On the variation of precessional flow instabilities with operational parameters in stirred vessels

Nikiforaki, L; Yu, Jiguo; Baldi, S; Genenger, Bernd; Lee, Ka-Lok; Durst, F.; Yianneskis, M.

doi:10.1016/j.cej.2004.05.002

Cited by 14 publications

(11 citation statements)

References 25 publications

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“…A second phenomenon that may impact intercompartment mixing is the presence of Macro-instabilities (MIs). These have been extensively studied in single-impeller geometries, both experimentally and numerically (Roussinova et al, 2003;Hartmann et al, 2004;Nikiforaki et al, 2004). Nikiforaki et al (2003) suggest jet instabilities or instabilities by precessing vortices are the dominant cause.…”

Section: Multi-impeller Systemsmentioning

confidence: 99%

Inter-compartment interaction in multi-impeller mixing: Part I. Experiments and multiple reference frame CFD

Haringa

Vandewijer

Mudde

2018

Chemical Engineering Research and Design

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CFD simulations of mixing in single-phase multi-Rushton stirred tanks based on the RANS methodology frequently show an over-prediction of the mixing time. This hints at an underprediction of the mass exchange between the compartments formed around the individual impellers. Some studies recommend tuning the turbulent Schmidt number to address this issue, but this appears to be an ad-hoc correction rather than physical adjustment, thereby compromising the predictive value of the method. In this work, we study the flow profile in between two Rushton impellers in stirred tank. The data hints at the presence of macroinstabilities, and a peak in turbulent kinetic energy in the region of convergent flow, which both may promote inter-compartment mass exchange. CFD studies using the steady-state multiple reference frame model (unsteady simulations are treated in part II) inherently fail to include the macro-instability, and underestimate the turbulent kinetic energy, thereby strongly overestimating mixing time. Furthermore, the results are highly mesh-sensitive, with increasing mesh density leading to a poorer prediction of the mixing time. Despite proper results for 1-impeller studies, we do not deem MRF-RANS models suitable for mixing studies in multi-impeller geometries.

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Section: Multi-impeller Systemsmentioning

confidence: 99%

Inter-compartment interaction in multi-impeller mixing: Part I. Experiments and multiple reference frame CFD

Haringa

Vandewijer

Mudde

2018

Chemical Engineering Research and Design

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“…The frequency of the macro-instabilities was found to be linearly related to the impeller speed with f' = f/N = 0.015-0.020, independently on impeller clearance and design. In a more recent work Nikiforaki et al (2004) studied the effect of operating parameters on macro-instabilities. In particular they observed the presence of other frequencies varying from f'= 0.04-0.15 , as the Reynolds number was reduced.…”

Section: Precessional Macro-instabilities (P-mis)mentioning

confidence: 99%

Flow Instabilities in Mechanically Agitated Stirred Vessels

Galletti¹,

Brunazzi²

2011

Hydrodynamics - Advanced Topics

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“…These MI differ in time scale from the circulation type MI resolved through LES simulations by Russinova et al9 Further, Nikiforaki et al13 report that turbulence levels are broadened up to 25% by superimposed macro instability flow patterns. Later studies by Nikiforaki et al16 show that RMS levels are increased by 23% by the presence of MI.…”

Section: Introductionmentioning

confidence: 93%

Flow instabilities, energy levels, and structure in stirred tanks

2006

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in Wiley InterScience (www.interscience.wiley.com).In this study, the spatial distribution and significance of macro instabilities in two cylindrical tanks of different size, and one square tank, stirred with standard six bladed Rushton impellers, are investigated. The instabilities of the flow structure are investigated by searching for significant frequencies using the Lomb algorithm on instantaneous velocity data collected using laser Doppler anemometry at various locations in the tanks. The investigation of the energy levels shows that the harmonics of the blade passage differ in both magnitude and spatial distribution for different tank sizes, geometries, and energy input. Several significant dimensionless frequencies, that is, frequency Strouhal numbers, are detected and identified both in relative magnitude and spatial range.

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On the variation of precessional flow instabilities with operational parameters in stirred vessels

Cited by 14 publications

References 25 publications

Inter-compartment interaction in multi-impeller mixing: Part I. Experiments and multiple reference frame CFD

Inter-compartment interaction in multi-impeller mixing: Part I. Experiments and multiple reference frame CFD

Flow Instabilities in Mechanically Agitated Stirred Vessels

Flow instabilities, energy levels, and structure in stirred tanks

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