The role of wall friction in the development, decay and breakage of an air turbulent confined confined vortex

Particle processing units such as cyclones swirl spray driers and coaters relay upon complex air swirling patterns to be maintained. Very rarely are there detailed experimental data available directly from production units so designs are often required to be scaled up from experience and computational fluid models of similar pilot designs. A number of effects linked to the operation at a full scale are disregarded: to date, spray drying models neglect the influence that the presence of deposits at the unit walls have in the vortex development, the structure of the turbulent boundary layer or the momentum lost to friction.
This work makes use of ultrasonic anemometry to characterise the swirl decay observed in counter-current spray drying towers across different scales, designs and deposit characteristics. The degree of deposition is found to have a remarkable impact in the development of the flow. The presence of deposits exerts a significant friction, reducing the air axial fluxes of momentum along the cylindrical section of the tower. Three distributions were studied, all resulting in a strong decay of the non-dimensional flux of angular momentum, or swirl intensity, as the vortex rises in the unit. The decay rate is found to be correlated with the distribution and morphology of the deposits. Smooth wall assumptions overestimate the carrier phase swirl and subsequently particle centrifugal inertia by up to one order of magnitude. They also fail to reproduce the axial reversion zone that develops at the vortex core for lower operating swirl intensities. The balance between the initial swirl intensity and that surviving at the top appears to be a critical factor determining the recirculation patterns observed in the cylindrical section.