Band 44, Heft 2

The knowledge of thermal transport of magnetohydrodynamic (MHD) flows across a stretching sheet plays a crucial role for transportation, fiber coating, heat exchangers, etc. Due to this fact, we scrutinize the heat transfer features of MHD micropolar fluid flow via a stretching surface in the neighborhood of the stagnation point with Joule heating, by taking advantage of the classical Fourier law. The flow equations are transformed into dimensionless form with the help of suitable similarity transformations. The Runge–Kutta-based shooting method is utilized to solve the converted non-linear coupled equations. Impacts of various physical parameters on the flow fields are represented via graphs. The heat transfer rate, couple stress coefficient and friction factor are presented in a separate table. Results anticipate that fluid temperature is an increasing function of Eckert number, radiation and magnetic parameters, whereas an opposite outcome is noticed for the Prandtl number. It is interesting to notice that the maximum velocity is attained in the absence of slip but maximum temperature is detected in the presence of slip.

Transfer of phonons through a silicon–diamond thin film pair with a nano-size gap at the interface is examined. The thin film pair is thermally disturbed by introducing 301 K at the silicon film left edge while keeping the other edges of the thin films at a low temperature (300 K). The radiative phonon transport equation is solved numerically to quantify the phonon intensity distribution in the combined films. The frequency dependent formulation of phonon transport is incorporated in the transient analysis. The thermal boundary resistance is adopted at the interface in the formulations. The near-field radiative heat transfer is also adopted at the gap interface, as the vacuum gap size falls within the Casimir limit. The predictions of thermal conductivity are validated through the thermocouple data. It is observed that predictions of thermal conductivity are in agreement with the experimental data. The ballistic phonons play a major role in energy transfer through the gap; their contribution is more significant than that of the near-field radiative heat transfer. Enlarging the size of the gap reduces the influence of the ballistic phonons on the energy transfer in the films. Increasing the silicon film thickness alters the energy transfer through the gap; in this case, the equivalent equilibrium temperature difference is increased at the interface.

We use the general formulation of irreversible thermodynamics and study the minimally nonlinear irreversible model of heat engines operating between a time-varying hot heat source of finite size and a cold heat reservoir of infinite size. We find the criterion under which the optimized efficiency obtained by this minimally nonlinear irreversible heat engine can reach the reversible efficiency under the tight coupling condition: a condition of no heat leakage between the system and the reservoirs. We assume the rate of heat transfer from the hot to the cold heat reservoir obeys Fourier’s law and discuss physical conditions under which one can obtain the reversible efficiency in a finite time with finite power. We also calculate the efficiency at maximum power for the minimally nonlinear irreversible heat engine under the nontight coupling condition.

Semi-clathrate hydrates are members of the class of clathrate compounds. In comparison with clathrate hydrates, where the networks are formed only by H 2 O molecules, the networks of semi-clathrate hydrates are formed by mixtures of H 2 O and quaternary ammonium salts (QASs). The addition of QASs to the solution enables to improve the formation of semi-clathrate hydrates at much milder conditions comparing to clathrate hydrates. In this work, we study the phase equilibria of semi-clathrate hydrates of CH 4 , CO 2 , and N 2 gas in an aqueous solution of tetra-n-butyl ammonium fluoride (TBAF). An extension of the Chen–Guo model is proposed as a thermodynamic model. The Peng–Robinson equation of state (PREOS) was applied to calculate the fugacity of the gas phase and in order to determine the water activity in the presence of TBAF, a correlation between the system temperature, the TBAF mass fraction, and the nature of the guest molecules has been used. These equations were solved simultaneously and through optimizing tuning parameters via the Nelder–Mead simplex algorithm. The results are compared to experimental data and good agreement is observed.

Relativistic kinetic theory is applied to the study of the balance equations for relativistic multicomponent mixtures, comparing the approaches corresponding to Eckart’s and Landau–Lifshitz’s frames. It is shown that the concept of particle velocity relative to the center of mass of the fluid is essential to establish the structure of the energy-momentum tensor in both cases. Different operational definitions of the center of mass velocity lead either to the inclusion of heat in the energy-momentum tensor (particle/Eckart frame) or to strictly relativistic contributions to the diffusion fluxes (energy/Landau–Lifshitz frame). The results here obtained are discussed emphasizing the physical features regarding each approach.

From the point of view of finite time thermodynamics, the performance boundaries of thermal machines are considered, taking into account the irreversibility of the heat exchange processes of the working fluid with hot and cold sources. It is shown how the kinetics of heat exchange affects the shape of the optimal cycle of a heat engine and its performance, with a focus on the energy conversion efficiency in the maximum power mode. This energy conversion efficiency can depend only on the ratio of the heat transfer coefficients to the sources or not depend on them at all. A class of kinetic functions corresponding to “natural” requirements is introduced and it is shown that for any kinetics from this class the optimal cycle consists of two isotherms and two adiabats, not only for the maximum power problem, but also for the problem of maximum energy conversion efficiency at a given power. Examples are given for calculating the parameters of the optimal cycle for the case when the heat transfer coefficient to the cold source is arbitrarily large and for kinetics in the form of a Fourier law.

Porous cellulose acetate (CA) flat sheet membranes containing 8-hydroxyquinoline (as complexing agent) coated with polyethylene glycol (PEG)/TiO 2 nanocomposite solution were fabricated. For the removal of Pb(II) ions from aqueous solutions, retention data were measured experimentally. To analyze and examine the practical effect of the coating layer on the rejection of metal ions, a quantum chemical calculation and modeling approach was employed. The simulations were conducted on (1) coating layers and (2) a substrate layer. In each simulation, (i) the lowest energy sites on layers were searched against their affinity for metal ions by performing Metropolis Monte Carlo and configurational bias Monte Carlo search of the ion–membrane configurational space according to a simulated annealing schedule, and (ii) the evolution of ions in each layer was examined by the mean square displacement through molecular dynamics simulations. The coating layer showed a potential to significantly interact with ions with higher energy requirements for the detachment from the coating layer in comparison to the substrate layer. This suggested that the metal ions would increasingly accumulate on the coating layer and its lateral surface. The mean square displacement data showed smaller values in the coating layer rather than in the substrate layer, indicating that the coating layer efficiently inhibits ion transfer and provides higher retention values. Thus, greater metal ion retention is expected on and around the coating layer. This is in agreement with the determined attachment and detachment energies. Finally, acceptable agreement was found between numerical and experimental results.

In the present study we aim to model and examine the impact of melting and double stratification on MHD flow of Carreau liquid over a stretching sheet. Incompressible non-Newtonian liquid is electrically conducting and exposed to a constant magnetic field. The exponential space dependent internal heat source effect is incorporated in the energy expression. Mass transfer is characterized in terms of activation energy and a binary chemical reaction. In addition, Soret and Dufour phenomena are included. The boundary layer concept is utilized to simplify the governing equations. A system of ordinary differential equations is obtained through proper transformations. The coupled non-linear system is then computed by the NDSolve technique. Plots and tabulated values are arranged for interpretation of different variables. The obtained results show that the flow field is influenced appreciably by melting, the heat source, the reaction rate and the activation energy. Moreover, Nusselt and Sherwood numbers are increased by the activation energy.