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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Time development of a perturbed-spherical nucleus in a pure supercooled liquid. I - Power-law growth of morphological instabilities. II - Nonlinear development

The linear growth stage of the morphological instabilities developing at the liquid-solid interface during crystal growth from an axisymmetric spherical nucleus is analyzed. The corresponding growth rate parameters are calculated numerically, and it is shown that morphological instabilities for free growth evolve according to a power law in agreement with WKB results and contrary to an exponential law found in a quasi-stationary approximation. The time evolution of an arbitrary perturbed liquid-solid interface from a linear into the nonlinear stage is studied. The initial perturbations include the eigenfunctions for the linear problem, localized perturbations of the grain-boundary type, and a stochastic noise. It is shown that the perturbations grow and spread in a wavelike manner. The formation of a predendritic growth stage is characterized by establishment of constant values of tip radius and velocity.

Pines, V.↗
NASA NTRS1991

An observational and theoretical study of highly supercooled altocumulus

The microphysics of two altocumulus clouds sampled at a temperature of about -30 C through the NCAR King Air aircraft and coincident lidar has been characterized. The clouds are structurally similar to stratocumulus and have extensive cloudtop entrainment, a capping temperature inversion, and a dry layer above. Microphysical and radiative properties of both clouds have been modeled numerically. When entrainment effects are incorporated in the model, calculations of droplet concentration and mean diameter profiles compare favorably with the measurements. Radiative transfer calculations indicate that radiative cooling causes sufficient negative buoyancy in cloudtop parcels for producing convective instability and reproducing the observed downdraft velocities. It is shown that entrainment of warmer, drier air near a cloudtop counteracts the radiatively induced negative buoyancy in the downdrafts.

Heymsfield, Andrew J.↗
NASA NTRS1993

Homogeneous ice nucleation and supercooled liquid water in orographic wave clouds

This study investigates ice nucleation mechanisms in cold lenticular wave clouds, a cloud type characterized by quasi-steady-state air motions and microphysical properties. It is concluded that homogeneous ice nucleation is responsible for the ice production in these clouds at temperatures below about -33 C. The lack of ice nucleation observed above -33 C indicates a dearth of ice-forming nuclei, and hence heterogeneous ice nucleation, in these clouds. Aircraft measurements in the temperature range -31 to -41 C show the following complement of simultaneous and abrupt changes in cloud properties that indicate a transition from the liquid phase to ice: disappearance of liquid water; decrease in relative humidity from near water saturation to ice saturation; increase in mean particle size; change in particle concentration; and change in temperature due to the release of latent heat. A numerical model of cloud particle growth and homogeneous ice nucleation is used to aid in interpretation of our in situ measurements. The abrupt changes in observed cloud properties compare favorably, both qualitatively and quantitatively, with results from the homogeneous ice nucleation model. It is shown that the homogeneous ice nucleation rates from the measurements are consistent with the temperature-dependent rates employed by the model (within a factor of 100, corresponding to about 1 C in temperature) in the temperature range -35 deg to -38 C. Given the theoretical basis of the modeled rates, it may be reasonable to apply them throughout the -30 to -50 C temperature range considered by the theory.

Heymsfield, Andrew J.↗
NASA NTRS1986

Supercooling Refrigerator

A Goddard/Philips research project resulted in a refrigeration system which works without seals, lubricants or bearings. The system, originally developed to cool satellite-based scientific instruments, has an extensive range of potential spinoffs. It is called the Stirling Cycle Cryogenic Cooler and eliminates friction by using electronically controlled linear magnetic bearings. Mechanical failure, contamination are eliminated.

Source record↗
NASA NTRSDate not supplied

Supercool(ed) Olivine Morphology in Shergottites Revealed Using X-ray Computed Tomography

Shergottites comprise most of the Martian meteorites and have a mafic to ultramafic bulk composition and a diversity of igneous textures [1].Olivine-phyric and poikilitic shergottites are sub-classes of shergottites and characterized by having a crystal cargo of large mafic minerals. Olivine-phyric shergottites have porphyritic textures of large olivinemegacrysts set in a fine-grained groundmass of pyroxene and maskelynite [2]. The poikilitic shergottites have assemblages of olivine chadocrysts poikilitically-enclosed by cm-sized pyroxene oikocrysts set in a coarse-grained groundmass of olivine and maskelynite [3,4]. The large crystal cargos in these samples are some of the first minerals to crystallize from their respective parent melts and have been extensively studied to understand shergottite petrogenesis. For instance, compositions of early-formed olivine and pyroxene in equilibrium with other phases (e.g. spinels) are used to establish depths, temperatures, and redox conditions of crystallization. There is increasing agreement that olivine megacrysts and poikilitic assemblages formed at or near the crust – mantle boundary (~85 km) before entrainment, ascent, and complete crystallization at or near the surface [1,3,4,6,7,8,9]. Additionally, linear olivine megacryst crystal size distribution (CSD)patterns and observations of polyhedral morphologies[8,9,10] are commonly interpreted to reflect steady-state crystallization under equilibrium conditions. However, oscillatory phosphorus zonation patterns in olivine from many samples indicate fluctuations in growth rates[10,12]. Recent studies measuring dendritic phosphorus zonation in terrestrial olivines [13] and experimental studies measuring how undercooling (undercooling is defined as the liquidus temperature minus the temperature being considered) controls changes in olivine morphology [14] have questioned the canonical view that large olivine megacrysts grow slowly and concentrically under equilibrium conditions. Here we present X-ray CT results from olivine-phyric and poikilitic shergottites that challenge the notion that early-formed minerals grew slowly under equilibrium conditions.

S A Eckley↗