On the application of LDA to bubbly flow in the wobbling regime |
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Authors: | J S Groen R F Mudde H E A Van Den Akker |
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Institution: | (1) Gist-brocades, P.O. Box 1, 2600 MA Delft The Netherlands, NL;(2) Kramers Laboratorium voor Fysische Technologie Delft University of Technology, Prins Bernhardlaan 6, 2628 BW Delft The Netherlands, NL |
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Abstract: | Considerations for applying LDA to bubbly flows with bubbles about 3 to 4 mm in diameter were investigated by means of detailed experiments in the model
geometry of a train of bubbles. Both forward scatter and backscatter LDA were studied. The validity of phase discrimination via burst amplitude was tested and special attention was paid to the impact
of bubble interface response to the laser beams.
Forward and backscatter measurements can be compared well. In both configurations, predominantly the liquid phase is “seen”
by LDA. A bubble itself only leads to a velocity realization in special conditions. In those cases the Doppler shift is determined
by the motion of the bubble interface which consists of the motion of the center of gravity of the bubble as well as shape
oscillations. In backscatter bubbles only give velocity realizations when their “cheeks” pass through the measuring volume
virtually perpendicularly.
It is shown that the bubble-caused velocity realization frequency is very low for bubbles of the size used. Phase discrimination
on burst amplitude does not hold. In ambient cases such as bubble columns one can assume that only the liquid phase is being
studied.
Received: 4 May 1998/Accepted: 30 September 1998 |
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