  This experimental study focuses on the characteristics of air-water two-phase
interfacial structure. Interfacial parameters including void fraction,
interfacial area concentration, and bubble interfacial velocity are measured
using four-sensor electrical conductivity probe on a 12.7 mm ID vertical tube.
The tube size is approximately equal to the maximum distorted bubble size.
Therefore, the bubbly-to-slug transition characteristics can be different from
in other sizes of tubes. Comparing with previous studies, this study provides
an experimental database with a wide range on the bubbly-to-slug transition
flows, with 4 different superficial liquid velocities (0.3, 0.5, 1.0, and 2.0
m/s) and void fraction ranging from 0.07 to 0.66. Experimental results show
that the wall-peak void distribution does not appear in a small diameter tube
under the bubbly-to-slug flow transition flow. The distribution is related with
both void fraction and the relative bubble size to the tube size. In this
sense, a new correlation of distribution parameter in the Drift Flux model is
proposed based on the previous studies by Ishii and Hibiki et al. This
experimental study can be a good reference for the model development of flow
regime transition and the Interfacial Area Transport Equation.
