Volume 28, Issue 2

This paper analyses physical limits of multistage production or consumption of mechanical energy (work) in sequential heat-mechanical operations characterized by finite rates. The benchmark system, where these limits are evaluated, is a cascade of imperfect stages through which a resource fluid flows with a finite rate. Each stage consists of a fluid at flow, an imperfect work generator or consumer and the environment. The problem investigated is that of limiting yield or consumption of work by the fluid that interacts sequentially with the environment in a finite time. A discrete, finite-rate model subsumes irreducible losses of work potential caused by thermal resistances. Dynamic limits on work are obtained which bound one-stage or multistage energy convertors with production or consumption of power. These limits are expressed in terms of classical exergy and a residual minimum of entropy generation. A discrete generalization of classical exergy is found for systems with finite number of imperfect stages and finite holdup times. For this generalized exergy a hysteretic property is valid, meaning a difference between the maximum work delivered from engine mode and the minimum work added to the corresponding heatpump mode of the system.

An attempt has been made in this paper to gather together theoretical results of important investigations of physical, chemical and mechanical properties of solid surfaces. In particular, we focus our attention on fundamental thermodynamical and mechanical quantities such as surface energy and stress which play a determining role in stability of micro- and nano-objects. Then, taking into account the non-autonomy of the surface, we define an extended pressure which includes irreversible effects due to mechanical and chemical processes in the surface and its underlying layer. This permits, more consistently, the proposition of a revisited Laplace formulation leading to a new interpretation of surface stress. Within the frame of homogeneous deformations, our theory now accounts for the aspect ratio and composition of the interfacial region as weighting factors of the variation of surface free energy with deformation of the surface.

Electrical circuits provide good examples of systems which gravitate towards the three possible thermodynamic conditions of equilibrium, metastability or dissipative steady state. Circuits are driven towards any one of the three conditions simply by increasing total entropy or decreasing available energy. The thermodynamic condition for steady state conduction in a number of simple LCR and hot filament circuits is found to be 1/( ds tot / dψ ) = 0 or 1/( de avail / dψ ) = 0 where s tot and e avail are the total entropy and available energy of the system and its surroundings, and ψ is any single valued variable which describes the condition or state of the total system. This is the inverse of the equilibrium and metastable conditions. It holds for all systems examined, whereas the Onsager-Prigogine principle, ds tot / dt = 0 at steady state, holds only in one case. This shows that the principle is not the causative principle of the steady state. As expected, the analysis also demonstrates that electric circuit theory is fully consistent with the second law of thermodynamics as well as with the first.

Convective boiling in sub-cooled water flowing through a heated channel is essential in many engineering applications where high heat flux needs to be accommodated. It has been customary to represent the heat transfer by the boiling curve, which shows the heat flux versus the wall-minus-saturation temperature difference. However it is a rather complicated problem, and recent revisions of two-phase flow and heat transfer note that calculated values of boiling heat transfer coefficients present many uncertainties. Quite recently, the author has shown that the average thermal gap in the heated channel (the wall temperature minus the average temperature of the coolant) was tightly connected with the thermodynamic efficiency of a theoretical reversible engine placed in this thermal gap. In this work, whereas this correlation is checked again with data taken by General Electric (task III) for water at high pressure, a possible connection between this wall efficiency and the reversible-work theorem is explored.

It is shown that a recently proposed sliplink theory for polymer melts is consistent with the two-generator, non-equilibrium thermodynamics formalism, GENERIC. The model was previously developed using the free energy as a generator for the dynamics. Its primary novelty is that it uses the monomeric density as a dynamic variable instead of as a parameter. By placing it in the GENERIC framework, I also derive an appropriate energy equation necessary for performing non-isothermal flow calculations. Using the algorithm already proposed and the results given here, it is possible to simulate the complete non-isothermal flow of linear, entangled polymeric fluids in complex geometries.