Author(s): N. E. Lima Neto; David Z. Zhu; Nallamuthu Rajaratnam
Linked Author(s): N. Rajaratnam
Keywords: Eration; Bubbles; Jets; Mixing; Slip velocity; Turbulent flow
Abstract: This study describes experiments on air-water bubbly jets in a relatively large water tank. The test conditions included gas volume fractions ranging from about 7 to 83% of the total mixture flow rate. Images of the bubbles were acquired using a high resolution CCD camera and measurements of bubble properties and mean liquid flow structure were obtained using a doubletip optical fiber probe and an electromagnetic propeller anemometer, respectively. Bubbles with approximately uniform diameters of 1-4 mm were generated in the tests with stronger water injection while bubbles with non-uniform diameters of 3-10 mm were generated in the tests with weaker water injection. Bubble properties such as bubble size, velocity and specific interfacial area as well as axial water velocity decreased from the jet centerline to its edge, resembling Gaussian or polynomial curves. The volumetric mass transfer coefficients estimated here were similar to those reported in the literature for fine bubble diffuser aeration. The liquid volume flux was found to increase with the gas volume fraction following approximately a power law. This was attributed to the additional entrainment into the wakes of the bubbles and turbulence due to interactions of the bubbles and their wakes. Bubble slip velocities were higher than the terminal velocities for isolated bubbles due to drag reduction and a new curve is proposed to describe bubble slip velocity as a function of bubble diameter. Overall, the results obtained here provide information for evaluation and validation of numerical models for air-water bubbly jets and similar vertical bubbly flows.
Year: 2007