فهرست مطالب afrasiab raisi

In this review article, the thermal performance of dual-tube heat exchangers with smooth walls is investigated in the presence of nanofluids. Important challenges in industrial and engineering processes, such as the failure of thermal devices to respond to higher capacities, conservation, saving, and optimization of energy, have been discussed in recent years. Heat exchangers are one of the types of thermal devices that are used in a wide range of engineering and industrial applications. The use of nanofluids is one of the most effective ways to enhance the thermal conductivity of heat exchangers in the industry. In this research, the types of heat exchangers are first introduced. Then, the methods of heat transfer enhancement (active, passive, and combined) are discussed. The introduction and method of preparing nanofluids are discussed, and finally, the studies on dual-tube heat exchangers in the presence of nanofluids are described. This review article examines previous studies on dual-tube heat exchangers and the use of nanofluids in them (216 references and 73 journals). The purpose of this article is to familiarize the readers with the types of heat exchangers and to understand the mechanisms of heat transfer in the context of using nanofluids in smooth dual-tube heat exchangers. It can be concluded that nanofluids are a very good substitute for other fluids because the use of nanofluids in heat exchangers leads to an improvement in their performance, a reduction in their energy consumption and costs, a decrease in their volume, a reduction in environmental effects, etc. Eventually, the challenges in the use of nanofluids in flat dual-tube heat exchangers are discussed. The most important ones include the economic costs of using nanofluids, deposition and accumulation of nanoparticles over time, stability of nanofluids, and lack of standardization among various researches and evaluations.

In this study, the natural convection heat transfer within a triangular cavity with elastic diagonal walls containing a cylindrical heat source is investigated. The assumed fluid inside the cavity is air. The flexible diagonal walls of the cavity are considered to be at a constant cold temperature of Tc and the cylindrical heat source is at the hot temperature of Th. The governing equations are discretized using Galerkin finite element method and to describe the motion of the fluid, the arbitrary Eulerian-Lagrangian approach is used. In this study, the interaction of fluid and solid fields and the effect of cylindrical heat source position on flow and temperature fields are examined. For this purpose, the effect of Rayleigh number and changing the position of the heat source along the vertical centerline on the deformation of flexible walls, flow and temperature fields, and heat transfer rate are investigated. The results show that for a fixed position of the heat source, an increase in the Rayleigh number increases the maximum of the stream function, the average Nusselt number, and the deformation of the flexible walls. Also, the results show that the position of the heat source depending on the Rayleigh number has different effects on the temperature and flow fields.

In this paper, mixed convection of Bingham fluid between two coaxial cylinders has been studied numerically without using any regularization method. The temperature of the inner rotating circle is higher than the temperature of the outer stationary circle. The finite volume method and non-iterative PISO algorithm have been employed to solve the problem. One of OpenFOAM solver, icoFoam, has been modified for solving the exact Bingham model. After validating the modified solver, it has been used to solve the problem for the following ranges of conditions: Reynolds number, Re=10, Prandtl number, Pr=10, Grashof number, Gr=500, Bingham number, 0≤Bn≤1000, and aspect ratio (AR) of 0.1. The effects of the Bingham number on flow and heat transfer characteristics such as the shape and size of the unyielded regions, streamline contours, the local and mean Nusselt number, and the torque coefficient have been investigated. The mean Nusselt number and the torque coefficient decreases and increases, respectively, when the Bingham number increases. The variation range of the local Nusselt number and dimensionless tangential stress on the inner wall decrease with Bn.

Liquid paraffin as a coolant fluid can be  applied in electronic devices as a result to its suitable capabilities such as electrical insulating, high heat capacity, chemical and thermal stability, and high boiling point. However, the poor thermal conductivity of paraffin has been confined its thermal cooling application. Addition of high conductor nanoparticles to paraffin can fix this drawback properly. In this article, the influence of the nanoparticles on the thermal conductivity of base material was assessed. Temperature (20-50°C) and volume fractions (0-3%) effect on the thermal conductivity of paraffin/alumina nanofluids have been considered. Nanofluid samples were prepared applying the two-step method. The thermal conductivity was measured by a KD2 pro instrument. The results indicated the thermal conductivity augments smoothly with an increase in volume fraction of nanoparticles as well as temperature. Moreover, it observed that for nanofluids with more volume-fraction the temperature affection is more remarkable. Thermal conductivity enhancement (TCE) and effective thermal conductivity (ETC) of the nanofluid was calculated and new correlations were reported to predict the values of them based on the volume fraction of nanoparticles and temperature of nanofluid accurately.

Abstract melting of Cyclohexane-Cu nano-material in a porous square cavity is studied numerically in this paper. At first initial temperature of the cavity is Ti that is equal to melting temperature of nano-material,Tm ,. The horizontal walls are adiabatic. Suddenly the left wall's temperature has changed to Th>Tm . The effective parameters in this case are and which appear in the nondimensionalized equations. Nondimensionalized governing equations are obtained based on the Darcy model; a control volume approach is used for solving these equations. The effect of the variation of mentioned parameters are investigated on the heat transfer rate, fluid flow, isotherms and melting time of nano-PCM. The results show that changing of any parameters will be effective on increase or decrease of heat transfer rate and melting process time. For example variation of has high effect on melt fraction in cavity with time. The results show that melting of PCM is prolonged when nano-particles are added. the increases of the Ra increases the natural convection heat transfer and therefore increases the melting rate, and deforms the melting line.

In this study, the natural convection heat transfer is numerically examined in a square enclosure filled with a non-Newtonia power-law fluid. Two fixed temperature baffles are mounted on the left wall of the enclosure. The left wall of the enclosure and the baffles installed on it, are at a constant temperature of T_h and the right wall of the enclosure is at a constant temperature of T_c, while its horizontal walls are thermally insulated. The governing equations for the power-law fluid flow are solved with the numerical finite difference method based on the control volume formulation and SIMPLE algorithm. The study investigates the effects of relevant parameters such as the Rayleigh number (〖10〗^3≤Ra≤〖10〗^6), the power-law index (0.8≤n≤1.4), the baffles length (0≤B≤0.5) and the baffles distance from each other (0.1≤D≤0.8) on flow and temperature fields and the rate of heat transfer. The results show that an increase in Rayleigh number, particularly when n

In this paper, the result of a numerical study on the natural convection in an inclined T shap cavity filled with Water-Cu nanofluid with the presence of a constant magnetic field was investigated. A heat source embedded on the bottom wall of enclosure, the upper wall is cold and the other walls are adiabatic. Discretization of the governing equations are achieved through a finite volume method and solved with SIMPLE algorithm. The Hartmann number has been varied from 0 to 80 and the cavity has been twisted under the angles between 0 to 90 degrees. The findings of study show that the effect magnetic field on the average Nusselt number is higher in high Reyleigh number. In Ra=105, the increase in nanofluid, to the Hartman number 20, contributes to decrease of the average number and in the Hartman number 40 and more, causes the average Nusselt number to increase. In Ra=106, the increase in nanofluid, to the Hartman number 20, contributes to increase of the average number and in the Hartman number 40 and more, causes the average Nusselt number to decrease. The results also indicate that, the maximum heat transfer, in Ra=105 and Ra=106 accurse at 67.5° angle. the minimum heat transfer, in Ra=105 and Ra=106 accurse at 0° and 22.5° angle respectively.

In this paper، the natural convection of water-Al2O3 nanofluid in a square enclosure exposed to a magnetic field is numerically investigated. The enclosure is bounded by two isothermal vertical walls at different temperaturesof Th and Tc. The two horizontals walls of the enclosure are thermally insulated. A vertical plate (membrane separator) with a negligible thickness and a variable height is located in the middle of the chamber. Discretization of the governing equations are achived through a finit method and are solved using the SIMPLE algorithm. Based on the results of the numerical solution، the effects of the relevant parameters such as the dimensionless height of the membrane separator، Rayleigh number، the solid volume fraction and the Hartmann number on the flow field and the heat transfer rate are investigated. The results show that the heat transfer rate decreases with an increase of the dimensionless height of the membrane separator and an increase of the Hartmann number. The heat transfer rate، however، increases as the Rayleigh number increases. Depending on the Rayleigh number، the thermal performance of the enclosure is either improved or deteriorated as the solid volume fraction is increased.