5,146 research outputs found
Quantum thermodynamic Carnot and Otto-like cycles for a two-level system
From the thermodynamic equilibrium properties of a two-level system with
variable energy-level gap , and a careful distinction between the Gibbs
relation and the energy balance equation , we infer some important aspects of the
second law of thermodynamics and, contrary to a recent suggestion based on the
analysis of an Otto-like thermodynamic cycle between two values of of
a spin-1/2 system, we show that a quantum thermodynamic Carnot cycle, with the
celebrated optimal efficiency , is possible in
principle with no need of an infinite number of infinitesimal processes,
provided we cycle smoothly over at least three (in general four) values of
, and we change not only along the isoentropics, but also
along the isotherms, e.g., by use of the recently suggested maser-laser tandem
technique. We derive general bounds to the net-work to high-temperature-heat
ratio for a Carnot cycle and for the 'inscribed' Otto-like cycle, and represent
these cycles on useful thermodynamic diagrams.Comment: RevTex4, 4 pages, 1 figur
Cables and fire hazards
Besides describing the experiments conducted to develop a nonflammable cable, this article discusses several considerations regarding other hazards which might result from cable fires, particularly the toxicity and opacity of the fumes emitted by the burning cable. In addition, this article examines the effects of using the Oxygen Index as a gauge of quality control during manufacture
Thermodynamic analysis of turbulent combustion in a spark ignition engine. Experimental evidence
A method independent of physical modeling assumptions is presented to analyze high speed flame photography and cylinder pressure measurements from a transparent piston spark ignition research engine. The method involves defining characteristic quantities of the phenomena of flame propagation and combustion, and estimating their values from the experimental information. Using only the pressure information, the mass fraction curves are examined. An empirical burning law is presented which simulates such curves. Statistical data for the characteristics delay and burning angles which show that cycle to cycle fractional variations are of the same order of magnitude for both angles are discussed. The enflamed and burnt mass fractions are compared as are the rates of entrainment and burning
A nonlinear model dynamics for closed-system, constrained, maximal-entropy-generation relaxation by energy redistribution
We discuss a nonlinear model for the relaxation by energy redistribution
within an isolated, closed system composed of non-interacting identical
particles with energy levels e_i with i=1,2,...,N. The time-dependent
occupation probabilities p_i(t) are assumed to obey the nonlinear rate
equations tau dp_i/dt=-p_i ln p_i+ alpha(t)p_i-beta(t)e_ip_i where alpha(t) and
beta(t) are functionals of the p_i(t)'s that maintain invariant the mean energy
E=sum_i e_ip_i(t) and the normalization condition 1=sum_i p_i(t). The entropy
S(t)=-k sum_i p_i(t) ln p_i(t) is a non-decreasing function of time until the
initially nonzero occupation probabilities reach a Boltzmann-like canonical
distribution over the occupied energy eigenstates. Initially zero occupation
probabilities, instead, remain zero at all times. The solutions p_i(t) of the
rate equations are unique and well-defined for arbitrary initial conditions
p_i(0) and for all times. Existence and uniqueness both forward and backward in
time allows the reconstruction of the primordial lowest entropy state. The time
evolution is at all times along the local direction of steepest entropy ascent
or, equivalently, of maximal entropy generation. These rate equations have the
same mathematical structure and basic features of the nonlinear dynamical
equation proposed in a series of papers ended with G.P.Beretta, Found.Phys.,
17, 365 (1987) and recently rediscovered in S. Gheorghiu-Svirschevski,
Phys.Rev.A, 63, 022105 and 054102 (2001). Numerical results illustrate the
features of the dynamics and the differences with the rate equations recently
considered for the same problem in M.Lemanska and Z.Jaeger, Physica D, 170, 72
(2002).Comment: 11 pages, 7 eps figures (psfrag use removed), uses subeqn, minor
revisions, accepted for Physical Review
Combustion and operating characteristics of spark-ignition engines
The spark-ignition engine turbulent flame propagation process was investigated. Then, using a spark-ignition engine cycle simulation and combustion model, the impact of turbocharging and heat transfer variations or engine power, efficiency, and NO sub x emissions was examined
Amino Acid Composition of Granules and Spots in Grana Padano Cheeses
Abstract Amino acids concentrated in Grana Padano cheese in two different physical forms, granules and spots. The major amino acid in the granules was tyrosine followed in concentration by phenylalanine and glutamic acid. Composition of spots was predominantly leucine and iso-leucine with tyrosine essentially absent. Composition of the free amino acids in the granules differed from that in the whole cheese. Bacterial populations were much higher in amino acid localization than in cheese as a whole, suggesting that bacterial action is a major contributing factor to the phenomenon of amino acid localization
Total synthesis of tetracyclic kynurenic acid analogues isolated from chestnut honey
A short and efficient synthesis of novel tetracyclic Kynurenic acid analogues, isolated from chestnut honey, is described. The crucial step of the strategy was a MW-assisted cyclization of enamines of ethyl dioxohexahydropyrrolizine and 2,3-dioxooctahydroindolizine carboxylates to obtain 2,3,6,11b-tetrahydro-1H-pyrrolizino[2,1-b]quinoline-5,11-dione and 5,8,91,011,11a-hexahydroindolizino[2,1-b]quinoline-6,12-dione, respectively. Because of its modular nature, the synthetic strategy can have value as a general method for the preparation of compounds containing these new heterocyclic scaffolds
A Note on the KKT Points for the Motzkin-Straus Program
In a seminal 1965 paper, Motzkin and Straus established an elegant connection between the clique number of a graph and the global maxima of a quadratic program defined on the standard simplex. Since then, the result has been the subject of intensive research and has served as the motivation for a number of heuristics and bounds for the maximum clique problem. Most of the studies available in the literature, however, focus typically on the local/global solutions of the program, and little or no attention has been devoted so far to the study of its Karush-Kuhn-Tucker (KKT) points. In contrast, in this paper we study the properties of (a parameterized version of) the Motzkin-Straus program and show that its KKT points can provide interesting structural information and are in fact associated with certain regular sub-structures of the underlying graph
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