Classification of the flow regimes in the flow-through cell

This paper examines the dissolution apparatus referred to as the flow-through cell from an engineering fluid mechanics viewpoint. The analysis demonstrates that laminar flow predominantly occurs in the standard operation of this apparatus. It is argued that fully turbulent conditions are unlikely. Consequently, the phrases ‘open column’ and ‘packed column’ are suggested as technically more accurate terms for its operational characteristics than the conventionally referenced ‘turbulent mode’ and ‘laminar mode’. Examples of flow profiles are given to show that the criterion of a “sinusoidal” input flow profile required by USP is not a sufficiently accurate characterization at the inlet as numerous profiles can be conceived which have the same average flow rate. The rationale of pulsating flows versus constant, steady flows is discussed. Examples of how references to turbulence in the dissolution-related literature can lead to ambiguities and/or inconsistencies are highlighted.