Flow patterns and energy dissipation rates in batch rotor-stator mixers

Utomo, Adi Tjipto (2009). Flow patterns and energy dissipation rates in batch rotor-stator mixers. University of Birmingham. Ph.D.

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Abstract

The flow pattern and distribution of energy dissipation rate in a batch rotor-stator mixer fitted with disintegrating head have been numerically investigated. Standard k-e turbulence model in conjunction with sliding mesh method was employed and the simulation results were verified by laser Doppler anemometry (LDA) measurements. The agreement between predicted and measured velocity profiles in the bulk and of jet emerging from stator hole was very good. Results showed that the interaction between stator and rotating blades generated periodic fluctuations of jet velocity, flowrate, torque and energy dissipation rate around the holes. The kinetic energy balance based on measured velocity distribution indicated that about 70% of energy supplied by the rotor was dissipated in close proximity to the mixing head, while the simulation predicted that about 60% of energy dissipated in the same control volume. Both simulations and measurements showed that jet velocity and total flowrate through holes were proportional to rotor speed, while the energy dissipation rate scaled with the cube of rotor speed. The effect of stator geometry on the flow pattern and energy dissipation rate was also numerically investigated using standard k-e model and sliding mesh method. The simulations showed that flow patterns in the holes were similar regardless of holes sizes and shapes, i.e. jets emerged in the proximity of the leading edge and they induced circulation flows behind them. The radial velocities of jets emerging from various stators plotted against normalized tangential coordinate were practically the same, however, jets tangential velocities were affected by hole width-to-depth ratio. Jets emerging from holes with large width to-depth ratio had negative tangential velocity component (the same as rotor rotation) while those from holes with small width-to-depth ratio had positive tangential velocity component (against rotor rotation). Jets emerging from stators with small hole spacing tended to merge and move tangentially, while those emerging from stators with large hole spacing tended to move radially as free jets. The power number correlated well with the total flowrate and the total flowrate correlated well with the total hole area. Both power number and flowrate were practically not affected by hole shape, hole spacing and stator thickness. For all stators investigated, high energy dissipation rate occurred in the regions around the leading and trailing edges due to stagnations in those regions. Stators with narrow holes generated more uniform energy dissipation rate profile around the holes than those with wide holes since the regions with high energy dissipation rate around the leading and trailing edges merged. The simulations also predicted that about 50 – 60% of total energy supplied by rotor was dissipated in the rotor swept volume regardless of stator geometries.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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