Showing papers on "Natural convection published in 2003"
TL;DR: In this article, an apparently paradoxical behaviour of heat transfer deterioration was observed in nano-fluid and its dependence on parameters such as particle concentration, material of the particles and geometry of the containing cavity have been investigated.
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.
906 citations
TL;DR: Numerical experiments showed that the simplified thermal model can keep the same order of accuracy as the thermal energy distribution model, but it requires much less computational effort.
Abstract: Considering the fact that the compression work done by the pressure and the viscous heat dissipation can be neglected for the incompressible flow, and its relationship with the gradient term in the evolution equation for the temperature in the thermal energy distribution model, a simplified thermal energy distribution model is proposed. This thermal model does not have any gradient term and is much easier to be implemented. This model is validated by the numerical simulation of the natural convection in a square cavity at a wide range of Rayleigh numbers. Numerical experiments showed that the simplified thermal model can keep the same order of accuracy as the thermal energy distribution model, but it requires much less computational effort.
385 citations
TL;DR: In this article, an experimental study of low-level turbulence natural convection in an air filled vertical square cavity was conducted, where both mean and fluctuation quantities are presented, i.e., local velocity and temperature were simultaneously measured at different locations in the cavity and both mean or fluctuation quantity are presented.
260 citations
TL;DR: In this paper, a finite volume-based computational study of steady laminar natural convection (using Boussinesq approximation) within a differentially heated square cavity due to the presence of a single thin fin is presented.
Abstract: A finite-volume-based computational study of steady laminar natural convection (using Boussinesq approximation) within a differentially heated square cavity due to the presence of a single thin fin is presented. Attachment of highly conductive thin fins with lengths equal to 20, 35 and 50 percent of the side, positioned at 7 locations on the hot left wall were examined for Ra=10 4 , 10 5 , 10 6 , and 10 7 and Pr=0.707 (total of 84 cases). Placing a fin on the hot left wall generally alters the clockwise rotating vortex that is established due to buoyancy-induced convection. Two competing mechanisms that are responsible for flow and thermal modifications are identified. One is due to the blockage effect of the fin, whereas the other is due to extra heating of the fluid that is accommodated by the fin. The degree of flow modification due to blockage is enhanced by increasing the length of the fin. Under certain conditions, smaller vortices are formed between the fin and the top insulated wall
208 citations
TL;DR: In this article, the mass, momentum and energy transfer governing equations were solved using a SIMPLER algorithm for the solution of the mass and momentum governing equations in air-filled, 2D rectangular enclosures with adiabatic sidewalls.
202 citations
TL;DR: In this paper, the entropy generation due to heat transfer and friction has been determined in transient state for laminar natural convection by solving numerically the mass, momentum and energy balance equations, using a control volume finite-element method.
200 citations
TL;DR: The physical mechanisms for the size effects on the micro-channel flow and heat transfer were divided into two classifications: (a) the gas rarefaction effect occurs when the characteristic length of the flow becomes comparable to the mean free path of the molecules; (b) Variations of the predominant factors influence the relative importance of various phenomena on the flow as mentioned in this paper.
188 citations
TL;DR: In this article, the Nusselt number was measured at the highest Rayleigh numbers for which Boussinesq conditions hold and sidewall forcing is negligible, and the results showed that the variation in the mean flow was approximately a 1/3 power of the Rayleigh number.
Abstract: New measurements of the Nusselt number have been made in turbulent thermal convection confined in a cylindrical container of aspect ratio unity. The apparatus is essentially the same as that used by Niemela et al. (2000), except that the height was halved. The measurement techniques were also identical but the mean temperature of the flow was held fixed for all Rayleigh numbers. The highest Rayleigh number was . Together with existing data, the new measurements are analysed with the purpose of understanding the relation between the Nusselt number and the Rayleigh number, when the latter is large. In particular, the roles played by Prandtl number, aspect ratio, mean wind, boundary layers, sidewalls, and non-Boussinesq effects are discussed. Nusselt numbers, measured at the highest Rayleigh numbers for which Boussinesq conditions hold and sidewall forcing is negligible, are shown to vary approximately as a 1/3-power of the Rayleigh number. Much of the complexity in interpreting experimental data appears to arise from aspects of the mean flow, including complex coupling of its dynamics to sidewall boundary conditions of the container. Despite the obvious practical difficulties, we conclude that the next generation of experiments will be considerably more useful if they focus on large aspect ratios.
177 citations
Book•
30 Apr 2003
TL;DR: In this article, the authors present a review of the history of CVD and its application in nuclear power plants, including tube and showerhead examples, as well as a discussion of the current state of the CVD process.
Abstract: Acknowledgements. Preface. 1: Introduction. 1. What's behind the facade? 2. Generic reactors and process considerations. 3. Tube and showerhead reactor examples. 2: Reactors without transport. 1. What goes in must go somewhere: Measuring gases. 2. Review: Kinetic theory. 3. The zero-dimensional reactor. 4. Zero-dimensional tube and showerhead examples. 3: Mass transport. 1. Introduction to transport. 2. Convection and diffusion. 3. Diffusion: Physics and math. 4. Fluid flow and convective transport. 5. When flows matter: The Knudsen number. 6. Tube and showerhead examples. 7. On to photons. 4: Heat transport. 1. What is heat (energy) transport? 2. Heat conduction and diffusion. 3. Convective heat transfer made (very) simple. 4. Natural convection. 5. Radiative heat transfer. 6. Temperature measurement. 7. Tube and showerhead examples. 5: Chemistry for CVD. 1. What does the C stand for anyway? 2. Volatility, the V in CVD. 3. Equilibrium: Where things are going. 4. Kinetics: The slowest step wins. 5. Real precursors for real films. 6. Tube reactor example. 7. A few final remarks. 6: Gas discharge plasmas for CVD. 1. Plasma discharges: An instant review. 2. The low-pressure cold-plasma state. 3. Key parameters for capacitive plasma state. 4. Alternative excitation methods. 5. Plasmas for deposition. 6. Plasma damage. 7. Technical details. 8. Ongoing example: Parallel plate reactor. 9. A remark on computational tools. 7: CVD films. 1. Why CVD? 2. Silicon dioxide. 3. Silicon nitride. 4. Tantalum pentoxide. 5. Metal deposition by CVD. 6. Concluding remarks. 8: CVD reactors. 1. CVD reactor configurations. 2. Tube reactors. 3. Showerhead reactors. 4. High density plasma reactors. 5. Injector-based atmospheric pressure reactors. 6. Reactor conclusions. Index.
168 citations
TL;DR: The study provides a direct confirmation that a rotatory mean wind indeed persists for the highest value of Ra reached in the experiment and reveals that the mean flow in the central region of the convection cell is of the shape of a coherent elliptical rotating core.
Abstract: The spatial structure of the velocity field in turbulent Rayleigh-Benard convection in water has been measured using the particle image velocimetry technique, with the Rayleigh number Ra varying from 9 x 10(8) to 9 x 10(11) and the Prandtl number remaining approximately constant (Pr approximately 4). The study provides a direct confirmation that a rotatory mean wind indeed persists for the highest value of Ra reached in the experiment. The measurement reveals that the mean flow in the central region of the convection cell is of the shape of a coherent elliptical rotating core for Ra below 1 x 10(10). Above this Ra, the orientation of the elliptical core changes by a 90 degrees angle and an inner core rotating at a lower rate inside the original bulk core emerges. It is further found that the rotation frequencies of the inner core and the outer shell have distinct scalings with Ra; the scaling exponent for the outer-shell is 0.5 and it is 0.4 for the inner core. From the measured rms and skewness distributions of the velocity field, we find that velocity fluctuations at the cell center are neither homogeneous nor isotropic. The turbulent energy production fields further reveal that the mean wind is not driven by turbulent fluctuations associated with Reynolds stress.
153 citations
TL;DR: In this paper, heat transfer by the natural convection of air layers within vertical, rectangular cavities with aspect ratios (A) of 20, 40 and 80 was investigated in relation to applications in building facade elements, such as insulating glazing units, double-skin facades, doors, etc.
TL;DR: In this paper, the authors measured the refraction index in a boundary layer in an unforced flowing state in a desalting cell, and evaluated ionic fluxes, current density, solution velocity and potential gradient in the boundary layer.
TL;DR: In this article, the integral forms of the governing equations are solved numerically using Finite Volume method and computational domains are divided into finite numbers of body fitted control volumes with collocated variable arrangement.
TL;DR: In this paper, a simple power-law fluid model, with the power law index n and the consistency coefficient K, is adopted to study transient buoyant convection in a square enclosure of a non-Newtonian fluid.
TL;DR: Local convective heat flux in turbulent thermal convection is obtained from simultaneous velocity and temperature measurements in an aspect-ratio-one convection cell filled with water.
Abstract: Local convective heat flux in turbulent thermal convection is obtained from simultaneous velocity and temperature measurements in an aspect-ratio-one convection cell filled with water. It is found that fluctuations of the vertical heat flux are highly intermittent and are determined primarily by the thermal plumes in the system. The experiment reveals a unique mechanism for the heat transport in turbulent convection.
TL;DR: In this article, the authors propose a numerical method to calculate unsteady flows of Bingham fluids without any regularization of the constitutive law, which is based on the combination of the characteristic/Galerkin method to cope with convection and of the Fortin-Glowinsky decomposition/coordination method to deal with the non-differentiable and non-linear terms that derive from constitutive laws.
TL;DR: In this article, an analytical and numerical study of the natural convection in a horizontal porous layer filled with a binary fluid is presented. But the authors assume that the solutal buoyancy forces are induced either by the imposition of constant fluxes of mass on the horizontal walls (double-diffusive convection, $a\,{=}\,0$ ) or by temperature gradients (Soret effects,$a \,{ =}\,1$ ).
Abstract: This paper reports an analytical and numerical study of the natural convection in a horizontal porous layer filled with a binary fluid. A uniform heat flux is applied to the horizontal walls while the vertical walls are impermeable and adiabatic. The solutal buoyancy forces are assumed to be induced either by the imposition of constant fluxes of mass on the horizontal walls (double-diffusive convection, $a\,{=}\,0$ ) or by temperature gradients (Soret effects, $a\,{=}\,1$ ). The governing parameters for the problem are the thermal Rayleigh number, $R_T$ , the Lewis number, $Le$ , the solutal Rayleigh number, $R_S$ , the aspect ratio of the cavity, $A$ , the normalized porosity of the porous medium, $\varepsilon$ , and the constant $a$ . The onset of convection in the layer is studied using a linear stability analysis. The thresholds for finite-amplitude, oscillatory and monotonic convection instabilities are determined in terms of the governing parameters. For convection in an infinite layer, an analytical solution of the steady form of the governing equations is obtained by assuming parallel flow in the core of the cavity. The critical Rayleigh numbers for the onset of supercritical, $R_{\hbox{\scriptsize\it TC}}^{\hbox{\scriptsize\it sup}}$ , or subcritical, $R_{\hbox{\scriptsize\it TC}}^{\hbox{\scriptsize\it sub}}$ , convection are predicted by the present theory. A linear stability analysis of the parallel flow pattern is conducted in order to predict the thresholds for Hopf bifurcation. Numerical solutions of the full governing equations are obtained for a wide range of the governing parameters. A good agreement is observed between the analytical prediction and the numerical simulations.
TL;DR: Direct numerical simulations of Rayleigh-Bénard convection in a plane layer with periodic boundary conditions at Rayleigh numbers up to 10(7) show that flow structures can be objectively classified as large or small scale structures because of a gap in spatial spectra.
Abstract: Direct numerical simulations of Rayleigh-Benard convection in a plane layer with periodic boundary conditions at Rayleigh numbers up to 10(7) show that flow structures can be objectively classified as large or small scale structures because of a gap in spatial spectra. The typical size of the large scale structures does not always vary monotonically as a function of the Rayleigh number but broadly increases with increasing Rayleigh number. A mean flow (whose average over horizontal planes differs from zero) is also excited but is weak in comparison with the large scale structures. The large scale circulation observed in experiments should therefore be a manifestation of the large scale structures identified here.
TL;DR: In this paper, a numerical study of mixed convection heat and mass transfer along a vertical wavy surface has been carried out numerically, including the developments of skin-friction coefficient, velocity, temperature, concentration, Nusselt number as well as Sherwood number.
TL;DR: In this paper, the transient thermal analysis of a natural convection solar air heater is presented, which consists of a single-glazed flat plate solar collector integrated with a paraffin type phase change material (PCM) energy storage subsystem and a rectangular enclosure which serves as the working chamber.
TL;DR: In this paper, the Arbitrary Lagrangian-Eulerian technique was used to solve the problem of sedimentation of two solid particles with thermal convection, including the energy equation.
Abstract: Based on the study of isothermal inert particle sedimentation, the Arbitrary Lagrangian-Eulerian technique was used to solve the problem of sedimentation of two solid particles with thermal convection, including the energy equation. The results show that because of the dynamic wake caused by thermal convection, the trajectory of particles are distinct from each other when settling in isothermal, cool and hot fluid; there is vortex shedding in hot fluid, while in cool fluid a strong upward thermal plume forms.
TL;DR: In this article, scaling laws for the breakdown of layered convection as well as the strength of convection are derived as a function of viscosity layering, the phase buoyancy parameter, and the thermal Rayleigh number.
Abstract: [1] We investigate the physics of multiscale convection in Earth's mantle, characterized by the coexistence of large-scale mantle circulation associated with plate tectonics and small-scale sublithospheric convection. In part 2 of our study, the temporal and spatial evolution of sublithospheric convection is studied using two-dimensional whole mantle convection models with temperature- and depth-dependent viscosity and an endothermic phase transition. Scaling laws for the breakdown of layered convection as well as the strength of convection are derived as a function of viscosity layering, the phase buoyancy parameter, and the thermal Rayleigh number. Our results suggest that layered convection in the upper mantle is maintained only for a couple of overturns, with plausible mantle values. Furthermore, scaling laws for the onset of convection, the stable Richter rolls, and the breakdown of layered convection are all combined to delineate possible dynamic regimes beneath evolving lithosphere. Beneath long-lived plates, the development of longitudinal convection rolls is suggested to be likely in the upper mantle, as well as its subsequent breakdown to whole mantle-scale convection. This evolutionary path is suggested to be consistent with the seismic structure of the Pacific upper mantle.
TL;DR: In this article, the lattice Boltzmann method is applied to problems of transport phenomena in a three-dimensional porous structure, and boundary conditions for the particle distribution function of a diffusing component are described in detail.
Abstract: SUMMARY The lattice Boltzmann method (LBM) for a binary miscible fluid mixture is applied to problems of transport phenomena in a three-dimensional porous structure. Boundary conditions for the particle distribution function of a diffusing component are described in detail. Flow characteristics and concentration profiles of diffusing species at a pore scale in the structure are obtained at various Reynolds numbers. At high Reynolds numbers, the concentration profiles are highly affected by the flow convection and become completely different from those at low Reynolds numbers. The Sherwood numbers are calculated and compared in good agreement with available experimental data. The results indicate that the present method is useful for the investigation of transport phenomena in porous structures. Copyright c � 2000 John Wiley & Sons, Ltd.
TL;DR: In this paper, the authors investigate numerically the heat transfer and fluid flow characteristics inside a wavy walled enclosure and apply second law of thermodynamics to predict the nature of irreversibility in terms of entropy generation.
TL;DR: In this paper, the effect of surface mass flux on mixed convective flow past a heated vertical flat permeable plate with thermophoresis has been considered, and the governing equations, reduced to local nonsimilarity boundary layer equations using suitable transformations, have been integrated employing an implicit finite difference method together with the Keller-box technique.
TL;DR: In this paper, the steady state natural convection in a square porous enclosure using a thermal non-equilibrium model for the heat transfer between the fluid and the solid phases is studied.
Abstract: The aim of the present paper is to study the steady state natural convection in a square porous enclosure using a thermal non-equilibrium model for the heat transfer between the fluid and the solid phases. The analysis assumes that the porous medium is homogeneous and isotropic. The present study also assumes the non-Darcy model of natural convection in porous media. It is assumed that the heat generation is only in solid phase. Two dimensional steady convection in a cavity bounded by isothermal walls at constant temperatures has been studied numerically by adopting a two-temperature model of microscopic heat transfer. Such a model. which allows the fluid and solid phases not to be in local thermal equilibrium, is found to modify the flow behaviour and heat transfer rates. Knowledge of this behaviour is very important for the design of the many engineering applications.
TL;DR: In this article, the effects of heterogeneity in permeability distributions on solute transport processes using a stochastic framework were investigated in homogeneous porous media, where a set of measurable indicator characteristics including solute fluxes, solute present, center of gravity and finger penetration depth were used in the quantitative analysis of output.
Abstract: [1] The situation of a dense fluid overlying a lighter one is potentially unstable and under certain conditions may result in fingers of dense contaminant freely convecting downward. Free convection causes increased contaminant transport over larger distances and over shorter timescales than is possible by diffusion alone. Unlike free convection in homogenous porous media, free convection in heterogeneous porous media has received relatively little attention. In this study, a well-understood problem of transient free convection in homogeneous porous media, theElder [1967b] “short heater” problem has been modified to study the effects of heterogeneity in permeability distributions on solute transport processes using a stochastic framework. A set of measurable indicator characteristics including solute fluxes, solute present, center of gravity and finger penetration depth are used in the quantitative analysis of output. Heterogeneity in the permeability distribution provides both the triggering mechanism for the onset of instabilities and also controls their subsequent growth and/or decay. Results show that (1) an increase in the standard deviation of the log permeability field results in a greater degree of instability at earlier times but promotes stability at later times, (2) an increase in the horizontal correlation length of the log permeability field creates laterally extensive low-permeability zones that dissipate upward and downward motion needed to maintain convection and therefore causes a reduction in the degree of instability, (3) a greater degree of heterogeneity causes greater uncertainty in predictions, and (4) traditional predictive methods such as the Rayleigh number (based upon an average permeability) do not generally work in their application to heterogeneous systems. Probability of exceedence analysis also demonstrates that analyses based upon homogeneous assumptions will typically underestimate, often quite significantly, the value of key measurable characteristics.
TL;DR: In this article, the authors estimate the impact of mixed convection effect on the cooling capacity of a ceiling radiant panel in mechanically ventilated spaces and find that the total cooling capacity can be enhanced by 5-35% under normal operating panel surface temperatures.
TL;DR: In this article, the effect of vertical confinement on the natural convection flow and heat transfer around a horizontal heated cylinder is investigated and the relationship between the flow pattern and the heat transfer characteristics at the cylinder surface is studied and the origin of the thermal plume is discussed.
TL;DR: In this article, a combination of natural and magnetic convective heat transfer through a ferrofluid in a cubic enclosure is simulated numerically, where the momentum equation includes a magnetic term that arises when a magnetic fluid is in the presence of a magnetic field gradient and a temperature gradient.