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Über dieses Buch

Drag Reduction of Turbulent Flows by Additives is the first treatment of the subject in book form. The treatment is extremely broad, ranging from physicochemical to hydromechanical aspects.
The book shows how fibres, polymer molecules or surfactants at very dilute concentrations can reduce the drag of turbulent flow, leading to energy savings. The dilute solutions are considered in terms of the physical chemistry and rheology, and the properties of turbulent flows are presented in sufficient detail to explain the various interaction mechanisms.
Audience: Those active in fundamental research on turbulence and those seeking to apply the effects described. Fluid mechanical engineers, rheologists, those interested in energy saving methods, or in any other application in which the flow rate in turbulent flow should be increased.

Inhaltsverzeichnis

Frontmatter

Chapter I. Introduction

Abstract
When Toms (1949) discovered that minute amounts of added long-chain polymer molecules could reduce the pressure losses in a turbulent pipe flow, this was not the first time that such an effect by additives was observed. It was already known that fibre suspensions, mainly fibres used in the paper making process, showed the same effect in concentrations, however, that were two orders of magnitude higher than in Toms’ experiment (for a review of these earlier papers see Radin et al. 1975). Toms however, showed for the first time that minute amounts, 5–10 ppm per weight, of polymers could have a really dramatic effect. In other words, the drag reducing effect became a promising candidate for partial solutions of the problem of energy saving.
A. Gyr, H.-W. Bewersdorff

Chapter II. Physico-Chemical Properties of Polymers in Solutions and Suspensions, Surfactants in Solutions; Characterization of Fibres

Abstract
In practice the used additives are polydisperse, and the result often difficult to interpret since it is never clear whether the effect is only the result of a subset of the used sample or if some subsets are more important with respect to their drag reducing efficiency. Although it is possible to produce a uniform sample of additives, it is not affordable. Since, as we will see later on, the drag reducing efficiency of the additives varies dramatically, it is evident that there is a need to characterize the additives by their physical and chemical properties as well as their geometry. However, it is not the intention of this presentation to completely characterize the additives but, rather, to discuss only properties which are relevant for describing the interaction of the additives with a turbulent flow. In other words, we will discuss the parameters which would also have to be considered in a dimensional analysis, such as the molecular weight and the aspect ratio of the additives, their extensibility and deformability. The main goal is to give information relevant for the additive-fluid interaction on a molecular level. Such information is essential to describe the competition between the bulk flow creating the distortion of the additives and the entropic restoring mechanism. Therefore a discussion of the self- and particle-particle interaction is omitted. The additives are thought to be present in a dilute concentration, a state which has to be defined for the various kinds of additives.
A. Gyr, H.-W. Bewersdorff

Chapter III. Rheology of Polymer and Surfactant Solutions, and of Fibre Suspensions

Abstract
In order to understand the rheological behaviour of different materials let us consider the following idealized experiment. A deformable fluid is found in the intervening space between two parallel plates which have indefinite dimensions in the spanwise direction, see Figure 3.1.
A. Gyr, H.-W. Bewersdorff

Chapter IV. Drag Reduction and Turbulence

Abstract
The subject of turbulence is by far too complicated to be handled in an introductionary chapter. Therefore, the goal of these notes is to introduce those elements of a description of turbulence which are relevant for representing the interaction of a turbulent flow field with dissolved or suspended additives. The intention is to understand the main elements of the drag reducing mechanism, with hope that in doing so we would also start to understand more about the turbulent mechanism itself.
A. Gyr, H.-W. Bewersdorff

Chapter V. Drag Reduction in Polymer Solutions

Abstract
As mentioned in the introduction, the effect manifests in a decrease of pressure loss over a fixed length of a straight pipe compared with a flow of Newtonian fluids through the same pipe section at the same Reynolds numbers for Reynolds numbers which are higher than a certain critical value. This fact is equivalent to a reduction of the friction factor due to the added polymers, see Fig. 1.2 & 1.3. In this gross flow behaviour of the friction factor the main characteristics of drag reducing flows by polymers can already be observed.
A. Gyr, H.-W. Bewersdorff

Chapter VI. Drag Reduction in Surfactant Solutions

Abstract
Although one of the first publications on drag reduction by additives deals with surfactant solutions, Mysels (1949), these types of additives have received less attention than others, especially polymers. However, in the past years this has changed because drag reducing surfactant solutions do not degrade over long periods of time. This is a definite advantage for practical applications, especially in closed flow circuits, and for studying the structure of turbulence under drag reducing conditions because the rheological solution properties and, consequently, the drag reduction do not change with time. Furthermore, systematic research studies in the chemical industry, documented by several European and U.S. patents, have led to the discovery of surfactant systems exhibiting drag reduction at surfactant concentrations below 100 ppm.
A. Gyr, H.-W. Bewersdorff

Chapter VII. Drag Reduction in Fibre- and Non-Fibrous Suspensions

Abstract
In the flow of paper pulp drag reduction was first observed six decades ago and in fine sand suspensions three decades ago. The drag reduction in fibre suspensions will be discussed with respect to fibre dimensions, fibre concentrations, fibre flexibility and fibre distribution in the flow. As for the other additives, their interaction with the structures of the turbulent flow, e.g., the drag reduction in fine sand suspensions, will be discussed in the frame of an inertial theory which shows that drag reduction can occur in a very limited range of concentration and size. This type of drag reduction also occurs in air flows.
A. Gyr, H.-W. Bewersdorff

Chapter VIII. Applications

Abstract
As soon as the first experiments in drag reduction by addition of small amounts of polymers, surfactants, and fibres were reported, several proposals were made for the possible applications of the effect. However, it is a long way from the idea of a possible practical application to a realization in a running installation. Several restrictions must be considered. Firstly, there has to be an ecomomic benefit besides the technical one. Thus it is essential to consider the following points:
(i)
One has to spend money on the additives. Even when the additive concentration needed to produce the desired drag reduction is very low, the consumption of additives can be considerable in case of high flow rates. In an economic study the costs for the additives are part of the variable costs.
Besides, one has to think of how to put the additives into the flow. This requires injection and/or mixing devices. The costs for these devices are part of the fixed costs. If pumps or stirrers are needed, they will consume energy, which again increases the variable costs.
 
(ii)
Any additive will harm our environment to some extent. Thus any possible application requires a study of the risks for the environment.
 
A. Gyr, H.-W. Bewersdorff

Chapter IX. References

Without Abstract
A. Gyr, H.-W. Bewersdorff

Backmatter

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