Polymer-induced drag reduction: Mechanistic insights for optimal polymer performance

The addition of low concentrations of high molecular weight polymers in turbulent flow can lead to significant reductions in fluid drag. With reported drag reduction (DR) magnitudes of up to 80%, operational costs go down while fluid throughput is increased. As a result, DR has found use in transport pipelines, heat exchangers, hydrofracking and fire-fighting. This research has undertaken a systematic study to improve understanding for optimum polymer performance. The contributions of this research in the area of DR are i) the evaluation of the double-gap concentric cylinder rheometer device as a reliable DR testing method, ii) demonstrating the influence of polymer conformation and “occupied volume” through a novel approach utilizing a simple thermo-responsive polymer system and iii) developing an understanding of how inorganic nanoparticles can impact the DR performance.
Polymer-induced DR was studied using a rheometer with a double-gap concentric cylinder geometry. Using two acrylamide-based polymers of contrasting rheology (non-ionic and anionic), good agreement between the rheometer method and pipe flow for DR-concentration was found. Therefore, the method was reliable to scrutinize the drag reduction performance of high molecular weight polymers, and to elucidate the governing parameters for the optimal polymer performance.
Three thermo-responsive linear polymers, poly N-isopropylacrylamide (pNIPAM) of varying molecular weights were synthesized. While not commonly considered as a DR polymer, its thermo-responsive behaviour allowed for a novel approach of studying the relationship between polymer conformation and performance, whereby the polymer coil size is tuned by adjusting the fluid temperature. Decreasing DR was observed with increasing temperature, and was related to a change in polymer coil size through the use of intrinsic viscosity [η] measurements. At equivalent volume fractions, adjusted through changes in polymer coil size, the three polymers induced the same DR %. Drop-off in DR performance with time was related to a decrease in the average molecular weight of the polymer, measured using size exclusion chromatography.
An organic-inorganic polymer composite was synthesized to keep the polymer molecular weight low but increase the “occupied volume” of the coils through incorporating nanoparticles in the polymer network. Small improvements in DR performance were attributed mainly to the nano-silica acting as an anchoring point for the polymer chains, however no apparent improvement was seen in the stability behaviour as compared to the linear polymer. With the underlying mechanisms not yet understood, possible explanations are critically discussed through the use of the literature.
The new insights of this research provide a new perspective to better understand the structure-performance relationship in DR that could lead to designing novel polymers to achieve optimal performance.