Abstract
There has been a strong demand to clarify the microscopic structure of bubbly flows. The authors attempted and succeeded in clarifying the flow structure around bubbles. The Ultrasonic Velocity Profile Monitor (UVP) was used to measure the instantaneous velocity profiles of both phases in bubbly countercurrent flows. From the UVP data, probability density function of the instantaneous velocities was obtained at each measuring point and the first to the fourth order statistical moments of the liquid velocity were calculated. From the distribution of these moments, existence of bubble boundary layers was found. The flow structure around bubbles was proposed to be divided into three regions:(1) a boundary layer, (2) a transition region and (3) a main flow region. In the boundary layer, liquid flow is strongly influenced by the bubble motion but is independent of the main flow. In the transition region adjacent to the boundary layer, liquid phase is influenced by both bubble motion and the liquid main flow. Liquid is dragged strongly by bubbles in the case where the flow rate is relatively high. In the main flow region, liquid phase receives much influence from the liquid main flow compared with other regions. Finally from the obtained data, some correlations were found between the layer thickness, void fraction and bubble Reynolds number.
