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.

An experimental investigation is carried out to study the transition from stratified to slug flow and the development of slug flow. The variation of Lockhart-Martinelli parameter with the non-dimensional liquid height is established based on the experimental data. A correlation is developed for the liquid height as a function of superficial gas and liquid Reynolds number. The liquid height is observed to increase up to some level depending on the mass flow rate, beyond which there is a sudden jump in the height leading to the formation of slug. This critical liquid height is the limiting condition for the evolution of slug. Below the critical height a stable stratified flow is observed. The critical height for stability limit of stratified flow is established experimentally for various combinations of mass flow rates of the primary and secondary phases.

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An experimental analysis towards establishing the effect of inclination on flow and thermal characteristics of air-water two-phase flow is presented. The experimental setup consists of a mixing section, a visualization section, a test section and an outlet section. The test section consists of two sets of pressure transducers, heater element and RTD sensors for surface temperature and inlet and outlet bulk fluid temperature measurements. The test setup can be inclined maximum up to 30º of inclination. Various flow patterns like stratified, slug, plug, wavy, annular and other mixed type are observed and captured at 0º, 2º, 5º and 7º for the range of the liquid (Re SL) and gas (Re SG) superficial Reynolds numbers of 500-10000 and 500-30000 respectively. Flow regime map is established based on this flow visualization. For inclined configuration the stratified regime is observed to be replaced by slug and slug/wavy type of flow regime and the transition between slug-plug and slug-slug/wavy takes place at higher Reynolds numbers. Surface and bulk fluid temperatures are recorded for constant wall heat flux condition, by varying the superficial Reynolds numbers of gas and liquid phases to establish the local and average two-phase heat transfer coefficient. It is observed that Re SL has a pronounced effect on heat transfer coefficient in comparison to Re SG. With the change in inclination this effect becomes more evident. A heat transfer correlation is deduced from the present experimental observation which includes the basic parameters like Reynolds number and Prandtl number, pressure drop and inclination effect.

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