Special attention is paid to the changes in turbulence structure brought about by polymer additives, and an effort is made to understand these in the light of recent advances in the study of shear flow turbulence in ordinary fluids. Also a simplified theoretical model of shear flow wall turbulence is presented, with the aid of which an exploratory study of the influence of some non-Newtonian fluid properties is carried out. A particularly intriguing feature is the remarkable effectiveness of extremely high-molecular-weight polymers by which only a few parts per million of weight of solvent are sufficient in some cases to lead to drag reductions of 50 per cent or more. (Modified author abstract).
An important practical aspect in the application and study of drag reduction by polymer additives is the degradation of the polymer, for instance due to intense shearing, especially in circulatory flow systems. Such degradation leads to a marked loss of the drag-reducing capability of the polymer. Polymers-Surfactant complex efficacy in reducing the drag and improving the degradation resistance is a new subject in drag reduction research. Turbulent drag reduction (DR) efficacy of ionic Sodium Polystyrene Sulfonate (NaPSS) and sodium Alkylbenzene sulfonate, complexes systems regarding polymer-surfactant interaction was examined under a turbulent flow in a rotating disk apparatus, in which the DR efficacy indicates how the torque is being reduced with a tiny amount of additives under a turbulent flow at a fixed rotational speed. It was found that the addition of the surfactant to the ionic increased the polymer chain dimensions via a conformational structural change, thus enhancing the DR efficacy. Polymer-surfactant system also shows that there exists a critical polymer concentration at which the drag reduction becomes a maximum, and then above the critical concentration, the DR efficacy decreases more rapidly than that of pure polymeric systems. On the other hand, it was found that the addition of the surfactant to the ionic polymer enhance its ability to resist the degradation caused by the high shear stress in the eddy flow. The addition of sodium Alkylbenzene sulfonate to the ionic polymer was found to have higher improvement than the addition of DDAB in degradation resistance. The DR and degradation resistance efficacy induced by the polymer-surfactant mixture is found to be obvious higher than of pure polymer. In addition, the maximum of DR efficacy versus polymer concentration occurred at 700 ppm.
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.
Experimental measurements were made to determine the effect of drag reducing polymer additives on the surface pressure fluctuations on smooth and rough surfaces in relative motion with water. Changes in surface pressure fluctuation intensity, caused either by the addition of drag reducing polymer or by changes in surface roughness, or both, were found to correlate with changes in surface shear. (Author).
Lthough extensive research work has been carried out on the drag reduction behavior of polymers and surfactants alone, little progress has been made on the synergistic effects of combined polymers and surfactants. A number of studies have demonstrated that certain types of polymers and surfactants interact with each other to form surfactant-polymer complexes. The formation of such complexes can cause changes in the solution properties and may result in better drag reduction characteristics as compared with pure additives. A series of drag-reducing surfactants and polymers were screened for the synergistic studies. The following two widely used polymeric drag reducing agents (DRA) were chosen: a copolymer of acrylamide and sodium acrylate (referred to as PAM) and polyethylene oxide (PEO). Among the different types of surfactants screened, a cationic surfactant octadecyltrimethylammonium chloride (OTAC) and an anionic surfactant Sodium dodecyl sulfate (SDS) were selected for the synergistic study. In the case of the cationic surfactant OTAC, sodium salicylate (NaSal) was used as a counterion. No counterion was used with anionic surfactant SDS. The physical properties such as viscosity, surface tension and electrical conductivity were measured in order to detect any interaction between the polymer and the surfactant. The drag reduction (DR) ability of both pure and mixed additives was investigated in a pipeline flow loop. The effects of different parameters such as additive concentration, type of water (deionized (DI) or tap), temperature, tube diameter, and mechanical degradation were investigated. The addition of OTAC to PAM solution has a significant effect on the properties of the system. The critical micelle concentration (CMC) of the mixed surfactant-polymer system is found to be different from that of the surfactant alone. The anionic PAM chains collapse upon the addition of cationic OTAC and a substantial decrease in the viscosity occurs. The pipeline flow behaviour of PAM/OTAC mixtures is found to be consistent with the bench scale results. The drag reduction ability of PAM is reduced upon the addition of OTAC. At low concentrations of PAM, the effect of OTAC on the drag reduction behavior is more pronounced. The drag reduction behavior of polymer solutions is strongly influenced by the nature of water (de-ionized or tap). The addition of OTAC to PEO solution exhibited a week interaction based on the viscosity and surface tension measurements. However, the pipeline results showed a considerable synergistic effect, that is, the mixed system gave a significantly higher drag reduction (lower friction factors) as compared with the pure additives (pure polymer or pure surfactant). The synergistic effect in the mixed system was stronger at low polymer concentrations and high surfactant concentrations. Also the resistance against mechanical degradation of the additive was improved upon the addition of OTAC to PEO. The mixed PEO/SDS system exhibited a strong interaction between the polymers (PEO) and the surfactant (SDS), Using electrical conductivity and surface tension measurements, the critical aggregation concentration (CAC) and the polymer saturation point (PSP) were determined. As the PEO concentration is increased, the CAC decreases and the PSP increase. The addition of SDS to the PEO solution exhibits a remarkable increase in the relative viscosity compared to the pure PEO solution. This increase is attributed to the changes in the hydrodynamic radius of the polymer coil. The pipeline flow exhibited a considerable increase in DR for the mixed system as compared to the pure PEO solution. The addition of surfactant always improves the extent of DR up to the PSP. Also the mixed PEO/ SDS system shows better resistance against shear degradation of the additive.
The addition of a small amount of polymers of high molecular weight can lead to a pressure drop decrease in turbulent flows. The polymers successively stretch and coil by interacting with the turbulent structures, which imposes a transient behaviour on the drag reduction (DR). As a result, DR undergoes three stages over time: A, B, and C. In stage A, DR departs from zero and assumes negative values due to a significant polymer stretching at the beginning of the process, which requires energy from the flow. After the minimum DR is reached, the polymers start their coil-stretch cycle and DR increases in response to the development of turbulent structures, achieving a maximum value, which makes for the beginning of stage B. However, during their coil-stretch cycle, polymers can be mechanically degraded as a result of an intense polymer stretching, which reduces their ability to act as energy exchange agents. Hence, when polymer degradation becomes pronounced, DR decreases until achieving a final value. The polymer degradation process characterizes the stage C. In the present work, numerical analyses are conducted aiming to investigate the stages A, B and C. The transient aspects of the polymer induced drag reduction phenomenon are explored with the aid of direct numerical simulations of turbulent plane Poiseulle and Couette flows of viscoelastic FENE-P fluids taking into account a large range of Reynolds number, Weissenberg number and maximum polymer molecule extensibility. Stages A and B are carefully studied from tensor, energy budget and spectral perspectives. A polymer scission model is developed in order to numerically reproduce the stage C.
The investigation of turbulent drag reduction, which is caused by the addition of a small amount of polymer or some other substances to the liquids flowing systems has been the focus of attention of many scientists for the last decades. Due to the reduction of the drag, pumping power for the pipeline will significantly reduced and thus will decrease the cost of electricity in total production cost. In this study, the effect of the presence of a drag reducing agent (DRA) and its variety of molecular weight on the torque produced in rotating disk apparatus containing water is investigated. The experimental procedure was divided into three parts; obtaining several different polymer molecular weights using ultrasonication method, testing the water using different polymer molecular weight at different polymer concentration and lastly is adding the different concentration of surfactant in the fixed concentration of water- polymer solution. Three polymer molecular weights are obtained by using ultrasonificator method with value of 11.7967 x106 g/mol, 4.830 x106 g/mol and 1.7179 x106 g/mol. A drastic reduction of drag in the turbulent flow of solutions as evaluated with torque differences in comparison to the pure solvent can be observed, even when only minute amounts of the additives are added. The percentage of drag reduction is relatively increases as we increase the polymer molecular weight and polymer concentration. A maximum drag reduction of 47.62% has been observed at polymer molecular weight of 11. 7697 x106 with polymer concentration of 200 ppm. In polymer- surfactant complex solution, 29% of drag reduction were reported with surfactant concentration of 2000ppm.
This book explains theoretical derivations and presents expressions for fluid and convective turbulent flow of mildly elastic fluids in various internal and external flow situations involving different types of geometries, such as the smooth/rough circular pipes, annular ducts, curved tubes, vertical flat plates, and channels. Understanding the methodology of the analyses facilitates appreciation for the rationale used for deriving expressions of parameters relevant to the turbulent flow of mildly elastic fluids. This knowledge serves as a driving force for developing new ideas, investigating new situations, and extending theoretical analyses to other unexplored areas of the rheology of mildly elastic drag reducing fluids.The book suits a range of functions--it can be used to teach elective upper-level undergraduate or graduate courses for chemical engineers, material scientists, mechanical engineers, and polymer scientists; guide researchers unexposed to this alluring and interesting area of drag reduction; and serve as a reference to all who want to explore and expand the areas dealt with in this book.
