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NASA NTRS1993

An Experimental and Theoretical Study of Radiative Extinction of Diffusion Flames

In a recent paper on 'Observations of candle flames under various atmospheres in microgravity' by Ross et al., it was found that for the same atmosphere, the burning rate per unit wick surface area and the flame temperature were considerably reduced in microgravity as compared with normal gravity. Also, the flame (spherical in microgravity) was much thicker and further removed from the wick. It thus appears that the flame becomes 'weaker' in microgravity due to the absence of buoyancy generated flow which serves to transport the oxidizer to the combustion zone and remove the hot combustion products from it. The buoyant flow, which may be characterized by the strain rate, assists the diffusion process to execute these essential functions for the survival of the flame. Thus, the diffusion flame is 'weak' at very low strain rates and as the strain rate increases the flame is initially 'strengthened' and eventually it may be 'blown out'. The computed flammability boundaries of T'ien show that such a reversal in material flammability occurs at strain rates around 5 sec. At very low or zero strain rates, flame radiation is expected to considerably affect this 'weak' diffusion flame because: (1) the concentration of combustion products which participate in gas radiation is high in the flame zone; and (2) low strain rates provide sufficient residence time for substantial amounts of soot to form which is usually responsible for a major portion of the radiative heat loss. We anticipate that flame radiation will eventually extinguish this flame. Thus, the objective of this project is to perform an experimental and theoretical investigation of radiation-induced extinction of diffusion flames under microgravity conditions. This is important for spacecraft fire safety.

Atreya, Arvind↗
NASA NTRS1993

Soot formation and radiation in turbulent jet diffusion flames under normal and reduced gravity conditions

Most practical combustion processes, as well as fires and explosions, exhibit some characteristics of turbulent diffusion flames. For hydrocarbon fuels, the presence of soot particles significantly increases the level of radiative heat transfer from flames. In some cases, flame radiation can reach up to 75 percent of the heat release by combustion. Laminar diffusion flame results show that radiation becomes stronger under reduced gravity conditions. Therefore, detailed soot formation and radiation must be included in the flame structure analysis. A study of sooting turbulent diffusion flames under reduced-gravity conditions will not only provide necessary information for such practical issues as spacecraft fire safety, but also develop better understanding of fundamentals for diffusion combustion. In this paper, a summary of the work to date and of future plans is reported.

Ku, Jerry C.↗
NASA NTRS1992

Comparison of diffusion and particle drift descriptions of radial transport in the earth's inner magnetosphere

The paper compares two approaches for describing the radial motion of charged particles in the equatorial plane of the earth's magnetosphere: the radial diffusion and the guiding-center drift. For this purpose, a time-dependent observation-based model electrostatic field is computed for the August 1990 storm event, and this field is used to (1) calculate the guiding-center drift of a ring of monoenergetic test particles and (2) compute the specific radial diffusion coefficient and solve the relevant diffusion equation. Generally, only fair agreement was found between the guiding-center and the diffusion calculation approaches. The best agreement could be obtained for the case of a series of small storms rather than for one substantial storm.

Riley, Pete↗
NASA NTRS1992

On ionized napthalene (C10H8(+)) as a carrier of diffuse interstellar bands

A recent laboratory study of ionized napthalene (C10H8(+)) has led to the suggestion that some of the unidentified diffuse interstellar bands may be formed by this molecular ion in the diffuse interstellar medium. In particular, there is a close match between a laboratory feature at 6741 A and a weak diffuse band at 6742 A, and some near-coincidences at other wavelengths. The same laboratory data show that other diffuse bands may be found, including one at 6151 A, if indeed C10H8(+) is responsible. This paper reports a search for the 6151 A feature, with negative results. The derived upper limits appear to be in conflict with the C10H8(+) hypothesis, but may not be, due to uncertainties in the expected width of the predicted interstellar feature, and due to the fact that the observations do not include data on the 6742 A band, so it is not known whether this putative C10H8(+) feature is present in the observed clouds.

Snow, Theodore P.↗
NASA NTRS1993

Diffuse gamma-ray emission from pulsars in the Large Magellanic Cloud

We investigate the contribution of pulsars to the diffuse gamma-ray emission from the LMC. The pulsar birth rate in the LMC is a factor of about 10 lower than that of the Galaxy and the distance to pulsars in the LMC is about 5-10 times larger than to Galactic pulsars. The resulting total integrated photon flux from LMC pulsars is thus reduced by a factor of about 100 to 1000. However, the surface brightness is not reduced by the same amount because of the much smaller angular extent of the LMC in comparison to the diffuse glow from the Galactic plane. We show that gamma-ray emission due to pulsars born in the LMC could produce gamma-ray fluxes that are larger than the inverse Compton component from relativistic cosmic-ray electrons and a significant fraction of the extragalactic isotropic background or the diffuse Galactic background in that direction. The diffuse pulsar glow above 100 MeV should therefore be included in models of high-energy emission from the LMC. For a gamma-ray beaming fraction of order unity the detected emissions from the LMC constrain the pulsar birth rate to less than one per 50 yr. This limit is about one order of magnitude above the supernova rate inferred from the historic record or from the star-formation rate.

Hartmann, Dieter H.↗
NASA NTRS1993

Pulmonary diffusing capacity, capillary blood volume, and cardiac output during sustained microgravity

We measured pulmonary diffusing capacity (DL), diffusing capacity per unit lung volume, pulmonary capillary blood volume (Vc), membrane diffusing capacity (Dm), pulmonary capillary blood flow or cardiac output (Qc), and cardiac stroke volume (SV) in four subjects exposed to nine days of microgravity. DL in microgravity was elevated compared with preflight standing values and was higher than preflight supine because of the elevation of both Vc and Dm. The elevation in Vc was comparable to that measured supine in 1 G, but the increase in Dm was in sharp contrast to the supine value. We postulate that, in 0 G, pulmonary capillary blood is evenly distributed throughout the lung, providing for uniform capillary filling, leading to an increase in the surface area available for diffusion. By contrast, in the supine 1-G state, the capillaries are less evenly filled, and although a similar increase in blood volume is observed, the corresponding increase in surface area does not occur. DL and its subdivisions showed no adaptive changes from the first measurement 24 h after the start of 0 G to eight days later. Similarly, there were no trends in the postflight data, suggesting that the principal mechanism of these changes was gravitational. The increase in Dm suggests that subclinical pulmonary edema did not result from exposure to 0 G. Qc was modestly increased inflight and decreased postflight compared with preflight standing. Compared with preflight standing, SV was increased 46 percent inflight and decreased 14 percent in the 1st week postflight. There were temporal changes in Qc and SV during 0 G, with the highest values recorded at the first measurement, 24 h into the flight. The lowest values of Qc and SV occurred on the day of return.

Prisk, G. K.↗
NASA NTRS1947

An Investigation of Convergent-Divergent Diffusers at Mach Number 1.85

An investigation has been conducted in the Cleveland 18- by 18-inch supersonic tunnel at a Mach number of 1.85 and angles of attack from 0 deg to 5 deg to determine optimum design configurations for a convergent-divergent type of supersonic diffuser with a subsonic diffuser of 5 deg included divergence angle. Total pressure recoveries in excess of theoretical recovery across a normal shock at a free-stream Mach number of 1.85 wore obtained with several configurations. The highest recovery for configurations without a cylindrical throat section was obtained with an inlet having an included convergence angle of 20 deg. Insertion of a 2-inch throat section between a 10 deg included angle inlet and the subsonic diffuser stabilized the shock inside the diffuser and resulted in recoveries as high as 0.838 free-stream total pressure at an angle of attack of 0 deg, corresponding to recovery of 92.4 percent of the kinetic energy of the free air stream. Use of the throat section also lessened the reduction in recovery of all configurations due to angle of attack.

Wyatt, Demarquis D↗
NASA NTRS1949

Effect of screens in wide-angle diffusers

An experimental investigation at low airspeeds was made of the filling effect observed when a screen or similar resistance is placed across a diffuser. The filling effect is found to be real in that screens can prevent separation or restore separated flow in diffusers even of extreme divergence and to depend principally on screen location and pressure-drop coefficient of the screen. Results are given for three different diffusers of circular cross section with a variety of screen arrangements. Effects of single screens and multiple screens are shown. The mechanics of the filling effect is explained, and possible efficiencies are discussed. Results of arrangements of multiple screens in wide-angle diffusers are given to show a possible application to damping screens as used in wind tunnels to reduce turbulence. (author)

Schubauer, G B↗
NASA NTRS1993

Limited subsolidus diffusion in type B1 CAI: Evidence from Ti distribution in spinel

Most models of calcium aluminum-rich inclusions (CAI) have focused on early stages of formation by equilibrium crystallization of a homogeneous liquid. Less is known about the subsolidus cooling history of CAI. Chemical and isotopic heterogeneties on a scale of tens to hundreds of micrometers (e.g. MacPherson et al. (1989) and Podosek, et al. (1991)) suggest fairly rapid cooling with a minimum of subsolidus diffusion. However, transmission electron microscopy indicates that solid state diffusion may have been an important process at a smaller scale (Barber et al. 1984). If so, chemical evidence for diffusion could provide constraints on cooling times and temperatures. With this in mind, we have begun an investigation of the Ti distribution in spinels from two type B1 CAI from Allende to determine if post-crystallization diffusion was a significant process. The type B1 CAIs, 3529Z and 5241 have been described by Podosek et al. (1991) and by El Goresy et al. (1985) and MacPherson et al. (1989). We have analyzed spinels in these inclusions using the electron microprobe. These spinels are generally euhedral, range in size from less than 10 to 15 micron and are poikilitically enclosed by millimeter-sized pyroxene, melilite, and anorthite. Analyses were obtained from both the mantles and cores of the inclusions. Compositions of pyroxene in the vicinity of individual spinel grains were obtained by analyzing at least two points on opposite sides of the spinel and averaging the compositions. The pyroxene analyses were obtained within 15 microns of the spinel-pyroxene interface. No compositional gradients were observed within single spinel crystals. Ti concentrations in spinels included within pyroxene, melilite, and anorthite are presented.

Meeker, G. P.↗
NASA NTRS1994

Active control of vaneless diffuser rotating stall

Experiments were carried out to study the feasibility of active stabilization of vaneless diffuser rotating stalls. Pressure fluctuation at the diffuser inlet was monitored and used to control the rotating stall. AC control flow, which is produced by a load speaker, was introduced into the diffuser at the inlet. It is shown that the rotating stall can be suppressed when the phase of the control flow has a certain relation with the phase of the rotating stall. By considering the energy flux due to the AC control flow, it is shown that the rotating stall is suppressed when the control flow has the phase such that the energy is subtracted out from the diffuser flow. Discussions are made on the relations between the energy flux and the amplitude of the pressure fluctuation due to the rotating stalls.

Yoshida, Yoshiki↗
NASA NTRS1994

Analysis and design of numerical schemes for gas dynamics. 2: Artificial diffusion and discrete shock structure

The effect of artificial diffusion on discrete shock structures is examined for a family of schemes which includes scalar diffusion, convective upwind and split pressure (CUSP) schemes, and upwind schemes with characteristics splitting. The analysis leads to conditions on the diffusive flux such that stationary discrete shocks can contain a single interior point. The simplest formulation which meets these conditions is a CUSP scheme in which the coefficients of the pressure differences is fully determined by the coefficient of convective diffusion. It is also shown how both the characteristic and CUSP schemes can be modified to preserve constant stagnation enthalpy in steady flow, leading to four variants, the E and H-characteristic schemes, and the E and H-CUSP schemes. Numerical results are presented which confirm the properties of these schemes.

Jameson, Antony↗
NASA NTRS1994

Experimental investigation of the Marangoni effect on the stability of a double-diffusive layer

Stability experiments were carried out in 4-cm-thick, salt-stratified fluid layer by heating from below and cooling from above. The bottom boundary was rigid while the top was either free or rigid. The initial solute Rayleigh number varied from 2.5 x 10(exp 6) to 4.6 x 10(exp 7). For the rigid-free case, at initial solute Rayleigh numbers R(sub s) greater than 5.4 x 10(exp 6), thermal Marangoni instabilities were observed to onset along the free surface at a relatively low thermal Rayleigh number, R(sub t). The convection was very weak, and it had almost no effect on the concentration and temperature distributions. Double-diffusive instabilities along the top free surface were observed to onset at a higher R(sub t), with much stronger convection causing changes in the concentration and temperature distributions near the top. At a yet higher R(sub t), double-diffusive convection was observed to onset along the bottom boundary. Fluid motion in the layer then evolved into fully developed thermal convection of a homogeneous fluid without any further increase in the imposed Delta T. For layers with R(sub s) less than 5.4 x 10(exp 6), Marangoni and double-diffusive instabilities onset simultaneously along the free surface first, while double-diffusive instabilities along the bottom wall onset at a higher R(sub t).

Tanny, Josef↗
NASA NTRS1994

Diffuse interstellar bands between 3900 A and 4200 A

Presented here are spectra and data on five probable diffuse bands with central wavelengths near 4180, 4070, 4040, 4000, and 3980 A which have full-width-half-maxima lying in the range 10-30 A. No narrow unidentified diffuse bands were found, but atomic and diatomic molecular absorptions, including CH and CH(sup +), were readily detected along some lines of sight. Although broad diffuse bands have now been found at longer wavelengths that appeared at one time to be the case (Herbig and Leka, 1991), it is clear that there are no prominent narrow bands at the shorter wavelength end of the diffuse band spectrum. This high-resolution spectra was reported for a number of reddened O- and B-typed and comparison stars, recorded with the 88 inch University of Hawaii telesecope in June of 1993. The sample included the heavily reddened star HD 183143.

Hibbins, R. E.↗
NASA NTRS1994

Role of pressure diffusion in non-homogeneous shear flows

A non-local model is presented for approximating the pressure diffusion in calculations of turbulent free shear and boundary layer flows. It is based on the solution of an elliptic relaxation equation which enables local diffusion sources to be distributed over lengths of the order of the integral scale. The pressure diffusion model was implemented in a boundary layer code within the framework of turbulence models based on both the kappa-epsilon-(bar)upsilon(exp 2) system of equations and the full Reynolds stress equations. Model computations were performed for mixing layers and boundary layer flows. In each case, the pressure diffusion model enabled the well-known free-stream edge singularity problem to be eliminated. There was little effect on near-wall properties. Computed results agreed very well with experimental and DNS data for the mean flow velocity, the turbulent kinetic energy, and the skin-friction coefficient.

Demuren, A. O.↗
NASA NTRS1994

Large-eddy simulation of flow through a plane, asymmetric diffuser

A challenge for traditional turbulence modeling, based on the Reynolds averaged Navier-Stokes equations, remains the accurate prediction of 'mild', adverse pressure-gradient driven separation from a smooth surface. With this study we want to explore the capability of large-eddy simulation to predict the separation which occurs on the deflected wall of an asymmetric, plane diffuser with opening angle of 10 deg. The flow through the plane diffuser exhibits some additional interesting physical phenomena which make it a challenging test case. In addition to 'mild' separation about halfway down the deflected ramp, the flow is characterized by a small backflow zone with stalled fluid in the rear part of the expanding section. The turbulent flow entering the diffuser is subject to combined adverse and radial pressure gradients stemming from the convex curvature. Finally the flow recovers into a developed, turbulent channel flow in the outlet section. Obi et al. provide measurements of mean flow, Reynolds stresses, and pressure recovery, which were obtained by means of LDV in a wind tunnel. The objective of this study is to investigate whether LES with the standard dynamic model is able to accurately predict the flow in the one-sided diffuser and to explore the resolution requirements and associated costs.

Kaltenbach, Hans-Jakob↗
NASA NTRS1995

Theoretical and experimental models of the diffuse radar backscatter from Mars

The general objective for this work was to develop a theoretically and experimentally consistent explanation for the diffuse component of radar backscatter from Mars. The strength, variability, and wavelength independence of Mars' diffuse backscatter are unique among our Moon and the terrestrial planets. This diffuse backscatter is generally attributed to wavelength-scale surface roughness and to rock clasts within the Martian regolith. Through the combination of theory and experiment, the authors attempted to bound the range of surface characteristics that could produce the observed diffuse backscatter. Through these bounds they gained a limited capability for data inversion. Within this umbrella, specific objectives were: (1) To better define the statistical roughness parameters of Mars' surface so that they are consistent with observed radar backscatter data, and with the physical and chemical characteristics of Mars' surface as inferred from Mariner 9, the Viking probes, and Earth-based spectroscopy; (2) To better understand the partitioning between surface and volume scattering in the Mars regolith; (3) To develop computational models of Mars' radio emission that incorporate frequency dependent, surface and volume scattering.

England, A. W.↗
NASA NTRS1994

Gamma-ray observations of Ophiuchus with EGRET: The diffuse emission and point sources

Observations of the Ophiuchus region made with the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) during the first 2 1/2 years of operation show the diffuse emission from the interstellar gas in Ophiuchus as well as variable emission from two point sources. The gamma-ray emission is modeled in terms of cosmic-ray interactions with atomic and molecular hydrogen in Ophiuchus and with low-energy photons along the line of sight. The model also includes the flux from the two point sources and an isotropic diffuse contribution. The cosmic-ray density is assumed to be uniform. The derived ratio of molecular hydrogen column density to integrated CO intensity is (1.1 +/- 0.2) x 10(exp 20) H-mols/sq cm (K km/s)(exp -1). At the sensitivity and resolution of the gamma-ray data, no variation of this ratio over the modeled region is discernible, nor are any regions of enhanced cosmic-ray density apparent. The model was fitted to seven narrow energy bands to obtain the energy depedence of the gamma-ray production function and the spectra of the point sources. The derived production function is in good agreement with theoretical calculations and the local cosmic-ray electron and proton spectra. The positions of the point sources were determined from maximum likelihood analysis of the gamma-ray emission observed in excess of the diffuse model. We identify one point source with the quasar PKS 1622-253, which has an average flux, E greater than 100 MeV, of (2.5 +/- 0.5) x 10(exp -7) photons/sq cm/s and photon spectral index -1.9 +/- 0.3. The other source, denoted GRO J1631-27, has not yet been identified at other wavelengths. Its average flux, E greater than 100 MeV, is (1.1 +/- 0.4) x 10(exp -7) photons/sq cm/s; however, its spectral index is poorly determined. The spectral index and intensity of the isotropic contribution to the model agree well with the extragalactic diffuse emission derived from the SAS 2 data.

Hunter, S. D.↗
NASA NTRS1995

First spectral observations of the diffuse background with the Extreme Ultraviolet Explorer

We present the first results from the analysis of the spectroscopic observations of diffuse extreme ultraviolet (EUV) emission taken with the Extreme Ultraviolet Explorer (EUVE) spectrometers in the wavelength range 160-740 A. Although not designed or optimized for diffuse observation, the EUVE spectrometers are the most sensitive diffuse EUV spectrometer in orbit. The spectral resolution for diffuse emission of the medium and long-wavelength spectrometers are 17 and 34 A FWHM, respectively. During the period from 1992 July 25 to 1992 August 19, the spectrometers surveyed a 2 x 20 deg field scanned from (l, b) = (24 deg, -28 deg) to (44 deg, -74 deg) with a total effective exposure time of 575,232 s. The only emission lines detected were those of He I and He II (584, 537, 304 A) with intensities consistent with local geocoronal and/or interplanetary scattering of solar radiation (584 A = 1.30 rayleighs; 537 A = 0.040 R; and 304 A = 0.029 R). Models of the soft X-ray background, which results from a 10(exp 6) K plasma (Local Bubble) surrounding the neutral gas near the Sun (Local Cloud), predict that most of the flux from the hot plasma appears as emission lines in the EUV. We have compared these spectral predictions with our observations to place limits on the emission measure versus temperature of the proposed hot plasma. Using the same plasma model, we derived emissions measures for our data and the C and B soft X-ray bands of the Wisconsin rocket survey. We find that our limits for the plasma emission measure are a factor of 5-10 below the C- and B-band emission measures over the temperature range from 10(exp 5.7) to 10(exp 6.4) K. We explore possible scenarios that could reconcile our results with the X-ray surveys and conclude that depletion or a nonequilibrium plasma state rather than absorption are the more likely explanations of the discrepancy. We also show that our spectrum is inconsistent with the spectrum from the approximately 10(exp 5) K gas at the conductive interface between the hot Local Bubble and the cooler Local Cloud given by Slavin (1989). In addition, we place new limits on the helium ionization parameter in the Sun's vicinity caused by the 10(exp 6) K plasma in the Local Bubble.

Jelinksy, P.↗