The present study deals with the effect of the bubble size, from small bubble scale to normal scale (d_(b)(velence)0.25approx2.6 mm), on the flow passing through a rectangular cylinder in an upward gas-liquid bubbly flow. Extensive visualization experiments are conducted and a digital camera and a high-speed camera analyzed the flow, while PIV analysis by the volume cross-correlation method is conducted to observe the differences in the flow pattern. In order to further understand the effect of bubble size, the pressure distribution along the pipe and the cylinder surface are measured. From the results taken, the drag force is calculated and compared to the case of single phase-flow. Furthermore, the fluctuation phenomena generating from the Karman vortex street downstream the cylinder are investigated, and how the intensity and frequency are affected by the bubble size and gas fraction is presented. The experiments are conducted under two different Reynolds number Re, and volumetric gas fraction ranging from (alpha)_(v)(velence)0approx5percent, giving valuable information regarding the changes that occur due to bubble size differences and the relation it has with volumetric gas fraction.
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