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Results for “Effective diffusivity”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

The effect of helium diffusion on the ages of globular clusters

Evolutionary tracks for halo stars were calculated, and isochrones which include the effects of microscopic diffusion of helium were constructed. The isochrones were fitted to a metal poor (M92) and a moderately metal rich (NGC 288) globular cluster using an updated version of the Revised Yale Isochrone color calibration. Ages of the two clusters were also determined using the difference between the turnoff magnitude and horizontal branch magnitude, and the difference in color between the main-sequence turnoff and lower giant branch. Considering all methods and constraints, diffusion is argued to reduce the derived ages of M92 and NGC 288 by 0.5-1 Gyr. The maximum age reduction that diffusion could cause is 3 Gyr. Age estimates including diffusion indicate that M92 is 16 +/- 2 Gyr old, and that M92 is about 3 Gyr older than NGC 288, assuming that the clusters have the same O/Fe of +0.4.

Chaboyer, Brian↗
NASA NTRS1996

Effects of Hydrodynamics of Soot Formation in Laminar Opposed-Jet Diffusion Flames

Effects of flow (hydrodynamic properties on the presence of soot in hydrocarbon-fueled laminar opposed-jet diffusion flames were studied experimentally at atmospheric pressure, emphasizing effects of velocities normal to the Rom sheet. These velocities were varied for conditions corresponding to combustion in air by transferring nitrogen from the oxidizer stream to the fuel stream which increases the stoichiometric mixture fraction of the flame and causes the stagnation plane of the flow to shift toward the fuel-rich side of the flame sheet. Fuels considered included acetylene, ethylene, ethane, propylene, propane, and 1-3 butadiene. Present measurements consisted of the critical strain rates for the flames to contain soot (the soot extinction limit) and for the flames to extinguish (the flame extinction limit) It was found that increasing the stoichiometric mixture fraction causes a progressive reduction of the critical strain rates for both flame and soot extinction; however, their ratio increases, and even becomes unbounded in most instances to yield a permanently-blue-flame regime. The results suggest that soot formation in nonpremixed flames can be controlled by varying velocities normal to the flame sheet. Nevertheless, definitive conclusions along these lines require evaluation of effects of corresponding variations of fuel and oxygen concentrations on soot formation when velocities normal to the flame sheet are changed by varying stoichiometric mixture fractions for laminar opposed-jet diffusion flames.

Lin, K.-C.↗
NASA NTRSDate not supplied

NASA HECC Geometry and Performance Review Part 3: A Numerical and Experimental Investigation of Tip Clearance Effects on the Vaneless Diffuser Configuration

Tip clearance effects in centrifugal compressors have been extensively investigated to understand the losses associated with the flow in the impeller tip clearance gap between the rotating blades and the stationary shroud. In Part 3 of this multipart investigation, experimental data and validated numerical simulations from Parts 1 and 2 were used to analyze the effects of the size of the tip clearance gap on the High Efficiency Centrifugal Compressor performance and aerodynamics. Four tip clearance gaps ranging from 0.012-in to 0.030-in (2.0% to 4.9% of the impeller exit blade height) were considered at both design and off-design operating conditions. The total pressure ratio and efficiency of the stage are found to decay linearly with increasing tip gaps. The sensitivity of the impeller performance to the tip gap was found to vary with the rotational speed of the compressor. Spanwise surveys of flow angle, total pressure, and total temperature collected at the impeller exit at design speed are used to validate numerical simulations at each experimental tip gap condition. Numerical simulations show that increased turbulence kinetic energy near the shroud at larger tip gaps leads to spanwise mixing of high entropy fluid near the impeller trailing edge which decreases the useful work input by the impeller. The data presented have been made available to the public in the HECC Data Archive located at https://storage.googleapis.com/hecc-data/NASA-HECC-Data-Archive.zip.

centrifugal compressor↗
NASA NTRSDate not supplied

NASA HECC Geometry and Performance Review Part 3: A Numerical and Experimental Investigation of Tip Clearance Effects on the Vaneless Diffuser Configuration

Tip clearance effects in centrifugal compressors have been extensively investigated to understand the losses associated with the flow in the impeller tip clearance gap between the rotating blades and the stationary shroud. In Part 3 of this multipart investigation, experimental data and validated numerical simulations from Parts 1 and 2 were used to analyze the effects of the size of the tip clearance gap on the High Efficiency Centrifugal Compressor performance and aerodynamics. Four tip clearance gaps ranging from 0.012-in to 0.030-in (2.0% to 4.9% of the impeller exit blade height) were considered at both design and off-design operating conditions. The total pressure ratio and efficiency of the stage are found to decay linearly with increasing tip gaps. The sensitivity of the impeller performance to the tip gap was found to vary with the rotational speed of the compressor. Spanwise surveys of flow angle, total pressure, and total temperature collected at the impeller exit at design speed are used to validate numerical simulations at each experimental tip gap condition. Numerical simulations show that increased turbulence kinetic energy near the shroud at larger tip gaps leads to spanwise mixing of high entropy fluid near the impeller trailing edge which decreases the useful work input by the impeller. The data presented have been made available to the public in the HECC Data Archive located at https://storage.googleapis.com/hecc-data/NASA-HECC-Data-Archive.zip.

centrifugal compressor↗
NASA NTRS1982

A numerical study of the effects of ambipolar diffusion on the collapse of magnetic gas clouds

The gravitational collapse of isothermal, nonrotating magnetic gas clouds have been calculated numerically, including the effects of ambipolar diffusion. The fractional ionization in the clouds is approximated by a power-law function of the gas density, f = K/n to the q-power, where K and q are adjustable parameters. Eleven numerical experiments were run, and the results indicate that the asymptotic character of collapse is determined mainly by the value of q and is largely independent of the other parameters characterizing a cloud (e.g., K, cloud mass). In particular, there is nearly a one-to-one correspondence between q and the slope, x, of the central magnetic field strength-gas density relationship. If q is no more than 0.8, a cloud collapses asymptotically, as though the magnetic field were 'frozen' to the neutral matter. The magnetic field strength at the center of a collapsing cloud is strongly amplified during collapse even for values of q of about 1, despite extremely low values of fractional ionization. A discussion of the theoretical basis for this unexpected behavior is given. Possible implications of our results for the problems of magnetic braking of rotating protostars and star formation in general are also presented.

Black, D. C.↗
NASA NTRS1980

The effect of velocity overshoot on the performance of magnetohydrodynamic subsonic diffusers

The effect of velocity overshoot, a velocity distribution encountered in magnetohydrodynamic channels with peak velocity near the wall instead of at the center, on the performance of two-dimensional subsonic diffusers was investigated. For the simplified conditions analyzed in this paper it was found that the ratio of peak-to-centerline velocity increases along the diffuser for a diffuser half angle greater than some critical value. It was also found that irrespective of the accompanying inlet temperature distribution the wall shear stress and the wall heat flux is substantially larger when the inlet velocity profile has an overshoot than that for a fully developed turbulent inlet velocity profile.

Greywall, M. S.↗
NASA NTRS1983

Evolutionary effects of helium diffusion in population II stars

Evolutionary calculations continuing until well past turnoff are presented for models of low-mass Population II stars which take into account the effects of the diffusion of helium relative to hydrogen. Evolutionary tracks, cluster isochrones and hydrogen distributions were obtained for stellar masses in the range 0.75 to 1.01 solar masses, both in the presence and absence of diffusion. It is found that for a star of a given mass, diffusion speeds up the evolutionary process on the main sequence, although after turnoff evolution is slowed with respect to the case without diffusion. As the stars ascend the red giant branch, their outer regions are remixed so that evidence of helium diffusion is erased, and the evolutionary tracks of the models with and without diffusion converge. Thus, if the age of a globular cluster is determined from the absolute magnitude at turnoff or from fitting isochrones, diffusion results in a 25% reduction in the derived age at a turnoff magnitude of 4.23, and a 14% reduction at a turnoff magnitude of 3.45.

Stringfellow, G. S.↗
NASA NTRS1990

Effects of Pressure on Microgravity Hydrocarbon Diffusion Flames

The effect of pressure on the propane diffusion flames burning in quiescent air under both normal-gravity and microgravity conditions (in the NASA's 2.2-Second Drop Tower at the Lewis Research Center) were studied at pressures of 0.5, 1.0, and 1.5 atm, with three fuel mass-flow rates for each pressure. Compared to normal-gravity flames, the microgravity tests showed enhanced sooting, tip-opening, and soot escape, accompanied with changes in color, size, and luminosity of the flames. At low pressures, less-efficient burning and a broader flame zone was found to exist due to the kinetics effects, while at high pressures, where was more soot and longer soot-burning zones. Results show that there exist a particular intermediate pressure at which both the microgravity and normal-gravity flame heights are minimized.

Bahadori, M. Yousef↗
NASA NTRS1978

Effect of concentration dependence of the diffusion coefficient on homogenization kinetics in multiphase binary alloy systems

Diffusion calculations were performed to establish the conditions under which concentration dependence of the diffusion coefficient was important in single, two, and three phase binary alloy systems. Finite-difference solutions were obtained for each type of system using diffusion coefficient variations typical of those observed in real alloy systems. Solutions were also obtained using average diffusion coefficients determined by taking a logarithmic average of each diffusion coefficient variation considered. The constant diffusion coefficient solutions were used as reference in assessing diffusion coefficient variation effects. Calculations were performed for planar, cylindrical, and spherical geometries in order to compare the effect of diffusion coefficient variations with the effect of interface geometries. In most of the cases considered, the diffusion coefficient of the major-alloy phase was the key parameter that controlled the kinetics of interdiffusion.

Tenney, D. R.↗
NASA NTRS1980

The coupled effects of chemistry and diffusion on the stratospheric ozone reduction

The effect of diffusive-mixing on O3-depletion is estimated using the five-step chemistry model. The coupled treatment uses an equilibrium turbulence field and retains the key features of the kinetics while keeping the chemistry simple. Results indicate that those calculations of Hilst and Donaldson (1973) employing the conservation of NO2/NO will yield erroneous results, and it is found that any significant level of turbulence will reduce the severity of the NOx catalytic cycle for O3 destruction.

Gupta, R. N.↗
NASA NTRS1960

Heat Diffusion in Gases, Including Effects of Chemical Reaction

The diffusion of heat through gases is treated where the coefficients of thermal conductivity and diffusivity are functions of temperature. The diffusivity is taken proportional to the integral of thermal conductivity, where the gas is ideal, and is considered constant over the temperature interval in which a chemical reaction occurs. The heat diffusion equation is then solved numerically for a semi-infinite gas medium with constant initial and boundary conditions. These solutions are in a dimensionless form applicable to gases in general, and they are used, along with measured shock velocity and heat flux through a shock reflecting surface, to evaluate the integral of thermal conductivity for air up to 5000 degrees Kelvin. This integral has the properties of a heat flux potential and replaces temperature as the dependent variable for problems of heat diffusion in media with variable coefficients. Examples are given in which the heat flux at the stagnation region of blunt hypersonic bodies is expressed in terms of this potential.

Hansen, C. Frederick↗
NASA NTRS1977

Effect of microstructure on Na/+/ diffusion in beta alumina

The effect of microstructural features, such as grain size and orientation, on sodium ion diffusion in polycrystalline beta alumina is investigated; the ionic solid is suitable for use in high-energy and power density batteries. Two types of hot-pressed polycrystalline samples are studied, one with grains highly oriented in the direction normal to the pressing direction, the other with significantly less preferred orientation normal to the pressing direction. Results of the experimental assessments indicate that grain size and diffusion path tortuosity have little effect on diffusion kinetics, while grain boundary type and grain aspect ratio are important factors in determining the diffusion rate.

Sarian, S.↗
NASA NTRS1950

Investigation of High-Subsonic Performance Characteristics of a 12 Degree 21-Inch Conical Diffuser, Including the Effects of Change in Inlet-Boundary-Layer Thickness

Investigations were conducted of a 12 degree 21-inch conical diffuser of 2:l area ratio to determine the interrelation of boundary layer growth and performance characteristics. surveys were made of inlet and exit from, longitudinal static pressures were recorded, and velocity profiles were obtained through an inlet Reynolds number range, determined From mass flows and based on inlet diameter of 1.45 x 10(exp 6) to 7.45 x 10(exp 6) and a Mach number range of 0.11 to approximately choking. These investigations were made to two thicknesses of inlet boundary layer. The mean value, over the entire range of inlet velocities, of the displacement thickness of the thinner inlet boundary layer was approximately 0.035 inch and that of the thicker inlet boundary layer was approximately six times this value. The loss coefficient in the case of the thinner inlet boundary layer had a value between 2 to 3 percent of the inlet impact pressure over most of the air-flow range. The loss coefficient with the thicker inlet boundary layer was of the order of twice that of the thinner inlet boundary layer at low speeds and approximately three times at high speeds. In both cases the values were substantially less than those given in the literature for fully developed pipe flow. The static-pressure rise for the thinner inlet boundary layer was of the order of 95 percent of that theoretically possible over the entire speed range. For the thicker inlet boundary layer the static pressure rise, as a percentage of that theoretically possible, ranged from 82 percent at low speeds to 68 percent at high speeds.

Copp, Martin R.↗
NASA NTRS1972

Effect of eddy diffusivity on wind-driven currents in a two-layer stratified lake

The steady state wind-driven circulation was numerically calculated in a rectangular stratified lake. The lake is composed of two layers having uniform but unequal densities and eddy diffusivities. The position in thermocline and the three-dimensional velocities in both layers calculated using shallow lake equations. The results show that, as the eddy diffusivity in the hypolimnion is increased, the thermocline tilt and hypolimnetic velocities increase. The effect of the other variables such as wind stress, density, basin length, and mean thermocline depth are also shown.

Gedney, R. T.↗
NASA NTRS1977

Theoretical effects of surface diffused region lifetime models on silicon solar cells

A computer simulation of silicon solar cells has indicated that the combination of band gap reduction due to heavy doping and certain spatial forms of lifetime dependence can combine to form severe limitations on the open-circuit voltage of such cells. The interaction of these effects tends to shift the active region of the diffused surface layer away from the injecting junction, resulting in an increase in the current density injected into the surface region. Reductions in open circuit voltage as great as 10% over models which do not include these effects can be seen.

Dunbar, P. M.↗