Biomedical subjects

A Calvo Hernández

Publications and source records attributed to A Calvo Hernández.

6 recordsLinked to original sources

PubMed2006

Linear irreversible thermodynamics and coefficient of performance.

Following the recent proposal by Van den Broeck for a heat engine [Phys. Rev. Lett. 95, 190602 (2005)], we analyze the coefficient of performance of a refrigerator in two working regimes using the tools of linear irreversible thermodynamics. In particular, one of the analyzed regimes gives a coefficient of performance which could be considered as the equivalent to the Curzon-Ahlborn efficiency. Also we consider the relation with the Clausius inequality, and some results for the relevant thermodynamic magnitudes in this formalism are confronted with those obtained using the finite-time thermodynamics framework.

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PubMed2005

Dynamical characterization of rotationally hindered species in liquids.

The rotational dynamics of HCl in liquid Ar has been studied by means of molecular-dynamics simulations. We calculate the lifetimes of weakly bound HCl-Ar dimers induced by the anisotropic pair interaction. It is shown that, although lifetimes are small with respect to the reorientational decorrelation, the time interval between the breaking down and formation of the next dimer is negligibly small. Thus, with respect to the rotational dynamics of the probe, the effect is similar to that and eventually would cause a time-stable complex. This provokes a peculiar hindered rotation of the diatomic in the liquid which is macroscopically embodied in the infrared spectrum of the solution as a Q-branch nonexistent otherwise.

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PubMed2005

Experimental analysis and modified rotor description of the infrared fundamental band of HCl in Ar, Kr, and Xe solutions.

We report an experimental study of the rotovibrational fundamental PQR-band shapes in the IR absorption spectra of HCl dissolved in condensed rare gases in a wide range of temperatures. The effective vibrational frequencies are determined from analysis of the fine rotational structure partially resolved in the band wings. The central Q-branch components appear redshifted with respect to the effective vibrational frequencies, their shifts in different solvents found to match the HCl stretching mode shifts in binary Rg...HCl van der Waals heterodimers. Theoretical quasi-free rotor and modified rotor models are applied to describe evolution of the band profiles at changing thermodynamic conditions. Both models are shown to reproduce equally well the observed spectral density distributions in the band wings. However, the modified rotor formalism that accounts for depopulation of the lower-energy rotational solute states provides better agreement with the experiment in the range of the P- and R-branch maxima. We surmise that the Q branches separated from the measured spectral profiles are formed by transitions between rotationally hindered states of diatomic molecules coupled to the solvent by the local anisotropy of the interaction potential.

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PubMed2004

Harmonic quantum heat devices: optimum-performance regimes.

The finite-time performance of a quantum-mechanical heat engine (or refrigerator) with a working fluid consisting of many noninteracting harmonic oscillators is considered in order to analyze three optimum operating regimes: maximum efficiency (maximum coefficient of performance), maximum work output (maximum cooling load) and a third one, Omega criterion, which represents a compromise between them. The reported results extend previous findings for macroscopic and mesoscopic energy converters to quantum heat devices and also endorse the Omega criterion as a unified, optimum working regime for energy converters, independent of their size and nature.

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PubMed2004

Infrared spectral profiles in liquids and atom-diatom interactions.

Molecular dynamics simulations of the infrared spectrum of a generic simple polar diatomic in a liquid nonpolar solvent allow to reproduce the different prototypical experimental line shapes of this kind of systems. This is feasible by using different solute-solvent anisotropic potentials at fixed thermodynamic conditions. In the limit cases, the rotation of the diatomic is explained in terms of a quasifree motion or a rotational diffusion evolution and the spectra show a doublet structure formed by P and R branches or a unique collapsed branch, respectively. When the profile contains three branches, including an intense Q branch in the vicinity of the center of the band, rotational evolution presents a particular hindering that can be understood by studying the influence on rotational spectral densities of the different time scales involved in rotational relaxation. Cancellation/enhancement effects among spectral density terms arising from intermediate and long times (0.4-1 ps) are essential to understand rotational hindering.

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PubMed2003

Adiabatic rocking ratchets: Optimum-performance regimes.

We analyze work and efficiency for an adiabatic rocking ratchet working under three operating regimes: maximum efficiency, maximum work, and a third one which represents a compromise between them. For all of these regimes the application of very concrete loads and external amplitudes is found necessary in order to obtain the maximum possible values of both efficiency and work. The reported results could be valuable to design efficient Brownian motors and compare their operation under different working regimes.

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