Effect of entrained air bubbles on micro bubbles generated by a pressurized dissolution method

Diameters of micro-bubbles are to range from about one to several hundreds urn, and therefore it is difficult to measure the correct diameter distribution using a single measurement method. In this study, diameters of bubbles generated by a pressurized dissolution method are measured by using phase Doppler anemometry (PDA) and an image processing method, which is based on the Sobel filter and Hough transform. The diameter distribution and the Sauter mean diameter of micro bubbles are evaluated based on the diameters measured by both methods. Experiments are conducted for several mass flow rates of dissolved gas and of air bubbles entrained in the upstream of the decompression nozzle to examine effects of the entrained bubbles on bubble diameter. The experimental result indicates that diameter distribution of micro bubbles can be accurately measured for a wide range of diameter by using PDA and the image processing method. The mean diameter of micro-bubbles generated by gasification of dissolved gas is smaller than that generated by breakup of air bubbles entrained in the upstream of the decompression nozzle. As the mass flow rate of the entrainment of air bubbles in the upstream of the decompression nozzle increases, the mean bubble diameter increases and the bubble number density decreases.