Volume 26, Issue 4

Recent developments in thermodynamic optimization are reviewed by focusing on the generation of optimal geometric form (shape, structure, topology) in flow systems. The flow configuration is free to vary. The principle that generates geometric form is the pursuit of maximum global performance (e.g., minimum flow resistance, minimum irreversibility) subject to global finiteness constraints (volume, weight, time). The resulting structures constructed in this manner have been named constructal designs. The thought that the same objective and constraints principle accounts for the optimally shaped flow paths that occur in natural systems (animate and inanimate) has been named constructal theory. Examples of large classes of applications are drawn from various sectors of mechanical and civil engineering: the distribution of heat transfer area in power plants, optimal sizing and shaping of flow channels and fins, optimal aspect ratios of heat exchanger core structures, aerodynamic and hydrodynamic shapes, tree-shaped assemblies of convective fins, treeshaped networks for fluid flow and other currents, optimal configurations for streams that undergo bifurcation or pairing, insulated pipe networks for the distribution of hot water and exergy over a fixed territory, and distribution networks for virtually everything that moves in society (goods, currency, information). The principle-based generation of flow geometry unites the thermodynamic optimization developments known in mechanical engineering with lesser known applications in civil engineering and social organization. This review extends thermodynamics, because it shows how thermodynamic principles of design optimization account for the development of optimal configurations in civil engineering and social organization.

Magnetorheological (MR) fluids have the unique ability to change properties from those of a fluid to those of a solid in the presence of a magnetic field. In this paper, we present a phenomenological theory for MR fluids within the framework of extended irreversible thermodynamics (EIT), which relates the dynamics of the fluxes to the form of the nonequilibrium equation of state and introduces the elasticity of MR fluids in a natural way. The Gibbs equation and the entropy inequality are derived to present the energy transfer and dissipative mechanisms. In particular, the viscoelastic behavior of the pre-yield regime and the fluid behavior of the post-yield regime are discussed in depth. Finally, according to the entropy inequality, we propose a mechanical model of MR fluids that can characterize the duplicate phenomena of the fluid-solid transition. The results of a numerical analysis of this model correspond with those of the functional tests of an MR damper.

The thermosolutal instability of a Rivlin-Ericksen fluid in a porous medium is considered in the presence of uniform vertical magnetic field to include the effect of Hall currents. For the case of stationary convection, the stable solute gradient and the magnetic field have stabilizing effects on the system, whereas the Hall currents have a destabilizing effect on the system. The medium permeability has both stabilizing and destabilizing effects on the system depending on the Hall parameter M. The kinematic viscoelasticity has no effect for stationary convection. Also, the dispersion relation is analysed numerically. The stable solute gradient and the magnetic field (and corresponding Hall currents) introduce oscillatory modes in the system, which were nonexistent in their absence. The sufficient conditions for the non-existence of overstability are also obtained.

The dependence of the electrophoretic coefficient (l 21 ) of a polystyrene model colloid on the composition of binary liquid mixtures (alcohol-alcohol and alcohol-water) has been studied. The results show that the presence of water in the liquid phase implies a non-linearity of the l 21 -liquid phase composition curves, while the dependence is linear when alcohol-alcohol mixtures form the liquid phase.