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Multiphase Science and Technology
SJR: 0.124 SNIP: 0.222 CiteScore™: 0.26

ISSN Imprimir: 0276-1459
ISSN On-line: 1943-6181

Multiphase Science and Technology

DOI: 10.1615/MultScienTechn.v15.i1-4.20
pages 21-31

FULLY DEVELOPED GAS-LIQUID FLOWS

Thomas J. Hanratty
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign Urbana, Illinois 61801, USA

RESUMO

The prediction of how phases distribute in a flow field is of first order importance in developing a scientific approach to multiphase flow. This involves the specification of the type of pattern and a quantitative description of where the phases are located for a given pattern. A striking feature of the physics is that macroscopic behavior is governed by small-scale interactions. A first step is to identify the critical microphysics by careful scientific (rather than empirical) analyses of results from studies in long pipelines. The "fully-developed" flows, that evolve far enough downstream, offer a simple system to test physical understanding. Furthermore, such studies provide results that can find direct use.
Striking examples of the influence of small scale behavior in multiphase systems are the sensitivity of the flow pattern in a gas-solid fluidized bed to the characteristics of the solid particles and the influence of polymers on the behavior of a gas-liquid flow. An annular flow pattern changes to a stratified pattern because polymers destroy disturbance waves on the wall film (Al-Sarkhi and Hanratty, 2001a, 2001b). This eliminates atonmation mid reduces the ability of the film to climb up the wall against gravity. Polymers have also been observed to delay the transition to slug flow by decreasing the stability of slugs (Soleimani et al, 2002). The shedding of liquid from a slug can be related to the velocity of the gas bubble behind it. Polymers cause an increase in the velocity of this bubble by damping turbulence and, thereby, changing the flow pattern in the slug and the bubble velocity.
This paper illustrates the approach outlined above by considering gas-liquid flows in horizontal or near-horizontal pipes. Stratified, slug and annular configurations are considered.
The following recommendations are made: (1) An understanding of the physics governing the transition from one regime to another should be a top priority. (2) Critical issues in annular flow are the prediction of the fraction of the liquid which is entrained as drops and the development of a physical understanding of how the drops and the wall film distribute asymmerically under the influence of gravity. (3) Several scientific issues that arise in slug flow need more attention. These include the mechanisms by which slugs are formed and the frequency of slugging (particularly, when the time intervals are stochastic). A better model for the flow pattern in a slug is needed to identify how aeration is occurring and to show how the velocity' of the bubble behind a slug depends on slug length (Bernicot and Drouffe. 1991; Fabre and Line, 1992; Barnea and Taitel, 1993). (4) The understanding of interfacial drag is a central problem in stratified, as well as annular flow. (5) Facilities are needed to carry out these integrative experiments.


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