124 research outputs found
Advanced Stirling receiver development program, phase 1
Critical technology experiments were designed and developed to evaluate the Stirling cavity heat pipe receiver for a space solar power system. Theoretical criteria were applied to the design of a module for containing energy storage phase change material while avoiding thermal ratcheting. Zero-g drop tower tests, without phase change, were conducted to affirm that the bubble location required to avoid ratcheting could be achieved without the use of container materials that are wetted by the phase change material. A full scale module was fabricated, but not tested. A fabrication method was successfully developed for the sodium evaporator dome, with a sintered screen wick, to be used as the focal point for the receiver. Crushing of the screen during hydroforming was substantially reduced over the results of other researchers by using wax impregnation. Superheating of the sodium in the wick under average flux conditions is expected to be under 10K. A 2000K furnace which will simulate solar flux conditions for testing the evaporator dome was successfully built and tested
The feasibility of using MgF2 for phase change heat storage in a spacecraft solar thermal power system
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1988.Includes bibliographical references.by Charles Alan Lurio.Ph.D
Dynamics in shear flow studied by X-ray Photon Correlation Spectroscopy
X-ray photon correlation spectroscopy was used to measure the diffusive
dynamics of colloidal particles in a shear flow. The results presented here
show how the intensity autocorrelation functions measure both the diffusive
dynamics of the particles and their flow-induced, convective motion. However,
in the limit of low flow/shear rates, it is possible to obtain the diffusive
component of the dynamics, which makes the method suitable for the study of the
dynamical properties of a large class of complex soft-matter and biological
fluids. An important benefit of this experimental strategy over more
traditional X-ray methods is the minimization of X-ray induced beam damage.
While the method can be applied also for photon correlation spectroscopy in the
visible domain, our analysis shows that the experimental conditions under which
it is possible to measure the diffusive dynamics are easier to achieve at
higher q values (with X-rays).Comment: 9 pages, 7 figures, to appear in Eur. Phys. J.
Torsional oscillator and synchrotron x-ray experiments on solid 4He in aerogel
X-ray diffraction experiments show that solid 4He grown in aerogel is highly
polycrystalline, with a hcp crystal structure (as in bulk) and a crystallite
size of approximately 100 nm. In contrast to the expectation that the highly
disordered solid will have a large supersolid fraction, torsional oscillator
measurements show a behavior that is strikingly similar to high purity crystals
grown from the superfluid phase. The low temperature supersolid fraction is
only ~3x10-4 and the onset temperature is ~ 100 mK
Curvature effects on the surface thickness and tension at the free interface of He systems
The thickness and the surface energy at the free interface of
superfluid He are studied. Results of calculations carried out by using
density functionals for cylindrical and spherical systems are presented in a
unified way, including a comparison with the behavior of planar slabs. It is
found that for large species is independent of the geometry. The obtained
values of are compared with prior theoretical results and experimental
data. Experimental data favor results evaluated by adopting finite range
approaches. The behavior of and exhibit overshoots
similar to that found previously for the central density, the trend of these
observables towards their asymptotic values is examined.Comment: 35 pages, TeX, 5 figures, definitive versio
Surface layering of liquids: The role of surface tension
Recent measurements show that the free surfaces of liquid metals and alloys
are always layered, regardless of composition and surface tension; a result
supported by three decades of simulations and theory. Recent theoretical work
claims, however, that at low enough temperatures the free surfaces of all
liquids should become layered, unless preempted by bulk freezing. Using x-ray
reflectivity and diffuse scattering measurements we show that there is no
observable surface-induced layering in water at T=298 K, thus highlighting a
fundamental difference between dielectric and metallic liquids. The
implications of this result for the question in the title are discussed.Comment: 5 pages, 4 figures, to appear in Phys. Rev. B. 69 (2004
Density functional calculations for 4He droplets
A novel density functional, which accounts correctly for the equation of
state, the static response function and the phonon-roton dispersion in bulk
liquid helium, is used to predict static and dynamic properties of helium
droplets. The static density profile is found to exhibit significant
oscillations, which are accompanied by deviations of the evaporation energy
from a liquid drop behaviour in the case of small droplets. The connection
between such oscillations and the structure of the static response function in
the liquid is explicitly discussed. The energy and the wave function of excited
states are then calculated in the framework of time dependent density
functional theory. The new functional, which contains backflow-like effects, is
expected to yield quantitatively correct predictions for the excitation
spectrum also in the roton wave-length range.Comment: 15 pages, REVTEX, 10 figures available upon request or at
http://anubis.science.unitn.it/~dalfovo/papers/papers.htm
Density of Superfluid Helium Droplets
The classical integral cross sections of large superfluid 4He_N droplets and
the number of atoms in the droplets (N=10^3-10^4) have been measured in
molecular beam scattering experiments. These measurements are found to be in
good agreement with the cross sections predicted from density functional
calculations of the radial density distributions with a 10-90 % surface
thickness of 5.7\AA. By using a simple model for the density profile of the
droplets a thickness of about 6-8\AA is extracted directly from the data.Comment: 27 pages, REVTeX, 5 postscript figure
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