Fluid Flow and Mixing Phenomena in Mechanically Agitated and Gas Stirred Ladle Systems and Their Comparisons

A sliding mesh-based LES (large eddy simulation) mathematical model has been developed to investigate fluid flow and mixing phenomena in a mechanically agitated (MA) water model ladle (D = 0.30 m) fitted with an impeller/paddle. Parallel to such, liquid velocity and 95% bulk mixing times were experimentally measured as a function of rotational speed of the impeller. These were applied to validate mathematical model predictions. On the basis of results derived from the present physical and mathematical modelling investigation and already published data on gas stirred (GS) ladle systems, a performance comparison between mechanical and gas agitated systems has been presented. It is shown that at specific stirring power, similar to those practised in ladle refining (similar to 10(-3) W/kg), while intensity of motion is more pronounced and mixing is considerably faster, corresponding flow establishment periods are relatively longer in the mechanically agitated system. Furthermore, while relationship between mixing time - specific stirring power in both the systems are found to be very similar e.g., tau(mix, 95%,)MA(epsilon M, MA)(infinity.-0.38) vs. tau(mix, 95%,)MA(epsilon M, GS)(infinity.-0.47), markedly different and contrasting dependence of mixing time on liquid depths i.e., tau(mix, 95%,MA)infinity H-1.8 vs. tau(mix, 95%,GS)infinity H-1.0 in the two systems have been observed. In addition to such, dynamic similarity criterion for mechanically agitated systems has been investigated and it is demonstrated that similar to gas stirred ladle systems, a Froude based modelling criteria suffices.