Hydrodynamic characteristics of an aerated coaxial mixing vessel equipped with a pitched blade turbine and an anchor

Abstract

Background In this work, computational fluid dynamics (CFD) simulation of an aerated coaxial mixing vessel composed of a centered impeller and a wall scraping anchor was conducted to investigate the effects of speed ratio, rotation modes (co‐rotating and counter‐rotating), and fluid viscosity on the local and global gas holdup values, flow pattern within the vessel, and turbulent kinetic energy. To validate the developed model, simulated gas holdup and gassed power uptake were compared with the measured experimental values. To gather experimental gas holdup values, an electrical resistance tomography technique was utilized. Results The results demonstrated that the co‐rotating coaxial mixer with a speed ratio higher than 10 provided a higher gas volume fraction within the vessel. It was also shown that the turbulent kinetic energy attained in the counter‐rotating mode was lower than those for the co‐rotating coaxial mixer in most regions within the mixing tank, especially near the vessel walls. The size and the number of circulation loops developed within the coaxial mixer were affected by the speed ratio. Conclusion It was demonstrated that speed ratio and rotation mode of the impellers affected the hydrodynamics developed within the aerated coaxial mixer. © 2017 Society of Chemical Industry

Local and overall gas holdup in an aerated coaxial mixing system containing a non‐Newtonian fluid

“…The net torque or corrected torque ( M c ) was obtained from the subtraction of the residual torque, which was the torque achieved by rotating the impeller inside the empty tank, from the torque measured when the impeller was rotated in the gas–liquid mixture ( M m ). The total power consumption of the upper and lower shafts was obtained separately using the following equation 34‐37,42,43 …”

“…The system was divided into four zones namely two central impeller regions, a sparger region and an anchor region. To better capturing the complexity of the fluid near the central impellers, a smaller mesh size was executed for the central impellers regions along with seven layers of inflation 34,48 . After performing the grid independency test, the optimum number of cells was determined to be 2,018,467.…”

“…The boundary conditions applied for different parts of the system were: non‐slip boundary condition for the wall of the vessel, central and anchor impellers as well as the sparger; mass flow rate boundary condition for the gas inlet on the top surface of the sparger; degassing boundary condition on the liquid surface; stationary wall for the sparger and tank wall, and moving wall boundary condition for the central impellers and anchor rotating with the speed relative to their zones. Simulation of the rotation of moving parts was performed through mesh motion method 34,53 . Two central impeller zones were rotating with the central impellers rotational speed and the anchor zone speed was adjusted to the anchor rotational speed.…”

“…In few studies, the effect of using a wall scraping anchor on the local and overall gas holdup was investigated in the aerated coaxial mixers. Hashemi et al 34 studied the radial and axial distributions of the gas holdup in a coaxial mixing system consisted of a central impeller and an anchor. It was reported that in the axial direction, the gas holdup was higher at the same level with the central impeller.…”

Application of CFD to Analyze the Hydrodynamic Behaviour of a Bioreactor with a Double Impeller

“…It should be noted that, despite the popularity of CFD in the simulation of biotechnological apparatus, it needs to be validated by experimental data as there are some assumptions and simplified models in CFD. Depending on the desirable parameters to study, numerical simulations can be performed for a single phase [20], two phases [21], or three phases [22]. Kaiser et al [20] used single-phase CFD simulations in order to compare the flow pattern of the 2-litre single-use bioreactor and its reusable counterpart cell-culture bioreactors.…”