2.1. INTRODUCTION
The flow and behavior of fluids is important in many of the separation processes in process engineering. A fluid may be defined as a substance that does not permanently resist distortion and, hence, will change its shape. In this text gases, liquids, and vapors are considered to have the characteristics of fluids and to obey many of the same laws.
In the process industries, many of the materials are in fluid form and must be stored, handled, pumped, and processed, so it is necessary that we become familiar with the principles that govern the flow of fluids as well as with the equipment used. Typical fluids encountered include water, air, CO2, oil, slurries, and thick syrups.
If a fluid is inappreciably affected by changes in pressure, it is said to be incompressible. Most liquids are incompressible. Gases are considered to be compressible fluids. However, if gases are subjected to small percentage changes in pressure and temperature, their density changes will be small and they can be considered to be incompressible.
Like all physical matter, a fluid is composed of an extremely large number of molecules per unit volume. A theory such as the kinetic theory of gases or statistical mechanics treats the motions of molecules in terms of statistical groups and not in terms of individual molecules. In engineering we are mainly concerned with the bulk or macroscopic behavior of a fluid rather than the individual molecular or microscopic behavior.
In momentum transfer we treat the fluid as a continuous distribution of matter, or a “continuum.” This treatment as a continuum is valid when the smallest volume of fluid contains a number of molecules large enough that a statistical average is meaningful and the macroscopic properties of the fluid, such as density, pressure, and so on, vary smoothly or continuously from point to point.
The study of momentum transfer, or fluid mechanics as it is often called, can be divided into two branches: fluid statics, or fluids at rest, and fluid dynamics, or fluids in motion. In Section 2.2 we treat fluid statics; in the remaining sections of Chapter 2 and in Chapter 3, fluid dynamics. Since in fluid dynamics momentum is being transferred, the term “momentum transfer” or “transport” is usually used. In Section 2.3 momentum transfer is related to heat and mass transfer.