Low frequency macroinstabilities in a stirred tank: scale-up and prediction based on large eddy simulations

Flow Instabilities in Mechanically Agitated Stirred Vessels

“…Since the remaining large-scale turbulent fluctuations are directly resolved, LES is well suited for capturing flow instabilities in stirred vessels, although it is very computationally expensive. This has been shown for both single-phase (for example Roussinova et al, 2003, Hartmann et al, 2004, Nurtono et al 2009) and multi-phase (Hartmann et al, 2006) flows. DNS consists on the full resolution of the turbulent flow field.…”

“…Paglianti et al (2006) analysed literature data on MIs as well as comprehensive data obtained from measurements of wall pressure time series, and develop a simple model (based on a flow number) for predicting the MI frequency due to impinging jets (J-MI). Also Galletti et al (2005b) investigated flow instability for a PBT and detected a f' = 0.187 (thus akin the Roussinova et al, 2003). Such instabilities were found to prevail in the region close to the impeller (just above it and below it in the discharged direction).…”

“…The authors provided an interpretation based on the Strouhal number. In a later work (Galletti et al, 2009) In such a case, resulting flow instabilities will show a frequency which is expected to increase with increasing the velocity of the impeller discharged stream (see the flow-instability analysis in terms of pumping number by Paglianti et al, 2006, and/or peak velocity by Roussinova et al, 2003), thus with decreasing the blade thickness (Rutherford et al, 1996b).…”

“…The same authors performed also a LES of the vessel stirred by a PBT and confirmed the above non dimensional frequency value. In a later work Roussinova et al (2003) identified three possible mechanisms triggering the above flow instabilities: the impingement of the jet-like impeller stream on either the vessel walls or bottom, converging radial flow at the vessel bottom from the baffles and shedding of trailing vortices from the impeller blades. For the resonant geometry, the first mechanism coincided with the impingement of the discharge stream on the vessel corner, generating pressure waves reflected back towards the impeller.…”

Long-time Solids Suspension Simulations by Means of a Large-Eddy Approach

“…In the first place because they allow for accurate predictions of flows in complex confinements such as stirred tanks (Derksen and Van den Akker, 1999;Derkson, 2001;Bakker and Oshinowo, 2004). This accuracy is not only reflected by the way the average flow field is represented, but also by the fluctuation levels, and the temporal behaviour of (macro) instabilities (Derksen and Van den Akker, 2000;Roussinova et al, 2003;Hartmann et al, 2004). In the second place, LES provides detailed, temporally resolved flow-field information.…”