Begell House Inc.
Heat Transfer Research
HTR
1064-2285
41
3
2010
Preface Special Issue dedicated to the International Symposium on Convective Heat and Mass Transfer in Sustainable Energy (CONV-09)
207-208
This special issue includes 9 papers selected from those presented at the International Symposium on Convective Heat and Mass Transfer in Sustainable Energy (CONV-09), held in Hammamet, Tunisia, April 26-May 1, 2009 and organized by the International Centre for Heat and Mass Transfer (ICHMT).
A total of 151 participants have attended the Symposium, including 66 Ph.D. students and 85 regular delegates. This makes the Symposium a successful one, as the objective of the Symposium is to bring together young scientists, researchers, and well-known experts in the field of heat and mass transfer. Distribution of the participants spanned over 30 countries.
8 invited keynote lectures and 113 papers have been presented at the Symposium. The keynote lectures have been delivered by well-known experts in the field: Professors Hans Muller-Steinhagen, Mohammed El Genk, Sadik Kakaç, Christophe Ménézo, Renato Cotta, Leonard Vasiliev, Leijin Guo, and Martinus Van Genuchten.
Recent and important advances in heat and mass transfer have been presented and discussed during the Symposium. A special focus has been given on the use and conversion of renewable energy, as well as energy production and protection of the environment. Conception and design of systems with low energy consumption are obviously becoming a great challenge for reducing pollution and emission of gases with greenhouse effects. They are a dominant factor for energy sustainability.
Lectures and papers presented at CONV-09 Symposium have included sustainable electricity and water for Europe and Mediterranean countries, convective heat and mass transfer in building and green houses, solar energy conversion and storage, cooling of fuel cells with micro-heat pipes and convection in microchannels. Dispersion of pollutants and their effects on ground and environment have been also discussed in depth. The contributions have highlighted different aspects and applications for electronic cooling, heat exchangers, jets, natural and forced convection as well as phase change and nanofluids.
The selected papers in this issue retain the flavor of the Symposium but have been expanded in some cases to include additional results. A wide range of topics were covered in the selected papers in this issue. It includes, the study of transient conjugated conduction-convection-radiation problems, determination of thermophysical properties of nanofluids, characterization of adsorption in metal hydrides, numerical modeling of steam reformers, and exergy consumption in heat exchangers. Convective heat transfer in solar conversion systems is analyzed in three papers that are concerned with natural convection from a constrained horizontal plate, theoretical and experimental study of a water phase change solar collector and instabilities in mixed convection in a vertical channel. Numerical study of conjugated heat transfer problems in parallel-plate micro-channels (including axial conduction in the fluid and the plate and also the viscous dissipation effects) is also presented here.
Experiments and Simulations in Transient Conjugated Conduction-Convection-Radiation
209-231
Carolina Palma
Naveira-Cotta
Laboratory of Nano- and Microfluidics and Microsystems, LabMEMS,
Mechanical Engineering Department and Nanotechnology Engineering Dept.,
POLI & COPPE, Universidade Federal do Rio de Janeiro, Cidade Universitária,
Cx. Postal 68503, Rio de Janeiro, RJ, CEP 21945-970, Brazil; Mechanical Engineering Department, University College London, UCL, United
Kingdom
Mohammed
Lachi
University of Reims Champagne-Ardenne GRESPI / Faculté des Sciences PB1039, 51687 Reims, France
Mourad
Rebay
University of Reims Champagne-Ardenne GRESPI / Faculte des Sciences PB 1039, 51687 Reims, France
Renato M.
Cotta
Laboratory of Nano- and Microfluidics and Microsystems, LabMEMS,
Mechanical Engineering Department and Nanotechnology Engineering Dept.,
POLI & COPPE, Universidade Federal do Rio de Janeiro, Cidade Universitária,
Cx. Postal 68503, Rio de Janeiro, RJ, CEP 21945-970, Brazil; Interdisciplinary Nucleus for Social Development—NIDES/CT, UFRJ, Brazil;
Mechanical Engineering Department, University College London, UCL, United
Kingdom
Experimental results and hybrid numerical-analytical simulations are critically compared for transient laminar forced convection over flat plates of non-negligible thickness, subjected to an applied wall heat flux at the fluid-solid wall interface. A conjugated conduction-convection-radiation problem is first formulated and then simplified through the employment of the Coupled Integral Equations Approach (CIEA) to reformulate the heat conduction problem on the plate by averaging the related energy equation in the transversal direction. A partial differential formulation for the transversally averaged wall temperature is obtained, and the boundary condition for the fluid in the heat balance at the solid-fluid interface is then rewritten. The coupled partial differential equations within the thermal boundary layer are handled by the Generalized Integral Transform Technique (GITT) under its partial transformation mode, combined with the method of lines implemented in the Mathematica 7.0 routine NDSolve. For the experiments, an apparatus was employed involving an air blower and flash lamps that heat a vertical PVC plate of 33 cm in length and 12 mm in thickness, while the temperature at the surface exposed to the cooling air is measured by infrared thermography. Thermocouple measurements are also utilized to provide estimates of heat losses at the back surface of the plate. The transient evolution of the measured surface temperatures along the plate length are then critically compared against the simulation results in order to verify the proposed model.
Numerical Modeling and Parametric Studies of Steam Reformers
233-245
C.
Ventura
Dep. Eng. Mecanica, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Joao Luis Toste
Azevedo
Technical University of Lisbon; Dep. Eng. Mecanica, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
This paper presents the application of a numerical model that was developed to describe the Steam Reforming Process (SRP model) in a small-scale reformer. The reformer consists of a burner enclosed in a vessel while the reforming reaction occurs in channels with multiple passages installed around the vessel. The SRP model was implemented in the C language and is used as a User Defined Function (UDF) in the commercial program Fluent.
This model is a one-dimensional representation of the reforming channels that surround the burner enclosure and was previously validated against experimental data and a Fluent-based simulation. The burner is simulated with a 2D approximation that was also previously evaluated against both experimental and 3D burner calculations. The reformer and burner model are coupled by the temperature distribution in the dividing wall that is updated by the SRP model.
The present paper presents applications of the burner model and the coupled reformer model. The objective of the study is to scale-up an existing reformer from 5 to 10 kg/day hydrogen production. Keeping the initial dimensions of the burner and doubling the flow rates, the model is used to analyze the position of the radiation shield on temperature and heat flux distribution. The simulations for the coupled model are used to analyze the effects of modifying the catalyst reaction length, position and the global dimensions of the reformer using a geometric scale-up factor.
Numerical Resolution of Conjugate Heat Transfer Problem in a Parallel-Plate Micro-Channel
247-263
Yassine
Kabar
Université de Jijel, Laboratoire d'Energétique Appliquée et des Matériaux, Faculté des Sciences et de la Technologie, BP. 96, 18000 Jijel, Algeria
Mourad
Rebay
University of Reims Champagne-Ardenne GRESPI / Faculte des Sciences PB 1039, 51687 Reims, France
Mahfoud
Kadja
Constantinel University, Laboratory of Applied Energetics and Pollution, Faculty of Technology Sciences, Department of Mechanical Engineering, Constantine 25000, Algeria
Colette
Padet
University of Reims Champagne-Ardenne, GRESPI/Laboratoire de Thermomécanique, Faculté des Sciences PB1039, 51687 Reims, France
The present paper deals with the characterization of the conjugated two-dimensional steady-state heat transfer problem in two parallel-plate micro-channel heat sinks. The fluid is assumed to be incompressible and with constant properties. Simultaneous hydrodynamic and thermal developing region is taken into consideration here. Axial conduction is also taken into account. An analysis is performed for constant wall temperature at the outer surfaces of the plates. The heat wave generated at these surfaces is crossing through the plates to reach the interface with the fluid in the micro-channel. Due to the fact that channel height (H) is of the same order of dimension of the plate thickness (E) in the micro-channel, the conduction in the plate cannot be assumed negligible. Therefore, the convective heat transfer in a micro-channel is conjugated with the conduction in the solid plates. The two-dimensional Navier-Stokes equations and the energy equation are solved by the finite-control-volume method. Detailed temperature profiles in the fluid and the solid, the fluid bulk temperature and the heat flux distributions on the fluid-solid interface are provided. The effects of the plate thickness and the solid to fluid thermal conductivities ratio (K = ks/kf) are studied for a water flow with a Reynolds number Re = 100. The results of different simulations are analyzed, and the axial distributions of the Nusselt number are deduced for each case. The results show that viscous heating of the fluid can significantly influence the heat transfer in the micro-channel heat sink. The thickness plate (E) may have an influence on both thermal developing length and the asymptotic Nusselt number value which corresponds to the fully developed flow. The bulk fluid temperature is shown to vary in a nonlinear form along the flow direction.
Minimum of Exergy Consumption in a Horizontal Fluidized Heat Exchanger
265-282
Artur
Poswiata
Warsaw University of Technology, Faculty of Chemical and Process Engineering, Poland
Z.
Szwast
Warsaw University of Technology, Faculty of Chemical and Process Engineering, Poland
The paper presents a methodology and results of calculation for optimization of a horizontal fluidized-bed heat exchanger. In the fluidized bed, fine solid particles are heated by flowing hot gas. The hydrodynamics of the bed and heat transfer kinetics are described by the Kunii and Levenspiel two-phase model. As the performance index the function describing the total cost of the process is applied. The process costs are expressed in exergy units. The continuous optimization algorithm of Maximum Principle was used for calculation optimization. For the considered problem, the profile of the inlet gas temperature as well as the inlet gas total flow are searched. The analytical solution describing the temperature optimal profiles for various hydrodynamic, kinetic, and economic parameters was obtained. The results of calculations were described and discussed.
Theoretical and Experimental Study of a Water Phase-Change Solar Collector
283-297
Ababacar
Thiam
Laboratoire d'Energétique Appliquée, Ecole Supérieure Polytechnique, Université Cheikh Anta Diop, BP 5085, Dakar, Sénégal
Youssouf
Mandiang
Laboratoire d'Energétique Appliquée, Ecole Supérieure Polytechnique, Université Cheikh Anta Diop, BP 5085, Dakar, Sénégal
Vincent
Sambou
Laboratoire d'Energétique Appliquée, Ecole Supérieure Polytechnique, Université Cheikh Anta Diop, BP 5085, Dakar, Sénégal
Dorothe
Azilinon
Laboratoire d'Energétique Appliquée, Ecole Supérieure Polytechnique, Université Cheikh Anta Diop, BP 5085, Dakar, Sénégal
Mamadou
Adj
Laboratoire d'Energétique Appliquée (LEA), Ecole Supérieure Polytechnique de l'université Cheikh Anta Diop de Dakar (UCAD), Sénégal
This work presents the theoretical and experimental study of a water phase-change solar collector. This solar collector is comprised of two contiguous sections of which one is filled with paraffin (40−42)°C and the other with water; this type of solar collector uses the combined storage of heat by water and paraffin. With a low thermal conductivity, the performances of solid paraffin are improved considerably by its semi-transparency in the vicinity of the melting point (40−42)°C, it becomes transparent in the liquid phase and while becoming solid during the night, it limits the thermal losses of stored water, thus water is preserved at the melting point of paraffin (40−42)°C. The experimental results show that the average water temperature reached 48°C at 5 p.m. and the thermal efficiency reaches 50% during four operating hours. A one-dimensional mathematical model based on the enthalpy formulation is applied to the system. It correctly describes the performance of the solar collector by predicting the changes of the temperature in paraffin and stored water. This model takes into account natural convection in molten paraffin. A water phase-change solar collector has a rate of recovery of about 10% to 50% which is higher than that of a slab of about 10% to 15%.
Experiments on Natural Convection Heat Transfer from an Isothermal Plate on a Semi-Infinite Surface and from a Constrained Horizontal Plate in Air
299-311
Janusz
Wojtkowiak
Poznan University of Technology, Institute of Environmental Engineering, 60965 Poznan, Piotrowo 3A, Poland
Czeslaw
Oleskowicz-Popiel
Institute of Environmental Engineering, Poznan University of Technology, 60 965 Poznan, Poland
Measurements were performed to determine the average natural convection heat transfer characteristics for an isothermal, upward-facing horizontal constrained rectangular plate, i.e., surrounded (fenced) by vertical adiabatic walls. Results revealed that for the Rayleigh number ranging from Ra = 1.8 × 106 to 1.5 × 107 and relative height of the walls in the range from about H/W = 0.25 to 1.0 the average Nusselt number is described by the equation Nu = 0.458 Ra0.271. These results are higher in comparison with the data obtained for an isothermal horizontal plate on a semi-infinite surface by about 15 to 18% in a laminar region and 12 to 11% in a turbulent region. The aspect ratio of the rectangular plate was Y/W = 2.548.
Pitchfork Bifurcation of the Mixed Convection in a Vertical Channel
313-323
Omar
Kholai
Department of Mechanical Engineering, University of Mentouri Constantine, Constantine, 25000, Algeria
Saadoun
Boudebous
Department of Mechanical Engineering, University of Mentouri Constantine, Constantine, 25000, Algeria
Zoubir
Nemouchi
Laboratoire d'Energetique Appliquee et de Pollution, Université des Frèses Mentouri, Constantine, Algeria
Mourad
Rebay
University of Reims Champagne-Ardenne GRESPI / Faculte des Sciences PB 1039, 51687 Reims, France
The steady laminar mixed convection in a two-dimensional vertical channel with locally and symmetrically heated walls is studied numerically. The fluid flows downward through the channel. The walls are thermally insulated in the upstream part, maintained at a constant temperature in the heated middle part, and thermally insulated in the downstream part. The present contribution deals, in particular, with the bifurcation from a symmetric to an asymmetric solution in such opposed flow conditions. In fact, the symmetry breaking occurs at a value of the Richardson number of about 4.5.
Dynamic Characterization of Adsorption in Metal Hydrides
325-337
C.
Sobrinho
Dep. Eng. Mecánica, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
N.
Santos
Dep. Eng. Mecánica, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Joao Luis Toste
Azevedo
Technical University of Lisbon; Dep. Eng. Mecanica, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
The paper presents results of the experiments on hydrogen absorption on metal hydrides using two metal structures: LaNi5 and La0.9Ce0.1Ni5. Different procedures were used for adsorption, submitting a constant pressure at the inlet, or increasing the inlet pressure in steps of 0.25 or 0.5 bar until achieving the target pressure. The mass flow rate was continuously monitored and analyzed. In some of the tests the flow rate was found to decay exponentially, as expected from the models. The flow rates and their decay were found to be similar for different intermetallic compounds, although, as expected, they occurred at different pressure levels. The flow rates for the tests done with pressure steps are comparable with those done with the imposed pressure procedure, but the final volume adsorbed was lower in the first case.
Experimental Study on Thermal Conductivity and Viscosity of Water-Based Nanofluids
339-351
Ismail
Tavman
Dokuz Eylul University
Alpaslan
Turgut
Mechanical Engineering Department, Dokuz Eylul University, 35100 Bornova Izmir, Turkey
Mihail
Chirtoc
Thermophysics Laboratory, GRESPI, University of Reims, BP 1039, 51687 Reims Cedex 2, France
Kliment
Hadjov
University of Chemical Technology and Metallurgy of Sofia, blv. Kl. Ohridski 8, 1756 Sofia, Bulgaria
Olivier
Fudym
Universite de Toulouse, Mines Albi, CNRS, Centre RAPSODEE, Campus Jarlard, F-81013
Albi cedex 09, France
Sebnem
Tavman
Food Engineering Department, Ege University, 35100 Bornova Izmir, Turkey
Thermal conductivity and viscosity of deionized water-based TiO2, SiO2, and Al2O3 nanofluids were investigated for various volume fractions of nanoparticles content and at different temperatures. A 3ω technique was developed for measuring thermal conductivity of nanofluids. The theory and the experimental setup of the 3ω measuring system is explained; a conductive wire is used as both heater and sensor in this system. At first, the system is calibrated using water with known thermophysical properties. Measured results showed that the effective thermal conductivity of nanofluids increases as the concentration of the particles increases but not anomalously as indicated in the majority of the literature and this enhancement is very close to the Hamilton-Crosser model; also this increase is independent of the temperature. The effective viscosities of these nanofluids increase by the increasing particle concentration and decrease with an increase in temperature, and cannot be predicted by the Einstein model.