Velocity field macro-instabilities in an axially agitated mixing vessel

Montes, J.-L.; Boisson, H. C.; Fořt, Ivan; Jahoda, M.

doi:10.1016/s1385-8947(97)00042-9

Cited by 56 publications

(44 citation statements)

References 8 publications

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“…Consequently, an explicit value of the dominant MI frequency could not be determined. Similar types of difficulties in determining the dominant MI peak have been noted in the works of Montes et al (1997), Hasal et al (2000), Nikiforaki et al (2003) and Khan (2005). However, this was not crucial in the analysis.…”

Section: Macro-instabilitiessupporting

confidence: 74%

PIV study of the flow field generated by a sawtooth impeller

Unadkat

Rielly

Nagy

2011

Chemical Engineering Science

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Stereoscopic and high-speed particle image velocimetry (PIV) techniques have been employed to study the flow field induced by a sawtooth (EkatoMizer) impeller, operated in the fully turbulent flow regime at an impeller speed of 1500 rpm.Ensemble-averaged mean flow fields and turbulence quantities were calculated for a region close to the impeller blades. The flow was found to be anisotropic near the impeller and exhibited return-to-isotropy behaviour further away from it. Macroinstabilities were found to have a high probability of occurrence in the discharge stream. All three velocity components from the stereo-PIV measurements were used to estimate the dissipation rate, by adopting a large eddy simulation (LES) analogy.Spurious vectors distorting the dissipation rate calculation were identified, and various standard deviation filters were applied for vector validation. By evaluating the filtered dissipation rate profiles against the multi-fractal intermittency model of Meneveau and Sreenivasan (1991), the global standard deviation filter was found to be the most suitable type. The ratio of the maximum to the mean dissipation rate for the EkatoMizer discharge stream was found to be similar to that reported for Rushton disk turbine and pitched-blade turbine impellers in the literature, raising questions about the reported high-shear advantage of sawtooth impellers.

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Section: Macro-instabilitiessupporting

confidence: 74%

PIV study of the flow field generated by a sawtooth impeller

Unadkat

Rielly

Nagy

2011

Chemical Engineering Science

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“…No flow-instabilities were noted for Re < 200 and an increase in f' was observed for 200 < Re < 5,000. Montes et al (1997) studied with LDA the flow instabilities in the vicinity of the impeller. induced by a 6-bladed 45° PBT of D = 0.33T set at C = 0.35T and observed different values for f' depending on the Reynolds number: f' = 0.09 for Re = 1140 and f' = 0.0575 for Re = 75,000.…”

Section: Jet Impingement Macro-instabilities (J-mis)mentioning

confidence: 99%

Flow Instabilities in Mechanically Agitated Stirred Vessels

Galletti¹,

Brunazzi²

2011

Hydrodynamics - Advanced Topics

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“…Once again the macroinstabilities scale linearly with the impeller rotational speed and the lowfrequency macroinstabilities are associated with changes in the circulation pattern, although the macroinstability measured with the tornadometer is not necessarily the same as that observed in the surface welling experiments. Montes et al (1997) used LDV measurements, flow visualization, spectral analysis, and wavelets to analyze the flow in a stirred tank equipped with a small six-bladed 45°PBT impeller (T ϭ 0.30 m, D ϭ T/3, C/T ϭ 0.35, four baffles). They suggest that the occurrence of macroinstabilities is accompanied by the presence of a large precessing vortex in the upper part of the vessel, and that the vortex precession is linearly coupled with the frequency of the impeller revolution.…”

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confidence: 99%

Resonant geometries for circulation pattern macroinstabilities in a stirred tank

Roussinova

Kresta

Weetman³

2004

AIChE Journal

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Published online in Wiley InterScience (www.interscience.wiley.com). Circulation pattern macroinstabilities and precessing vortices are known to exist in stirred tanks, but until recently they have eluded a full quantitative analysis. In this article, frequency analysis of the axial velocity reveals a resonant frequency and geometry for the 45°pitched-blade turbine (PBT) in a flat-bottom cylindrical tank. Unlike other axial impellers, such as the hydrofoils A310 and HE3, the PBT produces a large-scale circulation macroinstability at the scale of the vessel diameter. The macroinstability is superimposed on the turbulent random fluctuations with a timescale that is very long compared to the blade passage frequency and the inertial convective range of the turbulent cascade. In this article it is shown that the circulation pattern macroinstabilities generated by the 45°PBT are coherent and propagate throughout the tank only under very specific conditions (D ϭ T/2 and C/D ϭ 0.50). The normalized frequency of the coherent macroinstability, or Strouhal number, is f MI /N ϭ 0.186. This corresponds to a period of roughly five rotations of the impeller, or 20 individual blade passages. The laser Doppler velocimeter data used in this work are unevenly spaced because they are collected in burst detection mode. This severely complicates spectral or frequency analysis. Standard algorithms such as the fast Fourier transform, autocorrelation methods, and wavelet analysis all require evenly spaced data. The Lomb periodogram was successfully used to extract the low-frequency content of the unevenly spaced data with a higher accuracy than is possible with other methods. The Lomb method also eliminates a short time-averaging step that was required in earlier work. © 2004 American Institute ofChemical Engineers AIChE J, 50: 2986AIChE J, 50: -3005, 2004 IntroductionIn the cylindrical stirred tank used throughout the chemical process industries, macroinstabilites in the gross circulation pattern can cause strong vibrations, unsteady loading, and even mechanical damage to tank internals. The vibrations are a significant problem in plant operations. When these vibrations are large, tanks may have to be shut down until the problem can be resolved.Three ranges of frequencies are present in the tank and different flows are associated with each frequency range. First, at any instant the velocity field contains a whole spectrum of high frequencies arising from turbulent eddies and vortices. Second, there are trailing vortices close to the tip of the impeller blades. If the blade width is D/5, where D is the impeller diameter, then the trailing vortices are approximately D/10 in diameter. These trailing vortices are present only in the region surrounding the impeller. They do not extend out to the R. Weetman is currently at rjweetman.com (e-mail: ron@rjweetman.com). Correspondence concerning this article should be addressed to S. M. Kresta at suzanne.kresta@ualberta.ca. © 2004 American Institute of Chemical Engineers FLUID MECHANICS AND TRA...

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Velocity field macro-instabilities in an axially agitated mixing vessel

Cited by 56 publications

References 8 publications

PIV study of the flow field generated by a sawtooth impeller

PIV study of the flow field generated by a sawtooth impeller

Flow Instabilities in Mechanically Agitated Stirred Vessels

Resonant geometries for circulation pattern macroinstabilities in a stirred tank

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