3,444 research outputs found
Functioning of steam driven ejectors as a part of steam turbines
В статье рассмотрены вопросы функционирования пароструйных эжекторов с точки зрения повышения надежности на основе опыта авторов по разработке и модернизации более чем 80 аппаратов. Проведен анализ статистической информации по отказам эжекторов, времени восстановления. Показано, что отказ эжектора в большинстве случаев приводят к останову турбоагрегата. Представлен перечень выявляемых дефектов.In the paper the problems of functioning of steam driven ejectors are considered in the view of increasing the reliability and basing on the authors experience of design more than 80 devices. The analysis of statistic information about ejectors breakdowns and recovery periods is provided. It is shown, that in the majority of ejector breakdowns, the turbine has to be stopped. A list of revealed defects is presented.Работа выполнена при финансовой поддержке Правительства РФ; Постановление № 211, контракт № 02.А03.21.000
Exact Thermodynamics of Disordered Impurities in Quantum Spin Chains
Exact results for the thermodynamic properties of ensembles of magnetic
impurities with randomly distributed host-impurity couplings in the quantum
antiferromagnetic Heisenberg model are presented. Exact calculations are done
for arbitrary values of temperature and external magnetic field. We have shown
that for strong disorder the quenching of the impurity moments is absent. For
weak disorder the screening persists, but with the critical non-Fermi-liquid
behaviors of the magnetic susceptibility and specific heat. A comparison with
the disordered Kondo effect experiments in dirty metallic alloys is performed.Comment: 4 pages Late
Aging in Models of Non-linear Diffusion
We show that for a family of problems described by non-linear diffusion
equations an exact calculation of the two time correlation function gives
C(t,t')=f(t-t')g(t'), t>t', exhibiting normal and anomalous diffusions, as well
as aging effects, depending on the degree of non-linearity. We discuss also the
form in which FDT is violated in this class of systems. Finally we argue that
in this type of models aging may be consequence of the non conservation of the
"total mass".Comment: 4 pages, 1 figure, to appear in Phys.Rev.
Perturbative Linearization of Reaction-Diffusion Equations
We develop perturbative expansions to obtain solutions for the initial-value
problems of two important reaction-diffusion systems, viz., the Fisher equation
and the time-dependent Ginzburg-Landau (TDGL) equation. The starting point of
our expansion is the corresponding singular-perturbation solution. This
approach transforms the solution of nonlinear reaction-diffusion equations into
the solution of a hierarchy of linear equations. Our numerical results
demonstrate that this hierarchy rapidly converges to the exact solution.Comment: 13 pages, 4 figures, latex2
Renormalization Group Theory And Variational Calculations For Propagating Fronts
We study the propagation of uniformly translating fronts into a linearly
unstable state, both analytically and numerically. We introduce a perturbative
renormalization group (RG) approach to compute the change in the propagation
speed when the fronts are perturbed by structural modification of their
governing equations. This approach is successful when the fronts are
structurally stable, and allows us to select uniquely the (numerical)
experimentally observable propagation speed. For convenience and completeness,
the structural stability argument is also briefly described. We point out that
the solvability condition widely used in studying dynamics of nonequilibrium
systems is equivalent to the assumption of physical renormalizability. We also
implement a variational principle, due to Hadeler and Rothe, which provides a
very good upper bound and, in some cases, even exact results on the propagation
speeds, and which identifies the transition from ` linear'- to `
nonlinear-marginal-stability' as parameters in the governing equation are
varied.Comment: 34 pages, plain tex with uiucmac.tex. Also available by anonymous ftp
to gijoe.mrl.uiuc.edu (128.174.119.153), file /pub/front_RG.tex (or .ps.Z
Hidden non-Fermi liquid behavior due to crystal field quartet
We study a realistic Kondo model for crystal field quartet ground states
having magnetic and non-magnetic (quadrupolar) exchange couplings with
conduction electrons, using the numerical renormalization group method. We
focus on a local effect dependent on singlet excited states coupled to the
quartet, which reduces the non-magnetic coupling significantly and drives
non-Fermi liquid behavior observed in the calculated quadrupolar
susceptibility. A crossover from the non-Fermi liquid state to the Fermi liquid
state is characterized by a small energy scale very sensitive to the
non-magnetic coupling. On the other hand, the Kondo temperature observed in the
magnetic susceptibility is less sensitive. The different crystal-field
dependence of the two exchange couplings may be related to the different
dependence of quadrupolar and magnetic ordering temperatures in
CeLaB.Comment: 7 pages, 5 EPS figures, REVTe
Multiple Front Propagation Into Unstable States
The dynamics of transient patterns formed by front propagation in extended
nonequilibrium systems is considered. Under certain circumstances, the state
left behind a front propagating into an unstable homogeneous state can be an
unstable periodic pattern. It is found by a numerical solution of a model of
the Fr\'eedericksz transition in nematic liquid crystals that the mechanism of
decay of such periodic unstable states is the propagation of a second front
which replaces the unstable pattern by a another unstable periodic state with
larger wavelength. The speed of this second front and the periodicity of the
new state are analytically calculated with a generalization of the marginal
stability formalism suited to the study of front propagation into periodic
unstable states. PACS: 47.20.Ky, 03.40.Kf, 47.54.+rComment: 12 page
On an exponential attractor for a class of PDEs with degenerate diffusion and chemotaxis
In this article we deal with a class of strongly coupled parabolic systems
that encompasses two different effects: degenerate diffusion and chemotaxis.
Such classes of equations arise in the mesoscale level modeling of biomass
spreading mechanisms via chemotaxis. We show the existence of an exponential
attractor and, hence, of a finite-dimensional global attractor under certain
'balance conditions' on the order of the degeneracy and the growth of the
chemotactic function
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