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Fluids, that is gases and liquids, are self–evidently prerequisites for normal life. They also play a major role in the production of many artefacts and in the operation of much of the equipment upon which modern life depends. Occasionally, a fluid is the ultimate result of a technological process, such as a liquid or gaseous fuel, so that its existence impinges directly on the public consciousness. More often, fluids are intermediates in processes yielding solid materials or objects, and are then contained within solid objects so that their public image is very much less and their significance not fully appreciated. Nevertheless, every single component of modern life relies upon a fluid at some point and therefore upon our understanding of the fluid state.
The gross behavioral features of a fluid are well understood in the sense that it is easy to grasp that a gas has the property to completely fill any container and that a liquid can be made to flow by the imposition of a very small force. However, beyond these qualitative features lie a wide range of thermophysical and thermochemical properties of fluids that determine their response to external stimuli. This analysis concentrates exclusively on thermophysical properties and will not consider any process that involves a change to the molecular entities that comprise the fluid.
This book describes the most reliable methods available for evaluating the transport properties, such as viscosity, thermal conductivity and diffusion, of pure gases and fluid mixtures. Particular emphasis is placed on recent theoretical advances in our understanding of fluid transport properties in all the different regions of temperature and pressure. In addition to the important theoretical tools, the different methods of data representation are also covered, followed by a section which demonstrates the application of selected models in a range of circumstances. Case studies of transport property analysis for real fluids are then given, and the book concludes with a discussion of various international data banks and prediction packages. Advanced students of kinetic theory, as well as engineers and scientists involved with the design of process equipment or the interpretation of measurements of fluid transport properties, will find this book indispensable.
The Commission on Thermodynamics of the Physical Chemistry Division of the International Union of Pure and Applied Chemistry is charged by the Union with the duty to define and maintain standards in the general field of thermodynamics. This duty encompasses matters such as the establishment and monitoring of international pressure and temperature scales, recommendations for calorimetric procedures, the selection and evaluation of reference standards for thermodynamic measurements of all types and the standardization of nomenclature and symbols in chemical thermodynamics. One particular aspect of the commission's work from among this set is carried forward by two subcommittees: one on thermodynamic data and the other on transport properties. These two subcommittees are responsible for the critical evaluation of experimental data for the properties of fluids that lie in their respective areas and for the subsequent preparation and dissemination of internationally approved thermodynamic tables of the fluid state and representations of transport properties.
The Subcommittee on Transport Properties has discharged its responsibilities through the work of groups of research workers active in the field drawn from all over the world. These groups have collaborated in the preparation of representations of the viscosity, thermal conductivity and diffusion coefficients of pure fluids and their mixtures over wide ranges of thermodynamic states. The representations have almost always been based upon an extensive body of experimental data for the property in question accumulated over many years by the efforts of laboratories worldwide.
It is now estimated that there are some 50 million pure chemicals known of which some 20,000 are listed as high–volume, major chemicals by the European Economic Community (Forcheri & de Rijk 1981) some of which may be transported across national borders. For each pure fluid there are approximately 30 properties which are of technological significance of which twelve are functions of temperature and pressure. If just these twelve properties are considered and it is assumed that measurements at only ten pressures and ten temperatures are required then to provide the necessary information for only one pure fluid requires 1200 measurements. If all the pure species and all possible mixtures from among the set of bulk chemicals are included and composition is allowed as a variable then it is rather easy to estimate that, even for a generous estimate of the rate of experimental data acquisition in the world, the total effort required to fulfill the needs identified in Chapter 2 would exceed 100 billion man–years. This figure makes it immediately obvious that industry's needs for physical property data can never be met by measurement alone. It is therefore necessary to replace a complete program of measurements by an alternative strategy designed to meet the same objective. The philosophy and methods for the establishment of such a strategy have been discussed by many authors and have been updated regularly and most recently by Nieto de Castro & Wakeham (1992).
Transport coefficients describe the process of relaxation to equilibrium from a state perturbed by application of temperature, pressure, density, velocity or composition gradients. The theoretical description of these phenomena constitutes that part of nonequilibrium statistical mechanics that is known as the kinetic theory. The ultimate purpose of this theory is to relate the macroscopic (observable) properties of a system to the microscopic properties of the individual molecules and their interaction potentials.
The kinetic theory of dilute gases assumes a macroscopic system at densities low enough so that molecules most of the time move freely and interact through binary encounters only. Nevertheless, the densities are high enough to ensure that the effects of molecule–wall collisions can be neglected compared to those from molecule–molecule encounters. The first condition implies that the thermodynamic state of the fluid should be adequately described by a virial expansion up to and including the second virial coefficient. The second condition means that the mean free path of molecules is much smaller than any dimension of the vessel and that Knudsen effects play no significant role.
It is worth noting that in this context the terms ‘dilute’ or ‘low–density gas’ represent a real physical situation, whereas the frequently used expression ‘zero–density limit’ is related to results of a mathematical extrapolation of a density series of a particular transport property at constant temperature to zero density.
One of the triumphs of the simple kinetic theory of gases was the prediction by Maxwell that the viscosity at constant temperature should be independent of density (pressure), for systems of hard–sphere molecules and also molecules which repel with a force proportional to r−5 (Maxwell 1860, 1867). This somewhat surprising result was, contrary to expectation, found to be in good agreement with experiment up to moderate pressures. For example, the viscosity of argon at 298.15 K increases just slightly from 22.63 μPa s at 0.1 MPa to 22.81 μPa s at 1 MPa (Kestin et al. 1971). At 5 MPa, the viscosity is still only 23.87 μPa s and 25.77 μPa s at 10 MPa. However, as the pressure increases further, the rate of increase in viscosity becomes greater. At the marginally higher temperature of 301.15 K, the viscosity of argon is 104 μPa s at 149.9 MPa and 480 μPa s at 897.1 MPa (Trappeniers et al. 1980).
This difference in the pressure dependence of viscosity between a dilute gas and a dense gas arises because in a dilute gas it is the molecules themselves which transport the momentum; in a dense gas, however, transport of momentum occurs over nonzero distances on collision. The same is true for energy transport (thermal conductivity).
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