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To simulate the hydrodynamics in the flow-through (USP 4) dissolution apparatus and investigate the effects of hydrodynamics on mass transfer in a low velocity pulsing flow.

Computational fluid dynamics (CFD) was used to simulate the hydrodynamics and mass transfer in pulsing flow. Experimental flow visualisation was used to qualitatively confirm simulated hydrodynamic and mass transfer features. The experimental dissolution rate at 8 ml min⁻¹ (22.6 mm flow-through cell) was compared to the experimental dissolution rate in a free convection system.

Simulations revealed periods of low velocity at all flow rates, evidence of boundary layer separation, and, at higher flow rates, residual fluid motion during zero inlet velocity periods. The simulated diffusion boundary layer thickness varied in certain regions over the course of the pulse. The experimental dissolution rate in the free convection system was faster than that at 8 ml min⁻¹ in the flow-through apparatus.

A low velocity pulsing flow running counter to gravity inhibited the experimental dissolution rate compared to that in a free convection system. From the CFD simulations generated, simulation of both hydrodynamics and species transfer is recommended to characterise the influence of hydrodynamics on dissolution in a low velocity pulsing flow.