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This work provides new experimental data on high viscosity oil‐gas flow. A set of 170 downward flow experiments were carried out using a mixture of air and mineral oil (213 mPa · s, 50.8 mm internal diameter, 21.5 m long) for five different elevations: −45°, −60°, −70°, −80°, and −85°. Superficial liquid and gas velocities varied from 0.05 m/s to 0.7 m/s and 0.7 m/s to 7 m/s, respectively. Flow pattern transitions were identified and analyzed. The results helped to evaluate the effects of pipe inclination and liquid viscosity on the stratified‐annular transition, as well as the characterization of subclasses of annular flow pattern, named falling film, liquid slip, and wavy annular. Based on the experimental evidence, the study proposes a constant liquid‐fraction value as a transition between liquid slip and wavy annular flow patterns. A comparison of the present data with the flow pattern predictions of the available models were made. Results show disagreements, which justifies the need for a detailed study of the effects of viscosity and the inclination of the pipe in liquid‐gas downward flow mixtures. [ABSTRACT FROM AUTHOR]

• New experimental data of oil-air upward flow in inclined pipes are presented. • Experimental parameters are compared to the ones predicted by existing models. • The behavior of the pressure gradient components is studied. • Positive-frictional-pressure gradient occurs in all evaluated inclinations. • Positive-frictional-pressure happens when the power exceeds the energy dissipation. Two-phase flow models play a fundamental role in the design, improvement, and operation of oil and gas production systems. Considering very large high-viscosity oil reserves around the world, the understanding of two-phase flow systems, which involves high-viscosity-liquids (>100 mPa s), becomes a relevant topic. The behavior of this kind of flows is expected to be significantly different as compared with low viscosity oils. In this work, we report a new experimental data on high viscosity oil-gas flow. A set of 170 upward flow experiments were carried out using a mixture of air and mineral oil (213 mPa s, 50.8 mm ID, 21.5 m long) for five different elevations: 45°, 60°, 70°, 80°, and 85°. Superficial liquid and gas velocities were varied from 0.05 m/s to 0.7 m/s and from 0.5 m/s to 6 m/s, respectively. We measured and analyzed: flow pattern, pressure gradient, and average liquid holdup. These variables helped evaluate (a) the implications of a buoyancy-like term in the mechanical energy balance, and (b) the performance of two-phase flow models under the experimental conditions. As a result, we propose a practical relationship between the actual and homogenous densities through the slip and no-slip holdup fractions. This approach, supported by experimental evidence, shows that considering a buoyancy-like term is congruent with the physics that results from the interaction of gravitational and friction forces in inclined pipes. Moreover, the comparison of the present data with prediction models shows a performance which varies significantly depending on the predicted variable, the existing flow pattern as well as the inclination angle. In general, the model performance is good when the observed flow pattern is well predicted but fails when it is not, which typically falls into churn flow. This justifies the need to further understand churn flow and develop models consistent with the physics of transport. [ABSTRACT FROM AUTHOR]