Two-phase gas-liquid (and vapor-liquid) flows occur in a variety of process equipment such as petroleum production facilities, condensers and reboilers, power systems and core cooling of nuclear power plants during emergency operation. In addition to these normal gravity applications, two-phase flows also occur in many space operations such as active thermal control systems, power cycles, propulsion devices, and storage and transfer of cryogenic fluids.
For conditions of technological interest, there are a few major types of flow regimes observed for gas-liquid flows in pipes. Characteristics of these flow patterns and the conditions under which those flow patterns exist depends on the orientation of the pipe with respect to gravity. At low gas flow rates, a bubble flow pattern in which small gas bubbles are uniformly distributed in the liquid is obtained. Increasing the gas flow rate leads to slug flow. This flow pattern is characterized by large bullet shaped gas bubbles separated by liquid slugs. At even higher gas flow rates, a highly agitated churn flow is observed. Increasing the gas flow rate further leads to the annular flow regime in which the liquid moves along the pipe wall in a thin, wavy film and the gas flows in the core region.
The above description applies only to two-phase flows in vertical pipes. In horizontal pipes, chum flow does not exist. At low gas flow rates, smooth and wavy stratified flows exist in such pipes.
In the absence of gravity, there exist only three major flow patterns: bubble, slug, and annular (Figure 1). In microgravity, annular flows are obtained for a wide range of gas and liquid flow rates. Bubble and annular flow are the preferred flow pattern for the operation of two-phase systems in space. Slug flow is avoided, because vibrations caused by slugs result in unwanted accelerations. Therefore, it is important to be able to accurately predict the flow pattern which exists under given operating conditions of a two-phase flow system. Ever since the early work of Baker (19581, there have been attempts to predict the transitions between flow patterns for two-phase flows in pipes. Because of the large number of dimensionless groups (seven to nine) describing the phe-Figure 1. Flow patterns observed in microgravity two-phase flows (Bousman, 1994).