Pipe flow turbulent drag reduction

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 chemical additive injected into the crude oil line caused an appreciable amount of drag reduction [1165]. The effects of additive concentration and pipe diameter on drag reduction have been investigated. 

An exception to the generally observed drag reduction in turbulent channel flow of aqueous polymer solutions occurs in the case of aqueous solutions of polyacrylic acid (Carbopol, from B.F. Goodrich Co.). Rheological measurements taken on an oscillatory viscometer clearly demonstrate that such solutions are viscoelastic. This is also supported by the laminar flow behavior shown in Fig. 10.20. Nevertheless, the pressure drop and heat transfer behavior of neutralized aqueous Carbopol solutions in turbulent pipe flow reveals little reduction in either of these quantities. Rather, these solutions behave like clay slurries and they have been often identified as purely viscous nonnewtonian fluids. The measured dimensionless friction factors for the turbulent channel flow of aqueous Carbopol solutions are in agreement with the values found for clay slurries and may be correlated by Eq. 10.65 or 10.66. The turbulent flow heat transfer behavior of Carbopol solutions is also found to be in good agreement with the results found for clay slurries and may be calculated from Eq. 10.67 or 10.68. 

Basically, shear degradation of polymers in turbulent or laminar flow is caused by the stress imposed on the polymer molecules by the shearing motion of the liquid, whether in solution or a melt. When turbulent drag reduction is the objective, the degradation is primarily caused by the high shear stress region near the pipe wall or other surface where high shear stresses are produced. 

The discovery of turbulent drag reduction due to particle suspensions goes back to the 1930s. Forest and Grierson (3) reported the turbulent drag reduction in pipe flow of wood-pulp fiber suspensions of water. Vanoni (4) observed that water with suspended sand flowed more rapidly in an open channel. Toms (5) and Mysels (6) independently observed the striking reduction in turbulent drag in pipe flows... 

Therefore, in the case of turbulent flow, the drag reduction effect depends by the individual macromolecules stretching degree [1086]. By photographing J.W. Hoyt succeeded to visualise the elongated particles [1087]. The possible extrapolations and the limits of this effect have been defined by Hoyt starting from the friction velocity, u, caused by the friction to the pipe wall, which is expressed as ... 

As with Newtonians, it s a pretty safe bet that flow is laminar for Re < 2100, but drag-reducing additives often seem to delay the laminar-turbulent transition to higher Re s. The friction factor-Reynolds Number relation remains a function of pipe roughness. Unlike the Newtonian case, however, the Re curve seems to depend on pipe diameter when drag reduction is observed. 

A review on drag-reducing polymers is given in the literature [1359]. It has been suggested that drag reduction occurs by the interactions between elastic macromolecules and turbulent-flow macrostructures. In turbulent pipe flow, the region near the wall, composed of a viscous sublayer and a buffer layer, plays a major role in drag reduction. 

In the area of hydrodynamics the term drag reduction has become a familiar name for characterizing the reduction of friction in turbulent flow through pipes. This is not caused by an improvement in the wall properties as this would only lead to a decrease in friction of a few percent. It is possible, however, to reduce the degree of friction in turbulent flow by a considerable extent, if small amounts of a suitable additive in a concentration range of only a few parts per million by weight are used a reduction in friction of 80 percent can be reached. A tremendous change in the flow field will result, while turbulent flow still remains. 

The first reports on the drag reduction phenomenon are found in publications by Blatch (1906), Forrest (1931), as well as by Brautlecht (1933), who measured the flow behaviour of paper pulps. In independent studies made by Mysels (1949) and Toms (1948) this phenomenon was observed in the turbulent flow of gasoline in pipes when aluminum soaps were added and, in Toms experiments, when polymethylmethacrylate in monochlorobenzene was used. The reduction of friction is therefore often termed the Toms effect . This flow behaviour is also known in the literature as the Non-Newtonian- , visco-elastic- , Texas- or Texas-Toms-effect , due to the contributions made by Texan researchers. It is also more generally known, and this term will be used in this paper exclusively, as the drag-reduction effect of flow additives. 

A detailed discussion of the relationship between drag reduction and the random surface renewal in turbulent pipe flow is available in Fortuin s report. 

The main interest of this work, therefore, lay in the field of water-soluble polymers and their influence on drag reduction in turbulent pipe flow, as it is from this field that the major technological use is to be expected. The investigation of the influential parameters for this class of polymers, such as molecular weight and distribution thereof, thermodynamic quality of the solvent, to name but a few (see Sect. 6.3.3), must precede a clear-cut characterization (see Sect. 6.3.1) of the polymer used. 

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Frings B (1988) Heterogeneous drag reduction in turbulent pipe flows using various injection techniques Rheol Acta 27 92... 

McComb WD, Rabie LH (1982) Local drag reduction due to injection of polymer solutions into turbulent flow in a pipe AIChE J 28 547... 

Ogawa K, Kuroda C (1986) Experimental study on the effect of elasticity on drag reduction and turbulent fluctuations in the laminar-turbulent transition region in pipe flow of dilute polymer solutions Can J Chem Eng 64 497... 

Savins JG (1964) Drag reduction characteristics of solution of macromolecules in turbulent pipe flow Soc Petrol Eng J 4 203... 

Usui H, Kodama M, Sano Y (1987) Drag reduction with heterogeneous polymer injection into a turbulent pipe flow Technol Rep Yamaguchi Univ 4 51... 

For the Reynolds number range typical of drag reduction (Re 105), / is about 0.02 from the Moody chart (see Fig. 11.7). The typical turbulent intensity of gas in a pipe flow is about 5 percent. Using the Hinze-Tchen model (see 5.3.4.1), the ratio of the velocity fluctuation of the particles to that of the gas may be given by Eq. (5.196) as... 

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