Holdup
Because of the considerable differences in the physical properties (particularly
viscosity and density) of gases and liquids, the gas always tends to flow at a
higher average velocity than the liquid. Sometimes, this can also occur if the
liquid preferentially wets the surface of the pipe and therefore experiences a
greater drag. In both cases, the volume fraction (holdup) of liquid at any point
in a pipe will be greater than that in the mixture entering or leaving the pipe.
Furthermore, if the pressure falls significantly along the pipeline, the holdup of
.liquid will progressively decrease as a result of the expansion of the gas
Predictive methods for horizontal flow
Methods available for the prediction of the average value of liquid holdup fall
into two categories: those methods which are based on models which utilise
information implicit in the flow pattern and those which are entirely empirical.
Taitel and Dukler [1980] have developed a semi-theoretical expression for the
average liquid holdup and the two-phase pressure gradient for the stratified
flow of mixtures of air and Newtonian liquids. Although such analyses attempt
to give some physical insight into the flow mechanism, they inevitably entail
gross simplifications and empiricism. For instance, Taitel and Dukler [1980]
assumed the interface to be smooth and the interfacial friction factor to be
the same as that for the gas, but this model tends to underestimate the twophase
pressure drop. This methodology has subsequently been extended to the
stratified flow of a gas and power-law liquids [Heywood and Charles, 1979].
Similar idealised models are available for the annular flow of gas and powerlaw
liquids in horizontal pipes [Eissenberg and Weinberger, 1979] but most
of them assume the liquid to be in streamline flow and the gas turbulent
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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