4.1 Introduction
The flow problems considered in the previous chapter have concerned either
single phases or pseudo-homogeneous fluids such as emulsions and suspensions
of fine particles in which little or no separation occurs. Attention will
now be focussed on the far more complex problem of the flow of multi-phase
systems in which the composition of the mixture may show spatial variation
over the cross-section of the pipe or channel. Furthermore, the two components
may have different in-situ velocities as a result of which there is ‘slip’
between the two phases and in-situ holdups which are different from those in
the feed or exit stream. Furthermore, the residence times of the two phases
will be different.
Multiphase flow is encountered in many chemical and process engineering
applications, and the behaviour of the material is influenced by the properties
of the components, such as their Newtonian or non-Newtonian characteristics
or the size, shape and concentration of particulates, the flowrate of the
two components and the geometry of the system. In general, the flow is so
complex that theoretical treatments, which tend to apply to highly idealised
situations, have proved to be of little practical utility. Consequently, design
methods rely very much on analyses of the behaviour of such systems in
practice. While the term ‘multiphase flows’ embraces the complete spectrum
of gas/liquid, liquid/liquid, gas/solid, liquid/solid gas/liquid/solid and
gas/liquid/liquid systems, the main concern here is to illustrate the role of
non-Newtonian rheology of the liquid phase on the nature of the flow. Attention
is concentrated on the simultaneous co-current flow of a gas and a
non-Newtonian liquid and the transport of coarse solids in non-Newtonian
liquids.
Multi-phase mixtures may be transported horizontally, vertically, or at an
inclination to the horizontal in pipes and, in the case of liquid–solid mixtures,
in open channels. Although there is some degree of similarity between the
hydrodynamic behaviour of the various types of multi-phase flows, the range of
physical properties is so wide that each system must be considered separately
even when the liquids are Newtonian. Liquids may have densities up to three
orders of magnitude greater than gases, but they are virtually incompressible
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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