Effect of Macroinstabilities in Single- and Multiple-Impeller Stirred Tanks

Paglianti, Alessandro; Liu, Z.; Montante, Giuseppina Maria Rosa; Magelli, Franco

doi:10.1021/ie800253u

Cited by 7 publications

(9 citation statements)

References 41 publications

(131 reference statements)

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“…This gives f 1.1 /N = 0.020 ± 0.001 and f 2.1 /N = 0.061 ± 0.003 axially, and f 1.1 /N = 0.015 ± 0.007 and f 2.1 /N = 0.052 ± 0.015 radially. These values agree well with the precessing vortex and jet instability frequencies as reported by Paglianti et al (2008). The total MI kinetic energy, k MI,fit , reaches a maximum at y = 0 at all radial positions.…”

Section: Inter-compartment Dynamicssupporting

confidence: 89%

“…As for both phenomena frequency f scales linear with agitation rate N, MI frequencies are reported here in f/N. In the discharge stream of a 2 impeller system, frequencies of f/N ≈ 0.02 and f/N ≈ 0.055 are found for precessing vortices and jet instabilities, respectively (Paglianti et al, 2008). Guillard et al similarly identified instabilities with f/N ≈ 0.05 − 0.08 in the region above the top impeller (Guillard et al, 2000a,b).…”

Section: Multi-impeller Systemsmentioning

confidence: 87%

“…Nikiforaki et al (2003) suggest jet instabilities or instabilities by precessing vortices are the dominant cause. Paglianti et al (2006Paglianti et al ( , 2008 report both these effects are present in Rushton-stirred tanks. As for both phenomena frequency f scales linear with agitation rate N, MI frequencies are reported here in f/N.…”

Section: Multi-impeller Systemsmentioning

confidence: 91%

See 2 more Smart Citations

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: Inter-compartment Dynamicssupporting

confidence: 89%

Section: Multi-impeller Systemsmentioning

confidence: 87%

Section: Multi-impeller Systemsmentioning

confidence: 91%

See 1 more Smart Citation

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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“…4C), with a dominant frequency f/N = 0.058 and its harmonics. This is in excellent agreement with the jet instability frequency reported by Paglianti et al (2008), and the value observed experimentally in Part I of this work. Experimentally, we also observed a weak contribution of f/N = 0.02, and strong contribution of f/N ≈ 0.04 in the parallel flow region.…”

Section: 3supporting

confidence: 93%

Inter-compartment interaction in multi-impeller mixing. Part II. Experiments, sliding mesh and large Eddy simulations

Haringa

Vandewijer

Mudde

2018

Chemical Engineering Research and Design

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Steady state multiple reference frame-RANS (MRF-RANS) simulations frequently show strong over-predictions of the mixing time in single-phase, multi-impeller mixing tanks, which is sometimes patched by ad hoc tuning of the turbulent Schmidt-number. In Part I of this work, we experimentally revealed the presence of macro-instabilities in the region between the impellers, as well as a peak in the turbulent kinetic energy in the region where the flow from the individual impellers converges. The MRF-RANS method was found unable to capture both. In this second paper, we show that the sliding-mesh RANS (SM-RANS) approach does capture the effect of macro-instabilities, while still underestimating the turbulent kinetic energy. Consequently, the SM-RANS method mildly overestimates the mixing time, while being less sensitive to the exact mesh geometry. Large eddy simulations with the dynamic Smagorinsky model reasonably capture the kinetic energy contained in macroinstabilities, and properly assess the turbulent kinetic energy in the region between the impellers, even for crude meshes. Consequently, the mixing time is reasonably assessed, and even under-predicted at the crudest meshes. However, the turbulent kinetic energy and energy dissipation in the impeller discharge stream are poorly assessed by the dynamic Smagorinsky model.

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“…The research on MI phenomenon is of significant value for the better understanding of the flow pattern and mixing mechanism since the existence of MI help to enhance the turbulent flow to an extent, improve the mixing efficiency, moreover, it has the distinct effect on mass and heat transfer and local gas content distribution in a stirred tank [3][4]. In recent years, a number of studies have pointed to the likely effect of MI phenomenon in stirred tanks in different ways [5][6][7][8][9]. These results indicated that MI frequency is irrelevant to stirred tank size, impeller type and fluid property parameters, whereas MI frequency may produce a certain difference in condition of different Reynolds number and different ratio of kettle and impeller diameter.…”

Section: Introductionmentioning

confidence: 99%

Numerical simulation on macro-instability of coupling flow field structure in jet-stirred tank

Luan

et al. 2016

IOP Conf. Ser.: Mater. Sci. Eng.

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Abstract. The velocity field macro-instability (MI) can help to improve the mixing efficiency. In this work, the MI features of flow field induced by jet-stirred coupling action is studied by using computational fluid dynamics (CFD) simulations. The numerical simulation method of jet-stirred model was established based on standard turbulent equations, and the impeller rotation was modeled by means of the Sliding Mesh (SM) technology. The numerical results of test fluid (water) power consumption were compared with the data obtained by power test experiments. The effects of jet flow velocity and impeller speed on MI frequency were analyzed thoroughly. The results show that the calculated values of power consumption agree well with the experiment measured data, which validates the turbulent model, and the flow structure and MI frequency distribution are affected by both impeller speed and jet flow rate. The amplitude of MI frequency increases obviously with the increasing rotation speed of impeller and the eccentric jet rate, and it can be enhanced observably by eccentric jet rate, in condition of comparatively high impeller speed. At this time, the MI phenomenon disappears with the overall chaotic mixing. IntroductionThe fluid turbulent flow is highly unstable circulation pattern in a stirred tank with the existence of large-scale low-frequency phenomenon termed as macro-instability (MI). The frequency of MI is represented as a distinct peak for frequency oscillations in power spectrum of the region close to the impeller which is usually located between 0.01 and 1 HZ frequency band. The studies have reported that the impeller stream region contains a large number of the symmetry flow field structure, so it is difficult to finish dissipating energy to outward effectively. As a result, near 70% of the mechanical energy will be dissipated in this region [1], which leads to the reducing of mixing efficiency. The energy dissipation concerned with turbulent flow and the large eddy pattern of the flow field may be described well with MI phenomenon [2]. The research on MI phenomenon is of significant value for the better understanding of the flow pattern and mixing mechanism since the existence of MI help to enhance the turbulent flow to an extent, improve the mixing efficiency, moreover, it has the distinct effect on mass and heat transfer and local gas content distribution in a stirred tank [3][4].

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Effect of Macroinstabilities in Single- and Multiple-Impeller Stirred Tanks

Cited by 7 publications

References 41 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

Inter-compartment interaction in multi-impeller mixing. Part II. Experiments, sliding mesh and large Eddy simulations

Numerical simulation on macro-instability of coupling flow field structure in jet-stirred tank

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