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At least 127 records · Page 7

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NASA NTRS1989

Fluctuating pressures in pump diffuser and collector scrolls, part 1

The cracking of scroll liners on the SSME High Pressure Fuel Turbo Pump (HPFTP) on hot gas engine test firings has prompted a study into the nature of pressure fluctuations in centrifugal pump states. The amplitudes of these fluctuations and where they originate in the pump stage are quantified. To accomplish this, a test program was conducted to map the pressure pulsation activity in a centrifugal pump stage. This stage is based on typical commercial (or generic) pump design practice and not the specialized design of the HPFTP. Measurements made in the various elements comprising the stage indicate that pulsation activity is dominated by synchronous related phenomena. Pulsation amplitudes measured in the scroll are low, on the order of 2 to 7 percent of the impeller exit tip speed velocity head. Significant non-sychronous pressure fluctuations occur at low flow, and while of interest to commercial pump designers, have little meaning to the HPFTP experience. Results obtained with the generic components do provide insights into possible pulsation related scroll failures on the HPFTP, and provide a basis for further study.

Sloteman, Donald P.↗

NASA NTRS1981

Further determination of the characteristics of magnetospheric plasma vortices with Isee 1 and 2

Further studies of the vortices in magnetospheric plasma flow with the Los Alamos Scientific Laboratory/Max-Planck-Institut (LASL/MPI) fast plasma experiment on Isee 1 and 2 have revealed that the pattern of vortical flow has a wavelength of approximately 20-40 earth radii and moves tailward through the magnetosphere at speeds of several hundred kilometers per second. The tendency toward vorticity pervades the total breadth of the plasma sheet tailward of the dawn-dusk meridian. The sense of rotation of the plasma flow (as viewed from above the ecliptic plane) is clockwise in the morning side of the plasma sheet and counterclockwise in the evening side. The sense of rotation in the morning and evening boundary layers is reversed from that in the contiguous regions of the plasma sheet. The occurrence of vortical flow is independent of the level of geomagnetic activity but is associated with long-period geomagnetic pulsations.

Hones, E. W., Jr.↗

NASA NTRS1994

Boundary Conditions for Unsteady Compressible Flows

This paper explores solutions to the spherically symmetric Euler equations. Motivated by the work of Hagstrom and Hariharan and Geer and Pope, we modeled the effect of a pulsating sphere in a compressible medium. The literature available on this suggests that an accurate numerical solution requires artificial boundary conditions which simulate the propagation of nonlinear waves in open domains. Until recently, the boundary conditions available were in general linear and based on nonreflection. Exceptions to this are the nonlinear nonreflective conditions of Thompson, and the nonlinear reflective conditions of Hagstrom and Hariharan. The former are based on the rate of change of the incoming characteristics; the latter rely on asymptotic analysis and the method of characteristics and account for the coupling of incoming and outgoing characteristics. Furthermore, Hagstrom and Hariharan have shown that, in a test situation in which the flow would reach a steady state over a long time, Thompson's method could lead to an incorrect steady state. The current study considers periodic flows and includes all possible types and techniques of boundary conditions. The technique recommended by Hagstrom and Hariharan proved superior to all others considered and matched the results of asymptotic methods that are valid for low subsonic Mach numbers.

Hariharan, S. I.↗

NASA NTRS2015

Combustion Characteristics in a Non-Premixed Cool-Flame Regime of n-Heptane in Microgravity

A series of distinct phenomena have recently been observed in single-fuel-droplet combustion tests performed on the International Space Station (ISS). This study attempts to simulate the observed flame behavior numerically using a gaseous n-heptane fuel source in zero gravity and a time-dependent axisymmetric (2D) code, which includes a detailed reaction mechanism (127 species and 1130 reactions), diffusive transport, and a radiation model (for CH4, CO, CO2, H2O, and soot). The calculated combustion characteristics depend strongly on the air velocity around the fuel source. In a near-quiescent air environment (< or = 2 mm/s), with a sufficiently large fuel injection velocity (1 cm/s), a growing spherical diffusion flame extinguishes at ≈1200 K due to radiative heat losses. This is typically followed by a transition to the low-temperature (cool-flame) regime with a reaction zone (at ≈700 K) in close proximity to the fuel source. The 'cool flame' regime is formed due to the negative temperature coefficient in the low-temperature chemistry. After a relatively long period (≈18 s) of the cool flame regime, a flash re-ignition occurs, associated with flame-edge propagation and subsequent extinction of the re-ignited flame. In a low-speed (≈3 mm/s) airstream (which simulates the slight droplet movement), the diffusion flame is enhanced upstream and experiences a local extinction downstream at ≈1200 K, followed by steady flame pulsations (≈0.4 Hz). At higher air velocities (4-10 mm/s), the locally extinguished flame becomes steady state. The present axisymmetric computational approach helps in revealing the non-premixed 'cool flame' structure and 2D flame-flow interactions observed in recent microgravity droplet combustion experiments.

diffusion flame↗

NASA NTRS1995

Flame spread across liquids

Recent reviews of our understanding of flame spread across liquids show that there are many unresolved issues regarding the phenomenology and causal mechanisms affecting ignition susceptibility, flame spread characteristics, and flame spread rates. One area of discrepancy is the effect of buoyancy in both the uniform and pulsating spread regimes. The approach we have taken to resolving the importance of buoyancy for these flames is: (1) normal gravity (1g) and microgravity (micro g) experiments; and (2) numerical modeling at different gravitational levels. Of special interest to this work, as discussed at the previous workshop, is the determination of whether, and under what conditions, pulsating spread occurs in micro g. Microgravity offers a unique ability to modify and control the gas-phase flow pattern by utilizing a forced air flow over the pool surface.

Ross, Howard D.↗

NASA NTRS2003

Liquid Fuels: Pyrolytic Degradation and Fire Spread Behavior as Influenced by Buoyancy

This project was conducted by the Combustion and Emission Control Lab in the Engineering Department at Clark Atlanta University under NASA Grant No. NCC3-707. The work aimed at providing data to supplement the ongoing NASA research activities on flame spread across liquid pools by providing flow visualization and velocity measurements especially in the gas phase and gas-liquid interface. During this investigation, the detailed physics of flame spread across liquid pools was revealed using particle image velocimetry (PIV), 3-dimensional Laser Doppler velocimetry (LDV) and high-speed video imaging system (HSVS). Flow fields (front and side views) of both the liquid and gas phases were visually investigated for the three subflash regimes of flame spread behavior. Some interesting findings obtained from the front and side views on flame spread across butanol pools are presented. PIV results showed the size of the transient vortex in the liquid phase near the flame front varied with the initial pool temperature. The transient vortex ahead of the flame front in the gas phase was, for the first time, clearly observed located just within 0-3 mm above the liquid surface and its size was dependent on the initial pool temperature. We calculated the flow velocity at 1 mm below the liquid surface near the flame front and inferred the generation mechanism of the vortex in the gas phase. Finally, after comparison of the flow velocity of the liquid surface and the flame spread rate, a reasonable explanation to the formation mechanism of the pulsating characteristic was proposed. This explanation is compatible with the previous numerical calculations and deductions.

Ross, Howard D.↗

NASA NTRS1979

Sunspots and the physics of magnetic flux tubes. V - Mutual hydrodynamic forces between neighboring tubes

The mutual hydrodynamic forces between parallel cylinders in a moving fluid are illustrated through several formal examples. Parallel tubes in a uniform flow are attracted or repelled depending on whether they are side by side or one ahead of the other, respectively. A pulsating or undulating tube attracts all other neighboring tubes toward itself. These hydrodynamic effects suggest that the separate flux tubes beneath the sunspots exert significant attractive forces on each other.

Parker, E. N.↗

NASA NTRS1990

Toroidal standing waves excited by a storm sudden commencement - DE 1 observations

A 74-nT sudden commencement on July 13, 1982, was observed in the magnetosphere, with instruments on the Dynamics Explorer 1 satellite. Inbound, near L = 4.5, the satellite was located at 1524 magnetic local time and 20 deg magnetic latitude. The sudden commmencement established a strong, east-west oscillation, with 100-s period, which was observed in the magnetic field, the electric field, and the plasma flow velocity records. There was also a compressional component of this 100-s oscillation and a rapidly damped 300-s compressional pulsation. The compressional oscillations may be an evidence of cavity resonances, excited by the sudden commencement. The cavity waves may, in turn, couple to toroidal waves in field line resonance at the satellite location. In addition, the sudden commencement caused the onset of waves with frequencies from 0.1 up to at least 0.5 Hz. The observations are compared with similar reports from earlier pulsations related to sudden commencements.

Cahill, L. J., Jr.↗

NASA NTRS1991

ULF waves in the low-latitude boundary layer and their relationship to magnetospheric pulsations - A multisatellite observation

Combined data from five spacecraft have been analyzed to understand the structure of the low-latitude boundary layer (LLBL), the ULF waves in the LLBL, and the relation of the LLBL waves to magnetic pulsations in the magnetosphere. Both the intensity and the pitch angle distribution of the particles clearly show the presence of the LLBL for the time interval studied. The average magnetic field is rotated slightly at or near the LLBL/magnetosphere interface, which is consistent with a field-aligned current sheet with region 1 flow direction. For ULF waves, a 5-10 min compressional perturbation is present both in the LLBL and the magnetosphere. In the boundary layer, large-amplitude transverse oscillations are present. Magnetic Pc 4-5 pulsations are present in the magnetosphere with azimuthal perturbations and position-dependent frequency.

Takahashi, Kazue↗

NASA NTRS1986

An analysis for the sound field produced by rigid wide cord dual rotation propellers of high solidarity in compressible flow

An unsteady lifting service theory for the counter-rotating propeller is presented using the linearized governing equations for the acceleration potential and representing the blades by a surface distribution of pulsating acoustic dipoles distributed according to a modified Birnbaum series. The Birnbaum series coefficients are determined by satisfying the surface tangency boundary conditions on the front and rear propeller blades. Expressions for the combined acoustic resonance modes of the front prop, the rear prop and the combination are also given.

Ramachandra, S. M.↗

NASA NTRS1986

An approach to the calculation of the pressure field produced by rigid wide chord dual rotation propellers of high solidity in compressible flow

Ramachandra, S. M.↗

NASA NTRS1986

Multi-path peristaltic pump

The instant invention is directed to a peristaltic pump for critical laboratory or hospital applications requiring precise flow rates over an extended period of time. Within the cylindrical barrel pump housing is a single-piece, molded, elastometric, cylindrical liner with a multiplicity of flattened helical channels created therein from one end of the liner to the other. Three cylindrical rollers rotate about the center axis of the pump around the inside surface of the liner selectively compressing the liner, and hence the helical channels between the rollers and the barrel housing, creating a pumping action by forcing trapped fluid in the helical channels axially from one end of the liner to the opposite end. The novelty of the invention appears to lie in the provision of the special liner with multiple helical channels as the pumping chamber, rather than the standard single elastomeric tubing which is squeezed repeatedly by rollers to move the liquid through a typical peristaltic pump. Large, repeated deflections on the standard tubing causes a permanent set in the tubing, thus either changing the flow rate, or requiring a new section of tubing to be positioned in the pump head. Further, this configuration minimizes the amount of outflow pulsation which is characteristic of a typical single tubing peristaltic pump.

Chandler, Joseph A.↗

NASA NTRS1999

Opposed-Flow Flame Spread Across Propanol Pools: Effect of Liquid Fuel Depth

This computational study examines the effect of liquid fuel depth on flame spread across propanol pools with and without forced, opposed air flow. The initial pool temperature is below its closed- cup flash point temperature T(sub cc); so the liquid fuel must be heated sufficiently to create a combustible mixture of fuel vapor before ignition and flame spread can occur. Furthermore, in order for the flame to spread, an approximate rule is that the liquid fuel surface temperature ahead of the flame must be heated above T(sub cc) so that a flammable mixture just above the lean limit exists ahead of the flame. The depth of a liquid fuel pool would affect the heating of the liquid fuel pool and thus the liquid fuel surface temperature ahead of the flame. It has been observed experimentally and numerically that, at normal gravity without forced gas-phase flow and with the initial pool temperature T(sub 0) in a range well below T(sub cc), the flame periodically accelerates and decelerates (pulsates) as it propagates. The depth of a liquid fuel pool would change this range of T(sub 0) since it would affect the heating of the pool.

Kim, Inchul↗

NASA NTRS1999

Experimental Observations on a Low Strain Counter-Flow Diffusion Flame: Flow and Bouyancy Effects

Diffusion flames are of great interest in fire safety and many industrial processes. The counter-flow configuration provides a constant strain flow, and therefore is ideal to study the structure of diffusion flames. Most studies have concentrated on the high velocity, high strain limit, since buoyantly induced instabilities will disintegrate the planar flame as the velocity decreases. Only recently, experimental studies in microgravity conditions have begun to explore the low strain regimes. Numerical work has shown the coupling between gas phase reaction rates, soot reaction rates, and radiation. For these programs, size, geometry and experimental conditions have been chosen to keep the flame unaffected by the physical boundaries. When the physical boundaries can not be considered infinitely far from the reaction zone discrepancies arise. A computational study that includes boundary effects and accounts for the deviations occurring when the major potential flow assumptions are relaxed was presented by Borlik et al. This development properly incorporates all heat loss terms and shows the possibility of extinction in the low strain regime. A major constraint of studying the low strain regime is buoyancy. Buoyant instabilities have been shown to have a significant effect on the nature of reactants and heat transport, and can introduce instabilities on the flow that result in phenomena such as flickering or fingering. The counter-flow configuration has been shown to provide a flame with no symmetry disrupting instabilities for inlet velocities greater than 50 mm/s. As the velocity approaches this limit, the characteristic length of the experiment has to be reduced to a few millimetres so as to keep the Rayleigh number (Ra L = (β g 0 L 3 ∆T)/(𝑎v)) below 2000. In this work, a rectangular counter-flow burner was used to study a two-dimensional counter-flow diffusion flame. Flow visualisation and Particle Image Velocimetry served to describe the nature of the stagnation plane for strain rates smaller than 100 (1/s). These experiments were conducted with a non-reacting flow. Video images of a propane air diffusion flame were used to describe the behaviour of a diffusion flame in this regime. Flame geometry and pulsation frequency are described.

J. A. Sutula↗

NASA NTRS1996

Test Outline for Flutter Analysis of Rectangular Panels in Rarefied Flow Conditions

Jet plume impingement forces acting on large flexible space structures may precipitate dynamically unstable behavior during space flights. Typical operating conditions in space involve rarefied gas flow regimes which are intrinsically distinct from continuum gas flow and are normally modeled using the kinetic theory of gas flow. Docking and undocking operations of the Space Shuttle with the Russian Mir space laboratory represent a scenario in which the stability boundaries of solar panels may be of interest. Extensive literature review of research work on the dynamic stability of rectangular panels in rarefied gas flow conditions indicated the lack of published reports dealing with this phenomenon. A recently completed preliminary study for NASA JSC dealing with the mathematical analysis of the stability of two-degree-of-freedom elastically supported rigid panels under the effect of rarefied gas flow was reviewed. A test plan outline is prepared for the purpose of conducting a series of experiments on four rectangular rigid test articles in a vacuum chamber under the effect of continuous and pulsating Nitrogen jet plumes. The purpose of the test plan is to gather enough data related to a number of key parameters to allow the validation of the two-degree-of-freedom mathematical model. The hardware required careful design to select a very lightweight material while satisfying rigidity and frequency requirements within the constraints of the test environment. The data to be obtained from the vacuum chamber tests can be compared with the predicted behavior of the theoretical two-degree-of-freedom model. Using the data obtained in this study, further research can identify the limitations of the mathematical model. In addition modifications to the mathematical model can be made, if warranted, to accurately predict the behavior of rigid panels under rarefied gas flow regimes.

Akl, Fred A.↗

NASA NTRS1989

On the origin of pulsed emission from the young supernova remnant SN 1987A

To overcome difficulties in understanding the origin of the sub-msec optical pulses from SN 1987A, a model similar to that of Kundt and Krotscheck (1977) for pulsed synchotron emission from the Crab is applied. The interaction of the expected ultrarelativistic electron-positron pulsar wind with pulsar dipole EM wave or wind-carried toroidal magnetic field reflected from the walls of the expected pulsar cavity within the SN 1987A nebula can generate pulsed optical emission with efficiency at most 0.001. The maximum luminosity of the source is reproduced, and other observational constraints can be satisfied for an average wind energy flow of about 10 to the 38th erg/sec sr and for wind electron Lorentz factor gamma of about 100,000. This model applied to the Crab yields pulsations of much lower luminosity and frequency.

Ruderman, M.↗

NASA NTRS2003

Active Flow Separation Control of a Stator Vane Using Surface Injection in a Multistage Compressor Experiment

Micro-flow control actuation embedded in a stator vane was used to successfully control separation and improve near stall performance in a multistage compressor rig at NASA Glenn. Using specially designed stator vanes configured with internal actuation to deliver pulsating air through slots along the suction surface, a research study was performed to identify performance benefits using this microflow control approach. Pressure profiles and unsteady pressure measurements along the blade surface and at the shroud provided a dynamic look at the compressor during microflow air injection. These pressure measurements lead to a tracking algorithm to identify the onset of separation. The testing included steady air injection at various slot locations along the vane. The research also examined the benefit of pulsed injection and actively controlled air injection along the stator vane. Two types of actuation schemes were studied, including an embedded actuator for on-blade control. Successful application of an online detection and flow control scheme will be discussed. Testing showed dramatic performance benefit for flow reattachment and subsequent improvement in diffusion through the use of pulsed controlled injection. The paper will discuss the experimental setup, the blade configurations, and preliminary CFD results which guided the slot location along the blade. The paper will also show the pressure profiles and unsteady pressure measurements used to track flow control enhancement, and will conclude with the tracking algorithm for adjusting the control.

Culley, Dennis E.↗

NASA NTRS1997

Reactive Hydrodynamics in Rotating Spherical and Cylindrical Geometry

In turbulent spray combustion among many complex interactions between local flow structures called turbulent eddies and droplets are those associated with rotation of droplets. In general, for a complete statistical description of turbulent sprays, consideration of at least four degrees of freedom respectively associated with translational, rotational, vibrational (pulsational), and internal motions of the droplet are needed. Clearly the interactions between all degrees of freedom of the droplets and those for the gaseous background field will be exceedingly complex. For example, one type of interaction between the translational and the rotational velocity of droplets results in droplet helicity, H(d) = w(d).v(d), the significance of which in turbulent spray combustion is yet to be recognized. The role of droplet rotation in turbulent spray combustion modeling and its impact on the evaporation of liquid fuel droplets was recently investigated. Also, the impact of rotation on combustion of solid particles such as is encountered in pulverized coal combustion has been emphasized. The problem of viscous flow around a rotating sphere discussed above also occurs in other areas of physical sciences such as astrophysics and geophysics. Consequently, the subject has been addressed in many classical as well as more recent investigations. According to these investigations, the rotation of a rigid sphere in an otherwise quiescent, unconfined environment results in the motion of the fluid towards the poles. The polar flows from the northern and southern hemispheres move along helical trajectories towards the equatorial plane. Eventually, the polar flows collide at the equatorial plane, thus producing a sheet of rotating fluid that is radially ejected outward on this plane. Therefore, a droplet induces a strained flow field as a result of its rotation. Since the spatial extent of equatorial jets could easily exceed many droplet diameters, interactions between neighboring droplets are enhanced as a result of their rotation. Also, the equatorial jet substantially alters the spherical geometry of the diffusion flame surface that surrounds a rotating droplet. The objective of the research is to gain more knowledge about the hydrodynamics within and around rotating spherical and cylindrical body of fluid, and the behavior of diffusion or premixed flame surfaces that could surround such symmetric body of rotating fluid.

Sohrab, Siavash H.↗