Flow patterns of liquid-vapor (Gas) two-phase flow

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Two-phase flow refers to the interactive flow of two distinct phases – each phase representing a mass or volume of matter – with common interfaces in a channel. Two-phase flow can occur in a single-component or multicomponent system (Table 1.10). Possible phase combinations include: (1) solid-liquid, where solid particles are mostly dispersed in the liquid; (2) solid-gas, where the solid particles are carried by a stream of gas; (3) liquid-vapor (gas), where the volume fraction of one phase relative to the other results in different flow regimes; and (4) a combination of the above. While each of these modes represents a significant area of two-phase flow, liquid-vapor (gas) flow is by far the most common in various industries and thus has been investigated in greater depth. Therefore, the major emphasis of this chapter is on liquid-vapor (gas) flow.

Each regime in liquid-vapor (gas) two-phase flow has a characteristic flow behavior that can substantially affect both pressure drop and heat transfer. In the case of a single-component two-phase flow, such as forced convective condensation or evaporation, continuous mass transfer occurs between the vapor and liquid phases. Examples of liquid-gas flow processes include distillation, fractionation, flashing, spray-drying, stripping, and absorption. There are also two-component liquid-gas systems where the gas is noncondensable; these include air/water flow in aeration, deaeration, and humidification or dehumidification processes. A biomedical engineering example is the development of artificial lungs where absorption of oxygen or desorption of CO2 from blood is required.

Section 11.2 presents the important parameters for liquid-vapor (gas) two-phase flow, and flow patterns in vertical and horizontal tubes. The various two-phase flow models, as well as the prediction of pressure drop and void fraction, are presented in Section 11.3. The two-phase flow regimes and heat transfer characteristics for forced convective condensation and boiling are presented in Sections 11.4 and 11.5, respectively. The chapter is closed by a discussion of two-phase flow, condensation, and boiling heat transfer in micro/miniature channels.


11.2 Flow Patterns of Liquid-Vapor (Gas) Two-Phase Flow

Among the kinds of two-phase flows summarized in the preceding section, liquid-vapor (gas) flow is the most complex because the interfaces are deformable and the vapor or gas phase is more common. Furthermore, the interfacial configurations in two-phase flow are also very complicated due to heat and mass transfer and can vary over a wide range. The interfacial distribution in the liquid-vapor (gas) flow can be classified into a number of categories known as flow patterns or flow regimes. Flow regimes and flow regime transitions for vertical and horizontal tubes will be discussed. The flow patterns that will be presented in this section apply to two-phase flow in straight tubes only and are not applicable for two-phase flow in bends or coils. In addition, the flow patterns that will be addressed in this section are for cocurrent flow only and are not valid for countercurrent flow. However, one can easily obtain useful two-phase flow information in open literature concerning various configurations in geometry and/or countercurrent flow. Before introducing the flow patterns, it is useful to establish notations and to develop some fundamental concepts and relationships.


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