Hydrodynamics of pulsating flow type apparatus: Simulation and experiments

Hydrodynamics of pulsating flow type apparatus (PFA) in comparison with tubular turbulent apparatus (TTA) was investigated by use of CFD simulation and experimentally. Hydrodynamic parameters: hydraulic friction factor, turbulization of liquid flow and parameters of turbulence, relative volume of reverse flows, and pressure drop for both PFA and TTA are discussed. The level of turbulent energy in the predominant part of PFA volume (more than 70% of volume) is higher than 0.6 of the maximum value of turbulent energy, i.e. the distribution of turbulent energy is fairly even in PFA. Energy consumption is 13-16 lower in PFA in comparison with TTA at equal other conditions. Two scales of characteristic time exist in PFA: residence time and period of pulsations of pressure, shear stresses, local velocity and acceleration. These pulsation superimposed on turbulent flow may result in better energy allocation, which was observed by finer bubble dispersion at the same energy dissipation rate. PFA was also compared with Rushton turbine stirrer as a known benchmark for bubble/droplet dispersion. Gas bubble size in PFA is lower than in the stirrer with Rushton turbine at the same level of energy dissipation rate. More efficient bubble deformation and break-up in PFA can be attributed to the favorable combination of Rayleigh-Taylor and Kelvin-Helmholtz instabilities and to high level of shear stresses in the neck area which play more significant role than turbulent vortices delivering turbulence energy in free volume of stirrer with Rushton turbine.