(171ac) The Effect of Mixing Conditions on Drag Enhancement of Polymer Solutions in Pipes

In many industrial applications involving fluid flow at turbulent conditions, polymers are added at dilute concentrations to the flow causing significant reduction in the required differential pressure, leading to reduction of pumping energy cost [1,2]. This phenomenon, known as drag reduction, often results in positive values in the turbulent region. However, in the less studied laminar region, negative drag reduction or drag enhancement can be observed for certain additives and under a certain range of concentrations. In our previous work [3], it was suggested that this phenomenon, which negatively impacts the polymer’s performance, resuls from the incorrect use of the solvent viscosity in the conventional drag reduction formula, and can be eliminated if the polymer solution viscosity is used. This conclusion was found to be applicable only to low concentrations, as higher concentrations exhibited drag enhancement even when the corrected drag reduction formula was used. This suggests that at higher concentrations, physical factors may also be responsible for drag enhancement. In this work, we investigated how varying the degree of mixing can play a role in reducing or even eliminating drag enhancement at varying polymer concentrations.

Methods

An industrial-scale fluid flow loop (Charlton & Hill Welding LTD, Alberta, Canada) was used to introduce turbulent mixing of the polymer solutions, and to also test the drag reduction performance of polymer solutions of partially hydrolized polyacrylamide- HPAM at concentrations ranging from 5 ppm to 150 ppm. From each experiment, six samples were collected at varying time intervals, with the first sample experiencing minimal mixing, and the last sample experiencing maximum mixing in the flow loop device, as seen in Figure 1. Flow sweep rheology tests, spanning the laminar region and the early turbulent region, were then carried out using a rotational rheometer (TA Instruments, HR20) to find the viscosity profile against the shear rate. Using the methods developed in Hashlamoun et al. [3], whereby drag reduction can be caclualted using rheological measurements only, the level of drag enhancement was calculated for each sample. The effect of concentration on drag enhancement was also assessed. In this study, it was assumed that the effect of degradation over time, resulting from the mixing in the flow loop, could be neglected.

Results and Discussion. For low polymer concentrations (c < 50 ppm), the value of drag enhancement calculated using the methods of Hashlamoun et al. [3] was nearly zero in all tested samples. However, as the concentration was increased, drag enhancement was observed at low shear rates (or Re), especially in the samples that experienced minimal mixing. For a given flow loop test, as the mixing continued, the value of drag enhancement diminished. Based on this observation, it is suggested that the level of drag enhancement relies on physical factors such as the degree of mixing and on the polymer concentration.

References

[1] Burger, E. D., Munk, W. R., & Wahl, H. A. (1982). Flow increase in the Trans Alaska Pipeline through use of a polymeric drag-reducing additive. Journal of petroleum Technology, 34(02), 377-386.

[2] Savins, J. G. (1964). Drag reduction characteristics of solutions of macromolecules in turbulent pipe flow. Society of Petroleum Engineers Journal, 4(03), 203-214.

[3] Hashlamoun, K., Mheibesh, Y., Baakeem, S. S., & Nassar, N. N. (2023). Estimation of drag reduction by polymer additives at high reynolds numbers using rheological measurements. Industrial & Engineering Chemistry Research, 62(24), 9572-9586.