Oil-water flow pattern transitions in horizontal pipes have been studied both experimentally and theoretically. A new state-of-the-art, gas-oil-water test facility was designed, constructed and operated. A transparent test section (1.9735 in. ID {dollar}times{dollar} 51 ft long) can be inclined at any angle, to study both upward and downward flow simultaneously. Mineral oil and water were the working fluids ({dollar}musb{lcub}o{rcub}{dollar}/{dollar}musb{lcub}w{rcub}{dollar} = 29.6, {dollar}rhosb{lcub}o{rcub}{dollar}/{dollar}rhosb{lcub}w{rcub}{dollar} = 0.85 and {dollar}sigma{dollar} = 36 dynes/cm @ 78{dollar}spcirc{dollar}F). Only horizontal flow tests were conducted.; A new classification for oil-water flow patterns based on published and acquired data is proposed. Six flow patterns were identified and classified into two categories: Segregated flow and Dispersed flow. Stratified flow and stratified flow with some mixing at the interface (ST & MI) are segregated flow patterns. The dispersed flow can be either water dominated or oil dominated. A dispersion of oil in water over a water layer and an emulsion of oil in water are water dominated flow patterns. An emulsion of water in oil and a dual dispersion are oil dominant flow patterns. Pressure drop decreases when the transition to dispersed flow is crossed. Conductance probe data and high speed photographs are adequate flow pattern identification tools while wall pressure fluctuations are not. Slippage is only relevant for segregated flow patterns.; The oil-water flow pattern transitions for light oils are predicted using the two-fluid model and a balance between gravity and turbulent fluctuations normal to the axial flow direction. Linear and non-linear analyses reveal that the stratified/non-stratified transition must be addressed with the complete two-fluid model. Stratified flow is predicted by the viscous Kelvin-Helmholtz analysis while inviscid Kelvin-Helmholtz theory predicted the ST & MI flow pattern. Both the viscous Kelvin-Helmholtz analysis and structural stability criterion are satisfied simultaneously. For the dispersed flow pattern, the predicted drop sizes from the Hinze and Levich models are modified in order to account for the effect of the dispersed phase concentration. The controlling parameter for the coalescence phenomena is the water fraction. The model performance is excellent and compares well with published data. Moreover, the model gives reasonable predictions for inclined flow.
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