Begell House Inc.
Heat Transfer Research
HTR
1064-2285
47
8
2016
USING A THREE-EQUATION MODEL FOR PIPE FLOW WITH HEAT TRANSFER
701-706
Khalid
Alammar
Mechanical Engineering Department, King Saud University, Riyadh, Kingdom of Saudi Arabia
This study is aimed at evaluating a three-equation average turbulence model applied to flow and heat transfer through a pipe. Uncertainty is approximated by comparing with published direct numerical simulation results for fully developed pipe flow. The model is based on the Reynolds-averaged Navier–Stokes equations. The Boussinesq hypothesis is invoked for determining the Reynolds stresses. The solution is obtained for three local length scales based on which the eddy viscosity is calculated. There are only two constants in the model: one accounts for the surface roughness and the other is possibly attributed
to the fluid. Error in the mean axial velocity, wall temperature, friction, and heat transfer is found to be negligible.
NANOPARTICLE FRACTION IN AN ANNULUS IN THE JEFFREY FLUID MODEL
707-720
Noreen Sher
Akbar
DBS&H, CEME, National University of Sciences and Technology, Islamabad, Pakistan
Sohail
Nadeem
Department of Mathematics, Quaid-i-Azam University 45320, Islamabad 44000, Pakistan
Peristaltic flow of a nanofluid in an endoscope in the Jeffrey fluid model is analyzed. The flow is modeled in both fixed and wave frame of reference. The peristaltic wave moves with the speed c. The Jeffrey fluid model equations for the nanofluid are derived for the first time in the peristaltic literature. Energy and nanoparticle equations are coupled so that their exact solution is not possible and we have used the homotopy perturbation method (HPM) for temperature and nanoparticle equations, while exact solutions have been calculated for the velocity profile and pressure gradient. The expression of the pressure gradient is quite complicated, therefore to plot the pressure rise and frictional forces, numerical integration has been performed. The effects of various emerging parameters i.e., the amplitude ratio φ, thermophoresis parameter Nt, Jeffrey
fluid parameter l1, and the Brownian motion parameter Nb have been discussed for pressure rise, friction forces, pressure gradient, temperature profile, nanoparticle phenomena, and streamlines.
CHARACTERISTICS OF POOL BOILING ON BRONZE SINTERED POROUS SURFACE TUBES AT SUBATMOSPHERIC PRESSURES
721-732
Qiming
Men
School of Mechanical and Power Engineering, East China University of Science and Technology,
130 Meilong Road, Shanghai 200237, China
Xuesheng
Wang
Key Laboratory of Pressure System and Safety, Ministry of Education, East China University of
Science and Technology, Shanghai 200237, China
Xiangyu
Meng
School of Mechanical and Power Engineering, East China University of Science and Technology,
130 Meilong Road, Shanghai 200237, China
The pool boiling heat transfer performance of plain tubes and bronze sintered porous surface tubes immersed in deionized water were studied experimentally at subatmospheric pressures. Experiments were performed at reduced pressures from 3 to 50 kPa and heat flux values from 16.7 to 50 kW/m2. The heat transfer coefficients on both plain tubes and tubes with bronze sintered porous surface increase with the heat flux and pressure. The relationship between the heat transfer coefficient, heat flux, and pressure is described by equations. At subatmospheric pressures, the porous surface tube enhanced the boiling heat transfer signifi cantly. A factor for enhanced boiling of water on bronze sintered porous surface tubes was determined.
Moreover, the bundle effect at subatmospheric pressures was considered in the present work. The results confirmed
that the bundle effect remains in a vacuum and can improve the heat transfer coefficient on plain tubes as well as on bronze sintered porous surface tubes.
THERMAL CONTACT CONDUCTANCE AS A METHOD OF RECTIFICATION IN BULK MATERIALS
733-744
Robert A.
Sayer
Engineering Sciences Center, Sandia National Laboratories, Albuquerque, NM, USA
A thermal rectifier that utilizes thermal expansion to directionally control interfacial conductance between two contacting surfaces is presented. The device consists of two thermal reservoirs contacting a beam with one rough and one smooth end. When the temperature of reservoir in contact with the smooth surface is raised, a similar temperature rise will occur in the beam, causing it to expand, thus increasing the contact pressure at the rough interface and reducing the interfacial contact
resistance. However, if the temperature of the reservoir in contact with the rough interface is raised, the large contact resistance will prevent a similar temperature rise in the beam. As a result, the contact pressure will be marginally affected and the contact resistance will not change appreciably. Owing to the decreased contact resistance of the first scenario compared
to the second, thermal rectification occurs. A parametric analysis is used to determine optimal device parameters including surface roughness, contact pressure, and device length. Modeling predicts that rectification factors greater than 2 are possible at thermal biases as small as 3 K. Additionally, thin surface coatings are discussed as a method to control the temperature bias at which maximum rectification occurs.
EXPERIMENTAL INVESTIGATION OF OPPOSING MIXED CONVECTION HEAT TRANSFER IN A VERTICAL FLAT CHANNEL IN THE TRANSITION REGION. 2. ANALYSIS OF LOCAL HEAT TRANSFER IN THE CASE OF THE PREVAILING EFFECT OF BUOYANCY AND GENERALIZATION OF DATA
745-751
Robertas
Poskas
Lithuanian Energy institute; Kaunas Univerity of Technology, Kaunas, Lithuania
Arunas
Sirvydas
Lithuanian Energy Institute, Branduolinës inþinerijos problemø laboratorija, Breslaujos str. 3, LT-44403 Kaunas
Gytis
Bartkus
Lithuanian Energy Institute Breslaujos 3 Kaunas, 3035 Lithuania
This paper presents the results of experimental investigation of local opposing mixed convection heat transfer in a vertical flat channel in the transition region. Local heat transfer was analyzed at various air pressures (0.7-1.0 MPa), i.e., under the prevailing effect of buoyancy. Analysis of experimental results showed tendencies in heat transfer at higher pressures similar to those revealed at smaller pressures (0.2–0.4 MPa). But at higher pressures (i.e., when there is a higher buoyancy
effect), the transition from a vortical flow regime to a turbulent one does not cause such a drastic decrease in heat transfer as is the case at lower air pressures. This is due to the signifi cant increase in turbulent heat transfer with increase in the buoyancy effect in the case of a turbulent (nonvortical) flow. Also, it was determined that with increase in buoyancy the Re number also increases, with minimum heat transfer being observed (transition from a vortical to a turbulent flow). A correlation was suggested for calculating this critical Re number (Recr2) in the case of a stabilized flow.
TRANSIENT HEAT CONDUCTION IN WIRES WITH HEAT SOURCES; LUMPED AND DISTRIBUTED SOLUTION TECHNIQUES
753-765
Federico
Scarpa
University of Genoa, DIME/TEC, Division of Thermal Energy and Environmental Conditioning Via All'Opera Pia 15 A, 16145 Genoa, Italy
Mattia
De Rosa
University of Genoa, DIME/TEC, Division of Thermal Energy and Environmental Conditioning Via All'Opera Pia 15 A, 16145 Genoa, Italy
The aim of this study is to test the approximate lumped analysis against the more rigorous distributed parameter approach in the solution of 1D and 2D nonlinear heat conduction problems associated with electrical cables under a current load. The system of partial differential equations governing the distributed parameter model is replaced by a system of ordinary differential equations with total derivatives with respect to time. The number of temperature unknowns is highly reduced and the numerical solution, even though approximate, can be very fast. Furthermore, with the use of the thermal-electrical analogy, the lumped system can be described as an equivalent electric circuit composed of thermal resistances and capacitances properly connected and solved with dedicated software. In this work, the lumped approach is applied to the overheating effect of insulated electric cables under a current load both in a steady and a transient regime. For this application, the lumped methodology can be very useful owing to the presence of several regions of solution domain having negligible internal conductive resistance. The approximate solutions are compared with the distributed parameter approach obtained with a commercial FEM code. An empirical methodology is described for modeling single cables and composite flat cables in such a way as to minimize the difference between the approximate and the more rigorous solution, both in a steady state and in a dynamic regime. The difference can be limited to less than few percent and can be considered fully adequate for industrial design.
EXPERIMENTAL INVESTIGATION OF THE BUBBLE PUMP OF A SINGLE PRESSURE ABSORPTION REFRIGERATION SYSTEM UNDER THE EFFECT OF PRESSURE AND AMMONIA CONCENTRATION
767-780
Keng Wai
Chan
School of Mechanical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
Malcolm
McCulloch
Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK
Bubble pump plays a crucial role in single pressure absorption refrigerators as it drives the systems. Its performance is influenced by parameters such as the tube diameter, pressure, and ammonia concentration. In this study, the flow and performance under the effect of pressure and ammonia concentration were investigated. The increase in ammonia concentration leads to an increase in the size of bubbles and the mass flow rate. The heater temperature is independent of the tube diameter but is influenced by the system pressure and ammonia concentration. The bubble pump performance reduces the decrease in the ammonia concentration and contributes to the increase in the pressure or tube diameter. The timing equations of Chan and McCulloch were verified under various system conditions and dimensions. The average estimation error for the water-based 1-4 bar bubble pump is 12.8%. Meanwhile, the average estimation errors for the ammonia solution bubble pump with 6.5-9.6 mm tube diameter, and with 0.0-0.3 (mass fraction) ammonia concentration are 17.9% and 18.8%, respectively.
EXPERIMENTAL STUDY OF HEAT TRANSFER CONTROL AROUND AN OBSTACLE BY USING A RIB
781-795
Zahra
Ghorbani-Tari
Division of Heat Transfer, Department of Energy Sciences, Lund University, P.O. Box 118, SE-22100, Lund, Sweden
Bengt
Sunden
Division of Heat Transfer, Department of Energy Sciences, Lund University, P.O. Box 118,
SE-22100, Lund, Sweden
This paper investigates the effect of the presence of a rib on the end-wall heat transfer of an obstacle by liquid crystal thermography. An obstacle with a rectangular cross section is placed in a rectangular channel and blocks the entire height of it (AR = 4). A rib with a square cross section is placed at two positions, i.e., in the upstream and downstream areas of the obstacle, respectively. An important parameter in dealing with the control of heat transfer around an obstacle using a rib is the spacing between them. The spacing S, normalized by the spanwise width of the obstacle, has the values 1.25 and 0.625. The effect of the rib height, normalized by the channel hydraulic diameter, e/Dh, is also investigated by considering two values of it, i.e., 0.078 and 0.039, respectively. The results show that the local heat transfer especially in the upstream region of the obstacle is substantially modified by the upstream rib, e/Dh and S/d. The local heat transfer in the downstream region is more affected by the rib height, e/Dh. The local heat transfer in the upstream area of the obstacle is found nearly unaffected by the downstream rib regardless of the rib height e/Dh and S/d. It is found that the local heat transfer in the downstream area of the obstacle is modified differently and it is strongly affected by the rib height e/Dh and S/d. The heat transfer pattern due to the flow reattachment in the downstream area is significantly modified by the rib height e/Dh. The area influenced by the enhancement is found to be more affected by S/d. A larger enhancement area reflected a stronger impact associated with the heat transfer mechanism for e/Dh = 0.078.