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

Cees Haringa, Ruben Vandewijer, Robert F. Mudde
2018 Chemical engineering research & design  
You share, we take care!' -Taverne project Otherwise as indicated in Mixing Schmidt number CFD Multiple impellers Macro instability a b s t r a c t 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
more » ... ing 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 over-estimating 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. for single-impeller geometries (Coroneo et al., 2011) , but appear to consistently overestimate mixing times in multiimpeller geometries (Montante et al., 2005; Moštěk et al., 2005; Kukuková et al., 2005; Jahoda et al., 2007) ; this over-estimation may hold for other impeller types at large spacing (Montante et al., 2005) . Large Eddy simulations (LES) appear to adequately capture mixing behavior, but the required computation time prohibits routine application. We assess this overestimation of the mixing time in Rushton-stirred tanks with large impeller spacing by RANS simulations. Our original hypothesis was that of RANS simulations under-estimating t in the https://doi.
doi:10.1016/j.cherd.2018.06.005 fatcat:737een32t5aeljxi2rhuchutui