V. G. Pogrebnyak, Prof., Dr. Sci. (Tech.),
National Technical University of Oil and Gas,
Ivano-Frankivsk, Ukraine
I. V. Perkun, Ph.D. (Tech.),
National Technical University of Oil and Gas,
Ivano-Frankivsk, Ukraine
A. V. Pogrebnyak, Dr. Sci. (Tech.),
University of Customs and Finance ,
Dnipro, Ukraine
I. V. Adamchak, student
National Technical University of Oil and Gas,
Abstract: As a result of experiments it was confirmed the unfolding of molecules under wall-adjacent turbulence conditions, as well as it was proved that dynamic structure formation in polymer solutions is occured under the influence of supercritical longitudinal gradients of speeds. On the basis of data that characterize macromolecule dynamics in non-turbulence flow with stretching and the proved evidence of strong deformation effect on macromolecules in wall-adjacent turbulence, it has been established the molecular-and-supermolecular mechanism of effect reduction for flow resistance when injecting soluble polymer additives in a turbulence flow.
Keywords: Reduction of turbulent friction, polymer solution, macromolecule, deformation effect, dynamic structure formation, Toms effect.
The problem of reducing the energy intensity of the oil-trunk pipelines by increasing their productivity remains one of the key problems of economic development of different countries. It is obvious that the task of improving the hydrodynamic characteristics of the oil pipelines is to find the ways to reduce the fluid flow drag - fluid friction. Among the known methods of artificial influence on the boundary layer, in order to reduce the hydrodynamic drag in oil pipelines, special place is taken by a method, based on the injection of polymer solutions. This method is the only one in the development that has certain practical progress.
Laboratory and field tests of the influence of small polymer additions to reduce the drag of oil flow in pipelines began conducting since the late 70-ies of the XX century. The design development of oil pipeline by using polymeric additions for turbulent drag reduction was justified, because the actual increase of the Trans-Alaska oil-trunk pipeline capacity has amounted 20%. However, the resulting friction drag reduction effects by applying polymer solutions in oil pipelines are far from the theoretically predicted. Therefore, the technical task of reducing the hydrodynamic resistance of oil flow in the pipelines by applying the polymer solution into the boundary layer needs to be resolved. This applies to all questions above regarding the nature of turbulent drag reduction by using polymer additions.
In the hydrodynamics of polymer solutions there takes place the transition from accumulating experimental information to understanding the physical essence and establishing main regularities of manifestation of memory and elasticity effects. Toms effect revealed as an experimental fact in the late 40-ies, up to now has been causing great difficulties when interpreting it from the point of view of modern ideas of the hydrodynamics of turbulent flow.
Among the attempts to explain the nature of effect Toms', lying in drag reduction by the polymeric components, special place is taken by a hypothesis, based on strong deformation effect of a near-the-wall turbulence on macromolecules. For the substantiation of this hypothesis experimental proofs of presence of large degrees of deformation of macromolecules in a wall-adjacent zone of a turbulent flow are necessary. The skepticism concerning strong deformation effect of wall-adjacent turbulence on macromolecules is stipulated yet by the fact that, as a rule, shift effects wall-adjacent a turbulence are analyzed, and not the, jet flows (“explosions”) with a longitudinal gradient of speed which arise in the wall-adjacent area. It is possible to hope, that the way to understanding and describing phenomena reduction of turbulent friction by polymer additions lies through the study of hydrodynamic effects of big reversible (as well as non-reversible) deformations of molecular coils in flows with stretching.
Therefore the experiments proving the stretching of molecules in conditions of wall-adjacent turbulence have a fundamental character not only in point of developing the mechanism of drag reduction by polymer additions but also in point of more profound insight into the nature of turbulence itself.
Comparison of concentration dependence of the Toms’ effect and the data testifying to the formation of dynamic super-molecular structures in polymer solutions allows to state that in the vicinity of optimal concentration (C≥Сopt ), where the Toms’ effect reaches its maximum, solutions start generating anisotropic super-molecular forms having the lifetime 10-20 times longer than θc . Further increase of concentration leads to conditions favourable for interaction between individual polymer molecules even without the hydrodynamic field effecting on them. The longitudinal hydrodynamic field effect in polymer solution gives way to the formation of dynamic super-molecular structures with lifetime significantly exceeding (in several factors) the temporal scale of the wall-adjacent turbulence. That’s why unlike the super-molecular forms in semi-diluted solutions these super-molecular forms act as “stiff sticks”. As an outcome of this the Toms’ effect is decreased at a high polymer concentration in solution as well as at a high dynamic velocity.
On the basis of data that characterize macromolecule dynamics in non-turbulence flows with stretching and the proved evidence of strong deformation effect on macromolecules in wall-adjacent turbulence, and using data of model studies of turbulence peculiarities in a boundary layer there has been established the molecular-and-supermolecular mechanism of effect reduction for flow resistance when injecting soluble polymer additives in a turbulence flow. Mechanism of Toms’ effect lies in the occurrence of auto-fluctuating mode of reversible processes of macromolecule deformation caused by longitudinal velocity gradients that quasi-regularly originate in turbulence boundary layer and in macromolecule deformation effect both on molecular (when C<Сopt ) and super-molecular (when C>Сopt) levels on the wall-adjacent turbulence structure, i.e. as a result of macromolecule deformation oscillations and solubility of dynamic super-molecular forms brought about by the flow-with-stretching effect. All this leads to the increase of liquid ejection periods into the outer zone of the boundary layer and in consequence to the viscous sub-layer becoming thicker. As an outcome of this generation of primary turbulence gets reduced and general level of turbulence dissipation in the flow becomes lower. In case of sufficiently big molecular masses and concentrations the viscosity growth caused by both “common” intermolecular interaction and dynamic structure formation leads to sharp Toms’ effect decrease.
The considered experimental data prove the substantiation to transfer outcomes obtained during the study of macromolecule dynamics in non-turbulence flows with stretching onto jet currents of wall-adjacent ejections in turbulence flow, i.e. turbulence current (to macroscopic scale) is perceived as laminar one (to microscopic scale) when hydrodynamic field interacts with polymer molecules. Macromolecules may serve as an efficient tool of getting additional information about the structure of wall-adjacent turbulence.
The developed approach of explaining the turbulent drag reduction mechanism fits well into the general scheme of self-regulatory processes, which are dominated by negative feedbacks. It is typical for systems that can change their properties under the action of external physical effects, in this case, under the influence of jet currents ("explosions") with stretching which locally occurring in the boundary layer of the oil pipeline. The considered experimental data prove the substantiation to transfer outcomes obtained during the study of macromolecule dynamics in non-turbulence flows with stretching onto jet currents of wall-adjacent ejections in turbulence flow, i.e. turbulence current (to macroscopic scale) is perceived as laminar one (to microscopic scale) when hydrodynamic field interacts with polymer molecules.
Understanding the nature of reducing drag flow of oil in pipelines by small polymer additions will allow to develop recommendations on the choice of rational hydraulic regimes of oil pipelines, as well as to outline the ways for the directed synthesis of high-performance polymer additions that reduce friction in the turbulent oil pipelines.
