The development of two-phase flow structures in air–water planar bubble jets

Air–water two-phase upflow experiments are performed in a 200 × 10 mm2 rectangular duct to study the multi-dimensional development of two-phase interfacial structures. The air flow is injected into the test section with a uniform and a center peaked inlet profile for nine different flow rates covering bubbly, cap-turbulent and churn-turbulent flows. A four-sensor conductivity probe is used to measure local void fraction, interfacial area concentration, bubble velocity and Sauter mean diameter at three cross sections in a 3 m long test section. Experimental results show considerable axial and lateral developments of two-phase flow parameters in the test section. For certain conditions, the inlet profile has a significant effect on the phase distribution, phase velocity, interfacial structure and bubble interaction mechanisms. These results indicate that the conventional flow regime concept may not be applicable to developing flows and flows with a spatial distribution. Instead, the interfacial area transport equation (IATE) provides a dynamic and quantitative description of two-phase flow structures which can better serve the two-fluid model. A comparison of predictions by a one-dimensional (1D) two-group IATE and experimental data shows good agreement for uniform inlet flows. For center peaked flows, the 1D IATE systematically underestimates the interfacial area concentration of group 1 bubbles but overestimates that of group 2 bubbles. This is due to not accounting for the covariance terms in the 1D model such that certain bubble interaction mechanisms such as shearing-off and turbulent impact are under predicted. The pairing of uniform and center peaked planar jet experiments in this study present a unique benchmark for understanding separate effects in two-phase gas–liquid vertical flows.