Bubble columns
Structures or stability?
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Abstract
The aim of the thesis is to contribute to the understanding of the hydrodynamics of the gravity driven bubbly flow that can be found in bubble columns. Special attention is paid to the large scale structures that have a strong impact on several key parameters such as the degree of mixing, mass and heat transfer. An experimental investigation has been performed in a bubble column with a needle sparger that allows for gas injection in well controlled patterns. Experimental teechniques such as Laser Doppler Anemometry (LDA) and glass fiber probes are employed, and their accuracy for application to bubbly flows is investigated and improved. Signal processing techniques are evaluated and improved, with special attention for techniques to determine turbulence power spectra for LDA signals obtained in bubbly flows. The hydrodynamics for uniform gas injection have been studied, with particular attention for the stability of the flow regime. The importance of the lift force for stability is discussed. The importance of non-uniformities in the gas injection on the flow is investigated, focusing on the behavior of both the entrance region and the bulk region. Finally, liquid velocity flucuations are studied: both the strength of the pseudo-turbulence and the turbulence power spectra are explored.