Chapter One

Two-phase flow is generally understood as being a simultaneous flow of two different immiscible phases separated by an infinitesimal thin interface. Phases are identified as "homogeneous" parts of the fluid for which unique local state and transport properties can be defined. In most cases, phases are simply referred to as the state of matter, e.g. gas/vapor, liquid, or solid. Typical examples are the flow of liquid carrying vapor or gas bubbles, or the flow of gas carrying liquid droplets or solid particles. However, more complex flow processes may exist where the phase distribution is less well defined.

Two-phase flow is of large relevance for many scientific/technical disciplines ranging from environmental research to the modeling of normal operation or accident conditions in nuclear, chemical, or process engineering installations. For a long time, the analysis of two-phase flow processes was limited to mostly empirical correlations or to largely simplified engineering models and, therefore, two-phase flow was considered as a rather "dirty" branch of fluid dynamics. This situation has changed significantly during the last two decades when a large effort was spent for the analysis of two-phase flow systems and for the development of related numerical simulation methods. Much of this work was stimulated by the specific requirements for the safety analysis of pressurized water reactors which, for obvious reasons, relies largely on the prediction capability of computer codes for complex two-phase flow and heat transfer processes.

Many of the present advanced models for the description of nonhomogeneous nonequi