Abstracts - KTH Mechanics

Influence of inlet conditions on flow patterns in oil-water flows
in horizontal tubes at intermediate Eötvös number
B. Grassi ∗ , P. Poesio ∗ ,E.Piana ∗ , A. M. Lezzi ∗ and G. P. Beretta ∗
Liquid-liquid flows have been used in many industrial applications: for instance, in
petroleum industry water is added to oil to reduce the pressure drop in transportation
pipelines. In that respect, one of the flow patterns which appears to be most attractive
is the so called ’core-annular flow’, in which a central core of more viscous fluid (oil)
is surrounded by a thin annulus of less viscous one in contact with the pipe wall1 .It
is, therefore, of great importance to analyse the parameters to operatively create such
configuration, and to evaluate the most efficient combination of all the variables (in
particular the water holdup) in order to reduce the pressure drop as much as possible.
This paper focuses on the influence of inlet conditions on the development of
core-annular flow for a two-phase oil-water flow in a horizontal tube in the case of
Eo ≈ o(1), where the definition of the Eötvös number Eo = ∆ρ·g·D2
c·σ for a liquid-liquid
two-phase flow is given by Brauner2 with c =8.
From the viewpoint of the flow pattern maps, systems characterised by Eo ≪ 1
resemble reduced-gravity systems3 , while systems with Eo ≫ 1 show a behaviour
which can be considered similar to that of gas-liquid systems. In the present work,
two fluids with a consistent density difference and a small diameter (2.1 cm inner
diameter) glass tube have been chosen for the experimental campaign, so that both the
buoyancy and the superficial tension can have a strong influence on the establishing
flow pattern.
For Eo ≈ o(1), our experimental evidence shows that inlet conditions are crucial in
the development of a core-annular configuration: as an example, it has been observed
that a core-annular flow-pattern is promoted by injecting the oil into the water already
in the desired configuration, that is, by guiding the oil flux through an internal tube
concentric with the external glass pipe, while the water flows in the cavity between
the two conduits, in the initial stretch of the system. The injection of oil in water can
be realised in a number of ways, depending on which different results are obtained:
in particular four different types of injection procedures are studied.
The tests provide information on oil and water flow rates which are necessary to
generate a core-annular configuration (i.e., locus of core-annular existence on the flow
pattern map expressed in terms of superficial fluid velocities), on oil-water interface,
and on the pressure drop reduction with respect to the case of oil alone.
The present experimental investigation supports the conclusion that, for systems
characterised by intermediate Eo, the interpretation of flow pattern maps should be
related to the analysis of the inlet conditions, as speculated by Brauner4 .
∗ Università degli Studi di Brescia, Via Branze 38, 25123 Brescia, Italy.
1 Joseph, Powder Technology 94, 211-215 (1997).
2 Brauner, HEDU: The update journal of heat exchanger design handbook 5-1, 1-40 (1998).
3 Andreini, Greeff, Galbiati, Kuklwetter and Sotgia, International symposium on liquid-liquid
two-phase flow and transport phenomena, Antalya, Turkey, 3-7 (1997).
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