Showing papers on "Natural convection published in 1986"

Melting and Solidification of a Pure Metal on a Vertical Wall

Abstract: Etude experimentale de l'ecoulement de convection naturelle dans la phase liquide et de son influence sur le mouvement de l'interface liquide-solide et sur le transfert de chaleur au cours de la fusion et de la solidification de gallium a haute purete sur une paroi verticale

Engineering Heat Transfer

Abstract: Fundamental Concepts Mechanisms of Heat Transfer Dimensions and Units Fourier's Law of Heat Conduction Thermal Conductivity Convection Heat Transfer Convection Heat-Transfer Coefficient Radiation Heat Transfer Emissivity and Other Radiative Properties Combined Heat-Transfer Mechanisms Steady-State Conduction in One Dimension One-Dimensional Conduction Equation Plane Geometry Systems Polar Cylindrical Geometry Systems Spherical Geometry Systems Thermal Contact Resistance Heat Transfer from Extended Surfaces Steady-State Conduction in Multiple Dimensions General Conduction Equation Analytical Method of Solution Graphical Method of Solution Conduction Shape Factor Solution by Numerical Methods (Finite Differences) Numerical Method of Solution for Two-Dimensional Problems Methods of Solving Simultaneous Equations Unsteady-State Heat Conduction Systems with Negligible Internal Resistance Systems with Finite Internal and Surface Resistances Solutions to Multidimensional Geometry Systems Approximate Methods of Solution to Transient-Conduction Problems Introduction to Convection Fluid Properties Characteristics of Fluid Flow Equations of Fluid Mechanics Thermal-Energy Equation Applications to Laminar Flows Applications to Turbulent Flows Natural-Convection Problem Dimensional Analysis Convection Heat Transfer in a Closed Conduit Heat Transfer to and from Laminar Flow in Circular Conduit Heat Transfer to and from Turbulent Flow in Circular Conduit Heat-Transfer Correlations for Flow in Noncircular Ducts Convection Heat Transfer in Flows Past Immersed Bodies Boundary-Layer Flow Turbulent Flow over Flat Plate Flow Past Various Two-Dimensional Bodies Flow Past a Bank of Tubes Flow Past a Sphere Natural-Convection Systems Natural Convection on a Vertical Surface: Laminar Flow Natural Convection on a Vertical Surface: Transition and Turbulence Natural Convection on an Inclined Flat Plate Natural Convection on a Horizontal Flat Surface Natural Convection on Cylinders Natural Convection around Spheres and Blocks Natural Convection about an Array of Fins Combined Forced- and Natural-Convection Systems Heat Exchangers Double-Pipe Heat Exchangers Shell-and-Tube Heat Exchangers Effectiveness-Number of Transfer Units Method of Analysis Crossflow Heat Exchangers Efficiency of a Heat Exchanger Condensation and Vaporization Heat Transfer Condensation Heat Transfer Boiling Heat Transfer Introduction to Radiation Heat Transfer Electromagnetic Radiation Spectrum Emission and Absorption at the Surface of an Opaque Solid Radiation Intensity Irradiation and Radiosity Radiation Laws Characteristics of Real Surfaces Radiation Heat Transfer between Surfaces View Factor Methods for Evaluating View Factors Radiation Heat Transfer within Enclosure of Black Surfaces Radiation Heat Transfer within an Enclosure of Diff use-Gray Surfaces Bibliography and Selected References Appendices Index

Natural convection in an enclosed vertical air layer with large horizontal temperature differences

Abstract: Steady-state two-dimensional results obtained from numerical solutions to the transient Navier-Stokes equations are given for laminar convective motion of a gas in an enclosed vertical slot with large horizontal temperature differences. We present results for air using the ideal-gas law and Sutherland-law transport properties, although the results are also valid for hydrogen. Wide ranges of aspect-ratio, Rayleigh-number and temperature-difference parameters are examined. The results are compared in detail with the exact solution in the conduction and fully developed merged boundary-layer limits for arbitrary temperature difference, and to the well-established Boussinesq limit for small temperature difference. It is found that the static pressure, and temperature and velocity distributions are very sensitive to property variations, even though the average heat flux is not. In addition we observe a net vertical heat flux to be the same as that obtained from the Boussinesq equations. We concentrate on the boundary-layer regime, but we present a rather complete picture of different flow regimes in Rayleigh-number, aspect-ratio and temperaturedifference parameter space. We observe that, with increasing temperature difference, lower critical Rayleigh numbers for stationary and oscillatory instabilities are obtained. In addition we observe that in some cases the physical nature of the instability changes with increasing temperature difference.

Handbook of Heat Transfer Fundamentals (Second Edition)

Abstract: Recent advances in heat transfer are discussed, providing data and methodology to solve a wide range of heat transfer problems. The topics considered include: basic concepts of heat transfer, mathematical methods, thermophysical properties, conduction, numerical methods in heat transfer, natural convection, and internal duct flow and external flows in forced convection. Also addressed are: rarefied gases, electric and magnetic fields, condensation, boiling, two-phase flow, and radiation.

A numerical study of non-darcian natural convection in a vertical enclosure filled with a porous medium

Abstract: A numerical study of non-Darcian natural convection in a vertical enclosure filled with a porous medium is performed. The flow is modeled using the Brinkman-Forchheimer-extended Darcy equations. The governing equations are solved with the SIMPLER algorithm and good agreement with previously reported numerical and experimental results is found. An order of magnitude analysis and the numerical results demonstrate the importance of non-Darcian effects. For high Darcy numbers (Da > 10−4), both extensions are of the same order of magnitude and must be used simultaneously. In addition, Forch-heimer's extension must be included for Pr ≤ 1.0 and all Darcy numbers. Finally, Nusselt number correlations are presented for three different ranges of the Darcy number covering wide ranges of the governing parameters.

Turbulent thermal convection in wide horizontal fluid layers

Abstract: Turbulence in thermal convection is investigated for flows in which the production of turbulence energy is due solely to buoyancy, and the statistics of the flow are homogeneous in horizontal planes. New experimental results for high Rayleigh number unsteady turbulent convection in a horizontal layer heated from below and insulated from above are presented and compared to turbulent Rayleigh convection, convection in the planetary boundary layer, and laboratory penetrative convection. Mean temperature fields are correlated in terms of wall layer scales and convection scales. Joint statistics of turbulent temperature and horizontal velocity and vertical velocity through fourth order are presented for the core region of the convection layer.

Wildland Fire Spread by Radiation-a Model Including Fuel Cooling by Natural Convection

Abstract: The spread of a line fire through wildland fuel is modeled for situations in which unignited fuel is heated by radiation and cooled by radiation and convection. The fuel bed is idealized as a continuum in a homogeneous layer composed of uniformly distributed, approximately convex particles that are randomly oriented, thermally thin, and radiometrically black. The model is implemented as an algorithm that finds the temperature of the fuel particles every-where in the unignited bed and solves simultaneously for the rate of fire spread and the shape o f the surface (the ignition interface) upon which the particles are ignited. This algorithm is an extension of one presented earlier that neglects particle cooling by convection. Results predicted by the revised model are compared to measurements made on experimental fires. The ignition interface shape predicted for the experimental fires agrees reasonably well with measuremenls when two free parameters have values chosen so as to yield the measured ra...

Application of flux-corrected transport (FCT) to high Rayleigh number natural convection in a porous medium

Abstract: A numerical technique designed to solve a wide class of convectively dominated flow problems is applied to natural convective flow in a porous medium at large Rayleigh number. The technique is a finite difference method based on flux-corrected transport (FCT) and possesses four desirable numerical properties: stability, accuracy, monotonicity, and conservation. Steady natural convection is investigated for Rayleigh numbers as large as 10,000. An efficient methodology for obtaining steady state solutions is illustrated. A simulation is performed for transient thermal convection at a Rayleigh number of 2500. Transient natural convection involving both heat and mass transfer is illustrated for a Rayleigh number of 2500, Lewis number of 2, and buoyancy ratio of 0.1. All simulations are performed in a square cavity with heated vertical side walls. 17 refs.

Route to chaos in porous-medium thermal convection

Abstract: A pseudo-spectral numerical scheme is used to study two-dimensional, single-cell, time-dependent convection in a square cross-section of fluid saturated porous material heated from below. With increasing Rayleigh number R convection evolves from steady S to chaotic NP through the sequence of bifurcations S→P(1)→QP2→P(2)→NP, where P(1) and P(2) are simply periodic regimes and QP2 is a quasi-periodic state with two basic frequencies. The transitions (from onset of convection to chaos) occur at Rayleigh numbers of 4π2, 380–400, 500–520, 560–570, and 850–1000. In the first simply periodic regime the fundamental frequency f1 varies as . The chaotic states are characterized by spectral peaks with at least 3 fundamental frequencies superimposed on a broadband background noise. The time dependence of these states arises from the random generation of tongue-like disturbances within the horizontal thermal boundary layers. Transition to the chaotic regime is accompanied by the growth of spectral components that destroy the centre-symmetry of convection in the other states. Over-truncation can lead to spurious transitions and bifurcation sequences; in general it produces overly complex flows.

Flow through a porous medium in the presence of a magnetic field

Abstract: Unsteady free convective flow of an electrically conducting fluid through a porous medium bounded by an infinite vertical and porous plate is considered. The free stream velocity of the fluid vibrates about a mean constant value and the temperature of the plate is constant. The influences of the permeability parameter and the magnetic parameter on the velocity field are discussed.

Laminar Natural Convection in Shallow Open Cavities

Abstract: Etude experimentale realisee par anemometrie laser Doppler. La cavite est rectangulaire avec une paroi laterale ouverte

Flow past a suddenly heated vertical plate in a porous medium

Abstract: An analysis is presented for the steady free convection flow about a semi-infinite vertical flat plate that is embedded in a saturated porous medium at high Rayleigh numbers. Similarity solutions are obtained for a class of problems where the wall temperature varies as a power of the distance from the leading edge of the plate. The existence and uniqueness of the solutions are considered. The approach to this steady-state solution is also considered by investigating the temporal development of the flow when the temperature of the plate is impulsively increased from that of the surroundings. A numerical solution is presented that matches the small and large time solutions. For some temperature distributions on the plate it is found that the velocity achieves its maximum value within the boundary layer. For these the disturbance from the leading edge of the plate travels fastest within the boundary layer. An asymptotic solution valid at large times is presented and the approach of the numerical solution to this asymptotic solution is illustrated. For the situation in which the plate is impulsively heated to a constant temperature an analysis is presented for the early stages of the departure from the one-dimensional solution.

Convective instability in packed beds with throughflow

Abstract: Resultats des calculs numeriques des limites de stabilite lineaire de convection libre dans les lits de garnissage. Influence de la direction de l'ecoulement et de differentes conditions limites

Correlations for laminar mixed convection flows on vertical, inclined, and horizontal flat plates

Abstract: Local Nusselt numbers for laminar mixed convection flows along isothermal vertical, inclined, and horizontal flat plates are presented for the entire mixed convection regime for a wide range of Prandtl numbers, 0.1 ≤ Pr ≤ 100. Simple correlation equations for the local and average mixed convection Nusselt numbers are developed, which are found to agree well with the numerically predicted values and available experimental data for both buoyancy assisting and opposing flow conditions. The threshold values of significant buoyancy effects on forced convection and forced flow effects on free convection, as well as the maximum increase in the local mixed convection Nusselt number from the respective pure convection limits, are also presented for all flow configurations. It is found that the buoyancy or forced flow effect can increase the surface heat transfer rate from pure forced or pure free convection by about 20 percent.

Numerical study of high Rayleigh number convection in a medium with depth-dependent viscosity

Abstract: The equations of motion are solved numerically for a Boussinesq fluid with infinite Prandtl number in a square 2-D box where the viscosity increases with depth. Three heating modes are employed: bottom heating, internal heating, and half bottom and half internal heating. In all cases the boundaries are free slip. The range of Rayleigh numbers employed is 10^4-10^7. The viscosity increases as 10^(β(1-y)), where y is distance measured from the bottom upwards and β is a free parameter. In the bottom heated cases, the convective velocities slow near the bottom and result in a large temperature drop between the bottom boundary and interior compared with the top boundary and the interior. This results in increased buoyancy in the ascending limb. In the internally heated case, the flow in the top half of the box resembles Rayleigh-Benard convection and in the bottom half it approaches a conductive thermal regime for β greater than about 2. In this case the top surface heat flux decays from ascending to descending limb and the ascending and descending limbs become more equal in their buoyancy. Increasing β decreases the efficiency of heat transport, but has little effect on the exponents of Nu-Ra and Pe-Ra relations. There is a larger decrease in heat transport efficiency for a given β in the bottom heated case compared to the internally heated case.

Melting from a flat plate embedded in a porous medium in the presence of steady natural convection

Abstract: The problem of melting from a flat plate embedded in a porous medium is studied. The main focus is to determine the effect of natural convection flow in the liquid phase on the melting phenomenon. Two configurations an considered and modeled mathematically: a vertical plate and a horizontal plate. The final similarity equations art integrated numerically by use of the fourth-order Runge-Kutta method. Systematic “shooting” is required to satisfy the boundary conditions at infinity. Results are reported for the temperature and flow fields in the melt region. The melting phenomenon decreases the local Nusselt number at the solid-liquid interface.

Analysis of heat transfer during melting of a pure metal from an isothermal vertical wall

Abstract: A computational methodology is presented for the problem of melting of a pure metal from an isothermal vertical wall. The governing conservation equations of mass, momentum, and energy are solved with a control volume-based discretization scheme adapted for irregular geometries. The moving boundary is immobilized by employing the quasi-steady assumption with an algebraically generated nonorthogonal grid. All terms in the governing equations arising from the nonorthogonatity of the computational grid are retained in the solution algorithm. The model is validated by comparison with available experimental data for the effect of natural convection on melting heat transfer in a pure metal system. The influence of Rayleigh number on velocity and temperature fields is investigated, and sample results are presented for overall heat transfer and melting rate. The predictions indicate that buoyancy-induced fluid motion may have a less dominant role in energy transport in the transition to quasi-steady melting in th...