Antonio Calvo Hernández

Power and efficiency in a regenerative gas turbine

The effect of a regenerative heat exchanger in a gas turbine is analysed using a regenerative Brayton cycle model, where all fluid friction losses in the compressor and turbine are quantified by an isentropic efficiency term and all global irreversibilities in the heat exchanger are taken into account by means of an effective efficiency. This analysis, which generalizes that reported by Gordon and Huleihil for a simple, nonregenerative Brayton cycle, provides a theoretical tool for the selection of optimal operating conditions in a regenerative gas turbine. For a fixed ratio of the compressor inlet temperature to the turbine inlet temperature and in terms of the isentropic and regenerator efficiencies we present explicit results for the behaviour of the maximum efficiency, efficiency at maximum power, maximum power and power at maximum efficiency as well as for the behaviour of the pressure ratios required for maximum efficiency and maximum power.

On an irreversible air standard Otto-cycle model

We present a simplified model for an irreversible Otto cycle which accounts for the characteristic power-versus-efficiency curve of real heat engines. This model avoids the usual hypotheses of endoreversible heat engines and it considers an air Otto engine with internally dissipative friction and a pure sinusoidal law for the piston velocity on the adiabatic branches.

First vibrational overtone bandshape of HCl in fluid SF6 : An experimental and theoretical study

The first overtone band profiles are recorded in the IR spectra of the HCl molecular probes diluted in SF6, both below and above the critical point of the solvent. The spectral density distribution is considered as an additive superposition of the contributions due to the quasifree rotating and the rotationally-hindered probes. A recently proposed spectral theory is applied for description of the quasifree contribution. Good agreement is obtained between the experimental and calculated spectral profiles. The vibration–rotation coupling constant for the HCl molecules is found to decrease in high-density fluids. The central Q-branch components associated with the hindered solute states are separated from the measured spectra and their transformation is studied in a broad range of well-controlled thermodynamic conditions. We discuss the possible influence of the interaction-induced dipole transitions on the absorption in the vicinity of the band center.

Carnot-Like Heat Engines Versus Low-Dissipation Models

In this paper, a comparison between two well-known finite time heat engine models is presented: the Carnot-like heat engine based on specific heat transfer laws between the cyclic system and the external heat baths and the Low-Dissipation model where irreversibilities are taken into account by explicit entropy generation laws. We analyze the mathematical relation between the natural variables of both models and from this the resulting thermodynamic implications. Among them, particular emphasis has been placed on the physical consistency between the heat leak and time evolution on the one side, and between parabolic and loop-like behaviors of the parametric power-efficiency plots. A detailed analysis for different heat transfer laws in the Carnot-like model in terms of the maximum power efficiencies given by the Low-Dissipation model is also presented.

Infrared fundamental bandshape of HCl in fluid SF6: A modified-rotor description

The fundamental bandshape of HCl diluted in fluid SF6 has been studied in the temperature range 283–340 K at two constant densities of the solvent. It is found that the previously developed quasi-free rotor theory realistically reproduces the experimental band area-normalized intensity distribution in the band wings, but overestimates intensity near the maxima of the P- and R-branches. A concept of a potential barrier hindering rotation of the solute molecules is introduced and a new modified-rotor version of the spectral theory is formulated, which takes into account depopulation of the lower-energy rotational states. We show that the modified-rotor theory significantly improves agreement with the experiment. The rotationally-hindered contribution to the bandshape is separated from the measured spectral profiles and its temperature and density dependence is discussed.

Vibration-rotation spectra of hydrogen halides in rare-gas liquids: Q-branch absorption

Near-infrared spectra of HCl highly diluted in liquid Ar show intense absorption in the P-R interbranch region, so-called Q-branch absorption. In spite of its relevance for the shape of the bands, its physical origin has been elusive to date. We employ molecular dynamics simulations to study the influence of some physical effects that could contribute to Q-branch absorption. We check that multipole-induced dipole induction mechanisms are not quantitatively relevant in this spectral region. We show that the particular characteristics of accurate HCl-Ar anisotropic potentials and the peculiar hindered rotational motion they provoke on the diatomic probe are essential to understand Q-branch absorption.

Validating and Comparing with Experiments and Other Models

The objective of this chapter is to present and compare the performance parameters and the evolution of some process variables obtained by means of quasi-dimensional simulations with experimental results and with the corresponding estimations of other simulation or theoretical models. First, we summarize some of the basic performance parameters that are suitable for calculation. Second, some calculated parameters and functions obtained either from zero-dimensional or quasi-dimensional schemes are compared with engine test bench measurements. Finally, we analyze the evolution of a four-stroke Otto engine from a finite-time thermodynamics framework in order to elucidate to what extent a purely theoretical thermodynamic scheme is capable of reproducing realistic simulation results. It will be concluded that quasi-dimensional simulation models are capable to improve theoretical formulations in order to better approach theoretical predictions to experimental results.

Thermodynamic Engine Optimization

In this chapter, we show how the capabilities of numerical simulations and theoretical finite-time thermodynamics tools complement each other to optimize the performance of a spark ignition engine. Numerical simulations enable us to check the influence of specific parameters in engine operation, whereas theoretical techniques allow us to suggest possible optimization criteria and help to understand their consequences from a physical viewpoint in terms of the main sources of irreversibility. The analysis of power-efficiency curves plays a central role, because they provide a direct way to obtain maximum efficiency for any particular power requirement. In this optimization process, we consider both design and operation parameters such as the location of the ignition kernel, stroke-to-bore ratio, spark advance, fuel–air (equivalence) ratio, and cylinder wall temperature.

Physical Laws and Model Structure of Simulations

In this chapter, we present the Newton’s second law and the first principle of thermodynamics as a set or ordinary differential equations for the pressure and the temperature inside the combustion chamber. The solution of these equations with the appropriate initial conditions in each stroke allows to follow the evolution of the thermodynamic properties of the gas mixture performing the engine cycle. The solution of such equations requires the formulation of some additional submodels for the engine: heat transfer through cylinder walls, frictions, working fluid properties, combustion and chemical reactions, and several others. We present those basic models and how are they engaged in the structure of a computer simulation scheme. Special attention is paid to the use of alternative fuels. Detailed chemical reactions for the combustion of such fuels are presented in Appendix F. We include an up-to-date bibliographic survey on advanced submodels that could help the reader to build a particular advanced simulation scheme.

Cycle-to-Cycle Variability

In this chapter we shall analyze how to simulate, within a quasi-dimensional combustion model, the cyclic variability experimentally observed in spark ignition engines. Explicit qualitative comparison with experimental results will be shown. Moreover, we report a nonlinear dynamics analysis of cycle-by-cycle variations in heat release for the simulated engine with noisy components. Our approaches are based on nonlinear scaling properties of heat release fluctuations mainly, by means of correlation dimension, monofractal, and multifractal methods, and also by means of wavelet decomposition. We characterize the fluctuations for several fuel–air ratio values, \(\phi \), from lean mixtures to over stoichiometric situations by computing very long time series. Finally, we study the behavior of energetic functions when the presence of cyclic variability is considered. The fluctuating behavior of the net heat release, the power output, and the fuel conversion efficiency are simultaneously evaluated and analyzed.