In preparing the second edition of this book, the authors have been concerned to maintain or expand those aspects of the subject that are specific to chemical and process engineering. Thus, the chapter on gas-liquid two-phase flow has been greatly extended to cover flow in the bubble regime as well as to provide an introduction to the homogeneous model and separated flow model for the other flow regimes.
The chapter on nonoNewtonian flow has also been extended to provide a greater emphasis on the Rabinowitsch-Mooney equation and its modification to deal with cases of apparent wall slip often encountered in the flow of suspensions. An elementary discussion of viscoelasticity has also been given. A second aim has been to make the book more nearly self-contained and to this end a substantial introductory chapter has been written. In addition to the material provided in the first edition, the principles of continuity, momentum of a flowing fluid, and stresses in fluids are discussed.
There is also an elementary treatment of turbulence. Throughout the book there is more explanation than in the first edition. One result of this is a lengthening of the text and it has been necessary to omit the examples of applications of the Navier-Stokes equations that were given in the first edition. However, derivation of the Navier-Stokes equations and related material has been provided in an appendix.
The authors wish to acknowledge the help given by Miss S.A. Petherick in undertaking much of the word processing of the manuscript for this edition. It is hoped that this book will continue to serve as a useful undergraduate text for students of chemical engineering and related disciplines. Although gases and liquids consist of molecules, it is possible in most cases to treat them as continuous media for the purposes of fluid flow calculations.
On a length scale comparable to the mean free path between collisions, large rapid fluctuations of properties such as the velocity and density occur. However, fluid flow is concerned with the macroscopic scale" the typical length scale of the equipment is many orders of magnitude greater than the mean free path. Even when an instrument is placed in the fluid to measure some property such as the pressure, the measurement is not made at a point--rather, the instrument is sensitive to the properties of a small volume of fluid around its measuring element.
Although this measurement volume may be minute compared with the volume of fluid in the equipment, it will generally contain millions of molecules and consequently the instrument measures an average value of the property. In almost all fluid flow problems it is possible to select a measurement volume that is very small compared with the flow field yet contains so many molecules that the properties of individual molecules are averaged out. A material is isotropic if its properties are the same in all directions. Gases and simple liquids are isotropic but liquids having complex, chain-like molecules, such as polymers, may exhibit different properties in different directions. For example, polymer molecules tend to become partially aligned in a shearing flow.
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