13,921 research outputs found
Solar Tests of Aperture Plate Materials for Solar Thermal Dish Collectors
If a malfunction occurs in a solar thermal point-focus distributed receiver power plant while a concentrator is pointed at the Sun, motion of the concentrator may stop. As the Sun moves relative to the Earth, the spot of concentrated sunlight then slowly walks off the receiver aperture, across the receiver face plate, and perhaps across adjacent portions of the concentrator. Intense local heating by the concentrated sunlight may damage or destroy these parts. The behavior of various materials under conditions simulating walk-off of a parabolic dish solar collector were evaluated. Each test consisted of exposure to concentrated sunlight at a peak flux density of about 7000 kW/square meter for 15 minutes. Types of materials tested included graphite, silicon carbide, silica, various silicates, alumina, zirconia, aluminum, copper, steel, and polytetrafluoroethylene. The only material that neither cracked nor melted was grade G-90 graphite. Grade CS graphite, a lower cost commercial grade, cracked half-way across, but did not fall apart. Both of these grades are medium-grain extruded graphites. A graphite cloth (graphitized polyacrylonitrile) showed fair performance when tested as a single thin ply; it might be useful as a multi-ply assembly. High purity slipcast silica showed some promise also
Unnatural Acts: Unphysical Consequences of Imposing Boundary Conditions on Quantum Fields
I examine the effect of trying to impose a Dirichlet boundary condition on a
scalar field by coupling it to a static background. The zero point -- or
Casimir -- energy of the field diverges in the limit that the background forces
the field to vanish. This divergence cannot be absorbed into a renormalization
of the parameters of the theory. As a result, the Casimir energy of a surface
on which a Dirichlet boundary condition is imposed, and other quantities like
the surface tension, which are obtained by deforming the surface, depend on the
physical cutoffs that characterize the coupling between the field and the
matter on the surface. In contrast, the energy density away from the surface
and forces between rigid surfaces are finite and independent of these
complicationsComment: 10 pages, 3 figures, uses aipproc.cls which is included in upload.
allnew.eps replaces figures all.eps and section.ep
Color, Spin, and Flavor-Dependent Forces in Quantum Chromodynamics
A simple generalization of the Breit Interaction explains many qualitative
features of the spectrum of hadrons.Comment: 20 pp., 8 EPS figures, LaTeX2e using BoxedEPS, ws-p8-50x6-00.cls
macros; contribution to forthcoming published lectures of the Gregory Breit
Centennial Symposium, October 1999; email to [email protected]
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