Experimental Investigation of Large-Scale Three Dimensional Bubble Plume Dynamics

Simiano M.

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

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Table of contents :
Abstract……Page 8
Sintesi……Page 10
Motivation……Page 15
Introduction and State of the Art……Page 17
Part I……Page 20
Experimental setup……Page 21
Coordinate system……Page 22
PIV technique……Page 23
Photographic technique……Page 25
Double-tip Optical Probe technique……Page 27
Electromagnetic probe measurements……Page 29
Test matrix……Page 30
Void fraction measurements……Page 31
Bubble velocity measurements……Page 32
Mean bubble diameter and interfacial area concentration……Page 33
Global plume dynamics by image processing of the PIV pictures……Page 36
PIV results……Page 39
Global circulation in the vessel……Page 47
Comparisons between different techniques……Page 49
Conclusions……Page 53
Part II……Page 58
Experimental setup II……Page 59
Simultaneous two-phase PIV technique……Page 60
Two-perpendicular directions video recording system……Page 62
Test matrix II……Page 64
Void fraction measurements……Page 65
Image processing of the two-direction video recordings……Page 68
Conclusion……Page 78
Simultaneous two phase flow PIV results……Page 80
The average of the instantaneous relative velocity……Page 82
Hussain & Reynolds triple decomposition……Page 83
Ensemble-average data according to the plume-centreline position……Page 84
Bubble Liquid and Relative velocities close to the injector……Page 85
Globally time-averaged turbulence properties close to the injector……Page 90
Away from the injector, in the core flow region……Page 96
Buoyant turbulent intensity away from the injector……Page 98
Conclusion……Page 99
Ensemble averages according to the bubble plume diameter……Page 103
Bubble, liquid and relative velocities close and away from the injector……Page 104
Turbulence statistics……Page 107
Conclusions……Page 110
Summary, conclusions and perspectives……Page 112
Bibliography……Page 114
Curriculum Vitae……Page 0