Shear turbulence controllable synthesis of aggregated nano-particles using a swirling vortex flow reactor assisted by ultrasound irradiation

The mass transfer in the shear controllable synthesis process of nano-sized particles using a swirling vortex flow reactor is investigated. This type of a reactor is able to generate a strong ‘Rankine vortex-like’ flow with a great shear gradient in the radial direction, trapping the synthesised aggregated nano-particles into the vortex core to form aggregation. The aggregated particles are further agglomerated to form larger particles with high uniformity under the action of turbulent shear and subsequent mass transfer due to engulfment occurred in the turbulent eddies. The mass transfer is enhanced by engulfment between the slabs of the liquid streams tangentially entering into the reactor while the local shear in the turbulent eddies with the length scales down to the Kolmogorov scale shape the particles, yielding well spherical morphology and narrower size distribution. In addition, the mass transfer performance can be further improved by intensifying the local turbulent shear by applying the ultrasound to the synthesis process. The instantaneous collapse of cavitation bubbles in the vortex due to the introduction of ultrasound results in intensified micro-turbulent streaming. The particle characteristics, chemical reaction and mass transfer performance are found to be correlated with the local turbulence characteristics in the vortex. The mass transfer can be characterised using the film refresh model via the Sherwood number by considering the agglomerated particles response to the turbulent eddies. Both numerical simulation and experimental results clearly indicate the existence of the correlation in synthesis of aggregated nano-particles in swirling vortex reactor between the mass transfer and turbulence shear by using ultrasound irradiation.