An experimental study on wave characteristics and stratified to non-stratified transition for downward (-2°) two-phase flow and highly viscous oil (μO=181 cP) is presented in this study. Gravity and 2D waves have not been observed for high viscous oil-gas stratified flow. Irregular and breaking waves have been observed for high gas flow rates. Comparisons with wave formation criterion (Jeffrey, 1925) suggest that the sheltering coefficient is a function of the liquid viscosity. Finally, none of the criteria for stratified to non-stratified flow presents a fair agreement with experimental data.


When gas and liquid flow simultaneously in a pipe, the two phases can distribute themselves in a variety of flow configurations. The flow configuration strongly depends on pipe inclination. For horizontal and near horizontal configurations, flow patterns can be classified as stratified flow (ST), intermittent flow (I), which includes slug and elongated-bubble flow, annular flow (A), and dispersed-bubble flow (DB). Stratified flow occurs at relatively low gas and liquid flow rates. The two phases are separated by gravity, where the liquid phase flows at the bottom of the pipe and the gas phase on the top. Taitel and Dukler (1976) sub-divided the stratified flow into stratified-smooth (SS), where the gas liquid interface is smooth, and stratified-wavy (SW), occurring at relatively higher gas rates, at which stable waves form at the interface. Shoham (1982) performed a systematic experimental study on the effect of pipe inclination angle on the flow pattern transition for two phase water-air flow. He observed that downward inclinations affect the stratified to non-stratified transition boundary. In downward stratified flow, the liquid moves faster than the horizontal case owing to gravity forces. As a result, higher liquid velocities are required to have a transition from stratified to non-stratified.

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