首页|Multi-scale approaches in bubble column fluid dynamics
Multi-scale approaches in bubble column fluid dynamics
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Bubble columns are widely used as contacting devices and multiphase reactors in process and chemical industries.Their"baseline"layout involves a vessel wherein the gas phase is injected into a stagnant liquid phase in the form of"dispersed bubbles"or"coalescence-induced structures".This"baseline"layout can be modified by including internal heat exchangers(to control the heat transfer and reaction rates)and a solid-phase(a catalyst),and by applying co-current/counter-current liquid circulation(to increase the mass transfer rate).Even in the"baseline"layout,the coupling between the phases gives rise to rich,fascinating and mysterious fluid dynamics phenomena.Indeed,it is recognized that the"global-scale"is imposed by the"local-scale"and that the coupling between the two scales emerges in the flow patterns(flow regimes).The"bubble-scale"(i.e.,the bubble motion and turbulent eddies that control heat and mass transfer),influences the medium-scale(i.e.,turbulent eddies that transport the dispersed phase)and the large-scale(i.e.,circulation cells and central plume oscillations)circulation,thereby characterizing the"reactor-scale".Unfortunately,the precise definitions behind multi-scale connections have not been unveiled thus far.Although it is thought that the"bubble-scale"is determined by the connection of three local parameters(namely,the liquid velocity,void fraction,and bubble sizes),a precise and analytical description of the connections between the"local-scale"parameters as well as their upscaling towards the"reactor-scale"in the different flow regimes is elusive to date.For this reason,bubble columns are still modeled using macroscopic methods(i.e.,empirical or semi-empirical correlations),rather than physical-based approaches; this is a major shortcoming as empirical/semi-empirical correlations can hardly be applied beyond the range of operating conditions and system designs over which they were obtained.
Giorgio Besagni、Stoyan Nedeltchev、Thomas Ziegenhein
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Politecnico di Milano,Department of Energy,Via Lambruschini 4a,20156 Milano,Italy
Institute of Chemical Engineering,Polish Academy of Sciences,Baltycfea Str.5,44-100 Gliwice,Poland
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