Date

2021-7

Author

Taşkın, Ahmet Fırat

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Stirred tanks are widely used for a variety of processes in industry such as solid dispersion, crystallization and dissolution processes. Several parameters are used to optimize these stirred tank processes such as impeller type, tank geometry and impeller speed. The flow field in the stirred tank is complex and varies depending on these parameters. In these processes diffusion and chemical reaction rate depend on the flow field and velocity fluctuations; therefore, information about the flow field is valuable in process design. Even though stirred tank operations are commonly used there is not enough data about the flow field in concentrated slurries. The reason is that widely used velocity measurement techniques such as laser doppler velocimetry and particle image velocimetry are laser-based methods and optical accessibility is required for measurements; therefore, their application is limited to very dilute solid-liquid mixtures. Because of this limitation, there are no data about the velocity profile of the high concentration slurries. This problem can be solved by the applicability of ultrasound Doppler velocimetry to opaque systems. The objective of this study is using ultrasound Doppler velocimetry to determine the influence of solids loading and particle properties on the velocity and turbulence in stirred tanks, especially in the baffle-wall intersection area. Vertical wall jet formed in the baffle-wall intersection in stirred tanks was suggested as the main driving force in the formation of cloud height which is an important suspension performance indicator. Velocity profile in this vertical jet can be important to understand cloud height formation mechanism in future studies. Velocity measurements at high concentrations will provide insight into understanding mixing process in more detail. In this study, ultrasonic Doppler velocimetry was used to evaluate axial, radial and tangential velocities and turbulence levels. Four different particles were used to investigate the effect of concentration, particle size and density. Concentration was gradually increased up to 43 % v/v. It was observed that axial, radial and tangential mean velocities decreased as the concentration increased while root mean square velocities affected by the concentration depending on the measurement location. Also, it was observed that the flow pattern changed at high concentrations ( > 11 % v/v). Axial, radial and tangential velocities were found to decrease with increasing particle size and density while no significant effect was observed on root mean square velocities. Finally, measurement showed that the formation of cloud height changed velocity and turbulence profile in the tank substantially.

Subject Keywords

Ultrasound Doppler velocimetry, Multiphase flow, Mixing, Solids suspension, Stirred tanks

URI

https://hdl.handle.net/11511/91697

Collections

Graduate School of Natural and Applied Sciences, Thesis