ABSTRACT
The concurrent flow of gas/liquid in pipes poses a great challenge
due to the difficulty associated with the flow of fluid. The flow is
characterized by the existence of flow regimes which can be identified
by the geometrical arrangement of the phases in a pipe, with Churn flow
being the least understood flow pattern in vertical pipes because of the
controversies associated with its existence, therefore making it
difficult to be predicted. This work aims at investigating the behavior
of air–silicone oil flows in vertical and horizontal pipes for effective
gas–liquid transportation. To help predict the various flows that exist
in these pipes, a drift-flux model was developed for the efficient
calculation of void fraction. This model is often used to characterize
and predict flow regimes for lots of geometries. The model was developed
to calculate void fraction for the accurate prediction of flow regimes
that were observed in this work. The various flow patterns in existence
were identified, and the model generated for each of them by employing
Zuber and Findlay‟s correlation. Afterwards, the parameters obtained
from the drift-flux model, Co (distribution parameter) and Vd (drift
velocity) were fitted as linear functions, and their values were
obtained from the slope and the intercept respectively. The developed
model had better results for the void fraction as compared to the
existing correlations investigated.
CHAPTER 1
1.1 Introduction
The simultaneous flow of fluids is difficult in a safe and controlled
way, with the exception that various behaviors of the flow can be
predicted with adequate reliability. It deals with the concurrent flow
of fluids within different phases (i.e. gas, liquid and solid) or the
different chemical properties but in the same phase, for example
gas-liquid, gas-solid, liquid-solid, liquid-liquid and gas-liquid-solid
(Abdulkadir, 2015). Multiphase flows are encountered in industries like;
the petroleum, chemical, and nuclear industries. The transportation of
gas- liquid two-phase flow in the petroleum industry over long distance
is quite common. This simultaneous flow is encountered in instances like
the flow of oil from the reservoir to the separator, and to the process
facilities. As pressure decreases, gas starts to evolve, thereby
creating a two-phase flow in the pipeline. Various difficulties are
encountered in the flow of these fluids, some of which are phase
velocity differences and the existence of several flow regimes. These
flow regimes include; bubbly, slug, churn, plug, and stratified, among
many others. The existence of these flow regimes in transportation lines
poses certain challenges to the industry because they increase the
pressure drop, heat transfer, mass and corrosion rate in the pipeline.
Since the accurate prediction of these flow patterns is essential to the
success of designing multiphase flow systems in vertical and horizontal
flows, there is therefore, need to investigate the behavior of the
fluids in pipes of various inclinations, for effective transportation in
the industry.
1.2 Critics of Churn Flow
One may wish to know the reason for this particular subheading, the
critics of churn flow. The topic is discussed because of the various
schools of thoughts and ideologies from different research about the
existence of churn flow regime. The question is, does Churn flow exist
as a distinctive flow pattern or, it is just an extension of slug flow?
To address this, this work is yet to find out as Mao and Dukler, and
Hewitt and Jayanti presented different ideas about the existence of this
particular flow.
According to Mao and Dukler (1993) in their paper, “The Myth of Churn
Flow?” they presented evidence that proved that churn flow pattern is a
simple and continuous extension of the condition of slug flow and that
no transition actually existed. Therefore it is not a distinctive and
separate flow pattern on its own. In view of this, they presented two
different pieces of evidence to buttress their point. These are visual
evidence and instrumental evidence where experiments were performed to
support their findings.
1.2.1 Visual Evidence
This evidence is no different from what other researchers observed in
transparent pipes. Based on this, one can conclude that their
observations proved the existence of churn flow as a unique and separate
pattern that exists as a transition from, slug flow to annular flow.
According to Mao and Dukler, stable slug flow is an upward motion of a
quasi-periodic arrangement of alternating Taylor bubbles and liquid
slugs at a constant speed of propagation, and that the length of the
Taylor bubbles and the liquid slugs remain the same as they rise. The
velocity associated with the bubbles and liquid slugs is uniformly
upward and the same as that which exists in front of both the slug and
bubble. As the gas rate increases, the flow becomes chaotic, and the
size of the liquid slug and Taylor bubbles increase forming lumps of
bubbles as they move up and down the pipe. The flow then becomes
oscillatory and displays irregular periods. Again, the main
characteristic of slug flow is the falling of the liquid film around the
Taylor bubbles and it disappears.