A phase is simply one of the states of matter and can be either a gas, a liquid, or a solid. The general subject of two-phase flow includes gas-liquid, gas-solid, and solid-liquid flow. The term multicomponent is used to describe flows in which the phases do not consist of the same chemical substance. In the petrochemical industries many processes involve the evaporation (and condensation) of binary (n 2) and multicomponent mixtures. Pure single-component, two-phase flows are those during evaporation and condensation of the same chemical substance. For example, steamwater flow is a single-component, two-phase flow, while air-water is a two-phase, two-component flow. The main emphasis of the following presentation is on the two-phase flow of water.
Regimes of Gas-Liquid Flow
Description. Cocurrent, simultaneous flows of gases and liquids occur in numerous components of plant equipment such as steam generators, drain lines, and oil and natural gas pipelines. Ever since the earliest visual observations of two-phase flow, it has been recognized that there are natural varieties of flow patterns. In addition to the random character of each flow configuration, two-phase flows are never fully developed. In fact, the gas phase expands due to the pressure drop along a pipe leading to a modification of the flow structure. The flow pattern depends also upon the geometry changes of a flow channel (bends, valves, etc.). Flow patterns will be classified according to pipe geometry and flow direction (upward, downward, cocurrent, countercurrent), and several shown are discussed in the following subsections.
Upward Cocurrent Flow in Vertical Pipes.
The main flow patterns encountered in a vertical pipe are shown in Bubbly flow is certainly the most widely known configuration, although at high velocity its milky appearance prevents it from being easily recognized. Bubbles are spherical only if their diameters do not exceed 1 mm; whereas beyond 1 mm their shape is variable. Roumy distinguishes two bubbly flow patterns. In the independent bubble configuration, bubbles are spaced and do not interact with each other. On the other hand, in the packed configuration, bubbles are crowded together and strongly interact with each other. Slug flow is composed of a series of gas plugs. The head of a gas plug is generally blunt, whereas its end is flat with a bubbly wake. A simple visual observation reveals that the liquid film which surrounds a gas plug moves downward with respect to the pipe wall.
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