Drag reduction can occur if the transmission of eddies is damped by the viscoelastic properties of fluids. The transfer process of an isolated eddy in viscoelastic Maxwell fluids was studied, and expressions describing such phenomena were obtained (Li, 1991). The results of the study showed that eddy transmission was damped significantly with an increase of the viscoelastic properties of the fluids.

In the extensive literature on polymer drag reduction, it has occasionally been reported that a continuous thread of a high-concentration polymer solution, injected into the axis of a pipe, produces a drag reduction effect on the water flow in the pipe (Hoyt and Sellin, 1988). The thread seems to persist through the length of the pipe and little, if any, diffusion of polymer to the walls of the pipe is apparent.

A polyacrylamide (PAM) polymer was injected as a 0.5% solution from an axially placed nozzle at the bellmouth entrance. The experiments showed that the central thread provided drag reduction that was almost equivalent to premixed solutions of the same total polymer concentration flowing in the pipe. Overall concentrations of 1–20 ppm were used.

The effects were additive: 2 ppm overall thread concentration plus 2 ppm of premixed polymer gave drag reductions equivalent to 4 ppm of either type. Reynolds numbers of up to 300,000 were investigated. In other experiments, a number of different polymer fluids were injected on the centerline of a water pipe flow facility (Hoyt and Sellin, 1991). Two distinct flow regions were identified:

Drag reduction in turbulent flow is of great potential benefit to many industrial processes, including the long distance transportation of liquids, oil well operations, and transportation of suspensions and slurries, but it is complicated by the problem of polymer degradation.

A capillary rheometer was used to investigate the effect of various parameters on polymer degradation in turbulent flow (Moussa and Tiu, 1994). These parameters included polymer concentration, contraction ratio, pipe length, pipe diameter, number of passes, solvent weight, and molecular weight of polymer. A commercial organic drag reducer, two grades of PAM, and a high molecular weight polyisobutylene were used.

In turbulent flow, the polymer degraded more in a poor solvent at low Reynolds numbers, whereas an opposite effect was observed at high Reynolds numbers. The critical Reynolds number, Re_c or critical apparent shear extensional rate, V/d, was found to increase with polymer concentration and molecular weight, as represented by the dimensionless concentration c(η).

Polysaccharide guar gum is used as a turbulent drag reducer in aqueous systems. It reduces the friction drag tremendously in turbulent flow even in small amounts. A study on the mechanical degradation of guar gum has been presented, in which the effectiveness of drag reduction was measured as a function of time using a rotating disk apparatus.

Two different degradation models of a single-relaxation process and a stretched-exponential model were examined. The stretched-exponential model seemed to fit the experimental data better (Hong et al., 2010).

The drag-reducing properties of a PAM were tested in two-phase air/water flow, using a horizontal pipe of 31 mm diameter (Saether et al., 1989). The properties of the polymer were tested in single-phase water flow, and the results were found to comply with the reduction in pressure drop found by other workers. Positive effects in two-phase flow were found to depend on the Reynolds number of the liquid flow.

The drag reduction in stratified flow was found to be small or negative. In slug flow, the drag reduction seems to occur in the liquid slug, not in the layer below the bubble. The flow regime seems to be unaffected by the polymer. It has been established that in multiphase flow, drag reducers also act as corrosion inhibitors because they smooth the flow profile near the walls (Kang et al., 1998).

For storage or pipeline transportation of natural gas at pressures over 5.5 M Pa (800 psi), it is advantageous to add ammonia to the natural gas, but the ammonia should not create a liquid phase at the temperature and pressure used. A gaseous mixture of ammonia and natural gas can be compressed or pumped using less energy than would be needed for an equivalent volume of natural gas alone. When more than 4% by volume of ammonia is present, pumping through pipelines is also aided by the refrigerant effect of the ammonia, which reduces the temperature of the gas being transported (Morris and Perry, 2001).

Friction loss in liquids can be reduced by adding a predetermined amount of selected organo-polymeric microfibrils to a liquid (Shinomura, 1988). These microfibrils are insoluble but highly dispersible in the liquid.

An organo-polymeric microfibril is a solid, organic polymer in the form of microfibrils, which have an average diameter in the range of 100–1000 Å, of an average length of 1–500 μ, and an aspect ratio (length/diameter) of 10–1,000,000. Polymeric materials to be processed into microfibrils should be insoluble but highly dispersible in a given liquid.

The behavior of two types of drag-reducing surfactant solutions was studied under turbulent flow in pipes of different diameters (Bewersdorff and Ohlendorf, 1988). The surfactant systems contained rod-like micelles consisting of equimolar mixtures of n-tetradecyltrimethylammonium bromide, n-hexadecyltrimethylammonium bromide, and sodium salicylate.

The structure of the turbulence was studied using a laser-Doppler anemometer in a 50 mm pipe. In the regions of turbulent flow, both surfactant solutions exhibited characteristic flow regimes. In the regions of turbulent flow at low Reynolds numbers, velocity profiles similar to those observed for dilute polymer solutions were found, whereas at maximal drag reduction conditions, more S-shaped profiles that show deviations from a logarithmic profile occur.

Experiments have been conducted to investigate the effect of a soapy industrial cleaner on reducing the skin friction of a Jordanian crude oil flowing turbulently in pilot-scale pipes of different sizes. Experiments showed that a concentration of only 2 ppm of the additive injected into the crude oil line caused an appreciable amount of drag reduction (Mansour and Aldoss, 1988). The effects of additive concentration and pipe diameter on drag reduction were investigated.

An experimental study was conducted on the characteristics of frictional drag for a lyophobic surface made of polytetrafluoroethylene (PTFE), with a working media of water and machine oil (Saether et al., 1989). The test results indicate that, depending on the lyophobic performance of the lining material, the pipes lined with PTFE have a better drag-reducing effect than conventional steel pipes.

A drag reduction of approximately 12% is achieved if the working medium is water or 6% with machine oil, respectively. In other words, PTFE has a higher lyophobic performance against water than against machine oil.

The theoretical analysis of the flow mechanism on the lyophobic surface shows that treatment can lower the surface energy level to such a degree that the attraction of the solid wall to liquid molecules becomes weaker than the liquid molecular absorption. This effect causes a gliding flow adjacent to the pipe wall, thus reducing the drag.

It has been shown that hydrogen bonding-mediated interpolymer complexes can be powerful drag reducers. The drag reduction levels in such polymer systems are a factor of 2–6 greater than their nonassociating polymeric precursors. Their shear stability is also shown to be significantly enhanced (Malik and Mashelkar, 1995).

Hydrocarbon-soluble polymers containing small percentages of polar associating groups are used to determine the effects of polymer associations on solution drag reduction. Experimental data suggest that intrapolymer associations generally decrease the dilute solution drag reduction activities of single associating polymers with like polar groups (Kowalik et al., 1987).

Interpolymer complexes formed by a polymer with anionic groups and one with cationic groups can overcome this limitation and provide enhanced, dilute solution drag reduction activity as a result of favorable interpolymer associations, which build larger structures of higher apparent molecular weight. The latter associations may also increase the resistance of the polymers to degradation in turbulent flows.

The flow of liquid hydrocarbons can be enhanced by introducing a nonagglomerating suspension of ultra-high molecular weight polyethylene (UHMWPE) (Dindi et al., 1996; Smith et al., 1995) in water with small amounts of surfactant. The finely divided UHMWPE is prepared by polymerization and then cryogrinded below its glass transition temperature.

Several copolymers of α-olefins are used as drag reducers. Suggested recipes are summarized in Table 12.1 and monomers are shown in Figure 12.1.

Linear, low density polyethylene is a copolymer of ethylene and α-olefins, obtained by copolymerization utilizing Ziegler-Natta or metallocene catalysts. Concentrates may be prepared by precipitating the polymer from a kerosene solution with isopropanol (Fairchild et al., 1997). The resulting slurry concentrate dissolves rapidly in flowing hydrocarbon streams.

By coating poly-α-olefins with a fatty acid wax as a partitioning agent, and dispersing it in a long chain alcohol, a nonagglomerating, nonaqueous suspension can be obtained (Johnston and Lee, 1998).

Latex drag reducers comprise a polymer that is formed via an emulsion-polymerization reaction dispersed in a continuous phase. Subsequent modifications can be applied in order to increase the solubility of the polymer in hydrocarbons.

2-ethylhexyl methacrylate is polymerized by conventional emulsion polymerization techniques, details of which can be found elsewhere (Milligan et al., 2008). The emulsion polymerization reaction yields an initial latex composition, as a stable colloidal dispersion.

The dispersed phase is made up of up to 50% of colloidal particles of the high molecular weight polymer. The continuous phase is water and surfactant. The latex can be modified or formulated with additional surfactants and organic solvents to increase the viscosity. The drag reducer may be injected into the pipeline using conventional or umbilical delivery systems.

A liquid oil, an emulsifier, and a friction modifier, which includes certain polyether compounds, can be added to a drilling fluid consisting of a water-in-oil emulsion formed from a brine (Malchow, 1997). The friction modifier serves to decrease the coefficient of friction of the well drilling fluid.

Decreasing the coefficient of friction lowers the force required to turn the drill bit in the hole. Gravitational forces increase the coefficient of friction in deviated, horizontal, and extended-reach wells.

Tylose is not as effective in drag reduction as other substances described in the literature. Detailed mean velocity, normal Reynolds stress, and pressure drop measurements were performed with 0.4–0.6% aqueous solutions of tylose, a methylhydroxyl cellulose (molecular weight 6 k Dalton), after a selection process from a set of low molecular weight fluids (Bewersdorff and Ohlendorf, 1988).

The measurements of the viscosity of these solutions showed shear thinning behavior, and the oscillatory and creep tests measured elastic components of the stress in the order of the minimal detectable values by the rheometer. These low molecular weight polymer solutions delay the transition from laminar to turbulent regimes and show drag reductions of approximately half those which occur with other low elasticity, shear thinning, high molecular, aqueous polymer solutions.

Highly concentrated DRAs may be prepared by microencapsulating a polymer or a monomer, which may be performed before, during, or after polymerization. If the encapsulation is done before or during polymerization, a catalyst must be present, but little or no solvent is required. The result is bulk polymerization within the microcapsule.

The inert capsule or shell may be removed before, during, or after the introduction of the microencapsulated drag reducer into a flowing liquid. No injection probes or other special equipment should be required to introduce the drag-reducing slurry into the liquid stream, nor is grinding (cryogenic or otherwise) of the polymer necessary to form a suitable DRA (Kommareddi and Rzeznik, 1999 and Kommareddi and Rzeznik, 2000).

Aluminum-carboxylate-based DRAs are non-polymeric drag-reducing agents. These additives are not subject to shear degradation and do not cause undesirable changes in the emulsion or fluid quality of the fluid being treated, or undesirable foaming.

The compositions consists of an aluminum carboxylate and fatty acids. The aluminum carboxylates are selected from aluminum salts of fatty acids, including octoates, stearates, oleates, or naphthenates (Jovancicevic et al., 2007). The fatty acids are selected from long chain carboxylic acids. Aluminum salts of a combination of short and long chain carboxylic acids may provide an optimum balance between drag reduction and change in viscosity.