Rheological measurements drag reduction

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. 

Drag reduction of xanthan gum solutions is greatly reduced at high salt concentrations [Rochefort and Middleman, 1987]. From their rheological measurements, Rochefort and Middleman concluded that the xanthan molecules underwent a transition from a highly extended semiflexible coil with disordered backbone structure, to a collapsed, semirigid rod with an ordered backbone structure at high salt concentrations. 

The difference in the drag-reduction behavior of the fresh and the two presheared solutions was subsequently examined in terms of their rheological and molecular characteristics. These characteristics included the measurement of steady, dynamic, and extensional properties as well as molecular-weight distribution (MWD) (Moussa et al., 1990). The MWD measurements were conducted using the GPC technique and are shown in Fig. 3. The results showed that the fresh solution had two MW peaks, one at around... 

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