Volume 46, Issue 2

A numerical simulation of convective instability of a binary fluid with negative Soret effect in a cylindrical thermogravitational column is performed. The general problem statement, including equations of motion and heat/mass transfer with boundary conditions, are written in cylindrical coordinates. This is implemented in order to take into account the impact of the column geometry on the separation process and stability of the convective flow. The calculations for two cylindrical columns are performed in Ansys Fluent 14.5. The used parameters and values of applied temperature differences between the walls correspond to the reported experimental data. The considered binary fluid is an ethanol–water mixture at a concentration ratio of 0.2204/0.7796. At such a composition the mixture exhibits a negative Soret effect (the lighter component, ethanol, is enriched in the cold region). The results of simulation show that the convective flow in the column with a smaller gap between the walls is unstable for all applied temperature differences, while it remains stable in the column with a larger gap.

The importance of studying storage alterations in erythrocytes is highlighted by the need to understand changes that could potentially serve to optimize the storage system. With this aim, the non-equilibrium thermodynamic theory with internal variables was introduced, and perturbing the erythrocyte samples with a harmonic electric field some functions of the theory have been determined varying in the perturbation frequency. A frequency has been noted that acts as a separator element between two states showing a different entropy production above and below this frequency. In stored red blood cells compared to fresh ones, the increase in entropy production measured shows a greater state of disorder in the latter. Further alterations have been highlighted on the surface charge density of the stored erythrocyte membrane and on the speed of anionic kinetics. All these observations highlight the image of membrane structural and functional alterations of the stored erythrocytes and lead to the elaboration of a technique able to correlate a specific perturbation frequency with the aging time of red blood cells.

Traditional equilibrium thermodynamics can only solve a few equilibrium processes composed of continuous stable equilibrium states. However, the vacuum flash evaporation process is a typical unsteady process. The study of non-equilibrium thermodynamics of the two-phase flow model is helpful to improve our understanding of the basic law of the flash evaporation process. Based on the theory of non-equilibrium thermodynamics, the flash chamber in the vacuum flash ice making system was studied in this paper, and the possibility of non-equilibrium steady state evaporation with superheat was obtained. The chemical potential difference between liquid water and water vapor under non-equilibrium steady state conditions was determined, and the corresponding evaporation entropy was calculated. It is shown that the results obtained by equilibrium thermodynamics are only related to the temperature difference, while the results obtained by non-equilibrium thermodynamics are not only related to the temperature difference, but also the state of the evaporation process. This is because non-equilibrium thermodynamics considers the cooling of liquid water and the evaporation of water vapor as a whole, taking into account the interaction between the two processes. However, the traditional equilibrium thermodynamics theory divides the steady state evaporation process into two independent processes and ignores the influence of each other.

A model of a multielement thermoelectric refrigerator with another linear heat transfer law, the linear phenomenological heat transfer law Q∝Δ(1/T)Q\propto \Delta (1/T), is established. The refrigerating capacity and coefficient of performance (COP) are analyzed and optimized. The junction temperature solution equations are derived. The optimum electrical currents and thermal conductance allocation are discussed. The influences of thermoelectric element quantity and refrigerating temperature difference on the optimum performances and optimum electrical currents are analyzed. The results show that different optimization objectives have different requirements for the distribution of electrical current and thermal conductance. The refrigeration capacity is not proportional to the number of thermoelectric elements. It is found that the refrigerating capacity can be achieved only when the number of thermoelectric elements is matched for fixed external heat exchangers. The input electrical current and the allocation of the thermal conductance between the two heat exchangers can be optimized synchronously to achieve maximum refrigerating capacity or maximum COP. Performance is compared with that with a Newtonian heat transfer law. The influences of the Thomson effect are also examined. Performance of the refrigerator with Newtonian heat transfer law is higher than that of the refrigerator with linear phenomenological heat transfer law. The Thomson effect can improve the performance of the refrigerator.

The development of effective active thermal energy storage systems requires an understanding of how electrode geometry affects the electrofreezing process. This study aimed to observe the nucleation behavior of an inorganic phase-change material, CaCl 2 ·6H 2 O, using a DC electric field and various copper electrode geometries. The effects of both the electrode diameter (d=0.5d=0.5 and 0.7 mm) and the tip shape (flat and sharp end surfaces) were investigated. Data analysis was performed to reveal the nucleation temperature, freezing temperature, supercooling degree, supercooling time, and crystallization time period. The copper electrode with the larger diameter was found to result in a higher nucleation temperature, a smaller supercooling degree, faster nucleation, and a shorter crystallization time period. Moreover, changing from a flat tip to a sharp tip decreased the nucleation temperature and increased the supercooling degree. This study showed that the electrode geometry plays an important role in the phase-change behavior of CaCl 2 ·6H 2 O.

This paper establishes a model of an irreversible two-stage combined thermal Brownian refrigerator with an intermediate heat reservoir by combining finite time thermodynamics with non-equilibrium thermodynamics. The model is composed of two irreversible thermal Brownian refrigerators working in series. The combined thermal Brownian refrigerator works among three constant temperature heat reservoirs. There exist finite rate heat transfer processes among heat reservoirs and refrigerators. Considering heat leakage, heat transfer losses, and heat flows via kinetic energy change of particles, expressions of cooling load and the coefficient of performance (COP) are derived. The effects of design parameters on system performance are studied. The optimal performance of the irreversible combined thermal Brownian refrigerator is studied. The cooling load and COP are higher when the temperature of the intermediate heat reservoir is close to that of the bottom heat reservoir. Compared with the single-stage thermal Brownian refrigerator, which works between the heat source and sink with the same temperatures, the cooling load of the combined thermal Brownian refrigerator is greater, whereas the COP is smaller.

Our intention in this article is to investigate entropy optimization in nonlinear mixed convective unsteady magnetohydrodynamic flow of nanomaterials in porous space. An exponentially stretched sheet creates the liquid flow. Nanomaterial is considered electrically conducting. The concentration and energy expressions comprise viscous dissipation, Joule heating, thermophoresis and Brownian motion aspects. Arrhenius activation energy is considered. Computation of entropy generation based upon the second law of thermodynamics is made. Nonlinear partial expressions are obtained via suitable dimensionless variables. Resultant expressions are tackled by the OHAM technique. Features of numerous variables on entropy, temperature, velocity and concentration are graphically visualized. Skin friction and the temperature gradient at the surface are also elaborated. Comparative analysis is deliberated in tabulated form to validate the previously published outcomes. Velocity is reduced significantly via the suction parameter. The entropy rate increases for higher values of Brinkman, Biot and Hartmann numbers.

Thermal excitation of the low dimensional silicon film is introduced and an analytical approach is adopted for the solution of the transport equation. In the analysis, the phonon radiative transport equation is converted into an integral form of the Fredholm equation of the second kind. The analytical approach is extended to include the formulation of thermal stresses for the following cases: (i) stress-free at the edges and (ii) one edge is constrained to have maximum stress while the other edge is set to be stress-free. The analytical and numerical results are evaluated for comparisons. The findings demonstrate that both results are in good agreement. The dimensionless temperature rise at the film mid-thickness becomes sharp for small thickness film. The peak value of thermal stress at the film mid-thickness becomes larger as the film thickness is reduced further. Stress waves generated initially are compressive at the film mid-thickness and they become tensile at both ends of the stress-free film, which becomes more apparent as time increases. Two consecutive compressive and tensile stresses are generated at the mid-thickness of the film as the stress boundary condition is changed to the maximum stress at one edge of the film.