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 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  

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 numerical modeling of natural convection fluid flow and heat transfer in a quarter of gearwheel-shaped heat exchanger is carried out. The heat exchanger is included with internal active square bodies. These bodies have hot and cold temperatures with different thermal arrangements. Three different thermal arrangements are considered and showed with Case A, Case B and Case C. The CuO-water nanofluid is selected as operating fluid. The Koo-Kleinstreuer-Li (KKL) correlation is utilized to estimate the dynamic viscosity and thermal conductivity. In addition, the shapes of nanoparticles are taken account in the analysis. The Rayleigh number, nanoparticle concentration and thermal arrangements... 

In this paper, similarity solution of free convection from inclined permeable wall embedded in a stratified porous medium is investigated. It is assumed that the heated wall has constant temperature and permeability along the wall is variable. Energy equation and Darcy's law as momentum have been used. Due to similarity solution we consider only large wall inclination angle. Effects of mass flux, permeability variation function, and wall inclination angle on thermal and velocity boundary layer thickness and heat transfer at the wall are investigated. Copyright © 2005 by ASME  

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... 

Entropy generation due to heat transfer and fluid friction irreversibility has been investigated in a square cavity subjected to different side wall temperatures for compressible and incompressible natural convection flows. Based on the obtained velocity and temperature values, the distributions of local entropy generation, average entropy generation and average Bejan number are determined and compared for compressible and incompressible regimes. It is found that the entropy generated for compressible flow always is more than incompressible flow. The study is performed for Ra=10 4-10 8, j{cyrillic, ukrainian}=0.01(incompressible regime) and 0.6 (compressible regime), Ge=10 -5 and Pr=0.7  

Purpose: The purpose of this paper is to investigate the natural convection behavior of nanofluids in an enclosure. The enclosure is a 3D capsule with curved boundaries filled with TiO2-water nanofluid. Design/methodology/approach: In this paper, a multiple relaxation times lattice Boltzmann method (MRT-LBM) has been used. Two-component LBM has been conducted to consider the interaction forces between nanoparticles and the base fluid. Findings: Results show that the enhanced Nusselt number (Nu*) increases with the increase in volume fraction of nanoparticles (ϕ) and Ra number and decrease of nanoparticle size (λ). Additionally, the findings indicate that increasing volume fraction beyond a... 

One important heat transfer application in engineering is to predict the flow behavior and heat transfer rate in thin vertical air layers. There are numerous applications in engi- neering where high temperature gradients exist between the slot walls. In such cases, the methods based on simple Boussinesq approximations do not provide reliable predictions. Unfortunately, the compressibility effect in heat transfer rate through thin vertical slots has not been much investigated by the past investigators. In this work, a compressible algorithm is properly developed and utilized to solve compressible natural convection in vertical air layers. The current technique employs discretization equations... 

This note considers the effect of different Darcy numbers on the onset of natural convection in a horizontal, fluid-saturated porous layer with uniform internal heating. It is assumed that the two bounding surfaces are maintained at constant but equal temperatures and that the fluid and porous matrix are in local thermal equilibrium. Linear stability theory is applied to the problem, and numerical solutions obtained using compact fourth order finite differences are presented for all Darcy numbers between Da = 0 (Darcian porous medium) and Da → ∞ (the clear fluid limit). The numerical work is supplemented by an asymptotic analysis for small values Da. © 2007 Elsevier Masson SAS. All rights... 

This article applies a novel non-Boussinesq numerical algorithm to solve the free-convection problem in a wide range of thin to thick vertical cavities subject to different side-wall temperatures. In this regard, the compressible flow equations are solved using a primitive incompressible method. No Boussinesq approximation and low Mach number consideration are included in the formulation. To implement the compressibility effect, the density field is calculated via the equation of state for gas. The temperature gradient is suitably varied to generate different low to high thermobuoyant fields, where the Boussinesq approximation may or may not be valid. Contrary to published works on the thin... 

In this paper a three-dimensional steady state incompressible turbulent air flow is considered in a large single room. The buoyancy affected turbulent air flow is simulated by solving governing equations numerically. The turbulence modeling includes both k - ε and zero-equation models and their results are compared to the experimental data. The paper reviews several aspects such as displacement of radiator system performance, temperature and flow field distribution and comfort. The results show that the best temperature distribution and comfort obtain when radiator is installed under the window and its height be equal to or greater than that of the window. Copyright © 2006 by ASME  

The effect of the corona wind on the natural convection at a rectangular channel was investigated experimentally. The results indicate that the natural convection in the absence of electric/corona wind at obtuse angles outperforms than acute angles and keeps improving by increasing the angle. However, the efficiency of the electric/corona wind at acute angles is higher than obtuse angles. Generally, in the presence of electric/corona wind, heat transfer coefficient was increased. The effect of the electric/corona wind was decreased by raising heat flux. This mainly stems from the fact that the temperature gradient raises the thermal boundary layer and reduces the secondary flow power.... 

Proper orientation of walkways, based on regional winds and local shade, provides good control over walkway ventilation. The architecture of the old town of Sirjan had experimentally orientated the city's walkways in such a manner that the shadows in the walkways produced the maximum possible natural ventilation in this hot and dry town. This study has focused on the optimum design of Sirjan city considering a natural ventilation mechanism. At first, a typical walkway with certain geometric parameters is considered. Then, considering the symmetry of the shadows cast in the walkway, the natural ventilation rate is investigated at 7 different angles and at 15° intervals. The problem is modeled... 

Natural convection in cavity has become one of the classical heat transfer problems with a large volume of research performed both experimentally and numerically. There are several permutations of the cavity problem related to its shape, its boundary conditions, the properties of circulating fluid, etc. Among them, the most interesting one is that of a rectangular cavity, which is maintained at hot and cold temperatures on its opposing side walls while its horizontal walls are thermally insulated. Contrary to the past works, we study the cavity with small to high temperature differences. This produces a wide range of compressibility effects in the cavity, which need to be treated carefully... 

In current research, MAPLE software was utilized to scrutinize the heat transfer of copper–H2O nanomaterial migration over a sheet. Entropy production in existence of magnetic field was scrutinized, and Bejan number was reported as main outputs. Converting PDEs into ODEs was done via similarity transformation, and final ODEs were analyzed via RK4. The influence of different variables, including fraction of nanomaterial and Lorentz force on flow distribution and temperature field, also on surface tension and Nu was demonstrated. Besides, Be and NG were calculated for various ranges of scrutinized variables. © 2020, Akadémiai Kiadó, Budapest, Hungary  

Increasing the heat capacity of heat exchangers is a crucial need for modern devices. The thermal conductivity of the usual fluids and the Nusselt (Nu) number of flows containing such fluids are two bottlenecks in the way of increasing heat delivery in the heat exchangers. For this reason, nanofluids have been introduced. The effect of utilizing a Cu-water nanofluid as a coolant of two hot pipes in a square cavity is investigated numerically with a two-component lattice Boltzmann method. The volume fraction of nanoparticles is assumed to be constant (0.03) while the Richardson (Ri) number varies from 0.02 to 20. Results show that the effectiveness of nanoparticles is better observed in the... 

The temperature distribution in a finned tube bundle of an A-type air cooler is investigated. The heat transfer is governed by natural convection. By using a special steam condenser the effect of steam pressure on temperature fields formed outside the bank of finned tubes is studied. It is found that the distribution is approximately the same at various pressures with minor differences. The effects of two-phase flow on temperature distribution are also considered. All 3D temperature contours show maximum values, which highly depend on steam pressures. Moreover, asymmetric steam feeding and probable fouling inside the tubes are possible causes of contour shapes. The experimental Nusselt... 

Despite great advancement in the lattice Boltzmann method and its application in fluid flow problems, there are still major restrictions in treating either the solution domains with complex boundaries or buoyant flow problems. The past experience shows that the heat equation is a source for instabilities which jeopardizes the stable solution of the lattice Boltzmann method in solving fluid flow problems with heat transfer. The instabilities Increase with increasing buoyant force strength. In this work, we suggest a new approach to overcome the restrictions through implementing the advantages of finite volume method in LBM. In this regard, the lattice Boltzznann equation is incorporated with...