Thermal Irreversibilities Research Articles

Probing spin dynamics by conductance fluctuations and noise in mesoscopic spin-glass

Millikelvin magnetoconductance studies performed in mesoscopic spin-glass n +-Cd 1− x Mn x Te wires reveal strong effects of Mn spins upon universal conductance fluctuations. In particular, scattering potential of the frozen spins leads to magnetic and thermal irreversibilities of the fluctuation patterns. Slowly fluctuating spins, on the other hand, are an efficient source of conductance noise providing a real-time probe of spin dynamics.

Confinement effects in semimagnetic semiconductors

An overview is given of selected novel effects observed recently by various groups in modulated structures of Cd 1− x Mn x Te. Millikelvin studies of submicron wires doped with either indium or iodine have demonstrated the existence of a new mechanism, by which the universal conductance fluctuations can be generated in mesoscopic systems containing magnetic ions. Moreover, 1/ f conductance noise as well as thermal and magnetic irreversibilities have been observed, providing important information on spin-glass dynamics. Finite size effects in magnetic properties have been probed by direct static and dynamic SQUID measurements on superlattices consisting of few-monolayer spin-glass films separated by nonmagnetic barriers. The confined holes have been found to exert a strong influence upon the magnetic ions and to induce a ferromagnetic phase transition above 1 K in quantum wells modulation doped by nitrogen. Finally, it has been shown also that the giant spin-splitting of the bands offers a tool to tune the coupling between confined photon and exciton modes in photonic structures.

The performance of irreversible combined heat-pump cycles

The effects of thermal resistances and internal irreversibilities on the performance of combined heat-pump cycles (combined vapour compression - vapour compression heat pumping and combined vapour compression - absorption heat pumping) were investigated using a finite-time thermodynamic approach. Improved equations for the coefficient of performance of the systems under consideration were obtained. It was found that combined vapour compression heat pumping is more effective than combined vapour compression - absorption heat pumping for the same heating load regardless of its energy consumption being greater.

5544700 Method and apparatus for preferential cooling

This paper presents a novel method which can be helpful in assessing the optimal configuration of finned-tube heat exchangers. The method is an extension of the local irreversibilities method [17], and it is based on the determination on a local basis of the two components of the entropy generation rate: the one caused by viscous dissipations and the one due to thermal irreversibilities. Depending on the engineering purpose for which a technical device was designed, it can be argued that the optimal configuration will be that in which either one (or both) of these two entropy generation rates is minimized. For a heat exchanging device, it is important to minimize thermal irreversibilities, but more important is to minimize the mechanical power lost in achieving a prescribed heat-exchange performance: to this purpose, one can form a relative irreversibility index (named Bejan number here and in [17] because the original seed of this procedure can be found in [1]), and use it to assess the merit of a given configuration.In the procedure presented here, a circular, single-tube, finned heat exchanger configuration is considered: the velocity and temperature fields are computed (via a standard finite-element package, FIDAP) for a realistic value of the Reynolds number and for a variety of geometric configurations (various fin external diameters and fin spacing); then, the entropy generation rate is calculated from the flowfield, and is examined both at a local level, to detect possible bad design spots (ie, locations which correspond to abnormally high entropy generation rates, which could be cured by design improvements), and at an overall (integral) level, to assess the entropic performance of the heat exchanger. Optimal curves are given, and the optimal spacing of fins is determined using alternatively the entropy generation rate and the total heat transfer rate as objective functions: different optima arise, and the differences as well as the similarities are discussed in detail.

Optimum performance of heat engine-driven heat pumps: A finite-time approach

Classic thermodynamics, assuming the Carnot machine as the upper comparison limit in energy conversion systems analysis, considers thermal equilibrium during heat transfer interactions. Such an assumption requires either infinitely slow cycles or infinitely large heat exchanger surfaces. More realistic limits on the optimal operation of energy systems can be provided by finite-time thermodynamics, which takes into account the constraints represented by finite operation time and limited heat interaction area. This paper focuses on the search for the optimum heating performance of a heat engine-driven heat pump in which the waste engine heat is used for heating purposes. At first, only thermal irreversibilities are considered; then, in order to obtain a more realistic model, the so called “internal” heat leak irreversibilities are taken into account too. Finally, a comparison between the performances of the irreversible model and those of actual plants is carried out.

A minimum entropy generation procedure for the discrete pseudo-optimization of finned-tube heat exchangers

This paper presents a novel method which can be helpful in assessing the optimal configuration of finned-tube heat exchangers. The method is an extension of the local irreversibilities method [17], and it is based on the determination on a local basis of the two components of the entropy generation rate: the one caused by viscous dissipations and the one due to thermal irreversibilities. Depending on the engineering purpose for which a technical device was designed, it can be argued that the optimal configuration will be that in which either one (or both) of these two entropy generation rates is minimized. For a heat exchanging device, it is important to minimize thermal irreversibilities, but more important is to minimize the mechanical power lost in achieving a prescribed heat-exchange performance: to this purpose, one can form a relative irreversibility index (named Bejan number here and in [17] because the original seed of this procedure can be found in [1]), and use it to assess the merit of a given configuration. In the procedure presented here, a circular, single-tube, finned heat exchanger configuration is considered: the velocity and temperature fields are computed (via a standard finite-element package, FIDAP) for a realistic value of the Reynolds number and for a variety of geometric configurations (various fin external diameters and fin spacing); then, the entropy generation rate is calculated from the flowfield, and is examined both at a local level, to detect possible bad design spots (ie, locations which correspond to abnormally high entropy generation rates, which could be cured by design improvements), and at an overall (integral) level, to assess the entropic performance of the heat exchanger. Optimal curves are given, and the optimal spacing of fins is determined using alternatively the entropy generation rate and the total heat transfer rate as objective functions: different optima arise, and the differences as well as the similarities are discussed in detail.

Coefficient of performance for an irreversible combined refrigeration cycle

The effects of thermal resistances and internal irreversibilities on the performance of a combined (cascade) refrigerator are investigated. An improved equation is obtained for the coefficient of performance (COP) of an irreversible, combined refrigerator.

A comparative thermodynamic study of sorption systems: second law analysis

As a consequence of the phasing out of CFCs, sorption systems appear to be potential candidates to replace vapour compression systems. Amongst sorption systems there exists a choice between several systems, such as liquid absorption, solid adsorption and chemical reaction heat pumps. Nevertheless, few comparative studies between these systems have been undertaken so far. It is the aim of this paper to present such a study based on combined first and second law thermodynamical analysis of the different cycles. Simple entropy generation processes explain why the basic cycles for these systems yield performances much lower than the Carnot efficiency. The possibility of operating regenerative cycles with internal heat recovery and higher efficiencies has also been considered for typical common base conditions. Different entropy generation considerations have been visualised, such as thermal coupling (external/internal), non-uniform temperature component entropy production and other irreversible processes for the COP degradation in these systems. It is found that thermal coupling irreversibilities in solid sorption systems and other internal irreversibilities in liquid sorption systems with solution heat exchanger are dominant in the actual COP degradation with respect to the reversible Carnot COP.

First- and Second-Law Analysis of Steam-Turbine Cogeneration Systems

The paper presents an analysis of a cogeneration plant. The performance of the plant is compared to a conventional plant with separate production of process heat and power. The analysis is first- and second-law based and, therefore, quantifies the irreversibilities of the different components of each plant. In the cogeneration plant, the heat required in the boiler can be obtained either from fuel firing (condensing or back-pressure turbine plant) or from exhaust gases of a simple gas turbine (gas turbine cogeneration plant). The present study compares the two methods. The influence of the heat-to-power ratio and the process pressure on the thermal efficiency, utilization factor, and irreversibilities of the different components of each plant is presented. The results show that the total irreversibility of the cogeneration plant is lower by 38 percent compared to the conventional plant. This reduction in the irreversibility is accompanied by an increase in the thermal efficiency and utilization factor by 25 and 24 percent, respectively. The results show that most irreversible losses are due to boiler.

Regenerative Thermomagnetic Power

Thermomagnetic power devices utilize the temperature dependence of magnetic moment to convert heat into organized energy. In an analogous manner thermoelectrostatic devices would utilize changes of electric moment to perform the same function and the analysis of each can be given in a single treatment. This paper, which analytically explores new considerations relevant to these energy conversion methods, is motivated by a search for increased performance. To begin, a basis for the treatment of the thermodynamics of polarizable substances is developed in a manner that stresses material parameters. Explicit thermodynamic functions are then derived corresponding to both a simple and a realistic equation of state. With this information power cycles are devised and evaluation is made of their thermodynamic efficiencies. An optimum regenerative cycle emerges and criteria for its attainment are considered. Finally, a power estimate accounting for thermal irreversibilities predicts the possibility of low specific weight. Using more general conversion techniques than were admitted in the past, the prospect for thermomagnetic and thermoelectrostatic energy conversion is greatly improved.