This research work mainly investigates the implication of void
prediction in the estimation of total pressure gradient in vertical
pipes for multiphase flow systems. Experimental data was collected for a
multiphase flow system with silicone oil and air as the liquid and gas
phases. The void fraction prediction was carried out using Microsoft
Excel. Ten correlations were used for void estimation in chronological
order to include statistical analysis of correlation performance.
Nicklin et al. (1962) drift flux correlation gives the best void
fraction for bubble flow. The prediction from this correlation shows a
fairly constant average absolute error of about 20.98% for low gas rate
flow (bubble). Greskovich and Cooper (1975) give the best prediction for
void fraction in slug flow regime with about 4.84% average absolute
error in void fraction prediction. Hassan and Kabir (1989), show
progressively higher accuracy and stability in the direction of
increasing gas rate with an average absolute error of 6.99% in the churn
flow regime. Hence a good correlation for transitional flow region.
Pressure gradient prediction was carried out using two separate
approaches: the Homogeneous model and the Duns and Ros model (1963).The
statistical parameters used in this study are percentage absolute
average error, average absolute and relative error. The parameters
calculated were compared to determine the performance of the different
correlations evaluated. The realization of this work was used to develop
a quality assurance flow scheme for vertical sections.
Any fluid flow with more than one phase or flow species is termed
‘multiphase flow’. Most real life flow streams are multiphase. Common
examples are hydrocarbon movement either from the reservoir to the
wellbore or in transportation lines, blood flow streams in living
organisms, nuclear fluids in nuclear reactors, etc. Multiphase systems
may be two-phase, three-phase or more in no particular combination of
the states of matter (i.e. liquid-liquid such as in oil droplets in
water, solid-liquid such as in suspensions or gas-liquid-water found in
common hydrocarbon traps).
Multiphase flow is characterized by the simultaneous flow of the
components of the flow stream. Therefore the parameter to be accounted
for in any multiphase system design includes, the volumetric flow rate
(total and phase) [m3/s] , the mass flow rate [kg/s], the mass flux
[kg/m2], phase fraction, distribution term, flow velocities [m/s], slip
values, drift factor and variations of fluid properties as a result of
changes in flow stream (flow patterns).
The summation of the volumetric fraction of all the species in any
multiphase stream is unity and each phase moves with a superficial
velocity as a result of interference by the other phase(s). The mixture
velocity is obtained as the algebraic sum of the superficial velocities
of all the species.
A common subject of interest by investigators in the field of
multiphase streams are the flow regimes: prediction, identification, and
marching, liquid holdup (or void fraction), convective heat transfers
(due to mixing effects), pressure drop prediction and estimation, waxing
and hydrate formation. One of the most challenging factors in a
multiphase investigation or monitoring is the high tendency for flow
stream modifications (i.e. changes in flow regimes), this is because
each of the flow patterns has its unique impact on the flow parameters.
The flow pattern is also very sensitivity to flow line orientation.
Another important factor that affects the flow regime is the fluid
characteristics of the two phases. Most works in literature are reported
for air-water flow map, kerosene-air flow map, air-glycerin, and
air-oil flow map.
1.1.1 Bubble flow
This type of flow pattern is characterized by a small free-gas phase
with the pipe almost completely filled with the liquid phase. Hence a
liquid dominated flow. The gas phase is randomly distributed as small
bubbles with varying diameters. The individual gas bubble moves with
unique velocities as a function of its diameter8. In a riser, the liquid
moves up the pipe at a fairly uniform velocity and, except for its
density, the gas phase has little effect on the pressure-gradient.