Experimental visualization of two-phase flow patterns and transition from stratified to slug flow

The transition from stratified to slug flow generates oscillations in pressure and flowrates. Large liquid surges associated with slug flow are detrimental to the operation of process equipments involving two-phase flow. The characterization of two-phase flow regimes and their transition is thus an important area of research. In the present work, flow patterns for various regimes of air-water two-phase flow are captured experimentally. A flow pattern map is established based on the visualized images. The developed flow regime map is compared with that obtained by Ghajar and Tang. Slug frequency is recorded for a variety of superficial Reynolds number to show the instances of impact pressure. The development of slug and transition to slug flow from stratified flow are analysed using these captured images. It is observed that slug becomes highly chaotic with dispersion of air bubbles, when gas superficial Reynolds number is increased for a fixed value of liquid superficial Reynolds number. For lower gas superficial Reynolds number, the slug is observed to be very clear (without air entrainment). This is true for higher value of liquid superficial Reynolds number as well. The air entrainment increases with increasing gas superficial Reynolds number. This air entrainment might play a dominant role in deciding the flow and thermal characteristics of such two-phase flows.