Experimental study on interfacial area transport of two-phase bubbly flow in a vertical large-diameter square duct

In this paper, an experimental study on the atmospheric upward bubbly air-water flows in a vertical large-diameter square duct flow channel (cross-sectional sizes, 100 mm × 100 mm, hydraulic equivalent diameter, DH = 0.1 m and height, 3 m) have been performed by mainly using four-sensor probes. The four-sensor probes with the latest four-sensor probe method of Shen and Nakamura (2014) were applied in the local measurements at 3 axial positions (z/DH = 8.3, 18.3 and 28.3) to obtain interfacial area concentration (IAC), 3D bubble velocity vector and bubble diameter for the complex flow structure. The measurements were carried out locally at 66 points in an octant symmetric triangular zone in the cross-section at each axial position. The agreements between these local measurement results of the four-sensor probes and the measured results from the differential pressure gauges and the air flow meters are ± 7.97% in average relative measurement error for void fraction and ± 8.67% in average relative measurement error for superficial gas velocity. The obtained void fraction, IAC, 3D bubble velocity vector and bubble diameter serve as a valuable database relating to the cross-sectional local profiles and axial flow development and provide valuable insight into the local flow structure and behavior in the corresponding flow regimes. Although the interfacial area transport equation (IATE) and its bubble coalescence and breakup models have already played an important role in predicting the IAC in general two-phase flow fields now, they are mainly developed based on the two-phase flow experimental data taken in round pipes or small diameter channels with different shapes. To confirm their usability in the two-phase flow in large-diameter square duct, this study has evaluated the 1D one-group IATE with its 6 sets of bubble coalescence and breakup models with the presently-obtained database. The IATE with the bubble coalescence and breakup model of Sun et al. (2004a) has been concluded to give the best prediction for the IAC in the two-phase bubbly flow in the large-diameter square duct with a mean absolute relative error of 25.10%. It is highly desirable to develop a more accurate bubble coalescence and breakup model considering the complex turbulence in large-diameter square ducts.