The structure of turbulence in a drag reduced flat plate boundary layer has been studied with particle image velocimetry (PlV) and planar laser induced fluorescence (PLIF). Drag reduction was achieved by injection of a solution of water-soluble polymer through a spanwise slot near the leading edge of the flat plate. Velocity and concentration data were obtained using PrV and PLIF, respectively, in planes parallel to the wall (x-z plane) and perpendicular to the wall (x-y plane). Measurements of velocity, vorticity and streak spacing were obtained and trends analyzed. For increasing drag reduction, damping of streak oscillations, suppression of streak splitting and merging, streak stabilization and coarsening of the low speed streaks was observed. PLIF measurements of the injected polymer solution showed that regions of high polymer concentration are correlated with the low speed streaks. PW measurements in the x-y plane showed that at Maximum Drag Reduction (MDR) there are sighificant differences in the statistics of turbulence between boundary layers with polymer injection and channel flow with an ocean of polymer suggesting that in this sense, the achievement of MDR in flows with polymer addition is not unique.
Drag reduction by injection of high molecular weight polymers into boundary layers has been demonstrated repeatedly in the past. However, from a volume utilization tradeoff standpoint, the quantities of polymer required make the gains achieved by this process marginal. While indicating reduced polymer requirements for drag reduction, limited data obtained from pipe flow and external boundary layer flow experiments are conflicting and hard to interpret. Ambiguities in measurement techniques due to polymer effects on commonly used instrumentation and opposing features of varied flow facilities have also contributed to making these earlier works contradictory and difficult to resolve. Experiments performed in this research indicate that turbulence intensity distributions are altered by the addition of polymer in such a way that the peak of turbulence production is lowered and its location moved away from the wall. The transition region is delayed and extended by the addition of polymer to the boundary layer. The laminar sublayer of boundary layer profiles appears to have thickened due to the addition of polymer. When compared to the law of the wall corrected for developing flow, the velocity profiles also show evidence of a thickened sublayer.
Experimental studies of polymer drag-reduction were carried out in the laboratory of Thomas J. Hanratty. Companion studies were carried out in the polymer laboratory of Anthony J. McHugh so that the performance of drag-reducing polymer solutions can be related to their rheological and rheo-optical properties in simple non-turbulent flows. The work was influenced by the notion that aggregation of polymer molecules could be an important role. A remarkable finding is that molecular weight and molecular weight distribution need not change as the drag-reducing ability of the polymer solution degraded. Rheo-optical studies in a Couette device showed that effective polymer solutions develop turbidity over a range of shear rates characteristic of those used in the flow loop. This study shows that the drag reduction can be enhanced by using mixing and delivery procedures which enhance the formation aggregates.
Flow friction reduction by polymers is widely applied in the oil and gas industry for flow enhancement or to save pumping energy. The huge benefit of this technology has attracted many researchers to investigate the phenomenon for 70 years, but its mechanism is still not clear. The objective of this thesis is to investigate flow drag reduction with polymer additives, develop predictive models for flow drag reduction and its degradation, and provide new insights into the drag reduction and degradation mechanism. The thesis starts with a semi-analytical solution for the drag reduction with polymer additives in a turbulent pipe flow. Based on the FENE-P model, the solution assumes complete laminarization and predicts the upper limitation of drag reduction in pipe flows. A new predictive model for this upper limit is developed considering viscosity ratios and the Weissenberg number - a dimensionless number related to the relaxation time of polymers. Next, a flow loop is designed and built for the experimental study of pipe flow drag reduction by polymers. Using a linear flexible polymer - polyethylene oxide (PEO) - in water, a series of turbulent flow experiments are conducted. Based on Zimm's theory and the experimental data, a correlation is developed for the drag reduction prediction from the Weissenberg number and polymer concentration in the flow. This correlation is thoroughly validated with data from the experiments and previous studies as well. To investigate the degradation of drag reduction with polymer additives, a rotational turbulent flow is first studied with a double-gap rheometer. Based on Brostow's assumption, i.e., the degradation rate of drag reduction is the same as that of the molecular weight decrease, a correlation of the degradation of drag reduction is established, along with the proposal of a new theory that the degradation is a first-order chemical reaction based on the polymer chain scission. Then, the accuracy of the Brostow's assumption is examined, and extensive experimental data indicate that it is not correct in many cases. The degradation of drag reduction with polymer additives is further analyzed from a molecular perspective. It is found that the issue with Brostow's theory is mainly because it does not consider the existence of polymer aggregates in the flow. Experimental results show that the molecular weight of the degraded polymer in the dilute solution becomes lower and the molecular weight distribution becomes narrower. An improved mechanism of drag reduction degradation considering polymer aggregate is proposed - the turbulent flow causes the chain scission of the aggregate and the degraded aggregate loses its drag-reducing ability. Finally, the mechanism of drag reduction and degradation is examined from the chemical thermodynamics and kinetics. The drag reduction phenomenon by linear flexible polymers is explained as a non-spontaneous irreversible flow-induced conformational-phase-change process that incorporates both free polymers and aggregates. The entire non-equilibrium process is due to the chain scission of polymers. This theory is shown to agree with drag reduction experimental results from a macroscopic view and polymer behaviours from microscopic views. The experimental data, predictive models, and theories developed in this thesis provide useful new insights into the design of flow drag reduction techniques and further research on this important physical phenomenon.
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The experiments described in this report were performed within the framework of the overall ASTI Team effort in polymer science. The underlying predicate of this program has been to achieve a basis for understanding the shear viscosity of dilute polymer solutions through a detailed description of the molecular interactions. Interaction descriptions must include water/polymer, water/surface and polymer/surface contributions to provide understanding of the mechanisms determining the shear viscosity and related noise and drag reduction effects. Although certain anionic polymeric drag reduction effects observed herein are not dramatic compared to those achieved with the standard polyethylene-oxide, these types of polymers offer the potential for drastically altering the interaction between the surface and adjacent layers of water molecules. It can be assumed that the water layer/surface interaction is dependent on the nature of the surface potential and the presence of strongly interacting species in that region. Thus, any mechanism reducing the concentration of such active species near the surface should reduce the overall strength of the interaction. However, drag reduction actually increased for polyacrylamide changing from deionized water to 'instant ocean' (salt water environment). This suggests the role of ions in determining polymer effectiveness may be complex.
A large amount of studies have been carried out on pipeline flow with several kinds of drag reducing agents, especially polymers and surfactants. Drag reducing agents, by definition, are additives which help suppress or eliminate turbulence in a pipeline. The mechanism and methodology of polymer only or surfactant only as drag reducing additives have been fully discovered. Whether mixed drag reducers such as polymer-surfactant or polymer-polymer systems would be effective is still not clear. In our study, polymer-surfactant and polymer-polymer mixed additives are used in order to explore the synergistic effects and interactions in pipeline flow loops. The experimental work was divided into two sections: bench-scale experiments and pilot-scale experiments. In bench-scale experiments, the properties of prepared fluids such as, surface tension, conductivity and shear viscosity were measured. Several comparison methods and calculations were applied to give better understandings of the properties resulting from mixing of polymer with surfactant and polymer with polymer. After analysis of the properties, several combinations of concentrations were selected and solutions were prepared in the main tank of pilot plant and pumped into the pipeline set-up to test the pipeline flow behaviors. Turbulence structure/Reynolds number, pipe diameter, polymer-surfactant concentration were all considered as influencing factors. Critical micelle concentration, critical aggregation concentration, polymer saturation point, the onset of drag reduction, and the interactions between the mixed additives were discussed. A comparison between pipeline results and the predictions of Blasius Equation or Dodge-Metzner Equation were also discussed.. For polymer-surfactant studies, a commonly used polymer additive - carboxylmethylcellulose (referred to as CMC which is anionic) was selected as the drag reducing agent. The performance of this polymer was investigated in the presence of six surfactants respectively - Alcohol ethoxylate (referred to as Alfonic 1412-9 and Alfonic 1412-3 which are nonionic), Aromox DMC (nonionic surfactant), Stepanol WA-100 and Stepwet DF-95 (which mainly consist sodium lauryl sulfates, anionic surfactant) and Amphosol (which is zwitterionic).The experiments were first conducted with pure CMC solution with different concentrations (100ppm, 500ppm, 700ppm and 1000ppm) as a standard. The 500ppm CMC solution was selected as the best polymer concentration with highest drag reduction efficiency. For polymer-surfactant combinations, CMC-Alfonic 1412-9, CMC-Alfonic1412-3, CMC-Stepanol and CMC-Stepwet systems were found to have significant interactions. High surfactant concentration resulted in reduction in %DR. The addition of Aromox increased the drag reduction ability and onset point when concentration was higher than the polymer saturation points. Also, both hydrophobic and electrostatic interactions were thought to have an effect on critical micelle concentration, which led to the fluctuations in the %DR. For polymer-polymer studies, PAM-PEO system at two different polymer concentrations were investigated. Overall, Pure PAM solution had much higher drag reduction ability than pure PEO solutions. Mixing them together, strong interactions occurred when PEO fraction was high (over 50%) which affected %DR and shear viscosity substantially. Power-law constants n and k were also taken into account and found to exhibit opposite trends with the increase of PEO fraction.
