Two-phase flow in a large pipe with a 90° bend: experimental and numerical study

<p>Research on the two-phase flow in large pipes plays a vital role in the design, scale-up, optimisation, safety, and reliability of many industrial applications such as the applications in bubble columns, chemical processing, oil and gas transportation, wastewater treatment, and nuclear and biochemical reactors. The measurement and the investigation of the two-phase flow through pipes assist researchers in creating a reliable design and reducing the economic risk associated with pipeline failures. Failures in a two-phase flow pipeline often occur because of the interaction of different phases, which influences the flow stability, phase hold-up, phase distribution, and pressure drop. However, pipeline systems are not constrained to straight vertical and horizontal pipes. They are connected via bends and other types of fittings. Most of the research on the flow in bends is limited to the single-phase flow. Limited research has been carried out on the two-phase flow via bends, and even these studies are restricted to small pipe sizes. Therefore, it is necessary to design and build experimental rigs with several measuring techniques, to study the structure, behaviour, and distribution details of the gas and liquid flow in a large pipe with a bend.</p> <p>To address these requirements, two independent experimental rigs were designed and built at CSIRO Heavy Lab, Clayton, Australia. The first experimental rig consisted of a vertical to horizontal pipe with a diameter of 150 mm, connected via a 90° bend. Tomography techniques were utilised to measure the instantaneous void fraction, average cross-sectional void fraction, and the average bubble size distribution at the pipe length to diameter ratio of 8.4 before the bend and downstream of the bend. Four conditions of liquid superficial velocity in the range 1.02-2.00 m/s and gas superficial velocity in the range of 0.14-0.22 m/s were investigated. The second experimental rig consisted of a tall vertical pipe having a diameter of 150 mm with up to 25.9 times the diameter of the pipe after an air injection system and 12 times the diameter before the air injection system. The vertical pipe was jointed to the horizontal pipe by a 90° stainless steel bend with a centre-line radius of 228.5 mm. Measurement techniques, wire-mesh sensors, and pressure transducers were utilised to obtain the data for 16 two-phase flow conditions, for the liquid superficial velocity range of 0.78-2.00 m/s and the gas superficial velocity range of 0.08-0.30 m/s. The two-phase flow parameters were measured in the vertical pipe at the diameter ratios of 2.8, 10.5, and 25.9 after the air injection system and in the horizontal pipe at the diameter ratio of 0.4.</p> <p>The results from the first experimental setup demonstrated the use of instantaneous void measurement and an algorithm to plot a three-dimensional bubbly flow inside the pipe. The results highlighted the significance of liquid superficial velocities on the phase distribution after the bend. The results of the second experiment revealed the phase distribution at different heights of the vertical pipe and showed the effect of increasing the superficial gas and liquid velocities on the bubble size distribution. The results also showed the migration of small bubbles toward the pipe wall and the concentration of large bubbles at the pipe core. The results were discussed in terms of the average radial void fraction, average bubble velocity, and bubble size distribution. The results were also discussed in terms of several dimensionless numbers. A flow map was then developed to estimate the average bubble velocity and the average bubble diameter for specific conditions.</p> <p>The results of the second experiment were utilised to discuss the effect of a 90° bend on the two-phase distribution and the bubble size distribution. The results revealed the effects of the bend upstream of the flow, indicating several factors which influenced the two-phase distribution before the bend. The study also showed the two-phase secondary flow formation and its dependencies and complexity after the bend. The results before and after the bend indicated that the presence of the bend, the flow regime structure, and the flow rates contributed to the formation or dissipation of the secondary flow after the bend. The effect was also discussed in terms of the average slip ratio, and a basic empirical equation was used to estimate the averaged slip ratio after the bend. The bubble diameter distribution before and after the bend was analysed, considering the effect of the bend on the bubble coalescence and breakup mechanisms.</p> <p>In addition to the experimental study, a set of numerical study was carried out for several experimental vertical to horizontal gas-liquid flow through 90° bends. At the time of the design and build of the experimental rigs for the large pipe, experimental data after a 90° bend in large pipes were not existed or available. Therefore, the experimental data for 90° bend numerical study were first chosen from the research work of Qiao ( 2017) and Yadav (2013) for a small pipe of 50 mm in diameter and later a sets of numerical study were conducted for an experimental case in the large bend from this study.  The computation results were derived from the two-phase governing equations and multi-size group modelling (MUSIG) using commercial CFD packages CFX and ICEM. Several combinations of the interfacial forces in the CFD packages and the literature were modelled and compared with the experimental data for small bends and the large bend form this study in terms of the relative standard deviation, indicating the feasibility of adopting specific models in the prediction of a two-phase flow after a bend.</p>