A. Calvo Hernández

Optimum performance of a regenerative Brayton thermal cycle

The optimum performance of a regenerative Brayton cycle was analyzed. The model includes external and internal irreversibilities coming from four main sources: coupling to external heat reservoirs, turbine and compressor nonisentropic processes, pressure losses in the heater and the cooler, and the regenerator. In terms of the parameters accounting for each type of irreversibility, explicit numerical results are presented for the maximum efficiency, maximum power output, efficiency at maximum power output, power output at maximum efficiency, as well as for the pressure ratios required for maximum efficiency and maximum power. This analysis could provide a general theoretical tool for the optimal design and operation of real regenerative gas turbine power plants.

Regenerative gas turbines at maximum power density conditions

A new kind of power analysis has recently been presented which is based on the maximization of the power density and predicts smaller and more efficient non-regenerative Joule - Brayton engines than those designed at maximum power. In this paper we apply the power density maximization method to regenerative gas turbines using a theoretical framework where the optimal operating conditions of the heat engine are expressed in terms of the isentropic efficiencies of the compressor and turbine and of the heat exchanger efficiency. It is shown that, unlike non-regenerative results, real regenerative gas turbines are less efficient at maximum power density conditions than at maximum power conditions.

Theoretical and simulated models for an irreversible Otto cycle

We show in this work that a finite-time-thermodynamics model of an irreversible Otto cycle is suitable to reproduce performance results of a real spark ignition heat engine. In order to test our model we have developed a computer simulation including a two-zone combustion model and compared the evolution of the performance parameters of the simulated engine as functions of the rotational speed (ω) with those obtained from a simple theoretical scheme including chemical reactions. A theoretical Otto cycle with irreversibilities arising from friction, heat transfer through the cylinder walls, and internal losses properly reproduces simulation results by considering extreme temperatures and mass inside the cylinder as functions of ω. Furthermore we obtain realistic values for the parameters characterizing global irreversibilities, their evolution with ω, and a clearer understanding of their physical origin not always well established in theoretical models.

Feynman's ratchet optimization: maximum power and maximum efficiency regimes

The optimal performance of the Feynman ratchet-and-pawl engine is analysed by taking the power and the efficiency of the engine as objective functions. The power-efficiency curves are also obtained. These curves show a loop shape similar to those characteristic of some real heat engines. Explicit analytical expressions are reported in the so-called linear regime.

Optimizing the operation of a spark ignition engine: Simulation and theoretical tools

By performing quasidimensional computer simulations and finite-time thermodynamic analysis we study the effect of spark advance, fuel ratio, and cylinder internal wall temperature in spark ignition engines. We analyze the effect of these parameters on the power output and efficiency of the engine at any rotational speed, ω. Moreover, we propose the optimal dependence on ω of the spark advance angle and the fuel ratio with the objective to get maximum efficiency for any fixed power requirement. The importance of engine power-efficiency curves in order to perform this optimization procedure and also of the evaluation of macroscopic work losses in order to understand the physical basis of the optimization process is stressed. Taking as reference results from simulations with constant standard values of spark advance, fuel ratio, and cylinder internal wall temperature, the optimized parameters yield to substantial increases in engine performance parameters.

Optimization of heat engines including the saving of natural resources and the reduction of thermal pollution

The use of the new concept of a saving function as a measure of possible reductions of undesired side effects in heat engine operation is proposed. Two saving functions are introduced, one associated with fuel consumption and another associated with thermal pollution. Two optimization paths including the maximization of power output and these saving functions are presented. The first is based on a linear formalism and the second is based on a power-law formalism. When these optimization criteria are applied to a Curzon-Ahlborn heat engine, both criteria lead to a very similar optimum efficiency, opt = 1- 3/4 , where is the ratio between the temperatures of the cold and the hot external reservoirs. A numerical comparison with the efficiency of some modern nuclear power plants is reported.

Spectroscopic Studies of Diatomics in Dense Non-polar Fluids: an Overview

Abstract In this paper an overview of theoretical and experimental work in spectroscopic, mainly infrared, studies of diatomic molecules in dense nonpolar fluids is presented. Special attention is paid to some unsolved problems encountered in analyses of the band shapes and spectral moments.

Infrared spectra of diatomic polar molecules in rare‐gas liquids. I. Spectral theory

A theoretical model for the infrared (0–1) band of dilute solutions of diatomic polar molecules in nonpolar rare‐gas liquids is presented. The model is based on decomposition of the rotational motion of the diatomic molecule into two limiting cases, according to the Bratos model: quasifree rotation and rotational diffusion. Contribution to the infrared absorption coefficient due to quasifree rotation is analyzed within a non‐Markovian formalism using a stochastic directing intermolecular field (DIF) model to describe the diatomic molecule–solvent interaction. The P and R branches appear as a consequence of the quasifree contribution, which is also important in the Q‐branch region. The contribution due to rotational diffusion is calculated making use of the Debye model and is mainly significant in the Q‐branch region.

Vibration-rotation spectra of HCl in rare-gas liquid mixtures: Molecular dynamics simulations of Q-branch absorption

New experimental results are presented on the fundamental IR band shape of HCl dissolved in neat liquid Ar and Ar doped with Kr and Xe. A strong enhancement of the absorption in the range of a central Q-branch is observed in the spectra of doped solutions. Semiclassical molecular dynamics simulations of the spectral band profile are carried out using (12-6) Lennard-Jones site–site interaction potentials. The parameters of these model potentials were deduced by fitting the available anisotropic interaction surfaces, accurately describing the structure of binary rare-gas-HCl van der Waals complexes. Simulations realistically reproduce the observed triplet band structure and its evolution with changing thermodynamic conditions. The analysis of the influence of anisotropic interactions on the orientational dynamics of solutes and orientation-dependent radial distribution functions reveals the mechanisms that contribute to appearance of the Q-branches. It is shown that long-living solute-solvent spatial correl...

Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. I. Spectral theory

A unified non-Markovian theory for the vibrorotational Raman and infrared spectra of polar diatomic molecules diluted in nonpolar fluids is presented. From this theory, the physical basis of the spectra can be interpreted in terms of a few molecular properties of the isolated diatomic and of the time autocorrelation functions determining the collective effects of the solvent molecules on the vibrorotational dynamic of the diatomic. The spectrum is obtained as a diagonal part, constituted by an additive superposition of lines accounting for the integrated intensity, and an (exact) nondiagonal part accounting for the redistribution of intensity due to interbranch and intrabranch mixing effects. This theory generalizes previous theoretical frames based on a secular contribution modified by an (approximate) interference term. Also it allows the comparative analysis of the Raman and infrared spectra, and gives a clear and consistent interpretation of the theoretical lines building up the spectra.