Flow visualizations of refrigerant flows were made in a 0.5 mm glass channel. High speed videos of two-phase R134a flows were used to examine the secondary features within the two-phase flow and categorize the flow regimes present. Essentially, in small diameter channels, there are only two major observed flow regimes: inertia dominated (annular flow) and surface tension dominated (intermittent flow: slug or bubble). Image analysis was used to measure the velocities of the slugs and bubbles, and measurements of secondary features of the flow, such as the velocity of the surface waves that arose in slug-annular flow and the smaller bubbles that could be seen in the wakes of larger bubbles.
The secondary features of the two-phase flow are discussed with regard to their contribution to the flow regimes that are observed as the flow moves through the channel. Rings of liquid were observed to form into liquid slugs, dividing the longer bubbles into two. This is believed to be one of the transition mechanisms between the two flow regimes. Small droplets were also observed within the vapor core, which demonstrated velocity gradients within the vapor bubbles. Velocity measurements of the secondary features within the liquid films are used in the analysis of the dynamics within these flows that lead to pressure drop and flow regime transition.