Liquids, especially crude oils, refinery products and raw water, are always
transported under turbulent conditions in strategic pipelines. Massive amounts
of pumping power are lost during transportation due to the power dissipation,
which is caused by the turbulent structures that are formed in the flow media.
Due to the velocity difference between the laminar sub layer and the core of
the turbulent flow system, eddies are formed. Their shapes result from energy
absorptions from the main flow. Such energy absorptions lead to losses in the
pumping power that can be sensed as pressure drops across the piping system.
A solution to pumping power losses during transportation through pipelines
is the addition of certain soluble chemicals to the main flow. These chemicals,
which have viscoelastic properties, interfere with turbulent structures and
eventually suppress eddies. More specifically, they prevent eddies from absorbing
more energy from the main flow to reach their final shapes.
The drag reduction phenomenon is a combination of effects of various factors
that happen at a same time and at a given position. Examples of such factors
include pipe geometry, transported liquid properties and the degree of turbulence
inside the pipe. It is known that none of the above-mentioned factors act individually
but that many variables and factors may interact at a given moment to develop
certain turbulent flow structures inside the pipe. The mission of the additive
is to overcome the effect of turbulent flow structures and stop pumping power
Toms (1949) was the first to report the drag reduction
phenomenon and observed that the addition of few parts per million of long-chain
polymers in a turbulent flow produces a dramatic reduction of the friction drag.
This phenomenon has been the subject of extensive reviews by Mowla
and Nadari (2006), Ling and Hassan (2006), Li
et al. (2007), Wan et al. (2008),
Safri and Bouhadef (2008), Riccoa
and Quadrio (2008), Bari and Yunus (2009) Kamela
and Shah (2009), Al-Sarkhi (2010) and many others.
Drag Reducing Agents (DRAs) can be classified into three major categories:
polymers, surfactants and suspended solids (fibres). In general, polymers are
most effective from the industrial point of view and the most usable in the
industrial applications. It is known that, polymers are divided into two categories:
synthetic polymers and natural polymers. Synthetic polymers are derived from
petroleum oil, while natural polymers can be extracted from resources in nature.
Although synthetic polymers possess good mechanical properties and thermal stability
when used as flow improvers in pipelines, these materials biodegrade very slowly,
which is an environmental concern. Moreover, for similar molecular weights,
synthetic polymers are more expensive than natural polymers. On the other hand,
natural polymers are biodegradable and easily obtained. Indeed, these materials
are produced in the form of polysaccharides by microorganisms and plants.
Aloe Vera is an example of plant that contains a high amount of natural polymers.
It is often assumed that Aloe Vera belongs to the cactus family because of the
rosette-like arrangement of the long spiked leaves on the central stem. In fact,
Aloe Vera is a perennial plant from the lily family (Liliaceae), not the cactus
family. There are over 300 species of Aloe known and Aloe Vera L. is known as
the true Aloe Vera and is famous for its various applications and purported
healing virtues. Aloe Vera gel is the commercial name given to the fibre-free,
mucilaginous exudates that are extracted from the hydroparenchyma of the succulent
leaves of Aloe Vera. The viscous, pseudo plastic nature of Aloe Vera gel is
mainly due to the presence of polysaccharides. These polysaccharides are a mixture
of acetylated glucomannans that are lost shortly after extraction, apparently
due to enzymatic degradation.
MATERIALS AND MATHODS
A build-up system consisting of a horizontal test section and instrumentations
was used to fulfil the investigation requirements. The system included a pipe
made of transparent PVC. The pipe had a 0.0254 m Internal Diameter (ID) and
a length of 2 m to permit visual observation of the flow pattern. Each pipe
was divided into four pressure testing sections. The distance between two sections
was 0.5 m. The first pressure testing point for each pipe was located at a distance
corresponding to about 50 times the pipe diameter (50D) as shown in Fig.
1. This test was performed to ensure that the turbulent flows were fully
developed before the testing point.
The fluid flow rate in pipelines was measured by a Ultraflux Portable Flow
meter, Minisonic P, in which the ultrasonic flow measurement is sensitive to
changes in flow rate as low as 0.001 m sec-1. This exterior portable
ultrasonic measurement was used to avoid any disturbance in the flow pattern.
Baumer Differential Pressure Gauges were used to detect pressure drops in pipelines
with maximum differential pressure readings of up to 0.16 bar.
Experimental procedures: The experimental work was carried out in a
pipe having a length of 2.0 m and a diameter of 0.0254 m. Before the addition
of polymers in the tank, pressure drop was measured as a function of water flow
rate. The results were used in drag reduction calculations in which the drag
reduction in pipes is defined as:
where, ΔP1 is the pressure drop before the addition of drag reducers and
ΔP2 is the pressure drop after the addition of drag reducers.
|| Closed loop circulation system
Aloe Vera mucilage was tested at different concentrations. The concentrations
were 100, 200, 300 and 400 ppm. For each concentration, different flow rates
and different testing lengths were tested and the pressure drop readings were
taken and compared with the readings for pure water.
In this study, the Reynolds number is defined as:
The fanning friction factor was calculated from the equation of Yunus and Cimbala
where, ρ, V and μ are the density, velocity and viscosity of water,
respectively, D is the internal diameter of the pipe and L is the length of
RESULTS AND DISCUSSION
Figure 2 shows the relationship between drag reduction and
Reynolds number (NRe) with various polymer concentrations in a pipe having an
inner diameter of 0.0254 m and a length of 2.0 m. It can be noticed that the
%Dr generally increases with increasing NRe. Different patterns can be observed
depending on the factors that were tested in the experiments (e.g., additive
concentration and pipe geometry).
||Effect of the Reynolds number on the drag reduction percentage
for various additive concentrations, with D = 0.0245 m and L = 2.0 m
In most cases, the %Dr increases with increasing NRe until a certain value
is reached. The %Dr then decreases, which can be clearly seen for each concentration
(i.e., 100 to 400 ppm) in Fig. 2. The maximum value for the
%Dr was between 60 and 65% for the highest concentration (i.e., 400 ppm) and
for a NRe between 7000 and 10000. The minimum value for the %Dr was 1% and was
observed for a concentration of 100 ppm. Further, it can be observed that an
increase in the Re results in a gradual decrease in the %Dr.
When the NRe increases from 7000 to 10000, the degree of turbulence increases.
As the number of collisions between eddies is also increased, smaller eddies
are obtained. It is easier to suppress smaller eddies with polymer additives
than larger eddies as the amount of energy absorbed by smaller eddies is lower.
The decrease in %Dr after it has reached a maximum value at a NRe of 10000 is
due to a low ratio between additive concentration and degree of turbulence.
For a low ratio, the polymeric structure of polysaccharides is easy to break.
This results in a drastic reduction in the %Dr value. These results agrees well
with most of the published results for all the drag reducing agents introduced
where it agrees well with the results published by Virk (1975),
Ptasinski et al., (2001), Escudier
et al. (2009), Suali et al. (2010)
and Japper-Jaafar et al. (2009).
The results of the drag reduction experiment for Aloe Vera as a function of
additive concentration and NRe are shown in Fig. 3. It can
be seen that the drag reduction continuously increases until it reaches the
maximum concentration of 40 ppm. It can also be seen that for each concentration,
the highest drag reduction percentage was obtained for a NRe of 7736. The highest
drag reduction was obtained for a concentration of 400 ppm and a NRe between
7000 and 10000.
||Effect of the additive concentration on the drag reduction
percentage for various Reynolds numbers, with D = 0.0245 m and L = 2.0 m
The highest value was 62% and the lowest value was 2%, which was obtained
for a NRe of 30945.
These results indicate that increasing the additive concentration means increasing
the turbulence spectrum that is under the drag reducer effect. This may be due
to the increase in the number of polymeric molecules that influences the strength
of the degree of turbulence which will lead to the increase in the drag reduction
efficiency. These results agrees well with the results published by Virk
(1975), Japper-Jaafar et al. (2009), Cho
et al. (2007), Wei et al. (2009) and
Bari et al. (2008).
Figure 4 represents the %Dr as a function of the pipe length.
Thus, it is a representation of the relation between the stability of the extracted
polymer against the shear forces and the degree of turbulence inside the pipe.
It is clear that the efficiency of the additive reaches maximum operating values
after a certain distance. The optimum drag reduction (%Dr is 30%) was observed
at a pipe length of 1.0 m. The %Dr decreased for longer distances and the effect
of shear degradation started to appear. Indeed, the polymer molecules were subjected
to shear forces for a longer period of time, resulting in a loss of efficiency.
To explain this behaviour, it is very important to understand the relation
between the degree of turbulence, the pipe length, the additive concentration
and the additive type. It is believed that the degree of turbulence increases
by increasing the length due to the increase in eddy collisions inside the pipe.
That will lead to increase the effectiveness of the additives as mentioned earlier.
At this point, the concentration and type of the drag reducing agent will interfere
in deciding the effect of pipe length on the drag reduction behaviour. The combination
between all the factors mentioned above will lead to improve the flow in pipelines,
that, it is believed that all the factors will reach its optimum integrated
effect after 1 m length in the testing section where the concentration, NRe
and pipe diameter will show optimum conjugation.
||Effect of the pipe length on the drag reduction percentage
for various additive concentrations, with D = 0.0254 m and NRe = 41260.546
That does not mean that this must be the optimum condition for the maximum
%Dr and it must be an indication about how important controlling more than one
variable in the same time for better drag reduction performance.
Correlation: In the present investigation, a dimensional analysis was
performed to assemble significant quantities in a dimensionless group and reduce
the number of variables. It is rather difficult to choose the appropriate variables
that influence the friction factor (f) in the case of drag reduction. As this
factor is influenced by the solvent physical and flow properties, we started
with the following relation:
By applying the dimensional analysis using the Buckingham π theorem, the
following non-dimensional relation was proposed:
The values of the constants a, b and k that give the best fitting of the experimental
data were then determined with the least square method and a computer program
||Predicted versus observed values for the friction factor
The following equation was obtained:
Figure 5 represents the relation between the values for the
friction factor that were experimentally observed and those that were predicted
by mathematical correlation. It can be seen that most points are on or close
to the straight line, thereby indicating a good agreement between theoretical
and experimental data.
||Aloe Vera mucilage is introduced for the first time as natural
drag reducing agent in pipelines carrying aqueous media in turbulent flow
||The maximum drag reduction percentage that was achieved was 63%, indicating
that a power savings of up to 63% can be achieved by the addition of Aloe
Vera mucilage at a concentration as low as 400 ppm
||The %Dr was found to increase by increasing the liquid velocity to certain
extent when the degree of turbulence balances the effect of the additives
in the main flow and any further increase in the flow will reduce the effect
of the additive in the same concentration
||Generally, the %Dr was found to increase by increasing the addition concentrations
and pipe length
The University Malaysia Pahang is gratefully acknowledged for providing materials
||Pipe cross section area (m2)
||Drag reduction percentage (%)
||Pipe internal diameter (m)
||Volumetric flow rate (m3 sec-1)
||Average velocity (m sec-1)
||Pressure drop (bar)
||Density (kg m-3)
||Viscosity (kg m1 sec-1)