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Hagen-Poiseuille law Hagen-Rubens relation, between electrical and optical constants, Hall Taylor, N S, Halogenated hydrocarbons: Handley and Heggs equation for fixed bed pressure drop, Hankinson and Thomson method, for liquid density: Hardening (precipative) of stainless steels, Hardwick, R, Harris, D, Hausen equation for developing laminar flow, Hays, G F Headers in shell-and-tube heat exchangers, Heads, in heat exchangers: Heat and mass transfer: Heat exchanger design, introduction, Heat exchangers: Heat of vaporisation (see Enthalpy of vaporisation), of pure substances Heat pipes: Heat pumping, relation to heat exchanger network design, Heat storage (see Regenerators and thermal energy storage) entropy generation in, Heat transfer: Heat transfer coefficient: Heat transfer media, Heat transfer salt, Heat transfer regimes:
in boiling in a vertical tube, in free and forced single-phase convection,
Combined Free and Forced Convection in Passages
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Index

HEDH
A B C D E F G H
Hagen-Poiseuille law Hagen-Rubens relation, between electrical and optical constants, Hall Taylor, N S, Halogenated hydrocarbons: Handley and Heggs equation for fixed bed pressure drop, Hankinson and Thomson method, for liquid density: Hardening (precipative) of stainless steels, Hardwick, R, Harris, D, Hausen equation for developing laminar flow, Hays, G F Headers in shell-and-tube heat exchangers, Heads, in heat exchangers: Heat and mass transfer: Heat exchanger design, introduction, Heat exchangers: Heat of vaporisation (see Enthalpy of vaporisation), of pure substances Heat pipes: Heat pumping, relation to heat exchanger network design, Heat storage (see Regenerators and thermal energy storage) entropy generation in, Heat transfer: Heat transfer coefficient: Heat transfer media, Heat transfer salt, Heat transfer regimes:
in boiling in a vertical tube, in free and forced single-phase convection,
Combined Free and Forced Convection in Passages
Heat of vaporization, Heated cavity reflectometer, Heating media, for reboilers, Heavy water, physical properties of, Heggs, P J, Helical coils of circular cross section: Helical coils of rectangular cross section, Helical inserts, for enhancement of heat transfer in boiling, Helium: Helmholtz reciprocity principle, in radiative heat transfer, Henry, J A R, Henry-Fauske model, for critical two-phase flow, Henry's law, for partial pressure, Heptadecane: Heptadecene: Heptane: 1-Heptanol: 1-Heptene: Herman, K W, Hermes, C L L, Heterogeneous conveyance in horizontal pipes, Heterogeneous nucleation in boiling, Hewitt, G F Hexachloroethane (Refrigerant 116): Hexacyclopentane, superheated vapor properties, Hexadecane: Hexadecene: 1,5-Hexadiene: Hexagonal cells, in free convection, Hexamethylbenzene: Hexane: Hexanoic acid: 1-Hexanol: 1-Hexene: Hexylbenzene: Hexylcyclohexane: Hexylcyclopentane, Hicks equation, for fixed-bed pressure drop, High pressure closures, ASME VIII code guidance for, High-chrome steels, thermal and mechanical properties, High-finned tubes, correlations for single-phase heat transfer in flow over, Hills, P D Hohlraum cavity, Holdup, in liquid-liquid flow, Holland, guide to national practice for mechanical design of heat exchangers, Homogeneous condensation (fog formation), Homogeneous model: Homogeneous nucleation: Honeycombs: Hopkins, D, Horizontal condensers: Horizontal cylinders: Horizontal layers, of fluid, free convection heat transfer in, Horizontal pipes: Horizontal shell-side evaporator, Horizontal surfaces: Horizontal thermosiphon reboilers: Horizontal tube-side evaporator, Horizontal tubes: Hottel's rule, in absorption of radiation by gases, Hsu criterion, for onset of nucleate boiling, Hybrid cooling towers, Hydraulic conveyance: Hydraulic expansion, of tubes into tube sheets in shell-and-tube heat exchangers, Hydraulic turbine, lost work in, Hydraulic resistance, in flow of supercritical fluids, Hydraulically smooth surface, Hydrazine: Hydrocarbons: Hydrodynamic entrance length, in single-phase flow in ducts, Hydrogen: Hydrogen bromide: Hydrogen chloride: Hydrogen cyanide: Hydrogen fluoride: Hydrogen iodide: Hydrogen peroxide: Hydrogen sulfide: Hydrostatic testing of shell-and-tube heat exchangers, Hysteresis:
I J K L M N O P Q R S T U V W X Y Z
H Heat transfer regimes: in free and forced single-phase convection,
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Combined Free and Forced Convection in Passages

DOI 10.1615/hedhme.a.000177

2.5.10 Combined free and forced convection in passages

J. D. Jackson

General introduction

The term “combined forced and free convection” is used to describe the process of heat transfer in fluids where the flow field is modified significantly by the action of non-uniformity of gravitational body force as a consequence of the temperature dependence of fluid density. The influence of free convection is usually thought of in terms of the concept of fluid buoyancy. Another term commonly used to describe heat transfer under such conditions is “mixed convection”.

The effectiveness of heat transfer by forced convection as characterised by the Nusselt number NuF depends on Reynolds number and Prandtl number. In the case of free convection the corresponding parameter NuN depends on Grashof number and Prandtl number. Thus, for combined free and forced it is not surprising that Nusselt number depends on Reynolds number, Grashof number and Prandtl number.

In the early studies of convective heat transfer the forced and free convection modes were considered independently with only passing reference being made to any possible interaction between them. When combined free and forced convection did eventually begin to be investigated, attention was at first restricted to laminar and transitional flows. Later, it became clear that measurable influences of free convection could also be present in turbulent flows and that in some circumstances they were a dominant factor in determining the effectiveness of heat transfer under such conditions.

In the following review of combined free and forced convection in passages, attention is focussed on heat transfer in vertical and horizontal pipes. Clearly, the orientation of the pipe is an important parameter under conditions of buoyancy-influenced convective heat transfer. In the vertical case, the flow can be either aided by buoyancy (upward flow in a pipe with heating/ downward flow with cooling) or opposed by buoyancy (downward flow in a pipe with heating/upward flow with cooling). In the horizontal case buoyancy causes secondary, transverse motion to be superimposed on the axial flow. If a horizontal pipe is heated, the fluid tends to have an upward component of velocity over the sides and a downward component in the central region and the secondary flow pattern takes the form of two counterrotating vortices. Cooling, rather than heating, produces similar buoyancy-induced secondary circulation but with rotation in the opposite sense.

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