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Results for “isotherm model parameters”

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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Fracture mechanics criteria for turbine engine hot section components

The application of several fracture mechanics data correlation parameters to predicting the crack propagation life of turbine engine hot section components was evaluated. An engine survey was conducted to determine the locations where conventional fracture mechanics approaches may not be adequate to characterize cracking behavior. Both linear and nonlinear fracture mechanics analyses of a cracked annular combustor liner configuration were performed. Isothermal and variable temperature crack propagation tests were performed on Hastelloy X combustor liner material. The crack growth data was reduced using the stress intensity factor, the strain intensity factor, the J integral, crack opening displacement, and Tomkins' model. The parameter which showed the most effectiveness in correlation high temperature and variable temperature Hastelloy X crack growth data was crack opening displacement.

Meyers, G. J.↗
NASA NTRS1978

Does the emission measure decrease during the start of a soft X-ray flare

It is suggested that the apparent emission dip found in soft X-ray flare observations, interpreted as the emission from a single temperature plasma, is an artifact. According to this hypothesis, the flare phenomenon represents a combination of flare plus a stable active region plasma. A small amount of hot flare plasma will dominate the total emission, and the calculated isothermal plasma parameters will trend towards those of the hot flare plasma, so that a decrease in emission measure will be inferred. It is claimed that observed data fit the model of a stable plasma plus a flare plasma better than it does the model of a single plasma, in accordance with a chi square statistic. The model of stable plasma plus flare plasma is also supported by the observation that the plasma parameters in the postflare condition are indistinguishable from those during preflare.

Wolfson, C. J.↗
NASA NTRS1983

Turbulent Combustor Flowfield Investigation

The 2-D axisymmetric geometries under low speed, nonreacting, turbulent, swirling flow conditions were investigated. The effect of the parameters on isothermal flowfield patterns, time mean velocities and turbulence quantities is determined and an improved simulation in the form of a computer prediction code equipped with a suitable turbulence model is established. This is a prerequisite to the prediction of more complex turbulent reacting flows.

Lilley, D. G.↗
NASA NTRS1986

Pervasive small-scale structure in molecular clouds

An unbiased CO survey of molecular cloud cores was completed, and the profiles were analyzed within the context of a model for emission from clumpy clouds. It was found that all sources observed contain a significant amount of structure that is not resolved with our 2.3-arcmin beam, and that the parameters which describe the degree of clumping span a remarkably narrow range of the possible values. We studied two separate samples of cloud cores: a large sample of warm cores from the Massachusetts-Stony Brook 12CO survey of the first galactic quadrant, and a sample of cool cores in the Taurus dark clouds chosen primarily on the basis of H2CO emission. We observed all sources in the 1-0 transition of 12CO and 13CO with the 5-m telescope of the Millimeter Wave Observatory. The 12CO/13CO ratios can be explained if there is unresolved structure giving rise to significant variations of opacity across the beam. Our model cloud consists of a large number of identical clumps distributed randomly in the beam. These clumps have velocity widths v small compared to the width of the observed profile, which is determined by the relative motion of the clumps. The entire cloud is isothermal and in local thermodynamic equilibrium. With these assumptions the intensity and linewidth ratios depend on three parameters: the abundance ratio; the peak 13CO opacity through a single clump, tau(0); and the average number of clumps on a line of sight N. Small tau(0) and large N correspond to the microturbulent limit, which is indistinguishable from a uniform gas distribution. In the other extreme, large tau(0) and snall N, at a given velocity at most one clump contributes to the profile on each line of sight. A figure is presented which shows the model parameters which reproduce the measured intensity and linewidth ratios for the sample of warm cores, assuming an abundance ratio of 75.

Martin, B.↗
NASA NTRS2021

X-ray Emission from Magnetized Neutron Star Atmospheres at Low Mass Accretion Rates

Recent observations of X-ray pulsars at low luminosities allow, for the first time, to compare theoretical models for the emission from highly magnetized neutron star atmospheres at low mass accretion rates ( M ≲ 10(exp 15) g s(exp -1)) with the broadband X-ray data. The purpose of this paper is to investigate the spectral formation in the neutron star atmosphere at low M and to conduct a parameter study of physical properties of the emitting region. We obtain the structure of the static atmosphere, assuming that Coulomb collisions are the dominant deceleration process. The upper part of the atmosphere is strongly heated by the braking plasma, reaching temperatures of 30–40 keV, while its denser isothermal interior is much cooler (~2 keV). We numerically solve the polarized radiative transfer in the atmosphere with magnetic Compton scattering, free-free processes, and non-thermal cyclotron emission due to possible collisional excitations of electrons. The strongly polarized emitted spectrum has a double-hump shape that is observed in low-luminosity X-ray pulsars. A low-energy “thermal” component is dominated by extraordinary photons that can leave the atmosphere from deeper layers due to their long mean free path at soft energies. We find that a high-energy component is formed due to resonant Comptonization in the heated non-isothermal part of the atmosphere even in the absence of collisional excitations. The latter, however, affect the ratio of the two components. A strong cyclotron line originates from the optically thin, uppermost zone. A fit of the model to NuSTAR and Swift/XRT observations of GX304􀀀1 provides an accurate description of the data with reasonable parameters. The model can thus reproduce the characteristic double-hump spectrum observed in low-luminosity X-ray pulsars and provides insights into spectral formation.

X-Ray binaries↗
NASA NTRS1984

Investigations of flowfields found in typical combustor geometries

Studies are concerned with experimental and theoretical research on 2-D axisymmetric geometries under low speed, nonreacting, turbulent, swirling flow conditions. The flow enters the test section and proceeds into a larger chamber (the linear expansion ratio D/d = 2, 1.5 and 1) via a sudden or gradual expansion (side wall angle alpha = 90 and 45 degrees). A weak or strong nozzle (of area ratio A/a = 2 and 4) may be positioned downstream at x/D = 2 to form a contraction exit to the test section. Inlet swirl vanes are adjustable to a variety of vane angles with values of theta = 0, 38, 45, 60 and 70 degrees being emphasized. The objective is to determine the effect of these parameters on isothermal flow field patterns, time mean velocities and turbulence quantities, and to establish an improved simulation in the form of a computer prediction code equipped with a suitable turbulence model. The goal of the on going research is to perform experiments and complementary computations with the idea of doing the necessary type of research that will yield improved calculation capability. This involves performing experiments where time mean turbulence quantities are measured and taking input conditions and running an existing prediction code for a variety of test cases so as to compare predictions against experiment.

Lilley, D. G.↗
NASA NTRS2007

Transient Numerical Modeling of Catalytic Channels

This paper presents a transient model of catalytic combustion suitable for isolated channels and monolith reactors. The model is a lumped two-phase (gas and solid) model where the gas phase is quasi-steady relative to the transient solid. Axial diffusion is neglected in the gas phase; lateral diffusion, however, is accounted for using transfer coefficients. The solid phase includes axial heat conduction and external heat loss due to convection and radiation. The combustion process utilizes detailed gas and surface reaction models. The gas-phase model becomes a system of stiff ordinary differential equations while the solid phase reduces, after discretization, into a system of stiff ordinary differential-algebraic equations. The time evolution of the system came from alternating integrations of the quasi-steady gas and transient solid. This work outlines the numerical model and presents some sensitivity studies on important parameters including internal transfer coefficients, catalytic surface site density, and external heat-loss (if applicable). The model is compared to two experiments using CO fuel: (1) steady-state conversion through an isothermal platinum (Pt) tube and (2) transient propagation of a catalytic reaction inside a small Pt tube. The model requires internal mass-transfer resistance to match the experiments at lower residence times. Under mass-transport limited conditions, the model reasonably predicted exit conversion using global mass-transfer coefficients. Near light-off, the model results did not match the experiment precisely even after adjustment of mass-transfer coefficients. Agreement improved for the first case after adjusting the surface kinetics such that the net rate of CO adsorption increased compared to O2. The CO / O2 surface mechanism came from a sub-set of reactions in a popular CH4 / O2 mechanism. For the second case, predictions improved for lean conditions with increased external heat loss or adjustment of the kinetics as in the first case. Finally, the results show that different initial surface-species distribution leads to different steady-states under certain conditions. These results demonstrate the utility of a lumped two-phase model of a transient catalytic combustor with detailed chemistry.

Struk, Peter M.↗
NASA NTRS1982

Analysis of the effect of heated jet flow on the far field radiation from a noise source

The influence on jet noise radiation of the interaction with the heated moving flow of a parallel twin jet is investigated. An analytical model of jet shielding is developed which consists of the sound field emitted from a stationary, discrete frequency point source impinging on a cylinder of locally parallel flow. The zones in which the various shielding mechanisms dominate are analyzed using the model. The effects of parameters such as jet temperature and flow speed are examined. It is found that the results obtained using the model are comparable to the experimental results for a point noise source impinging on a subsonic isothermal air jet.

Gerhold, C.↗
NASA NTRS1972

Isothermal elastohydrodynamic theory for the full range of pressure-viscosity coefficient.

The isothermal, elastohydrodynamic (EHD) solutions in the inlet region of line contacts are extended to cover the full range of pressure-viscosity parameter, G, and the region of extremely heavy loads. The effect of a composite exponential model for the pressure-viscosity dependence on the film thickness is also studied. Results of the film thickness are compared with those based on work by Grubin, Dowson-Higginson, Bell and Kannel, and Herrebrugh. Comparison is also made between the theoretical results with the recently obtained X-ray film thickness measurements.

Cheng, H. S.↗
NASA NTRS1986

Crystal growth and furnace analysis

A thermal analysis of Hg/Cd/Te solidification in a Bridgman cell is made using Continuum's VAST code. The energy equation is solved in an axisymmetric, quasi-steady domain for both the molten and solid alloy regions. Alloy composition is calculated by a simplified one-dimensional model to estimate its effect on melt thermal conductivity and, consequently, on the temperature field within the cell. Solidification is assumed to occur at a fixed temperature of 979 K. Simplified boundary conditions are included to model both the radiant and conductive heat exchange between the furnace walls and the alloy. Calculations are performed to show how the steady-state isotherms are affected by: the hot and cold furnace temperatures, boundary condition parameters, and the growth rate which affects the calculated alloy's composition. The Advanced Automatic Directional Solidification Furnace (AADSF), developed by NASA, is also thermally analyzed using the CINDA code. The objective is to determine the performance and the overall power requirements for different furnace designs.

Dakhoul, Youssef M.↗
NASA NTRS1984

Simulation of solidification in a Bridgman cell

The Hg/Cd/Te solidification in a Bridgman cell was thermally analyzed by Continuum's VAST code. The energy equation is solved in an axisymmetric, quasi steady domain for both the molten and solid alloy regions. Alloys composition is calculated by a simplified one dimensional model to estimate its effect on melt thermal conductivity and, consequently, on the temperature field within the cell. Solidification is assumed to occur at a fixed temperature of 979 K. Simplified boundary conditions are included to model both the radiant and conductive heat exchange between the furnace walls and the alloy. Calculations are performed to show how the steady state isotherms are affected by: (1) the hot and cold furnace temperatures; (2) boundary condition parameters; and (3) the growth rate which affects the calculated alloy's composition.

Dakhoul, Y. M.↗
NASA NTRS1995

Sensitivity analysis of the non-linear dynamic viscoplastic response of 2-d structures with respect to material parameters

A computational procedure is presented for evaluating the sensitivity coefficients of the viscoplastic response of structures subjected to dynamic loading. A state of plane stress is assumed to exist in the structure, a velocity strain-Cauchy stress formulation is used, and the geometric non-linearities arising from large strains are incorporated. The Jaumann rate is used as a frame indifferent stress rate. The material model is chosen to be isothermal viscoplasticity, and an associated flow rule is used with a von Mises effective stress. The equations of motion emanating from a finite element semi-discretization are integrated using an explicit central difference scheme with an implicit stress update. The sensitivity coefficients are evaluated using a direct differentiation approach. Since the domain of integration is the current configuration, the sensitivity coefficients of the spatial derivatives of the shape functions must be included. Numerical results are presented for a thin plate with a central cutout subjected to an in-plane compressive loading. The sensitivity coefficients are generated by evaluating the derivatives of the response quantities with respect to Young's modulus, and two of the material parameters characterizing the viscoplastic response. Time histories of the response and sensitivity coefficients, and spatial distributions at selected times are presented.

Kulkarni, Makarand↗
NASA NTRS1992

The crystallization and crystalline properties of LARC-TPI

LARC-TPI, a thermoplastic polyimide, has been studied in order to develop an understanding of its crystalline phase transition. Our experiments suggest that samples synthesized in different laboratories apparently had different degrees of imidization and their thermal behaviors differed accordingly. When the most crystalline of these polyimides was studied in some detail, we found that it melted irreversibly in that once a sample was completely melted it would not recrystallize. A polymer that did not recrystallize displayed a glass transition, which increased in temperature upon subsequent cooling and reheating. Solubility experiments indicated that heating above the crystalline melting temperature led to network formation in the polymer, a conclusion that is consistent with other behavior just mentioned. Differential calorimetric studies revealed that annealing at slow heating rates or under isothermal conditions resulted in dual melting transitions. These studies, supported by X-ray diffraction results, strongly indicate that the annealing process involves a solid-liquid-solid transformation. From an existing phenomenological model for the kinetics of phase transitions, kinetic parameters for these crystallizations have been evaluated. The Avrami exponents n increased with the annealing temperature in the protocol used in this study. Their values were about 2 or lower, thus indicating that crystallization may have followed a mechanism that included heterogeneous nucleation of a low dimensional order in which all the embryonic crystallites formed at the beginning of the process. A positive temperature coefficient for these crystallizations indicated that diffusion may have had a rate controlling influence and affected the values of n.

Theil, Michael H.↗
NASA NTRS1984

Dilution Jets in Accelerated Cross Flows

Results of flow visualization experiments and measurements of the temperature field produced by a single jet and a row of dilution jets issued into a reverse flow combustor are presented. The flow in such combustors is typified by transverse and longitudinal acceleration during the passage through its bending section. The flow visualization experiments were designed to examine the separate effects of longitudinal and transverse acceleration on the jet trajectory and spreading rate. A model describing a dense single jet in a lighter accelerating cross flow is developed. The model is based on integral conservation equations, including the pressure terms appropriate to accelerating flows. It uses a modified entrainment correlation obtained from previous experiments of a jet in a cross stream. The flow visualization results are compared with the model calculations in terms of trajectories and spreading rates. Each experiment is typified by a set of three parameters: momentum ratio, density ratio, and the densimetric Froude number. When injection velocities are large or densities are small, the Froude number becomes very large and hence, unimportant. Therefore, the Froude number is generally significant in describing liquid experiments but is unimportant for the gas experiments in the combustor. Agreement between test and calculated results is encouraging but tends to become poorer with increasing momentum ratio. The temperature measurements are presented primarily in the form of consecutive normalized temperature profiles. Some interpolated isothermal contours are also shown. The single jet trajectories are consistently found to be swept towards the inner wall of the bend, whether injection is from the outer or the inner wall. This behavior is explained by a drifting effect which consists of a transverse velocity component across the combustor due to the developing nature of the flow along it. Plots of lateral temperature distributions of the jet indicate that under longitudinal acceleration conditions the thermal spreading of the jet may be completely suppressed. Comparison between combustor experimental results and model calculations shows poor agreement due to the drifting effect which is not taken into consideration in the model calculations. The row of jets experiments are characterized by two additional parameters: spacing ratio and confinement parameter. The results are shown in the form of consecutive normalized temperature profiles. The confinement parameter appears to become increasingly important with decreasing spacing ratio, in particular when its effect is enhanced by the drifting phenomenon and associated pressure field. A tightly spaced row of jets injected from the inner wall, prior to the bend, is surprisingly kept attached to the inner wall in spite of the strong turning. A similar attachment for a jet injected from the outer wall is not observed.

Lipshitz, Abraham↗
NASA NTRS1975

The effect of intermediate-scale motions on line formation

The problem of LTE and non-LTE line formation in the presence of nonthermal velocity fields with geometric scales between the microscopic and macroscopic limits is investigated in the cases of periodic sinusoidal and sawtooth waves. For a fixed source function (the LTE case), it is shown that time-averaged line profiles progress smoothly from the microscopic to the macroscopic limits as the geometric scale of the motions increases, that the sinusoidal motions produce symmetric time-averaged profiles, and that the sawtooth motions cause a redshift. In several idealized non-LTE cases, it is found that intermediate-scale velocity fields can significantly increase the surface source functions and line-core intensities. Calculations are made for a two-level atom in an isothermal atmosphere for a range of velocity scales and non-LTE coupling parameters and also for a two-level atom and a four-level representation of Na I line formation in the Harvard-Smithsonian Reference Atmosphere (1971) solar model. It is found that intermediate-scale velocity fields in the solar atmosphere could explain the central intensities of the Na I D lines and other strong absorption lines without invoking previously suggested high electron densities.

Shine, R. A.↗
NASA NTRS1976

Nonhomologous contraction and equilibria of self-gravitating, magnetic interstellar clouds embedded in an intercloud medium: Star formation. II - Results

The paper studies the equilibrium of self-gravitating isothermal models of interstellar clouds with a frozen-in magnetic field linked smoothly to the field of a hot tenuous intercloud medium. Equilibrium states are determined which can be reached by clouds contracting nonhomologously from a spherical uniform initial state. Three free parameters characterize the problem: a dimensionless initial radius related to the Jeans length of the cloud, the initial ratio of the magnetic and gas pressures in the cloud, and the initial ratio of the intercloud and cloud pressures. The dependence of the solutions on each of these parameters is investigated. It is found that: (1) the frozen-in field causes the cloud to become oblate with its major axis normal to the field lines; (2) the flattening increases as the magnetic-field strength, gravitational forces, or intercloud pressure increases; (3) increasing intercloud pressure eventually leads to gravitational collapse; and (4) the cloud can reach equilibrium only if its radius does not exceed some critical value. The observed inefficiency of the star-formation process within massive clouds is examined and explained in terms of magnetic phenomena in a collapsing cloud.

Mouschovias, T. C.↗
NASA NTRS1982

Evidence for the three-stage cooling history of olivine-porphyritic fluid droplet chondrules

A three-stage model for the cooling of olivine-porphyritic fluid droplet chondrules found in ordinary chondrites is proposed, in light of an experimental examination of three haplochondritic melts to determine thermal parameters in a pre-accretionary chondrule environment. The model encompasses: (1) initial continuous cooling from at least liquidus temperatures, at 300-4000 C/hour, to about 1300 C, over approximately 5-60 min; (2) interruption of the initial cooling by a short-duration isothermal event at about 1300 C; and (3) quenching. This thermal model, in which the entire history of fluid droplet chondrules is only a brief process in the early history of the solar system, suggests a specific and restricted physical origin for chondrules which is not likely to have been achieved by such repetitive, small-scale events as planetary surface impact splattering or volcanism. A single, large-scale event is postulated.

Planner, H. N.↗
NASA NTRS1982

Modeling of polymer photodegradation for solar cell modules

It was shown that many of the experimental observations in the photooxidation of hydrocarbon polymers can be accounted for with a computer simulation using an elementary mechanistic model with corresponding rate constants for each reaction. For outdoor applications, however, such as in photovoltaics, the variation of temperature must have important effects on the useful lifetimes of such materials. The data bank necessary to replace the isothermal rate constant values with Arrhenius activation parameters: A (the pre-exponential factor) and E (the activation energy) was searched. The best collection of data assembled to data is summarized. Note, however, that the problem is now considerably enlarged since from a theoretical point of view, with 51 of the input variables replaced with 102 parameters. The sensitivity of the overall scheme is such that even after many computer simulations, a successful photooxidation simulation with the expanded variable set was not completed. Many of the species in the complex process undergo a number of competitive pathways, the relative importance of each being often sensitive to small changes in the calculated rate constant values.

Somersall, A. C.↗