Summary Oil/gas pipe flows are expected to exhibit significantly different behavior at high oil viscosities. Effects of high-viscosity oil on flow pattern, pressure gradient, and liquid holdup are experimentally observed, and differences in flow behavior of high- and low-viscosity oils are identified. The experiments are performed on a flow loop with a test section of 50.8-mm ID and 18.9-m-long horizontal pipe. Superficial liquid and gas velocities vary from 0.01 to 1.75 m/s and from 0.1 to 20 m/s, respectively. Oil viscosities from 0.181 to 0.587 Pa·s are investigated. The experimental results are used to evaluate the performances of existing models for flow pattern and hydrodynamics predictions. Comparisons of the data with the existing models show significant discrepancies at high oil viscosities. Possible reasons for these discrepancies are carefully examined. Some modifications are identified and implemented to the closure relationships employed in the Zhang et al. (2003) model. After these modifications, the model predictions provide better agreement with experimental results for flow pattern transition, pressure gradient, and liquid holdup. Introduction Gas/liquid two-phase flow in pipes is a common occurrence in the petroleum, chemical, nuclear, and geothermal industries. In the petroleum industry, it is encountered in the production and transportation of oil and gas. Accurate prediction of the flow pattern, pressure drop, and liquid holdup is imperative for the design of production and transport systems. High-viscosity oils are discovered and produced all around the world. High-viscosity or "heavy oil" has become one of the most important future hydrocarbon resources, with ever-increasing world energy demand and depletion of conventional oils. Almost all flow models have viscosity as an intrinsic variable. Two-phase flows are expected to exhibit significantly different behavior for higher viscosity oils. Many flow behaviors will be affected by the liquid viscosity, including droplet formation, surface waves, bubble entrainment, slug mixing zones, and even three-phase stratified flow. Furthermore, the impact of low-Reynolds-number oil flows in combination with high-Reynolds-number gas and water flows may yield new flow patterns and concomitant pressure-drop behaviors. The literature is awash with two-phase studies addressing mainly the flow behavior for low-viscosity liquids and gases. However, very few studies in the literature have addressed high-viscosity multiphase flow behavior. In this literature review, the state-of-the-art of two-phase flow is first summarized. Then, the studies addressing the effects of liquid viscosity on two-phase oil/gas flow behavior are reviewed.

A unified model of multiphase heat transfer is developed for different flow patterns of gas-liquid pipe flow at all inclinations from -90° to +90° from horizontal. The required local flow parameters are predicted by use of the unified hydrodynamic model for gas-liquid pipe flow recently developed by Zhang et al.1,2 The model prediction of the pipe inside convective heat transfer coefficients are compared with experimental measurements for a crude oil/natural gas system in horizontal and upward vertical flows, and good agreement is observed. Introduction As oil and gas production moves to deep and ultra-deep waters, flow assurance issues such as wax deposition, hydrate formation, and heavy oil flow become very crucial in transportation of gas, oil and water to processing facilities. These flow assurance problems are strongly related to both the hydraulic and thermal behaviors of the multiphase flow. Therefore, multiphase hydrodynamics and heat transfer need to be modeled properly to optimize the design and operation of the flow system. Compared to experimental and modeling studies of multiphase hydrodynamics, very limited research results can be found in the open literature for multiphase heat transfer. Davis et al.3 presented a method for predicting local Nusselt numbers for stratified gas-liquid flow under turbulent liquid/turbulent gas conditions. A mathematical model based on the analogy between momentum transfer and heat transfer was developed and tested, using heat transfer and flow characteristics data taken for air/water flow in a 63.5-mm inside diameter (ID) tube. Shoham et al.4 measured heat transfer characteristics for slug flow in a horizontal pipe. The time variation of temperature, heat transfer coefficients, and heat flux were reported for the different zones of slug flow. Substantial difference in heat transfer coefficient was found to exist between the bottom and top of the slug. Most previous modeling studies were aimed at developing heat transfer correlations for different flow patterns.5–10 Kim et al.11 evaluated 20 heat transfer correlations against experimental data collected from the open literature, and made recommendations for different flow patterns and inclination angles. However, these recommended correlations did not give satisfactory predictions when compared with experimental results by Matzain12. Manabe13 developed a comprehensive mechanistic model for heat transfer in gas-liquid pipe flow. The overall performance was better than previous correlations in comparison with experimental data. However, some inconsistencies in the hydrodynamic model and the heat transfer formulations for stratified (annular) and slug flows need to be improved. A unified hydrodynamic model has been developed for gas-liquid pipe flow at the Tulsa University Fluid Flow Projects (TUFFP)1,2. The major advantage of this model compared with previous mechanistic models is that the predictions for both flow pattern transition and flow behavior are incorporated into a single unified model based on slug dynamics. Multiphase heat transfer depends on the hydrodynamic behavior of the flow. The objective of this study is to develop a unified heat transfer model for gas-liquid pipe flow that is consistent with the unified hydrodynamic model.