Showing papers on "Heat transfer published in 2003"

Investigation on Convective Heat Transfer and Flow Features of Nanofluids

Abstract: Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.

Temperature dependence of thermal conductivity enhancement for nanofluids

Abstract: Usual heat transfer fluids with suspended ultra fine particles of nanometer size are named as nanofluids, which have opened a new dimension in heat transfer processes. The recent investigations confirm the potential of nanofluids in enhancing heat transfer required for present age technology. The present investigation goes detailed into investigating the increase of thermal conductivity with temperature for nano fluids with water as base fluid and particles of Al 2 O 3 or CuO as suspension material. A temperature oscillation technique is utilized for the measurement of thermal diffusivity and thermal conductivity is calculated from it

Heat transfer handbook

Abstract: Preface. Contributors. 1. Basic Concepts (Allan D. Kraus). 2. Thermophysical Properties of Fluids and Materials (R. T Jacobsen, E. W. Lemmon, S. G. Penoncello, Z. Shan, and N. T. Wright). 3. Conduction Heat Transfer (A. Aziz). 4. Thermal Spreading and Contact Resistances (M. M. Yovanovich and E. E. Marotta). 5. Forced Convection: Internal Flows (Adrian Bejan). 6. Forced Convection: External Flows (Yogendra Joshi and Wataru Nakayama). 7. Natural Convection (Yogesh Jaluria). 8. Thermal Radiation (Michael F. Modest). 9. Boiling (John R. Thome). 10. Condensation (M. A. Kedzierski, J. C. Chato, and T. J. Rabas). 11. Heat Exchangers (Allan D. Kraus). 12. Experimental Methods (Jose L. Lage). 13. Heat Transfer in Electronic Equipment (Avram Bar-Cohen, Abhay A. Watwe, and Ravi S. Prasher). 14. Heat Transfer Enhancement (R. M. Manglik). 15. Porous Media (Adrian Bejan). 16. Heat Pipes (Jay M. Ochterbeck). 17. Heat Transfer in Manufacturing and Materials Processing (Richard N. Smith, C. Haris Doumanidis, and Ranga Pitchumani). 18. Microscale Heat Transfer (Andrew N. Smith and Pamela M. Norris). 19. Direct Contact Heat Transfer (Robert F. Boehm). Author Index. Subject Index. About the CD-ROM.

Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer

Abstract: The present study is to enhance the critical heat flux (CHF) in pool boiling from a flat square heater immersed in nanofluid (water mixed with extremely small amount of nanosized particles). The test results show that the enhancement of CHF was drastic when nanofluid is used as a cooling liquid instead of pure water. The experiment was performed to measure and compare pool boiling curves of pure water and nanofluid at the pressure of 2.89 psia (Tsat=60 °C) using 1×1 cm2 polished copper surfaces as a boiling surface. The tested nanofluid contains alumina (Al2O3) nanoparticles dispersed in distilled and deionized water. Tested concentrations of nanoparticles range from 0 g/l to 0.05 g/l. The measured pool boiling curves of nanofluids saturated at 60 °C have demonstrated that the CHF increases dramatically (∼200% increase) compared to pure water case; however, the nucleate boiling heat transfer coefficients appear to be about the same.

Natural convection of nano-fluids

Abstract: Fluids with nano size solid particles suspended in them have been given the name nano-fluid which in recent studies have shown tremendous promise as heat transfer fluids. However, before suggesting such fluids for applications a thorough knowledge of physical mechanism of heat transfer in such fluids is wanted. The present study deals with one such aspect of natural convection of nano fluids inside horizontal cylinder heated from one end and cooled from the other. An apparently paradoxical behaviour of heat transfer deterioration was observed in the experimental study. Nature of this deterioration and its dependence on parameters such as particle concentration, material of the particles and geometry of the containing cavity have been investigated. The fluid shows characters distinct from that of common slurries.

Evolution of Microchannel Flow Passages-Thermohydraulic Performance and Fabrication Technology

Abstract: This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, the advantages of using microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Single-phase performance for liquids is still expected to be describable by conventional equations; however, the gas flow may...

Heat Transfer Analysis and Modeling of a Parabolic Trough Solar Receiver Implemented in Engineering Equation Solver

Abstract: This report describes the development, validation, and use of a heat transfer model implemented in Engineering Equation Solver. The model determines the performance of a parabolic trough solar collector's linear receiver, also called a heat collector element. All heat transfer and thermodynamic equations, optical properties, and parameters used in the model are discussed. The modeling assumptions and limitations are also discussed, along with recommendations for model improvement.

The Specific heat of matter at low temperatures

Abstract: Basic Concepts and Relations Lattice Specific Heat Electronic Specific Heat Specific Heat of Cryogenic Liquids Specific Heat Anomalies Magnetic Specific Heat Experimental Methods.

Heat Transfer in Friction Stir Welding—Experimental and Numerical Studies

Abstract: In the friction stir welding (FSW) process, heat is generated by friction between the tool and the workpiece. This heat flows into the workpiece as well as the tool. The amount of heat conducted into the workpiece determines the quality of the weld, residual stress and distortion of the workpiece. The amount of the heat that flows to the tool dictates the life of the tool and the capability of the tool for the joining process. In this paper, we formulate the heat transfer of the FSW process into two boundary value problems (BVP)—a steady state BVP for the tool and a transient BVP for the workpiece. To quantify the physical values of the process the temperatures in the workpiece and the tool are measured during FSW. Using the measured transient temperature fields finite element numerical analyses were performed to determine the heat flux generated from the friction to the workpiece and the tool. Detailed temperature distributions in the workpiece and the tool are presented. Discussions relative to the FSW process are then given. In particular, the results show that (1) the majority of the heat generated from the friction, i.e., about 95%, is transferred into the workpiece and only 5% flows into the tool and (2) the fraction of the rate of plastic work dissipated as heat is about 80%.@DOI: 10.1115/1.1537741#

Surface modes for near field thermophotovoltaics

Abstract: Thermal radiative energy transfer between closely spaced surfaces has been analyzed in the past and shown not to obey the laws of classical radiation heat transfer owing to evanescent waves and, more recently, electromagnetic surface modes. We have analyzed the energy transfer between layered media, one of the layers being the thermal source, using a Green’s functions method and the fluctuation-dissipation theorem. Based on the analysis, we propose a structure that can utilize the surface modes to increase the power density and efficiency of low temperature thermophotovoltaic generators.

A 3D, Multiphase, Multicomponent Model of the Cathode and Anode of a PEM Fuel Cell

Abstract: A computational fluid dynamics multiphase model of a proton-exchange membrane ~PEM! fuel cell is presented. The model accounts for three-dimensional transport processes including phase change and heat transfer, and includes the gas-diffusion layers ~GDL! and gas flow channels for both anode and cathode, as well as a cooling channel. Transport of liquid water inside the gas-diffusion layers is modeled using viscous forces and capillary pressure terms. The physics of phase change is accounted for by prescribing local evaporation as a function of the undersaturation and liquid water concentration. Simulations have been performed for fully humidified gases entering the cell. The results show that different competing mechanisms lead to phase change at both anode and cathode sides of the fuel cell. The predicted amount of liquid water depends strongly on the prescribed material properties, particularly the hydraulic permeability of the GDL. Analysis of the simulations at a current density of 1.2 A/cm 2 show that both condensation and evaporation take place within the cathode GDL, whereas condensation prevails throughout the anode, except near the inlet. The three-dimensional distribution of the reactants and products is evident, particularly under the land areas. For the conditions investigated in this paper, the liquid water saturation does not exceed 10% at either anode or cathode side, and increases nonlinearly with current density. The operation of proton-exchange membrane ~PEM! fuel cells depends not only on the effective distribution of air and hydrogen, but also on the maintenance of an adequate cell operating temperature and fully humidified conditions in the membrane. The fully humidified state of the membrane is crucial to ensuring good ionic conductivity and is achieved by judicious water management. Water content is determined by the balance between various water transport mechanisms and water production. The water transport mechanisms are electro-osmotic drag of water ~i.e., motion of water molecules attaching to protons migrating through the membrane from anode to cathode!; back diffusion from the cathode ~due to nonuniform concentration!; and diffusion and convection to/from the air and hydrogen gas streams. Water production depends on the electric current density and phase change. Without control, an imbalance between production and removal rates of water can occur. This can result in either dehydration of the membrane, or flooding of the electrodes, which are both detrimental to performance. A common water management technique relies on the humidification of the air and hydrogen gas streams. At higher current densities, the excess product water is removed by convection via the air stream, and the rate of removal is controlled by adjusting moisture content in concert with pressure drop and temperature in the flow channels. Thermal management is also required to remove the heat produced by the electrochemical reaction in order to prevent drying out of the membrane, which in turn can result not only in reduced performance but also in eventual rupture of the membrane. Thermal management, which is performed via forced convection cooling in larger stacks, is also essential for the control of the water evaporation or condensation rates. The operation of a fuel cell and the resulting water and heat distributions depend on numerous transport phenomena including charge-transport and multicomponent, multiphase flow, and heat transfer in porous media. The complexity and interaction of these processes and the difficulty in making detailed in situ measurements have prompted the development of a number of numerical models. The theoretical framework was laid out in early one-dimensional numerical models of the membrane-electrode. 1-3 A quasi-twodimensional model based on concentrated solution theory was also proposed by Newman and Fuller, 4 and a full two-dimensional model including flow channels but no electrodes was also presented by Nguyen and White. 5 This model was refined in a number of subsequent studies to account for the porous electrodes and interdigitated

Heat-transfer and solidification model of continuous slab casting: CON1D

Abstract: A simple, but comprehensive model of heat transfer and solidification of the continuous casting of steel slabs is described, including phenomena in the mold and spray regions. The model includes a one-dimensional (1-D) transient finite-difference calculation of heat conduction within the solidifying steel shell coupled with two-dimensional (2-D) steady-state heat conduction within the mold wall. The model features a detailed treatment of the interfacial gap between the shell and mold, including mass and momentum balances on the solid and liquid interfacial slag layers, and the effect of oscillation marks. The model predicts the shell thickness, temperature distributions in the mold and shell, thickness of the resolidified and liquid powder layers, heat-flux profiles down the wide and narrow faces, mold water temperature rise, ideal taper of the mold walls, and other related phenomena. The important effect of the nonuniform distribution of superheat is incorporated using the results from previous three-dimensional (3-D) turbulent fluid-flow calculations within the liquid pool. The FORTRAN program CONID has a user-friendly interface and executes in less than 1 minute on a personal computer. Calibration of the model with several different experimental measurements on operating slab casters is presented along with several example applications. In particular, the model demonstrates that the increase in heat flux throughout the mold at higher casting speeds is caused by two combined effects: a thinner interfacial gap near the top of the mold and a thinner shell toward the bottom. This modeling tool can be applied to a wide range of practical problems in continuous casters.

Prediction of temperature distribution and thermal history during friction stir welding: input torque based model

Abstract: A novel three-dimensional thermal model is proposed to study the transient temperature distributions during the friction stir welding of aluminium alloys. The moving heat source engendered by the rotation and linear traverse of the pin tool has been correlated with the actual machine power input. This power, obtained from experimental investigation, has been distributed to the different interfaces formed between the tool and the weldpiece based on the torques generated at different tool surfaces. Temperature dependent properties of the weld material have been used for the finite element based numerical modelling. Good agreement between the simulated temperature profiles and experimental data has been demonstrated. The effects of various heat transfer conditions at the bottom surface of the workpiece, thermal contact conductances at the interface between the workpiece and backing plate and different backing plate materials on the thermal profile in the weld material have also been investigated nume...

Assessment of erosion of the ITER divertor targets during type I ELMs

Abstract: This paper presents the results of a preliminary assessment conducted to estimate the thermal response and erosion lifetime of the ITER divertor targets clad either with carbon-fibre composite or tungsten during type I ELMs. The one-dimensional thermal/erosion model, used for the analyses, is briefly described. It includes all the key surface heat transfer processes such as evaporation, melting, and radiation, and their interaction with the bulk thermal response, and it is based on an implicit finite-difference scheme, which allows for temperature-dependent material properties. The cases analysed clarify the influence of several ELM parameters on the heat transfer and erosion processes at the target (i.e. characteristic plasma ELM energy loss from the pedestal, fraction of the energy reaching the divertor, broadening of the strike-points during ELMs, duration and waveform of the ELM heat load) and design/material parameters (i.e. inclination of the target, type and thickness of the armour material, and for tungsten only, fraction of the melt layer loss). Comparison is made between cases where all ELMs are characterized by the same fixed averaged parameters, and cases where instead the characteristic parameters of each ELM are evaluated in a random fashion by using a standard Monte Carlo technique, based on distributions of some of the variables of interest derived from experiments in today's machines. Although uncertainties rule out providing firm quantitative predictions, the results of this study are useful to illustrate trends. Based on the results, the implications on the design and operation are discussed and priorities are determined for the R&D needed to reduce the remaining uncertainties.

Thermal modeling of porous insertion electrodes

Abstract: Isothermal calorimetry was performed on Li|LiPF 6 in ethylene carbonate:dimethyl carbonate|LiAl 0.2 Mn 1.8 O 4-δ F 0.2 cells. The measured rate of heat generation varied substantially with time. To understand why, we investigated the entropy, irreversible resistance, and heats of mixing. Two methods for computing the heat of mixing, one computational and one analytic, are derived. We demonstrate how the energy balance of Rao and Newman accounts for heat of mixing across electrodes, but neglects heat of mixing within particles and in the electrolyte, which may be of equal magnitude. In general, the magnitude of the heat of mixing, which is the amount of heat released during relaxation after interruption of the current, will be small in materials with transport properties sufficiently high to provide acceptable battery performance, with the possible exception of heat of mixing within the insertion particles if the particle radius is large. Comparing simulations of heat generation to calorimetry measurements reseals that the entropic heat is significant and accounts for much of the variation of the rate of heat generation. The rate of irreversible heat generation is larger when the open-circuit potential varies steeply with lithium concentration, because of diffusion limitations within the solid.