Measurement of local two-phase flow parameters of downward bubbly flow in mini pipes

In order to extend a precise database on local two-phase flow parameters in mini pipes, experiments were conducted for adiabatic gas–liquid bubbly flows flowing down in vertical mini pipes with inner diameters of 1.03, 3.00, and 5.00 mm. A stereo image-processing was applied to observe the phase distribution characteristics in pipe cross-section. The local flow parameters including profiles of void fraction, Sauter mean bubble diameter, and interfacial area concentration in pipe cross-section were obtained at three axial locations in the test pipes with various flow conditions: superficial gas velocity of 0.00508–0.0834 m/s and superficial liquid velocity of 0.208–3.00 m/s. The axial developments of the local flow parameters were discussed in detail based on the obtained data and the visual observation. It was confirmed that the core peak distributions were formed at low liquid flow rate conditions in which the buoyancy force dominated while the wall peak distributions were formed at high liquid flow rate conditions in which the body acceleration due to the frictional pressure gradient dominated. The result indicated the existence of lift force pushing the bubbles towards the pipe wall even in vertical downward flows. The database obtained through the present experiment is expected to be useful in modeling the interfacial area transport terms, the validation of the existing lift force models as well as the benchmarking of various CFD simulation codes.