Investigation of low-frequency phenomena within flow pattern in standard mixing vessel induced by pitched blade impeller

An experimental study of the flow pattern dynamics in a standard mixing vessel with radial baffles filled with water and induced by a pitched blade turbine (PBT) pumping downward is presented. Investigation is focused on detection and analysis of quasi-periodical or periodical low-frequency phenomenon connected with time- and length-scales considerably exceeding the blade passage frequency (BPF) and common turbulent eddies. This phenomenon, which is expressed as large-scale mean-flow variations, is known as flow macro-instability (MI) . It could break-down just below the liquid surface, or it crashes to the liquid surface and causes its macro-swelling (MS) Our investigation was based on two- dimensional (2D) particle image velocimetry (PIV) within selected vertical and horizontal planes as well and analysis of the velocity field. The dominant frequencies evaluated in the selected points and overall analysis of the quasi-periodical macro-flow pattern behavior is to be shown. Identification of the quasi-periodical substructures appeared within the flow pattern was performed using the oscillation pattern decomposition (OPD) method. Observation of the macro-flow patterns confirmed presence of the macro-flow structures detected within flow pattern at the identical mixing pilot plant setup by previous investigations of the MIs phenomenon.

Important contribution of the presented work is the investigation of both flow pattern within the baffles vicinity and in the middle of the sector far from the baffle, which a significant difference. Low-frequency periodical (or quasi-periodical) behavior of the investigated macro-structures was qualitatively confirmed by the presented results and it was quantified using the velocity dominant frequencies evaluation. This frequency analysis brings insight into detected interconnections between dynamics of the adjacent flow structures. Detected different flow patterns within the main vertical plane near the baffles and in the inclined plane reveal a strong influence of the baffle presence. According by PIV measurement within appropriate horizontal plane, quite different flow pattern appears tangentially in front of and behind the baffle, where a wake is revealed, indicating significant influence of the baffle. The new findings represent a contribution to better understanding the physical phenomena behind the mixing process and therefore can help to optimize it, reduce mixing time as well as dynamics force strain on mixing equipment.