Volume 31, Issue 3

Phase transitions in fluids can be drastically altered by shear flow. Bubble deformation and thermal growth are investigated numerically by means of a three-dimensional volume-of-fluid method. Furthermore, the asymptotic case of nucleation in extremely superheated liquids is treated in the framework of a continuum theory based on the Cahn–Hilliard gradient model. It is found that there is always an interesting distinction of the value π/4 for the flow-induced orientation angle in the case of small shear rates. For a great variety of viscosity and density ratios, the bubble deformation and even the break-up radius as a function of the corresponding capillary number are obtained. The growth process including non-equilibrium contributions is calculated, too. The results indicate that shear is able to completely suppress classical nucleation under certain circumstances.

We show that non-equilibrium thermodynamics theory for surfaces combined with electrochemical impedance spectroscopy can be used to derive the excess surface concentrations of reactants and products of an electrochemical reaction at an electrode. We predict the equivalent circuit for a postulated reaction using this theory, and derive expressions for the excess surface concentrations. The method is illustrated with experimental data for the following hydride reaction to hydrogen at a Zn anode in a molten eutectic mixture of LiCl and KCl at 673 K: The results support a two-step mechanism for hydrogen evolution via the hydrogen atom. We calculate the excess surface concentrations of the hydride ions and the hydrogen atoms at the metal surface, and find that the hydride ions cover a fraction of the surface while the hydrogen atoms are present in large excess. The excess surface concentration of the hydride ions varies largely with the polarized state of the surface, and so does its mean activity coefficient at the surface. The results contribute to a better understanding of the system in question. The method is general and is expected to give similar information for other electrodes.

The heat integral in the entropy equation is separated into heat-absorbing and heat-emitting parts. Defining the effective heat-absorbing and heat-emitting temperatures for the boundary of the system over which the heat flows, we derive a general formula for the real working power in terms of effective temperatures with corresponding entropy generation rates. The method gives an explicit solution to the classical problem of how to determine the correct temperature T _ in each case for the power loss = (temperature T _) × (entropy generation rate σ). For different types of heat exchangers, we derive mathematical presentations for the effective heat-absorbing and heat-emitting temperatures; we thus distinguish the entropy generation rates, or exergy losses, for each fluid flow separately. The mathematical formulae for combined-flow systems are illustrated by a district heating system. As an example of chemical processes, we analyze the entropy generation rate in a combustion chamber, and we solve the problem of the real power loss of isobaric combustion. We also show that an adiabatic reversible combustion of carbon in air chambers could be self-pressurized theoretically, even up to several hundred bar, without compressors – and that the efficiency of the gas turbine process could be the same as when fuel cells are used.

Endoreversible thermodynamics is concerned with reversible sub-systems that are in irreversible interaction with each other. Consequently, endoreversible thermodynamics represents the analogue for discrete systems to the local equilibrium hypothesis in continuum thermodynamics. Here a real cyclic 2-reservoir process is simulated by different endoreversible model processes. Simulation means that the simulating process has the same net heat exchanges, cycle time, power, entropy production, and efficiency as the original one. By introducing process-independent simulation parameters as constraints for the irreversible interaction, a family of comparative endoreversible processes is generated including the simulation of the original process. This procedure allows the process parameters of the family of comparative processes to be compared to those of the original one. The fraction “power of the real process over the maximal power inbetween the comparative family” is introduced as a parameter describing the process excellence.