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. The importance of predicting different flow patterns in two-phase flows is well established. Annular flow is obtained for a wide range of gas and liquid flow rates in microgravity and it is the preferred flow pattern for the operation of two-phase systems in space. Therefore, it is important to be able to accurately predict the flow pattern which exists under given operating conditions of a two-phase system. We present a flow pattern transition criteria based on dimensionless parameters and validate it.
Annular flow is the most common and widely occurring of the two-phase flow patterns.
It is known that the wavy liquid film in annular flow has a profound influence on the transfer of momentum and heat between the gas and the liquid. Thus, an understanding of the characteristics of the wavy film is essential for developing accurate correlations that may be used to predict the transfer rates. One goal of our research program is to determine the characteristics of wavy films in microgravity. The absence of gravity reduces the number of observed flow patterns to three. It also reduces the number of dimensionless groups required to characterize two-phase flows by two, because of absence of the magnitude of gravity and the angle of orientation with respect to gravity.