1. Introduction

Single-phase or multiphase flow in chemical engineering apparatus and reactors such as evaporators, columns, fixed and fluidized beds, and stirred vessels is decisive for the efficiency and capacity of these equipment units. The mixing of two or more liquid components without the formation of a second liquid phase leads to a one-phase flow, for instance in a stirred vessel. In the case of an evaporative crystallizer a three-phase system is moved by the stirrer and the rising bubbles. In many cases two phases are flowing in countercurrent direction, e.g., in columns for absorption, rectification, and extraction. Two phases are also moving in bubble and drop columns, froth layers on plates, fluidized beds, and stirred vessels used for suspension flow or the breakup of gases and liquids. Dealing with packed or film columns, the liquid phase runs down in films and rivulets in countercurrent to the gas. In many apparatus of chemical engineering, internal pieces of equipment are mounted and the fluid flow is passing around or through these internals.