Sharif Digital Repository

The complex nature of the convective terms in the fluid flow governing equations has caused many researchers to work on estimating more accurate advection term magnitudes by employing better approximate expressions. The trend of progress has been to include more physics of flow in the approximations. However, there is no archival evidence that reports the direct implementation of buoyancy as a physical influence in the advection term modeling. In this work, the influence of buoyancy in an efficient advection term expression is investigated in a control volume context with a collocated grid arrangement. The accuracy of the results obtained by both excluding and including the buoyancy term is... 

In this paper, an analytical model is presented for forced convective film condensation inside smooth vertical tubes, in which Prandtl mixing length theory, Van Driest hypothesis and Reynolds analogy are used. Comparison of this model with the available experimental data and three other correlations reported in the literature demonstrates that the results are in satisfactory and close agreement  

The effect of a magnetic field on the fully developed forced convection of Fe3O4 flow inside a copper tube is experimentally and numerically investigated. The flow is assumed to be under uniform heat flux. This study aims to examine the effects of the nanoparticle volume fraction, as well as alternating magnetic field strength and frequency, on the convective heat transfer for different Reynolds numbers. To ensure accuracy, the numerical results are validated by empirical results with similar geometry and boundary conditions. A satisfying agreement was achieved. The results show that the heat transfer increases with increase in alternating magnetic field frequency but... 

In the second part of the paper, we compare the solutions produced in the framework of the conference "Mathematical and numerical aspects of low Mach number flows" organized by INRIA and MAB in Porquerolles, June 2004, to the reference solutions described in Part 1. We make some recommendations on how to produce good quality solutions, and list a number of pitfalls to be avoided. © EDP Sciences, SMAI 2005  

In this paper, with the aim of enhancing the thermal conductivity of the fluid, a nanofluid is prepared based on SiO2. A series of experimental tests were carried out for both laminar and forced convection regimes in a horizontal tube with two different geometric shapes (circular and square cross section) subjected to constant wall heat flux (4735 W m−2). A comparative study has been done to investigate the effect of the geometry on the convective heat transfer. Moreover, the effect of the volume concentration on the behavior of the nanofluid and the base fluid was evaluated by comparing various volume concentrations (0.05, 0.07 and 0.2%). The experiments were done under two different... 

In this paper, with the aim of enhancing the thermal conductivity of the fluid, a nanofluid is prepared based on SiO2. A series of experimental tests were carried out for both laminar and forced convection regimes in a horizontal tube with two different geometric shapes (circular and square cross section) subjected to constant wall heat flux (4735 W m−2). A comparative study has been done to investigate the effect of the geometry on the convective heat transfer. Moreover, the effect of the volume concentration on the behavior of the nanofluid and the base fluid was evaluated by comparing various volume concentrations (0.05, 0.07 and 0.2%). The experiments were done under two different... 

This work presents an experimental study of the effect of a magnetic field on laminar forced convection of a ferrofluid flowing in a tube filled with permeable material. The walls of the tube are subjected to a uniform heat flux and the permeable bed consists of uniform spheres of 3-mm diameter. The ferrofluid synthesis is based on reacting iron (II) and iron (III) in an aqueous ammonia solution to form magnetite, Fe3O4. The magnetite is mixed with aqueous tetra methyl ammonium hydroxide, (CH3) 4NOH, solution. The dependency of the pressure drop on the volume fraction, and comparison of the pressure drop and the temperature distribution of the tube wall is studied. Also comparison of the... 

In this work, the thermal and hydrodynamic performance of ferromagnetic fluid, which flows through a copper tube in thermal entrance region, has been studied. The flow in the tube is laminar and subjected to constant heat flux. A part of the tube contains a porous medium with paramagnetic properties and porosity of 0.46. Ferrofluid is composed of Fe3o4 and water with (CH3)4NOH as a surfactant that is prepared in three different volume fractions. The effects of constant and oscillating magnetic fields on convective heat transfer coefficient were examined for various Reynolds numbers, frequencies and volume fractions. The results show that the maximum enhancements of average heat transfer... 

Concerning the geometrical effect of inner cylindrical hot pins, the natural convective heat transfer of nanofluid in a homogenous porous medium in a squared enclosure is numerically studied, using lattice Boltzmann method (LBM). In order to investigate the arrangement of inner cylinders for better heat transfer performance, five different configurations (including one, three, and four pins) were compared, while the total heat transfer area of inner pins were held fixed. Squared cavity walls and inner cylinder's surfaces were constantly held at cold and warm temperatures, respectively. In our simulation, Brinkman and Forchheimerextended Darcy models were utilized for isothermal... 

The Elder problem is one of the well-known examples of an unstable density-driven flow (DDF) and solute transport in porous media. The goal of this research is to investigate the influence of fracture networks on this benchmark problem due to the great importance of the fractured heterogeneity effect on unstable DDF. For this aim, the fractured Elder problem is solved using COMSOL Multiphysics, which is a finite element method simulator. Uniform and orthogonal fracture networks are embedded to analyze free convective flow and development of unstable salt plumes. The results indicate that the mesh sensitivity of the fractured Elder problem is greater than the homogeneous case. Furthermore, it... 

Hybrid nanofluid gains attention of scientists due to its dynamic properties in various fields, and thus, hybrid nanofluids can be taken as an innovative form of nanofluids. Even though analysts acquire tremendous results in the field of hybrid nanofluids but yet no study has been carried out to predict magnetohydrodynamic effects in such fluid models. In this present analysis, influence of MHD has been investigated for the micro hybrid nanofluid over a stretched surface under convective conditions. Combine boundary layer equations for the flow have been altered into a suitable form via boundary layer approximations. Further, complete nonlinear system of equations has been numerically solved... 

This paper reports the results of a study of the transient removal of contaminant particle from enclosures containing an obstacle. We study specially a phenomena occur sometimes called the step effect. This phenomenon may occur if the size of the obstacle is small enough in comparison with the length or height of the enclosure. These results are the basic instruments for finding a model for contaminant particle removal from an enclosure containing an obstacle. A numerical CFD code is developed and validated with different cases, and then proper two- and three-dimensional cases are modeled. The size of the obstacle affect the order of magnitude of the convection-diffusion terms in the... 

This research study presents an experimental investigation on forced convection heat transfer of an aqueous ferrofluid flow passing through a circular copper tube in the presence of an alternating magnetic field. The flow passes through the tube under a uniform heat flux and laminar flow conditions. The primary objective was to intensify the particle migration and disturbance of the boundary layer by utilizing the magnetic field effect on the nanoparticles for more heat transfer enhancement. Complicated convection regimes caused by interactions between magnetic nanoparticles under various conditions were studied. The process of heat transfer was examined with different volume concentrations... 

This paper deals with the mixed convective heat transfer of nanofluids through a concentric vertical annulus. Because of the non-adherence of the fluid-solid interface in the presence of nanoparticle migrations, known as slip condition, the Navier's slip boundary condition was considered at the pipe walls. The employed model for nanofluid includes the modified two-component four-equation non-homogeneous equilibrium model that fully accounts for the effects of nanoparticles volume fraction distribution. Assuming the fully developed flow and heat transfer, the basic partial differential equations including continuity, momentum, and energy equations have been reduced to two-point ordinary... 

This is a theoretical study that extends a classical method of treating the convection heat transfer in complex geometries to gaseous slip flow forced convection in microchannels with H1 thermal boundary condition. Through this line, the momentum and energy equations in cylindrical coordinates are made dimensionless. Afterward, solutions are presented that exactly satisfy the dimensionless differential equations along with the symmetry condition and finiteness of the flow parameter at the origin. The first-order slip boundary conditions are then applied to the solution utilizing the least squares matching method. Though the method is general enough to be applied to almost any arbitrary cross... 

This is a theoretical study dealing with longitudinal gaseous slip flow forced convection between a periodic bunch of microcylinders arranged in regular array. The selected geometry has applications in microscale pin fin heat sinks used for cooling of microchips. The flow is considered to be hydrodynamically and thermally fully developed. The two axially constant heat flux boundary conditions of H1 and H2 are considered in the analysis. The velocity and temperature discontinuities at the boundary are incorporated into the solutions using the first order slip boundary conditions. The method considered is mainly analytical in which the governing equations and three of the boundary conditions... 

Both experimental and numerical studies are unanimous for enhancing Nusselt number for forced convection of nanofluids with slight difference, but there is inconsistency for natural convection heat transfer of nanofluids. In this paper attempt is made to study the effects of nanoparticles migration on the natural convection behavior of nanofluids. For analysis, a mixture model is used by including important phenomena such as Brownian motion and thermophoresis effects. These two mechanisms are taken into account to compute the slip velocities between the base fluid and nanoparticles. The governing equations are solved numerically and good agreements are observed in comparison with... 

Convection heat transfer below a horizontal, hot, and isothermal strip of infinite length and width of 2L embedded in fluids with different Prandtl number (Pr) and Nusselt number (Nu) is analyzed with the aid of integral method. A new concept is utilized to determine the boundary layer thickness at the strip's edge, which is based on matching the flow rate of the boundary layer below the strip at its edge and the flow rate of the plume, which forms after the heated fluid detaches from the strip's edge. In addition to these novelties, a numerical model is developed to verify the analytical framework, and an excellent agreement is observed between the analytical and numerical models  

In this research convective heat transfer coefficient enhancement of nanofluids prepared from high surface area graphene has been investigated in laminar flow in the developing region. The nanofluid has been prepared from nanoporous graphene with high surface area and with concentration of 0.025 to 0.1 wt.%. Deionized water has been used as the base fluid and a type of Ter-Polymer has been utilized as the surfactant. The results indicate that thermal conductivity of nanofluid with concentration of 0.1 wt.% remains quite constant, with only 3.8% enhancement, while convective heat transfer coefficient improves significantly, with 34% enhancement. The behavior of this enhancement related to the...