The study concerning drag reduction in pipes is rapidly becoming an attraction because of its capability to increase the power saving in transporting fluids through pipes. The term of drag usually refer to the mechanical force that resist the movement of fluid in transporting system and the addition a minute amount of surfactant could dramatically reduce this drag. Any chemical additives that capable to reduce the drag in turbulent flow are known as Drag Reducing Agent (DRA). For the past few years, surfactants are well known as a good DRA. Surfactants are usually organic compounds that are amphiphilic, which contain both hydrophobic groups and hydrophilic groups. Therefore, they are soluble in both organic solvents and water. The main reason of choosing surfactant as DRA is probably because of their ability to form micelle.
Lu et al. (1998) found that the Arquad 16-50
capable to reduce drag up to 70% and Kawaguchi et al.
(2002) showed that the 40 ppm of CTAC surfactants capable to achieved 60%
of drag reduction, while Myska and Mik (2004) proved
that CTAC capable to achieve up to 72% drag reduction. The study of CTAC as
drag reducing agent also mentioned in study of Kang et
al. (2001), Zhou et al. (2006), Sung-Hwan
(2007), Ge et al. (2008) and Li
et al. (2008) found that the pressure drop is reduced by 25 to 40%
in a presence of 400 ppm of drag reducing anionic surfactant, Sodium Dodecyl
Sulfate (SDS). This material also mentioned by Cheng et
al. (2007), Varade et al. (2007) and
Duangprasert et al. (2008) study.
There are a lot of mechanisms been proposed in literature behind the drag reduction
in turbulent flow. Li (1991) defined that the structure
of polar group at one end bonds onto side inner wall of pipeline and a non-polar
group at the other side smoothes the gas-solid interface between wall and flowing
gas thereby reducing gas turbulence at side interface while conducting an experimental
work on gas pipelines using drag reducing surfactant, oxylated fatty acid amine.
Lu et al. (1998) suggested that drag reduction
by surfactant additive is probably caused by suppression of turbulent eddies.
Extensional motions dominate in the bursting and growth of these eddies. Kawaguchi
et al. (2002) believe the drag reducing surfactant changes the structure
of turbulent flow and the mechanism of drag reduction in a presence of surfactants
additive probably also occurred near the wall. Myska and
Mik (2004) studied the behavior of drag reducing agent, which CTAC, Arquad
SV-50, DR-0205 and SPE 98330 in term of degradation by age in a simple built
up system. They believe degradation behavior of surfactants is the key to understand
the mechanism of drag reduction.
In the other hand, Zhang et al. (2005) showed
that the rheology properties and the micelle formation in drag reducing surfactant
probably the key source to understand the mechanism of drag reduction in a presence
of surfactants. Li et al. (2008) observed that
the near-wall vortex structures near the walls in the drag reducing surfactant
solution flow are changed gradually with drag reduction level and the viscoelasticity
of solution. This result has a good agreement with the proposed mechanisms in
surfactants solution by Kawaguchi et al. (2002)
and Zhou et al. (2006). Most of the stated mechanisms
agree that the micelle formation in surfactant solution damped the structure
of turbulent flow and the mechanism is likely occurred near the wall.
MATERIALS AND METHODS
Anionic surfactant: Glycolic acid ethoxylate 4-nonylphenyl ether (GAENE) in liquid form supplied by Sigma-Aldrich (M) Sdn. Bhd also known as nonoxynol-8 carboxylic acid is used as an investigated anionic surfactant in this study. It is non-hazardous and contains impurities of ≤1 wt.% NaCl. The viscosity of this material is independent with the solution concentration and used as supplied. The average-number molecular weight (Mn) of GAENE is 600 and 358.47 g mol-1 of Molecular weight (Mw). In this present study, the transported fluid was used is water.
Closed loop system: The built up closed loop circulation system was
used to study the performance of GAENE as drag reducing agent in turbulent flow.
The closed loop made up from PVC pipes with diameter equal to 0.0254 m and the
length is equal to 4 m including the length of testing section, 2 m. The schematic
flow of this circulation system is shown in Fig. 1. This system
consists of pump, tanks, flow meter and pressure gauges. T1 with maximum load
420 L used as a storage solution before the experiment begin. T2 is temporary
tank for solution before the recirculation process. Ultraflux Portable Flow
Meter Minisonic P used to measures the flow rate of fluid in pipe. This portable
flow meter was placed about 1 m before the testing section to ensure the fluid
flow rate are fully turbulent and capable to detects changes in flow rates as
low as 0.001 m sec-1. Baumer pressure gauges used to detect the pressure
drop in pipes before and after the addition of GAENE in closed loop circulation
Each pipe section was completed with control valve while, the bypass pipe was used to control the flow rate in circulation system where, the fluid was pumped through the entire system by Precision pump with maximum load 6.5 m3 h-1. The discharge pipes were prepared for cleaning purpose.
The most important in closed loop circulation is the testing section which consists of three pressure gauges. Each pressure gauge was placed with a gap in length about 0.5 m.
Experimental procedure: An experimental run consisted of establishing the fluid Reynolds number (Re) with and without the anionic surfactant. For each run, the pressure drop measurement were carried out based on surfactant concentration (Vppm) at ratio of pipe length to pipe diameter (L/D) equal to 59.
The friction factor of water alone in pipes was recorded before the measurement for pressure drop with the chemical additives. This is to verify the appropriateness of the experimental system.
The investigated solution was pumped from T1 by circulation pump (P2) through entire closed loop system. The pressure drop in pipe with the addition of surfactant was conducted for 200, 300, 400, 500 and 600 ppm concentration varied with fluid Re from 11235 to 78645. Each experiment was conducted at room temperature.
||The schematic of closed loop system. V1 to V7: Value, P1,
P2: Pump, T1: Solution tank, T2: Temporarily tank, G1, G2, G3: Pressure
gauge, D1, D2, D3: Discharge pipe, D4: Byypass pipe and F1 Flowmeter
RESULTS AND DISCUSSION
Basic mathematical description: The fluid flow in pipe was measured using Eq. 1. Equation one is universally used to distinguish the laminar and turbulent flow, where the relation of Re with the velocity and viscosity of fluid are defined as:
The symbol ρ is fluid density, μ is kinematics viscosity of fluid, V is velocity and L is the length characteristic, in fully filled pipe, L usually the pipe diameter. The friction factor values of tap water in pipe was calculated based on fanning friction factor equation and defined as
where, defined that the wall shear stress in a fully developed pipe flow is related to the pressure drop by the following equation:
where, D is pipe diameter and ΔP is the pressure drop in pipe while, Blasius developed an asymptote to define the relation of friction factor and Re for fully developed turbulent flow in pipes. The relation is defined by the Eq. 4
It is observed that the maximum possible of friction factor in pipes where the drag reduction is insensitive to the drag reducing agent properties is given by the following equation:
In the other hand, the friction factor of laminar flow in pipe defined as:
The drag reduction of pipe flow for incompressible fluid system is obtained based on pressure drop and defined as:
where, ΔPb is the value of pressure drop of tap water before the addition of chemicals additives and ΔPa is the value of pressure drop after the addition of chemical additives in turbulent flow. The drag reduction in turbulent flow for surfactant solutions always had been related to Critical Micelle Concentration (CMC). In analyzing the CMC, equation below are needed to converts the molarity of solution into concentration in ppm unit:
where, M (mol L-1) is molarity, C (ppm) is concentration, Mw (g mol-1) is molecular weight and ρ (kg L-1) is the fluid density.
Verification of circulation system: Figure 2 shows the friction factor of water in pipe compared with friction factor of Blasius and Virk asymptote. It is clearly shown that the fanning friction factor of water lies near the Blasius asymptote and far from the Virk asymptote and laminar equation. This is proved that the experimental system appropriate and suitable to use to investigate the performance of GAENE as drag reducing agent. In addition, this calibration also shows that the flows in testing section are fully turbulent.
Drag reduction in pipe: Figure 3 shows the drag reduction
obtained of GAENE solution against Re with concentration varied from 200 to
600 ppm in pipe. The optimum fluid Re for maximum drag reduction to occur is
within the range of 22470 and 33705. It is clearly shown that the drag reduction
is initially increases as the fluid Re increases but began to decreases when
fluid Re reach to optimum value.
|| Friction factor of water without additive
|| The drag reduction in L/D equal to 59
In addition, the drag reduction obtained in pipe varied with the concentration
of solution. At 200 ppm of solution the maximum drag reduction obtained is 6%.
The drag reduction increased about 2% when the concentration increased to 100
ppm. At 600 ppm, the maximum drag reduction obtained is 14%. From the observation
of figure, the drag reduction valued probably reach to minimum if the fluid
Re keep increasing without any changes on solution concentration.
CMC of GAENE solution: For the past few years, there are a lot of opinion been discussed about the mechanism behind this phenomena. It is believed that the key source of drag reduction in surfactant solution is the formation of micelle in solution. Where, the presence of micelle in solution can be analyzed through graphing method. The Critical Micelle Concentration (CMC) of GAENE can be found by plotting the specific conductivity obtained from GAENE solution against the concentration as shown in Fig. 4. The CMC is the concentration of solution for first appearance of micelle. From figure, it is clearly shown that the second slope occurred at 0.00045 mmol L-1 or 120 ppm. The second slope in figure indicates the CMC of GAENE solution. Any changes of slope occur after the second slope of surface tension against concentration plot indicates the changes of micelle structure in solution. Since the surface tension varied with the specific conductivity of surfactant solution, it is worth presumed that the changes of slope in specific conductivity may indicate the same meaning as in surface tension. Thus, the third slopes occurred in GAENE solution at 0.00165 mmol L-1 or equal to 450 ppm proved there are changes of micelle structure in solution.
|| The micelle behavior of GAENE solution
The changes of micelle structure effect the drag reduction in pipe. However this statement needs more attention in the future.
The maximum drag reduction increased about 5% at 500 ppm compared to 400 ppm, where the maximum drag reduction increased from 300 to 400 ppm is only 1 to 2%. The possible reason of this phenomenon is the changes of micelle structure at 450 ppm of solution improved the performance of GAENE as drag reducing agent.
Drag reduction mechanism: The turbulent flows are formed from random and rapid fluctuations of swirling regions known as eddies. These fluctuations provide additional mechanism for momentum and energy transfer. Each eddy dissipates its energy through viscous shear with its surroundings and eventually disappears. This is the reason of energy loss in pipelines where always interpreted as pressure drop in pipe. The decreased of drag reduction after the optimum value of fluid Re related to capability of micelle to suppress the eddies occurred in turbulent flow. When fluid Re increasing, the eddies in turbulent flow become stronger thus required more micelle to suppress the eddies. As the concentration of solution increases, the network of micelle became stronger thus more eddies can be damped.
When fluid circulated through the closed loop system, the micelle align with the eddies in turbulent flow. As the concentration of surfactants increases, the length of micelle formed also increases. As a result, the skin friction between the flowing fluid and the solid boundary is reduced. Thus less energy required to transfer the fluid to its receptacle. The extensional motions dominate in the bursting and growth of eddies. With the addition of GAENE solution, the apparent extensional viscosity is greatly increased, thus increasing the resistance of the fluid in eddies to extensional flow and suppressing eddy growth.
The rheology properties of micelle such as its capability to reform after the degradation process also important in understanding the mechanism of drag reduction. This is because, most of the micelle network after the circulation process became weaker and this affects its performance as drag reducing agent. Some of the study shows that the mechanisms of drag reduction related to wall shear stress, the velocity fluctuation in turbulent flow, the effect of co-solvent on solution and the experimental condition such as temperatures. In the future, this issue may become interesting.
From the experimental study it can be concluded that glycolic ethoxylate lauryl 4-nonyl phenyl ether can be consider as a good drag reducing agent. The result of discussion leads to conclusion that the concentrations of solution play important role in reducing the drag in turbulent flow. The optimum fluid Reynolds number for maximum drag reduction to occur is within the range 22470 and 33705. The critical micelle concentration occurred at 100 ppm and at 450 ppm the structure of micelle began to changes.
The present study can be improved toward more excellent results. It is highly recommended to study the performance of this anionic surfactant in term of solution ages, the effect of co-solvent and the effect of pipe scale. It is also recommend to keeps the concentration of anionic surfactant below 1000 ppm to avoid the environmental loads.
The author would like to thanks University Malaysia Pahang for providing grant to support this research.