the majority of light blockage over the microshutter and also ensures that the shutter can be operated over a wide temperature range by maintaining its flatness. These improvements were motivated by the requirements dictated by the James Webb Space Telescope NIRSpec instrument. The science goals of the NIRSpec require observing some of the very faintest objects in a given field of view that also may contain some very bright objects. To observe the faint objects, the light from the bright objects - which could be thousands of times brighter - must be completely blocked. If a closed microshutter is even slightly transmissive, a very bright object will still transmit a small signal, which can be larger than a signal from a very faint object transmitted through an open shutter. Since this situation can completely corrupt the results, it was necessary that the closed shutters be able to attenuate light by at least a factor of 2,000. There currently exist four flight-quality microshutter arrays that have been fully or are currently undergoing testing and the results support that the three improvements described above have successfully led to contrast levels greater than 50,000 in over 99 percent of the microshutters at an operating temperature of 35 K. Applications for these high-contrast microshutters are in the photomask generation and stepper equipment used to make integrated circuits and microelectromechanical (MEMS) devices. Since microshutters are a reconfigurable optical element, their versatility in these industries provides an improvement over printed masks and fixed projection alignment systems.

Jhabvala, Murzy↗

NASA NTRS2018

Investigation of Turbulence Generated by Upwind Blockages at the Inlet of a 1/50th-Scale Model of the 80- by 120-Foot Wind Tunnel

A 1/50th scale model of the 80- by 120-Foot Wind Tunnel (hereafter referred to as the 80x120) was utilized to determine the magnitude of turbulence caused by buildings located upstream of the wind tunnel inlet. The 1/50th scale models of existing and proposed buildings were constructed to act as blockage for the test. Various inlet locations were sampled for turbulence intensity levels under a variety of blockage conditions including simple three-dimensional rectangular bodies creating quasi two-dimensional physics along the tunnel centerline, existing building structures in the vicinity of the full-scale wind tunnel inlet flow field, and proposed building structures that may someday be constructed at NASA Ames Research Center upwind of the inlet. Therefore, the testing performed and reported in this report can be considered representative of quiescent atmospheric conditions that exist when operating the full-scale 80x120 at night. At quiescent atmospheric conditions there is a measurable increase in turbulence intensity produced by upstream blockages. The blockages examined produced an average turbulence intensity level between 2% and 5% when measured at the inlet. Previous research has shown that the flow control of the 80x120 is capable of reducing this turbulence to less than 0.5% when measured in the test section. Additional research will need to be conducted to determine the influence of atmospheric wind on relative turbulence intensity at the inlet. These results show that for future buildings lying more than 1,000 ft upstream of the full-scale 80x120 inlet, these new buildings will have a negligible effect on the flow quality of the air entering the 80x120 test section under strictly quiescent atmospheric conditions. The Googleplex buildings modeled and tested in this experiment are located approximately 2,450 ft upstream and, as seen in this test campaign, have a negligible influence on the turbulence levels measured at the inlet under quiescent atmospheric conditions.

80- by 120-Foot Wind Tunnel↗