Time-spacial fluctuation of void fractions in bubbly flows within a large diameter vessel

Time-series void fractions were obtained by using a four-tip optical fiber probe in a large diameter vessel of 400 mm in diameter and 1500 mm in height. The diameters and vertical-velocity components of the bubbles were measured simultaneously. The experimental setup was equipped with an acrylic water jacket to accurately visualize large scale structures of the bubbly flows. It was found that the flows have meandering of the bubble clusters and fluctuation of vortical structures. To quantitatively clarify those characteristics, we measured the local-time-series gas void fraction. After pre-processing the raw data of the void fraction appropriately, we analyze the processed data via FFT. As a result, short- to long-periods fluctuations in local void fractions have been extracted. As compared the extracted fluctuations with the results obtained from visualization, they well correspond reciprocally. (author)