Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
17
3
2010
Study of Forced Convection in a Pipe Partially Filled with a Porous Medium
205-221
10.1615/JEnhHeatTransf.v17.i3.10
Hossein
Shokouhmand
University of Tehran
Habib-Olah
Sayehvand
Mechanical Engineering Department, Engineering Faculty of Bu-Ali Sina University, Hamedan, Iran
forced convection
porous medium thickness
pressure drop
Nusselt number
The objective of the present work was to perform a detailed numerical study of laminar fully developed forced convection in a pipe partially filled with a porous medium. The Brinkman-Forchheimer-extended Darcy model is adopted for the porous matrix region, which is attached to the pipe wall and extends inward, toward the centerline. The effects of the Darcy number, Da, the thickness of a porous layer, S, and the thermal conductivity ratio, Rk, are
investigated. The results showed that as the thickness of the porous layer increases or the Darcy number decreases, the pressure drop grows. It was found that the dependence of the Nusselt number Nu on S is not straightforward. For a high Rk, the Nusselt number increases monotonically with increasing S while for a low Rk a minimum exists in the distributions. The obtained results show that increasing either the Darcy number or the thermal conductivity of the porous material improves the convective heat transfer. Further, for highly permeable and conducting porous media, it may not be necessary to fill the pipe cross section completely to attain maximum heat transfer.
Heat Transfer and Flow Characteristics of Fluid Jets Impinging on a Surface with Cavities
223-229
10.1615/JEnhHeatTransf.v17.i3.20
Tilak T.
Chandratilleke
Department of Mechanical Engineering, Curtin University, GPO Box U1987, Perth WA6845, Australia
Andrew
King
Curtin University of Technology
R.
Narayanaswamy
Department of Mechanical Engineering, Curtin University of Technology, GPO Box U 1987, Perth, Western Australia
jet impingment
cavity
heat transfer enhancement
This paper examines the thermal enhancement potential of a novel technique for cooling a heated surface by fluid jet impingement. Instead of using a conventional flat impinging surface, the fluid jet is directed into a coaxial cylindrical cavity embedded in the heated surface. A numerical study is performed to obtain the heat and fluid flow characteristics and to examine the parametric influence arising from the modified surface geometry. Comparisons are made with the thermal performance of conventional fluid jet impingement at a smooth heated surface. The results indicate a several-fold increase in heat transfer due to the presence of a cavity beneath the jet. It is noted that the degree of enhancement is primarily dependent on the cavity depth and jet Reynolds number.
Numerical Study on Heat Transfer Enhancement in a Mist/Air Impingement Jet
231-242
10.1615/JEnhHeatTransf.v17.i3.30
Hossein
Shokouhmand
University of Tehran
Mohammad Mahdi
Heyhat
School of Mechanical Engineering, University College of Engineering, University of Tehran, Tehran, Iran
heat transfer enhancement
jet impingement
mist/air
two-phase
numerical study
In this paper, a laminar mist/air slot jet impingement has been considered to enhance the heat transfer rate from an isothermal target flat plate surface. The droplets are modeled as heat sinks, dispersed in the continuous gas phase and the governing equations for two-phase gas–vapor-droplet flow have been solved numerically. The effect of thermal and flow parameters such as the Reynolds number, target temperature, droplet size, and mass concentration of the liquid phase on local heat transfer rate of the region near impingement has been determined. The results obtained indicate that by adding a relatively low mass concentration of liquid droplets, the rate of heat transfer in mist/air impingement jet enhances drastically.
Heat Transfer and Pressure Drop Characteristics of Fin-Tube Heat Exchangers with Different Types of Vortex Generator Configurations
243-256
10.1615/JEnhHeatTransf.v17.i3.40
Levent
Bilir
Department of Mechanical Engineering, Izmir Institute of Technology, Urla, 35430, Izmir, Turkey
Baris
Ozerdem
Department of Mechanical Engineering, Izmir Institute of Technology, Urla, 35430, Izmir, Turkey; Department of Energy Systems Engineering, Bahcesehir University, Besiktas 34353 Istanbul, Turkey
Aytunc
Erek
Mechanical Engineering Department, Dokuz Eylul University, 35397 Buca/Izmir, Turkey
Zafer
Ilken
Department of Mechanical Engineering, Izmir Institute of Technology, Urla, 35430, Izmir, Turkey
heat transfer
computational fluid dynamics
heat exchangers
vortex generator
A fin-and-tube heat exchanger with three different types of vortex generators is investigated in this study in order to observe the effects of these vortex generators on heat transfer and pressure drop characteristics. The individual as well as the cumulative influences of the vortex generators on the performance of a heat exchanger are examined. The numerical analyses are performed using a computational fluid dynamics (CFD) program named "Fluent". Firstly, each vortex generator type is placed at four different locations on the fin to determine its best location in terms of heat transfer and pressure drop values. After the determination of the best location on the fin for a vortex generator of each type, two different models with all three types of vortex generators are created and analyzed numerically. The investigation of the cumulative effect of three different vortex generators is the novelty of the study. The results of the study show that the use of three different vortex generators together increases heat transfer rate with a moderate increase in pressure drop value. The comparison of the present study results with an experimental and numerical study showed also a good agreement.
A Three-Dimensional Analytical Model for the Effective Thermal Conductivity and Porosity in High-Porosity Metal Foam Using a Nonisotropic Unit Cell
257-270
10.1615/JEnhHeatTransf.v17.i3.50
Norollah
kasiri
CAPE Center, School of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran
M.
Haghighi
CAPE Center, School of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran
effective thermal conductivity
porosity
porous media
nonisotropic geometry
A comprehensive and precise analytical mesoscale model for effective thermal conductivity and porosity calculation for nonisotropic, open-cell metal foam are developed. These models are extended using an elongated nonisotropic tetrakaidecahedron as a representative unit cell. The unit cell consists of eight hexagons and six quadrilateral faces, with 36 edges and 24 vertices connecting them. The solid lump at the intersection of ligaments is represented as a spherical node and has a key role in effective thermal conductivity calculations. A general description of one unit cell is defined with five independent dimensions consisting of cell edge lengths L and b, inclination angle θ, diameter of fiber ligaments, and spherical node. The effect of porosity on the fiber cross-section shape was taken into consideration. It was found that the change in the cross-section shape affects strongly the porosity and keff. It was also confirmed that porosity increases with increasing b/L and decreasing θ. keff is developed for two domains, high-porosity domain with a hypocycloid cross-section shape of ligaments and low-porosity domain with a circular cross-section shape. For both cases, conductivity decreases with increasing porosity and results are in very good agreement with experimental data. As aforementioned model was developed using a general nonisotropic representative elementary volume, then it is easily extended to a wide range of geometrical characteristics.
Effect of Tube Spacing on Heat Transfer Performance of Staggered Tube Bundles in the Presence of Vortex Generators
271-291
10.1615/JEnhHeatTransf.v17.i3.60
Shaligram
Tiwari
Department of Mechanical Engineering, Indian Institute of Technology Madras,
Chennai, 600036, India
S.
Jayavel
Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai-600036, India; Department of Mechanical Engineering, Indian Institute of Information Technology Design and Manufacturing Kancheepuram, Chennai - 600127, India
staggered tube bundle
vortex generator
longitudinal and transverse tube spacing
enhancement in heat transfer
reduction in pressure drop
Effect of longitudinal and transverse tube spacings has been investigated on the performance
of staggered tube-bundles employed in fin-tube heat exchangers. The three-dimensional numerical
study compares flow and heat transfer characteristics of tube bundles in the staggered
arrangement confined between channel walls and subject to cross flow of air by using a finite
volume-based developed computational code. For a steady laminar flow (ReD = 400), heat
transfer and pressure drop characteristics in the presence of multiple pairs of vortex generators
are presented. For fixed channel confinement, the flow Reynolds number and tube separations
are varied. The objective is to identify the longitudinal and transverse tube separations
of the staggered tube-bundle in the presence of vortex generators which would be associated
with maximum enhancement in heat transfer and minimum pressure drop.