In addition to these normal gravity applications, such flows also occur in many space operations such as active thermal control systems, power cycles, propulsion devices and storage and transfer of cryogenic fluids. Two-phase flows can be grouped into a number of different flow patterns. The gravitational body force acting on the phases with different densities causes stratification. In addition to the magnitude of that body force, orientation of it with respect to the direction of flow has a strong impact on the flow patterns observed and also on the flow pattern transition boundaries.
Experimental data indicate that two-phase flow patterns are somewhat simplified in microgravity.
In addition, it is found that annular flow is obtained for a wide range of gas and liquid flow rates in microgravity. It is 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. In many cases, bubbly flow should also be avoided because of the difficulty of preventing the transition from bubbly flow to slug flow. Therefore, it is important to be able to accurately predict the flow pattern which exists under given operating conditions of a two-phase system.