Development of local two-phase flow parameters for vertical bubbly flow in a pipe with sudden expansion

Experimental data are presented for vertical air-water bubbly flow obtained by fiber-optic sensors in a pipe with sudden expansion (40 mm to 90 mm). The local void fraction, local bubble velocity, local bubble frequency, bubble chord length, bubble size, and local interfacial area concentration were determined, using decentralized processing of the signals of multiple sensors. Main emphasis is put on the local interfacial area concentration, here defined as the ratio of the total surface area of all bubbles in a reference volume to the size of that volume. The calculation of the surface area was based not only on the assumption of spherical bubbles, as is commonly done for bubbly flow, but on different bubble shapes. The calculated distribution of bubble sizes depends on the chord length resulting from the product of measured bubble velocity and bubble residence time at the sensor tip. The values for the interfacial area concentration for flows with low void fractions ( alpha less than or equal to 0.04) agree well with those measured by assuming only spherical bubbles. The existing deviations for flows with void fractions above alpha identical with 0.04 seem reasonable according to the differently assumed bubble shapes in the determination methods. This leads to an increase in the values for the interfacial area concentration.