7,717 research outputs found
Erosion/corrosion of turbine airfoil materials in the high-velocity effluent of a pressurized fluidized coal combustor
Four candidate turbine airfoil superalloys were exposed to the effluent of a pressurized fluidized bed with a solids loading of 2 to 4 g/scm for up to 100 hours at two gas velocities, 150 and 270 m/sec, and two temperatures, 730 deg and 795 C. Under these conditions, both erosion and corrosion occurred. The damaged specimens were examined by cross-section measurements, scanning electron and light microscopy, and X-ray analysis to evaluate the effects of temperature, velocity, particle loading, and alloy material. Results indicate that for a given solids loading the extent of erosion is primarily dependent on gas velocity. Corrosion occurred only at the higher temperature. There was little difference in the erosion/corrosion damage to the four alloys tested under these severe conditions
The erosion/corrosion of small superalloy turbine rotors operating in the effluent of a PFB coal combustor
Superalloy turbine rotors in a single stage turbine with 6 percent partial admittance were operated in the effluent of a pressurized fluidized bed coal combustor for up to 164 hours. Total mass flow was 300 kg/hr and average particulate loadings ranged from 600 to 2800 ppm for several coal/sorbent combinations. A 5.5 atm turbine inlet gas pressure and inlet gas temperatures from 700 to 800 C yielded absolute gas velocities at the stator exit of about 500 m/s. The angular rotation speed (40,000 rpm) of the six inch diameter rotors was equivalent to a tip speed of about 300 m/s, and average gas velocities relative to the rotating surface ranged from 260 to 330 m/s at mean radius. The rotor erosion pattern reflects heavy particle separation with severe (5 to 500 cm/yr) erosion at the leading edge, pressure side center, and suction side trailing edge at the tip. The erosion distribution pattern provides a spectrum of erosion/oxidation/deposition as a function of blade position. This spectrum includes enhanced oxidation (10 to 100 x air), mixed oxides in exposed depletion zones, sulfur rich oxides in deposition zones, and rugged areas of erosive oxide removal
An exactly solvable model of a superconducting to rotational phase transition
We consider a many-fermion model which exhibits a transition from a
superconducting to a rotational phase with variation of a parameter in its
Hamiltonian. The model has analytical solutions in its two limits due to the
presence of dynamical symmetries. However, the symmetries are basically
incompatible with one another; no simple solution exists in intermediate
situations. Exact (numerical) solutions are possible and enable one to study
the behavior of competing but incompatible symmetries and the phase transitions
that result in a semirealistic situation. The results are remarkably simple and
shed light on the nature of phase transitions.Comment: 11 pages including 1 figur
An algebraic approach to problems with polynomial Hamiltonians on Euclidean spaces
Explicit expressions are given for the actions and radial matrix elements of
basic radial observables on multi-dimensional spaces in a continuous sequence
of orthonormal bases for unitary SU(1,1) irreps. Explicit expressions are also
given for SO(N)-reduced matrix elements of basic orbital observables. These
developments make it possible to determine the matrix elements of polynomial
and a other Hamiltonians analytically, to within SO(N) Clebsch-Gordan
coefficients, and to select an optimal basis for a particular problem such that
the expansion of eigenfunctions is most rapidly convergent.Comment: 19 pages, 8 figure
Reversable heat flow through the carbon nanotube junctions
Microscopic mechanisms of externally controlled reversable heat flow through
the carbon nanotube junctions (NJ) are studied theoretically. Our model
suggests that the heat is transfered along the tube section by
electrons () and holes () moving ballistically in either in parallel or
in opposite directions and accelerated by the bias source-drain voltage (Peltier effect). We compute the Seebeck coefficient , electric
and thermal conductivities and find that their magnitudes
strongly depend on and . The sign reversal of
versus the sign of formerly observed experimentally is interpreted
in this work in terms of so-called chiral tunneling phenomena (Klein paradox)
Richardson-Gaudin integrability in the contraction limit of the quasispin
Background: The reduced, level-independent, Bardeen-Cooper-Schrieffer
Hamiltonian is exactly diagonalizable by means of a Bethe Ansatz wavefunction,
provided the free variables in the Ansatz are the solutions of the set of
Richardson-Gaudin equations. On the one side, the Bethe Ansatz is a simple
product state of generalised pair operators. On the other hand, the
Richardson-Gaudin equations are strongly coupled in a non-linear way, making
them prone to singularities. Unfortunately, it is non-trivial to give a clear
physical interpretation to the Richardson-Gaudin variables because no physical
operator is directly related to the individual variables. Purpose: The purpose
of this paper is to shed more light on the critical behavior of the
Richardson-Gaudin equations, and how this is related to the product wave
structure of the Bethe Ansatz. Method: A pseudo-deformation of the quasi-spin
algebra is introduced, leading towards a Heisenberg-Weyl algebra in the
contraction limit of the deformation parameter. This enables an adiabatic
connection of the exact Bethe Ansatz eigenstates with pure bosonic multiphonon
states. The physical interpretation of this approach is an adiabatic
suppression of the Pauli exclusion principle. Results: The method is applied to
a so-called "picket-fence" model for the BCS Hamiltonian, displaying a typical
critical behavior in the Richardson-Gaudin variables. It was observed that the
associated bosonic multiphonon states change collective nature at the critical
interaction strengths of the Richardson-Gaudin equations. Conclusions: The
Pauli exclusion principle is the main responsible for the critical behavior of
the Richardson-Gaudin equations, which can be suppressed by means of a pseudo
deformation of the quasispin algebra.Comment: PACS 02.30.Ik, 21.10.Re, 21.60.Ce, 74.20.F
Vector coherent state representations, induced representations, and geometric quantization: II. Vector coherent state representations
It is shown here and in the preceeding paper (quant-ph/0201129) that vector
coherent state theory, the theory of induced representations, and geometric
quantization provide alternative but equivalent quantizations of an algebraic
model. The relationships are useful because some constructions are simpler and
more natural from one perspective than another. More importantly, each approach
suggests ways of generalizing its counterparts. In this paper, we focus on the
construction of quantum models for algebraic systems with intrinsic degrees of
freedom. Semi-classical partial quantizations, for which only the intrinsic
degrees of freedom are quantized, arise naturally out of this construction. The
quantization of the SU(3) and rigid rotor models are considered as examples.Comment: 31 pages, part 2 of two papers, published versio
Vector coherent state representations, induced representations, and geometric quantization: I. Scalar coherent state representations
Coherent state theory is shown to reproduce three categories of
representations of the spectrum generating algebra for an algebraic model: (i)
classical realizations which are the starting point for geometric quantization;
(ii) induced unitary representations corresponding to prequantization; and
(iii) irreducible unitary representations obtained in geometric quantization by
choice of a polarization. These representations establish an intimate relation
between coherent state theory and geometric quantization in the context of
induced representations.Comment: 29 pages, part 1 of two papers, published versio
Liver transplantation for arteriohepatic dysplasia (Alagille's syndrome)
Thirteen out of 268 children (<18 years old) underwent hepatic transplantation (OLT) for end-stage liver disease (ESLD) associated with arteriohepatic dysplasia (AHD). Seven children are alive and well with normal liver function. Six children died, four within 11 days of the operation and the other two at 4 and 10 months after the OLT. Vascular complications with associated septicemia were responsible for the deaths of three children. Two died of heart failure and circulatory collapse, secondary to pulmonary hypertension and congenital heart disease. The remaining patient died of overwhelming sepsis not associated with technical complications. Seven patients had a portoenterostomy or portocholecystostomy early in life; five of these died after the OLT. Severe cardiovascular abnormalities in some of our patients suggest that complete hemodynamic monitoring with invasive studies should be performed in all patients with AHD, especially in cases of documented hypertrophy of the right ventricle. The improved quality of life in our surviving patients confirms the validity of OLT as a treatment of choice in cases of ESLD due to AHD. © 1992 Springer-Verlag
The Skyrme energy functional and low lying 2+ states in Sn, Cd and Te isotopes
We study the predictive power of Skyrme forces with respect to low lying
quadrupole spectra along the chains of Sn, Cd, and Te isotopes. Excitation
energies and B(E2) values for the lowest quadrupole states are computed from a
collective Schroedinger equation which as deduced through collective path
generated by constraint Skyrme-Hartree-Fock (SHF) plus self-consistent cranking
for the dynamical response. We compare the results from four different Skyrme
forces, all treated with two different pairing forces (volume versus
density-dependent pairing). The region around the neutron shell closure N=82 is
very sensitive to changes in the Skyrme while the mid-shell isotopes in the
region N<82 depend mainly on the adjustment of pairing. The neutron rich
isotopes are most sensitive and depend on both aspects
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