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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Absorbing media, interaction phenomena in, Absorption of thermal radiation: Absorption coefficient, Absorption spectra in gases, Absorptivity: Acentric factor: Acetaldehyde: Acetic acid: Acetic anhydride: Acetone: Acetonitrile: Acetophenone: Acetylene: Acetylenes Ackerman correction factor in condensation, Acoustic methods, for fouling mitigation, Acoustic vibration of heat exchangers, Acrolein: Acrylic acid: Active systems for augmentation of heat transfer: Additives: Adiabatic flows, compressible, in duct, Admiralty brass, Advanced models for furnaces, Agitated beds, heat transfer to, Agitated vessels, Ahmad scaling method for critical heat flux in flow boiling of nonaqueous fluids, Air: Air-activated gravity conveyor, Air-cooled heat exchangers: Air preheaters, fouling in, Albedo for single scatter in radiation, Alcohols: Aldehydes: Aldred, D L, Allyl alcohol: Allyl chloride (-chloropropane) Alternating direction (ADR) method, for solution of implicit finite difference equations, Aluminum, spectral characteristics of anodized surfaces, Aluminum alloys, thermal and mechanical properties, Aluminium brass, Ambrose-Walton corresponding states method, for vapour pressure, Amides: Amines: Ammonia: tert-Amyl alcohol: Analogy between heat and mass and momentum transfer Analytical solution of groups, for calculation of thermodynamic Anelasticity, Angled tubes, use in increasing flooding rate in reflux condensation, Aniline: Anisotropy of elastic properties, Annular distributor in shell-and-tube heat exchangers, Annular ducts: Annular (radial) fins, efficiency Annular flow (gas-liquid): Annular flow (liquid-liquid), Annular flow (liquid-liquid-gas), Anti-foulants, Antoine equation, for vapour pressure, Aqueous solutions, as heat transfer media, Arc welding of tubes into tube sheets: Archimedes number, Area of tube outside surface in shell-and-tube heat exchangers: Argon: Arithmetic mean temperature difference, definition, Armstrong, Robert C Aromatics: ASME VIII code, for mechanical design of shell-and-tube heat exchangers: Assisted convection: Attachment, of fouling layers, Augmentation of heat transfer Austenitic stainless steels, Average phase velocity in multiphase flows,
Introduction and Fundamentals
Axial flow reboilers, Axial wire attachments, for augmentation of condensation, Azeotropes, condensation of

Index

HEDH
A
Absorbing media, interaction phenomena in, Absorption of thermal radiation: Absorption coefficient, Absorption spectra in gases, Absorptivity: Acentric factor: Acetaldehyde: Acetic acid: Acetic anhydride: Acetone: Acetonitrile: Acetophenone: Acetylene: Acetylenes Ackerman correction factor in condensation, Acoustic methods, for fouling mitigation, Acoustic vibration of heat exchangers, Acrolein: Acrylic acid: Active systems for augmentation of heat transfer: Additives: Adiabatic flows, compressible, in duct, Admiralty brass, Advanced models for furnaces, Agitated beds, heat transfer to, Agitated vessels, Ahmad scaling method for critical heat flux in flow boiling of nonaqueous fluids, Air: Air-activated gravity conveyor, Air-cooled heat exchangers: Air preheaters, fouling in, Albedo for single scatter in radiation, Alcohols: Aldehydes: Aldred, D L, Allyl alcohol: Allyl chloride (-chloropropane) Alternating direction (ADR) method, for solution of implicit finite difference equations, Aluminum, spectral characteristics of anodized surfaces, Aluminum alloys, thermal and mechanical properties, Aluminium brass, Ambrose-Walton corresponding states method, for vapour pressure, Amides: Amines: Ammonia: tert-Amyl alcohol: Analogy between heat and mass and momentum transfer Analytical solution of groups, for calculation of thermodynamic Anelasticity, Angled tubes, use in increasing flooding rate in reflux condensation, Aniline: Anisotropy of elastic properties, Annular distributor in shell-and-tube heat exchangers, Annular ducts: Annular (radial) fins, efficiency Annular flow (gas-liquid): Annular flow (liquid-liquid), Annular flow (liquid-liquid-gas), Anti-foulants, Antoine equation, for vapour pressure, Aqueous solutions, as heat transfer media, Arc welding of tubes into tube sheets: Archimedes number, Area of tube outside surface in shell-and-tube heat exchangers: Argon: Arithmetic mean temperature difference, definition, Armstrong, Robert C Aromatics: ASME VIII code, for mechanical design of shell-and-tube heat exchangers: Assisted convection: Attachment, of fouling layers, Augmentation of heat transfer Austenitic stainless steels, Average phase velocity in multiphase flows,
Introduction and Fundamentals
Axial flow reboilers, Axial wire attachments, for augmentation of condensation, Azeotropes, condensation of
B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A Average phase velocity in multiphase flows,
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Introduction and Fundamentals

DOI 10.1615/hedhme.a.000153

2.3.1 Introduction and fundamentals

G. F. Hewitt

A. Classification of multiphase flows

Surveys carried out on industrial heat exchanger systems have indicated that more than half of these involve multiphase flow in one form or another. Multiphase flow’s are ubiquitous in the power generation and process industries and have a very wide range of applications. Such flows are often extremely complex in nature and it should be stated at the outset that many of the relationships used for multiphase flows are of an essentially empirical nature, are of limited applicability, and reflect the poor physical understanding of many two-phase flow phenomena.

This part of the handbook deals with a variety of multiphase flows in which the phases passing through the system may be solid (denoted by the subscript s), liquid (denoted by ℓ ), or gas 1 (denoted by g ). Some of the characteristic features associated with the behavior of each of these phases in multiphase flows are as follows:

  1. Solids: Normally, the solid phase is in the form of lumps or particles. To all intents and purposes, the solid phase can be regarded as incompressible and to have a nondeformable interface with the fluid phase or phases with which it is flowing. The flow characteristics are strongly dependent on the size of the individual solid elements and on the motions of the associated fluids. Very small particles follow the fluid motions whereas larger particles are less responsive to turbulent eddies in the fluid. Normally, the size is nonuniform and a knowledge of the particle size distribution is of great significance in studying such flows. More often than not, the solid is denser than the associated fluid phases and, in horizontal flow systems, this can give rise to gravitational separation or stratification. Solid particles may adhere to channel walls as permanent fouling layers, and these layers can often be very significant resistances to heat transfer. Examples here would be the deposition of magnetite particles on the tubes of a boiler or deposition of crystalline solids in a cooler crystallizer.

  2. Liquid: In multiphase flows containing a liquid phase, the liquid can be the continuous phase, containing dispersed elements of solids (particles), gases (bubbles), or other liquids (drops). The liquid phase can also be discontinuous, for example, in the form of drops suspended in a gas phase or in another liquid phase. With the exception of some special kinds of non-Newtonian liquids, liquids differ greatly from solids in their response to deforming forces. In solids, provided the force is not too high, a small reversible deformation (elastic) occurs, allowing an equal and opposite force to be transmitted through the solid to balance the imposed force, if the solid is to remain at rest. As a fluid, a liquid does not have this property and a balancing force can only exist if the liquid is in motion. A liquid also differs from a solid insofar as its interface with other fluids (gases or other liquids) is readily deformable. The existence of interfacial tension (which may be regarded as the energy required to form a unit area of interface) tends to limit the deformation. For example, there is a tendency to form spherical droplets when the liquid is the discontinuous phase, such droplets representing the minimum interfacial energy per unit volume of the liquid.
    Another important property of liquid phases relates to wetting. When a liquid phase is in contact with a solid phase (such as the channel wall) and is adjacent to another phase which is also in contact with the wall, there exists at the wall a triple interface, and the angle subtended at this interface

  3. Gas: As a fluid, a gas lias the same properties as a liquid in its response to forces. However, it has the important additional property of being (in comparison to liquids and solids) highly compressible. Notwithstanding this property, many multiphase flows containing gases can be treated as essentially incompressible, particularly if the pressure is reasonably high and the Mach number with respect to the gas phase is low (e.g., < 0.2).

Having made some general statements about the properties of the various phases that make up multiphase flows, the common forms of multiphase flow will now be considered and examples given of their applications.

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