Showing papers on "Combined forced and natural convection published in 1991"

Turbulent Compressible Convection

Abstract: Numerical simulations with high spatial resolution (up to 96-cubed gridpoints) are used to study three-dimensional, compressible convection. A sequence of four models with decreasing viscous dissipation is considered in studying the changes in the flow structure and transport properties as the convection becomes turbulent. 39 refs.

Numerical simulations of three-dimensional thermal convection in a fluid with strongly temperature-dependent viscosity

Abstract: Numerical calculations are presented for the steady three-dimensional structure of thermal convection of a fluid with strongly temperature-dependent viscosity in a bottom-heated rectangular box. Viscosity is assumed to depend on temperature T as exp ( --ET), where E is a constant ; viscosity variations across the box r (= exp (E)) as large as lo5 are considered. A stagnant layer or lid of highly viscous fluid develops in the uppermost coldest part of the top cold thermal boundary layer when r > rcl, where r = rcl = 1.18 x 103R,0~308 andR, is the Rayleigh number based on the viscosity at the top boundary. Three-dimensional convection occurs in a rectangular pattern beneath this stagnant lid. The planform consists of hot upwelling plumes at or near the centre of a rectangle, sheets of cold sinking fluid on the four sides, and cold sinking plume concentrations immersed in the sheets. A stagnant lid does not develop, i.e. convection involves all of the fluid in the box when r rc2 = 3.84 x 106R;1.36. The planform of the convection is rectangular with the coldest parts of the sinking fluid and the hottest part of the upwelling fluid occurring as plumes at the four corners and at the centre of the rectangle, respectively. Both hot uprising plumes and cold sinking plumes have sheet-like extensions, which become more well-developed as r increases. The whole-layer mode of convection occurs as two-dimensional rolls when r < min (rcl, rc2). The Nusselt number Nu depends on the viscosity at the top surface more strongly in the regime of whole-layer convection than in the regime of stagnant-lid convection. In the whole-layer convective regime, Nu depends more strongly on the viscosity at the top surface than on the viscosity at the bottom boundary.

Numerical simulation of thermal transport associated with a continuously moving flat sheet in materials processing

Abstract: The thermal transport that arises due to the continuous motion of a heated plate or sheet in manufacturing processes such as hot rolling, extrusion, continuous casting, and drawing is numerically investigated. The resulting temperature distribution in the solid is influenced by the associated flow in the ambient fluid, which is taken as stationary far from this moving surface, and is of particular interest in this work. A numerical study is carried out, assuming a two-dimensional, steady circumstance with laminar flow in the fluid. The full governing equations, including buoyancy effects, are solved, employing finite-difference techniques. The effect of various governing parameters, such as the Peclet number, Pe, the mixed convection parameter, Gr/Re{sup 2}, and the conductivity parameter, K{sub f}/K{sub s}, which determine the temperature and flow fluids, is studied in detail. Also, the effect of the boundary conditions, particularly at the location of the emergence of the plate, on the downstream thermal transport is investigated. The penetration of the conductive effects, upstream of the point of emergence, is found to be significant. The effect of buoyancy is found to be more prominent when the plate is moving vertically upward than when it is moving horizontally. The appropriate boundary conditions andmore » their imposition in the numerical scheme are discussed for a variety of practical circumstances.« less

Mixed convection from an isolated heat source in a rectangular enclosure

Abstract: The mixed convection transport from an isolated thermal source, with a uniform surface heat flux input and located in a rectangular enclosure, is studied numerically. The enclosure simulates a practical system such as an oven or an air-cooled electronic device, where an airstream flows through the openings on the two vertical walls. The heat source represents a heater or an electronic component located in such an enclosure. The interaction of the cooling stream with the buoyancy-induced flow from the heat source is of interest in this work. Laminar, two-dimensional flow is assumed, and the problem lies in the mixed convection regime, governed by the buoyancy parameter GrIRe1 and the Reynolds number Re. Other significant variables include the locations of the heat source and the outflow opening. The inflow is kept at a fixed position. The mathematical model is developed with vorticity and stream function, along with temperature, as the dependent variables. The unsteady terms are retained in the vo...

Non-Darcy Mixed Convection Along a Vertical Wall in a Saturated Porous Medium

Abstract: In most of the previous studies of either natural or mixed convection, the boundary-layer formulation of Darcy's law and the energy equation were used. However, the inertial effect is expected to become very significant when the pore Reynolds number is large. This is especially true for the case of either the high Rayleigh number regime or for high-porosity media. In spite of its importance in many applications, the non-Darcy flow effect has not received much attention. In this note, non-Darcy flow effects, which include the inertial and thermal dispersion effects, are closely examined. Steady-state non-Darcy convection, in the form of natural, mixed, and forced convection, is considered for a heated vertical surface embedded in a saturated porous medium.

Unsteady mixed convection flow in stagnation region adjacent to a vertical surface

Abstract: The unsteady two-dimensional laminar mixed convection flow in the stagnation region of a vertical surface has been studied where the buoyancy forces are due to both the temperature and concentration gradients. The unsteadiness in the flow and temperature fields is caused by the time-dependent free stream velocity. Both arbitrary wall temperature and concentration, and arbitrary surface heat and mass flux variations have been considered. The Navier-Stokes equations, the energy equation and the concentration equation, which are coupled nonlinear partial differential equations with three independent variables, have been reduced to a set of nonlinear ordinary differential equations. The analysis has also been done using boundary layer approximations and the difference between the solutions has been discussed. The governing ordinary differential equations for buoyancy assisting and buoyancy opposing regions have been solved numerically using a shooting method. The skin friction, heat transfer and mass transfer coefficients increase with the buoyancy parameter. However, the skin friction coefficient increases with the parameter λ, which represents the unsteadiness in the free stream velocity, but the heat and mass transfer coefficients decrease. In the case of buoyancy opposed flow, the solution does not exist beyond a certain critical value of the buoyancy parameter. Also, for a certain range of the buoyancy parameter dual solutions exist.

Convective heat and mass transfer in porous media

Abstract: Transport Processes in a Rapidly Changing World.- Modelling of Transport Phenomena in Porous Media.- Fundamentals of Mechanics of Saturated Porous Media: Basic Equations and Waves.- The Stability of Convective Flows in Porous Media.- Free Convection Heat and Mass Transfer in a Porous Medium.- Natural Convection in a Vertical Porous Annulus.- Non-Darcy Natural Convection in Saturated Porous Media.- Mixed Convection in Saturated Porous Media.- Forced Convective Flow and Heat Transfer Through a Porous Medium Exposed to a Flat Plate or a Channel.- Forced Convection Heat Transfer in a Porous Medium.- Radiation Transport in Porous or Fibrous Media.- Fundamentals of Drying of Capillary-Porous Bodies.- Heat Transfer During Unsaturated Flow in Porous Media.- Buoyancy-Induced Flow and Heat Transfer in Saturated Fissured Media.- Effect of Randomness on Heat and Mass Transfer in Porous Media.- Analytical Solutions to Transient Convective Mass Transfer Within Porous Media.- Natural Convection in Porous Media with Variable Porosity and Thermal Dispersion Effects.- Convective Flow Interaction and Heat Transfer Between Fluid and Porous Layers.- Temperature Distribution in a Porous Slab with Random Thermophysical Characteristic.- Forced Convection in Packed Tubes and Channels with Variable Porosity and Thermal Dispersion Effects.- Transient Double Diffusive Convection in a Horizontal Fluid Layer Situated on top of a Porous Substrate.- Drying of Wood Residues in a Fixed Bed.- Heat and Mass Transfer in Adsorbent Beds.- Solidification of a Binary Mixture Saturating a Bed of Glass Spheres.- Melting in the Presence of Natural Convection in a Saturated Porous Medium.- Air-Water Two-Phase Flow Pressure Drop in Large Scale Porous Media.- Boiling and Dryout in Unconsolidated Porous Media.- Heat Transfer from a Surface Covered with Hair.- Measurements of Thermal Conductivity in Porous Media.- Determination of Velocity Vectors in Porous Media with Fluorescent Particle Image Velocimetry (FPIV).- Non Invasive Measurement Techniques in Porous Media.- Flash Method of Measuring Thermal Diffusivity and Conductivity.- Mechanics, Heat and Mass Transfer in Saturated Porous Media. Application to Petroleum Technology.- Drying Complex Porous Materials-Modelling and Experiments.- Some Geophysical Problems Involving Convection in Porous Media.- Porous Surface Boiling and Its Application to Cooling Microelectronic Chips.- Heat and Mass Transfer in Spouted Beds.- Liquid Seeping into Porous Ground.- Future Research Needs in Convective Heat and Mass Transport in Porous Media.

A two‐phase, two‐component model for natural convection in a porous medium

Abstract: A numerical study of natural convection in a two-phase, two-component flow in a porous medium heated from below is presented. Interphase mass and energy transfer, latent heat and bouyancy effects are major physical features. This study extends earlier studies of natural convection based on single-phase, saturated porous medium models. The appearance of two-phase heat pipe zones in the flow has a marked effect on the fluid and heat flows as well as on the performance of the numerical methods. The numerical techniques for handling phase change, Jacobian construction and time step selection are discussed.

Effects of geometry and orientation on laminar natural convection from isothermal bodies

Abstract: The effects of body shape and orientation on laminar natural convection heat transfer from isothermal, two-dimensional, and axisymmetric bodies are investigated. A new form of the body-gravity function is presented and its behavior is examined for various body shapes and orientations. The square root of the total surface area is obtained from the analysis as the charateristic body length. A definition of the aspect ratio is introduced, which can be used consistently for both two-dimensional and axisymmetric bodies. The resulting body-gravity function is observed to be weakly dependent on geometry and orientation of the bodies for a range of the aspect ratio from approximately 0.2 to 5, over which the body-gravity function resulted in a good agreement with values obtained from the existing experimental data.

Mixed Convection in Saturated Porous Media

Abstract: Recent developments in mixed convection in saturated porous media are reviewed. Flow geometries include external flows (flat plates), horizontal layers and annuli, and vertical layers and annuli. Heat transfer results are generally presented in the form of Nusselt numbers as a function of Rayleigh and Peclet numbers. Generally, theoretical advances have left many key problems uninvestigated in the laboratory, and some areas remain where basic knowledge is needed to both test modelling assumptions and reveal fundamental flow phenomena. Nevertheless, a sufficient base of information exists so that correlations of practical value now exist for thermal engineering design for several flow geometries.

Forced and free mixed convection boundary layer flow with uniform suction or injection on a vertical flat plate

Abstract: A laminar forced and free mixed convection flow on a flat plate with uniform suction or injection was theoretically investigated. Nonsimilar partial differential equations are transformed into nonsimilar ordinary ones by means of difference-differential method. The solutions of the resulting equations are obtained with integral forms, and are calculated by the method of successive iteration. The velocity profiles, temperature profiles, friction coefficient and heat transfer coefficient are obtained for various values of suction/injection parameter and buoyancy parameter.

Mixed Convection to Power-Law Type Non-Newtonian Fluids from a Vertical Wall

Abstract: The mixed convection boundary layer flow on a vertical stationary or moving plate to a power-law non-Newtonian fluid is analyzed. An exact similarity solution is derived for the case when the surface temperature is inversely proportional to the distance from the leading edge of the plate. A discussion is provided on the effects of the flow index and buoyancy parameter on the velocity and temperature fields.