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Название: Experimental Investigation of Large-Scale Three Dimensional Bubble Plume Dynamics

Автор: Simiano M.

Аннотация:

The report is divided in two parts, reflecting the chronology of the research work performed. The first part presents an extensive study of the most important hydrodynamic characteristics of fairly large-scale bubble plumes, created in a cylindrical vessel having a diameter of 2 m with water depth of 1.5-2 m. The bubbles were produced near the bottom of the vessel by a multi-needle circular injector having a diameter of 0.3 m. Several measurement techniques and a variety of tools were used to measure and to analyze the data. Particle Image Velocimetry (PIV), double-tip Optical Probes (OP), photographic techniques and three-dimensional Electro-Magnetic Probes (EMP) were extensively applied to measure bubble and liquid velocities, void-fraction, interfacial area concentration, bubble size and liquid re-circulation rates. PIV measurements in a vertical plane crossing the centre of the injector provided the instantaneous velocity fields for both phases, including selected hydrodynamic parameters, such as the movement of the plume axis and its instantaneous cross-sectional width. Statistical studies were performed using image processing to determine the diameter and the instantaneous centreline position distributions and their fluctuations in time. An important finding was that there is not much instantaneous spreading of the plume. The stress tensor distributions obtained from the instantaneous data indicate that for the continuous phase, these stresses scale linearly with the local void-fraction in the range of 0.5%< a <2.5%. The bubbles were found to be approximately ellipsoidal, with shape factor e=0.5 an equivalent bubble diameter (Sauter) of about 2-2.5 mm.
In the second part, the limitations imposed by measuring successively the flow parameters in each phase were eliminated. Indeed, since the velocities of the continuous and discrete phases are strongly correlated, simultaneous velocity measurements are required to estimate correctly the relative velocity. The PIV system was expanded with a second camera, used to measure simultaneously the velocity fields of the two phases in the vertical plane crossing the injector. In addition, a two-camera video recording system, storedsimultaneous images of the 3D bubble plume structure so that the PIV data could be categorized according to the state of the plume (position and width).
The instantaneous bubble and liquid velocity vector plots were correlated with the corresponding instantaneous bubble plume images. This allowed sampling and ensemble averaging of according to the bubble plume position or bubble plume diameter. The three dimensional plume dynamics was intensively investigated. The large-scale instability of the bubble plume resulted from meandering of the plume structure around its axis with superimposed horizontal cross-section fluctuation, as already suggested from the results obtained in the first part of the work.
The plume meandering and the horizontal cross-section fluctuation, supplied the instantaneous measured quantities with a coherent contribution which appeared as a fluctuation over the mean value.
The coherent contribution to the globally time-averaged quantities such as velocities, Reynolds stress terms and therefore to the turbulence intensity was evaluated; the results showed a negligible contribution close to the injector and a substantial one at higher elevations. The coherent contribution was significant in both phase velocities, but stronger in the Reynolds stress terms and therefore in the turbulence properties.