Nature of Multiphase Flows and Basic Concepts

Abstract

This chapter introduces the reader to multiphase flows and to phenomena that are unique to them. A special but most common case of multiphase flows are the two-phase, gas-liquid flows. Multiphase flows are present whenever there is heat transfer accompanied with phase change such as boiling and condensation. The unique nature of multiphase flows is made evident by a few examples. The definitions and the bases necessary for dealing with such flows are given: the various definitions of the void fraction (generically speaking, the fraction of space or time occupied by the gas phase), the phase flow rates, velocities, etc., and the flow quality (the ratio between the mass flow rate of the gas to the total mass flow rate). The void fraction has a major influence on the flow regimes that characterize the topological arrangement of the two phases in the flow channel. Averaging of the flow properties in time and/or space is usually needed to deal analytically with multiphase flows. The various types of averaging needed are reviewed. The cross-sectional-average velocities of the two (or more) phases are usually not equal. The cross-sectional-average void fraction, the ratio of the average phase velocities and the quality are linked by a so-called triangular relationship. In the case of homogeneous flow, the phase velocities are assumed to be equal. The difference between the actual flow quality and the thermal-equilibrium quality is clarified. A few non-dimensionless numbers or groups of variables that are very often used are introduced in anticipation of their actual use in the following chapters. The most often used sources of information on multiphase flows are listed at the end of the chapter. Appendices at the end of the volume provide a tutorial for the reader not familiar with some needed notions of fluid mechanics and heat transfer, define the usual nomenclature and provide conversion factors.