The simultaneous gas-liquid flow in horizontal and vertical pipes has been a subject of investigation over a long period of time. Results of this research endeavor are of general interest to petroleum, chemical and nuclear industries: flow of hydrocarbon mixtures in pipelines and wellbores are standard examples of multiphase applications in oil industry. (Operations of oil and gas gathering systems, compressors and pumps depend on accurate estimation of pressure drop.) Increasing dependence on hydrocarbon production from offshore fields where much of the production is transported as two-phase and three-phase mixtures further enhances the need for better understanding of these physically complex systems.
The importance of inclined flow has been mostly limited to moderate angles of inclination; cross-country pipelines though rarely horizontal are usually not featured by steep angles. With offshore production, however, the occurrence of multiphase flow in highly inclined systems is fairly common.
Beggs and Brill l developed a model for predicting a pressure drop and holdup in pipes inclined at any angle between 0 ° and 90 °. The method is flow regime dependent although the flow pattern map provided by the authors is strictly of an empirical nature.
Of other studies, only two are mentioned in this place:
Mechanistic modeling of flow pattern transition by Barnea et. al.2; models of flow patterns transition developed earlier for horizontal and vertical tubes were modified for upward flow at any angle of inclination. A study by Hasan and Kabir3, in which the drift flux model was modified for the inclined bubbly and slug flows whereas the entrainment concept was used for the annular flow.
Given rather limited information on gas-liquid flow behavior in highly deviated systems, an experimental study was conducted to investigate the flow characteristics in pipes at high angles of inclination.
The main component of the flow loop is a 4 m long pipe on an inclinable trescale which can be set at angles from 0 ° to 9 ° (see Fig. 1). Test section is made of a transparent acrylic tube of 25.8 mm ID. This fluid system used in all experiments consists of air-oil mixture at room temperature and pressure of up to about 250 kPa. The compressor provides a maximum of 8.5 scm3/s. The oil used is a light refined machine oil of 6.5 mPas viscosity and 860 kg/m3 density at 23 °C.
Single phase oil and air flaws are metered separately using bath orifices and and rotameters. The two phases are brought together at the bottom of the test section. Upon leaving the test section the mixture enters a 2.5 m high separator tank where the air is vented out of the building through an exhaust valve manifold and the oil is returned in a closed loop to storage tanks and the pumping system. Overall test section pressure is controlled via the air exhaust valves on the separator tank.
The liquid system consists of a pair of steel storage tanks of 1.4 m3 capacity with three pumps mounted in parallel to feed the test section (0.75, 5.25 and 18.7 m3/s pumps).