PSEUDO-LAMINARIZATION OF MICRO-BUBBLE CONTAINING MILKY BUBBLY FLOW IN A PIPE

Abstract

Micro bubble technology has become to attract people's concerns due to its wide potential in practical applications to a variety of advanced and conventional science and technologies. However, our knowledge of micro bubbles containing bubbly two-phase flow is almost nothing. We developed a specially designed nozzle which generates micro air bubbles with high bubble number density (mean diameter ≈ 40microns, number density higher than 2×105/cc). Using this micro-bubble generator, we carried out a measurement of two-phase frictional pressure drop, cross-sectional average void fraction, local void fraction profiles and liquid velocity profiles. The range of cross-sectional average void fraction covered was up to 0.6 % which is high enough to realize milky bubbly flow. The most exciting result we found is that the two-phase flow becomes pseudo-laminarized by injecting such ultra small bubbles into the water flow. For 0.3 ≈ 0.5% void fractions, pseudo-laminar-to-turbulent transition occurred at Re= 10,000 ≈ 20,000, showing a significant reduction in wall friction. The radial liquid velocity profiles show typical turbulent 1/7 th power law profile. However, the liquid velocity profiles in milky bubbly flow obviously shifted from von Karman's universal velocity profile towards lower values of the non-dimensional distance from the wall. The cross sectional averaged void fraction correlates well with a homogeneous flow model, which is verified by uniform profiles of the local void fraction distribution over the whole channel cross section. The mechanisms of pseudo-laminarization and flow structures have been discussed.