Volume 42, Issue 2 -

Cross-plane energy transport in aluminum and silicon films pair with presence of minute vacuum gap in between them is investigated. Laser short-pulse heating is introduced in the aluminum film and energy transfer in the films pair is formulated using the Boltzmann equation. Energy exchange between the electron and lattice subsystems is expressed in terms of the electron–phonon coupling. The vacuum gap size is considered to be less than the mean-free path silicon and the Casimir limit is applied to incorporate the thermal radiation contribution to the overall energy transport across the vacuum gap. It is found that ballistic phonon contribution to energy transfer across the vacuum gap is significant and the contribution of the thermal radiation, due to Casimir limit, to energy transfer is small. The vacuum gap size has significant effect on the energy transfer from aluminum film to the silicon film; in which case, increasing vacuum gap size enhances temperature difference across the interface of the vacuum gap.

The conditions of existence of extra mass flux in single-component dissipative nonrelativistic fluids are clarified. By considering Galilean invariance, we show that if total mass flux is equal to total momentum density, then mass, momentum, angular momentum and booster (center of mass) are conserved. However, these conservation laws may be fulfilled also by other means. We show an example of weakly nonlocal hydrodynamics where the conservation laws are satisfied as well although the total mass flux is different from momentum density.

A new version of extended irreversible thermodynamics (EIT) satisfying a Hamiltonian structure is proposed. For pedagogical purpose, the simple problem of linear heat conduction in a rigid body is investigated to illustrate the general framework. In contrast with earlier versions of EIT wherein the heat flux was upgraded to the status of state variables, we select here its conjugate dual and higher order fluxes as new independent variables. Their time–evolution equations are formed of reversible and irreversible terms but they cannot take any arbitrary form. Restrictions are placed on the reversible terms by imposing a Hamiltonian structure while the irreversible contribution is subject to the requirement to satisfy the second law of thermodynamics. Explicit expressions of the temperature and heat flux waves are also derived.

Using the method of adjoint equations described in Ref. [1], we have calculated the maximum thermal efficiencies that are theoretically attainable by free-piston Stirling and Carnot engine generators by considering the work loss due to friction and Joule heat. The net work done by the Carnot cycle is negative even when the duration of heat addition is optimized to give the maximum amount of heat addition, which is the same situation for the Brayton cycle described in our previous paper. For the Stirling cycle, the net work done is positive, and the thermal efficiency is greater than that of the Otto cycle described in our previous paper by a factor of about 2.7–1.4 for compression ratios of 5–30. The Stirling cycle is much better than the Otto, Brayton, and Carnot cycles. We have found that the optimized piston trajectories of the isothermal, isobaric, and adiabatic processes are the same when the compression ratio and the maximum volume of the same working fluid of the three processes are the same, which has facilitated the present analysis because the optimized piston trajectories of the Carnot and Stirling cycles are the same as those of the Brayton and Otto cycles, respectively.

Electric circuits with transient elements can be good examples of systems where non-steady irreversible processes occur; so in the same way as a steady-state energy converter, we use the formal construction of the first-order irreversible thermodynamic to describe the energetics of these circuits. In this case, we propose an isothermal model of two meshes with transient and passive elements, besides containing two voltage sources (which can be functions of time); this is a non-steady energy converter model. Through the Kirchhoff equations, we can write the circuit phenomenological equations. Then, we apply an integral transformation to linearize the dynamic equations and rewrite them in algebraic form, but in the frequency space. However, the same symmetry for steady states appears (cross effects). Thus, we can study the energetic performance of this converter model by means of two parameters: the “force ratio” and the “coupling degree”. Furthermore, it is possible to obtain characteristic functions (dissipation function, power output, efficiency, etc.). They allow us to establish a simple optimal operation regime of this energy converter. As an example, we obtain the converter behavior for the maximum efficient power regime.

The well-known Gouy–Stodola theorem states that a device produces maximum useful power when working reversibly, that is with no entropy production inside the device. This statement then leads to a method of thermodynamic optimization based on entropy production minimization. Exergy destruction (difference between exergy of fuel and exhausts) is also given by entropy production inside the device. Therefore, assessing efficiency of a device by exergy analysis is also based on the Gouy–Stodola theorem. However, assumptions that had led to the Gouy–Stodola theorem are not satisfied in several optimization scenarios, e.g. non-isothermal steady-state fuel cells, where both entropy production minimization and exergy analysis should be used with caution. We demonstrate, using non-equilibrium thermodynamics, a few cases where entropy production minimization and exergy analysis should not be applied.