Band 30, Heft 1

Diffusions of free and adsorbed molecules of subcritical hydrocarbons in activated carbon were investigated to study the influence of adsorbed molecules on both diffusion processes at low pressures. A collision reflection factor, defined as the fraction of molecules undergoing collision to the solid surface over reflection from the surface, is incorporated into Knudsen diffusivity and surface diffusivity in meso/macropores. Since the porous structure of activated carbon is bimodal in nature, the diffusion of adsorbed molecules is contributed by that of weakly adsorbed molecules on the meso/macropore surfaces and that of strongly adsorbed molecules in the small confinement of micropores. The mobility of adsorbed molecules on the meso/macropore surface is characterized by the surface diffusivity D μ 2 , while that in the micropore is characterized by D μ 1 . In our study with subcritical hydrocarbons, we have found that the former increases almost linearly with pressure, while the latter exhibits a sharp increase at a very low-pressure region and then decreases beyond a critical pressure. This critical pressure is identified as a pressure at which the micropores are saturated.

We present linear perturbation analysis of the mass, momentum and energy conservation equations for a compressible fluid to obtain quantitative bounds for the validity of the Boussinesq approximation for a rotating flow. We have used the methodology developed by Gray and Giorgini [11] and generalized it to include the effects of rotation. We consider cases where the axis of rotation is contained in the body of fluid and also those where the axis of rotation is located far away from the region occupied by the fluid.

Thermocapillary convection has been observed both in capillaries and at the surface of evaporating sessile droplets. The present paper aims to give a model of the onset of these convective patterns. The model accounts for the self-induced surface gradient of temperature linked to the stronger mass transfer observed near the triple line region. The role of the apparent contact angle is also evidenced. Dimensionless quantities such as the Marangoni number and the partition coefficient are the key factors of the present analysis. The calculations show that for the same environmental conditions the convection can be influenced by the geometrical considerations. Indeed, the calculated Marangoni numbers are found to be greater in the capillary than in the drop for a wetting contact angle of less than π /4 ; for an angle greater than π /4 , the results are inverted.

A positive definite thermodynamic Lyapunov function, L s , has been defined as the magnitude of excess rate of entropy production. The total time derivative of L s in thermodynamic perturbation space then determines the stability or instability of a process. In the present paper the thermodynamic stability of deformation in visco-elastic fluids has been investigated using the above proposed thermodynamic Lyapunov function and Lyapunov's direct method of stability of motion. For this purpose, in the present case the thermodynamic space based on extended irreversible thermodynamics (EIT) has been used to construct the thermodynamic perturbation space. Three different models of deformation in viscoelastic fluids have been tested for the stability that establishes the regions of asymptotic stability and the stability under constantly acting small disturbances in each case. The case of instability also surfaces out.

While endoreversible heat-to-power conversion systems operating between two heat reservoirs have been intensely studied, systems with several reservoirs have attracted little attention. Here we analyse the maximum power processes of such systems with stationary temperature reservoirs. We find that independent of the number of reservoirs the working fluid uses only two isotherms and two infinitely fast isentropes/adiabats. One surprising result is that there may be reservoirs that are never used. This feature is explained for a simple system with three heat reservoirs.

Crack growths processes of various types are widely covered by power laws. Especially, both creep deformation of creep-resistant bulk materials and creep crack growth in such materials can be covered by power laws and possess close exponents in many cases [1]. This paper focuses on the microscopic thermodynamic mechanisms of the correspondence and the power law itself. In this paper, it is shown that the power laws can be considered as certain homogenous kinetic rate laws of local or microscopic internal variables within the thermodynamic framework of Rice [2, 3] and correspond to a certain macroscopic requirement of maximum dissipation. It is revealed that nonlinear phenomenological equations and Onsager reciprocal relations emerge naturally from the framework if each rate is a monotonic increasing and homogeneous function of the same degree in its conjugate force. The homogeneity property transfers exactly from local internal variables to global internal variables. On the basis of the remarkable properties, it is shown that the power laws of crack growth directly lead to the refined Griffith criterion by Rice [4], and both exponents of creep deformation and creep crack growth can be related by a simple linear relation.