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In the present work, the detached eddy simulation (DES) model was used to gain an insight into theliquid-phase mixing processes in stirred tanks agitated by centrically and eccentrically located fourpitched-blade turbine. The impeller rotation was modeled using the sliding mesh (SM) method. Basedon the time variation of the scalar concentration, the mixing patterns and mixing time were analyzedand compared with the published planar laser induced fluorescence (PLIF) experimental data 33260
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Resultsshow that the combination of DES model and SM method can characterize the transient mixing state instirred tanks and can provide a detailed spatial and temporal evolution of the scalar concentration. Theconcentration recorded at several locations inside the domain revealed different mixing patterns underconcentric and eccentric agitation configurations. The differences in mixing time at different monitoringpoints were small under eccentric agitations, which exhibit better mixing performance than the concen-tric agitation. Overall, the predicted mixing time compared well, on average within 20%, with the resultsobtained by the PLIF technique. The agreement shows that DES is a reliable tool to investigate theunsteady mixing characteristics in stirred tanks. 1. IntroductionStirred tanks are widely used in the chemical, mineral process-ing, wastewater treatment and several other process industries.Among these operations, the mixing of single and multiphase flu-ids is one of the most common unit operations and is fundamentalto most aspects of process performance.
The most crucial parame-ter used to evaluate the mixing efficiency is mixing time and typ-ically, the t95 mixing time is used. It is defined as the time requiredfrom a non-equilibrium condition to achieve within a value of ±5%of the final concentration. Extensive investigations of the fluidmix-ing in stirred tanks have been carried out over the past several dec-ades based on experimental techniques. Nere et al. [1] reviewedsuch experimental techniques and pointed out their main featuresand limitations. Some of these disadvantages, such as the difficultapplicability in the case of non-transparent stirred tanks, the se-vere experimental conditions, and the fail to obtain very detailedevolution of the scalar concentration, have confined their applica-tion in the process industries and can only be used in the labora-tory scale experimental measurements.In recent years, owing to the great progress in the computertechnique, the computational fluid dynamics (CFD) technique isbeing increasingly used as a substitute for experiments to carryout detailed studies of the mixing process. The accurate prediction of the liquid flow field, including its turbulent characteristics, playsan important role in the numerical predictions of the mixing time[2]. Previous studies have demonstrated that numerical simula-tions based on the Reynolds-averaged Navier–Stokes (RANS)approach can provide satisfactory results of the mean flow instirred tanks [3–7]. However, it is difficult to obtain accurate pre-dictions of the turbulent quantities and fails to capture the instan-taneous nature of the turbulent structures because of theassumption of isotropy and ‘time-averaged’ modification of theNavier–Stokes equations [4,6–8]. This will certainly affect the pre-dictions of the mixing time in stirred tanks. As a matter of fact, ithas been identified that, in the turbulent flow regions, even whenthe fully predictive simulation strategies, such as the sliding mesh(SM) and multiple reference frame (MRF) method, were employed,the prediction of mixing time based on RANS approach is not accu-rate when compared with the result determined from experimentsor empirical correlations reported in the literature. For example,Osman and Varley [9] studied the mixing time in an unbaffled stir-red tank agitated with a Rushton turbine using the MRF method.The predicted mixing time was found to be two times higher thanthe experimental value. Jaworski et al. [10] studied the homogeni-zation in a baffled stirred tank agitated with a dual Rushton impel-ler.
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