Comparison of local interfacial characteristics between vertical upward and downward two-phase flows using a four-sensor optical probe

To model the interfacial transfer terms more accurately in two-fluid model as well as to further comprehend the intrinsic mechanism for downward two-phase flows, more information that involves local interfacial parameters is required. The local interfacial characteristics in upward and downward bubbly flows were investigated separately in a 50.8 mm inner-diameter round pipe. A four-sensor optical probe was used in the measurement of local interfacial parameters, including void fraction, interfacial area concentration (IAC), bubble frequency, interfacial velocity and Sauter mean diameter. The radial profiles of these parameters in downward flows were presented and compared with that in upward flows. In general, the void fraction shows a core-peaked distribution for the downward flow at low void fraction, but shows a wall-peaked distribution for the upward flow; while at high void fraction, it has an off-center-peaked distribution for the downward flow but a core-peaked distribution for the upward flow. Also, under relative high void fraction conditions, the profile of Sauter mean diameter presents a pronounced maximum in the channel center for the upward flow whereas near the channel wall for the downward flow. Additionally, with an increase in the gas flow rate, the flat profiles of interfacial velocity transform to power-law shapes in the upward flows but to that with the velocity increasing along the radius for the downward flows.