Experimental Investigation Of Turbulence Characteristics In Stirred Vessels

Stirred vessels are widely used in many industry processes, such as petrochemical, biochemical, pharmaceutical and food industry. Hence, the hydrodynamics of the flow is very important for process design and optimization. In this thesis, different flow patterns in vessels stirred by multiple impellers were investigated by using PIV and TRPIV (Time-resolved PIV).TR PIV and 2D PIV were employed to measure velocity field in a HEDT impeller stirred tank with a diameter of 0.192 m. Db4 wavelet was used to analyze fluctuated velocity with continue wavelet transform (CWT) and discrete wavelet transform (DWT). The primary aim was to separate the periodic fluctuation and turbulent fluctuation. The turbulent kinetic energy obtained by wavelet transform agreed with that obtained by angle-resolve method. It proved that wavelet transform was able to separate the turbulent kinetic energy successfully. By observing results of CWT, it was found that a range of eddies exist in turbulence, and big eddies were broken up into small eddies with frequency increasing. DWT was able to decompose the turbulent fluctuation to different scales. The energies of turbulent scales increased with frequency decreasing. TRPIV were also employed to measure velocity field in stirred tanks with Rushton turbine and CBY impeller. In the impeller region, PSD (Power Spectral Density) at f0 (blade passage frequency) produced one large peak as well as at 2f0 in the two tanks. However, in the stirred tank with HEDT impeller, PSD at f0 produced one large peak, but no peak at 2f0.The flow field of impeller region in a Rushton impeller stirred tank with a diameter of 0.476 m was measured using PIV. Average velocity and turbulent kinetic energy k distribution were obtained. Turbulence kinetic energy dissipation rateεwas estimated by using a large eddy PIV approach. Impeller stream inclines slightly upwards, accompanied with two vortices on both sides of the stream. The incline angle of impeller stream and the distance between the centers of two vortices changes as the stream moves towards the tank wall. The incline angle increases before the phase angleθ=40°, then decreases with the maximal value 13.2°atθ=40°. The distance between the centers of two vortices decreases beforeθ=20°, then increases with the minimum value 0.0387 (normalized by the tank diameter T) atθ=20°. The peaks of turbulent kinetic energy and turbulence kinetic energy dissipation are both located in the vortex region near the jet flow.The experiments were carried out in a perspex vessel of 0.476m in diameter. Three pairs of impellers were used in the experiments with diameters of D1=0.33T, D2=0.40T and D3=0.50T respectively. The multi-block and 360°ensemble-averaged PIV approaches were used in the PIV measurement. Three typical flow patterns, named merging flow, parallel flow and diverging flow, were obtained by changing the clearance of the lower impeller above the tank base (C1) and the spacing between the impellers (C2) from radial and axial angle-resolved velocity distributions. For the impellers with diameters 0.33T,0.40T and 0.50T, the results show that the parallel flow is generated in the value of C2≥0.40T, C2≥0.38T and C2≥0.32T under the condition that Cl is set to D constantly, and the merging flow in the value of C2≤0.38T, C2≤0.36T and C2≤0.27T in the D1, D2 and D3 systems, respectively. When C2 is equal to D constantly, the diverging flow occurs in the value of C1≤0.15T in all three systems. The flow number NQ of the three pairs of impellers were calculated in parallel flow at the same Re. The characteristics of trailing vortex and its trajectory were described in detail for those three flow patterns. About 37% of the total energy is dissipated in the dual impeller jet flow region.PIV and fluorescence particle were used to quantify the hydrodynamics of solid-liquid suspension in a square stirred tank. Solid particle spheres with 750μm diameter were employed as the dispersed phase with up to the volumetric concentration of 0.9% in water. The magnitude of continuous phase mean axial velocity decreased in the impeller region and the near-wall region with the solid concentration increasing. The relationship between the speed drop and the particle concentration could be discribed as△v*∝Cv0.776 and Av*∝Cv1.474 in the impeller region and the near-wall region respectively. The turbulent kinetic energy distribution was complex since it increased in the impeller region with the solid volumetric concentration up to 0.5% and decreased beyond that. The overall turbulent kinetic energy remained decreased with the particle concentration increasing from 0.2% to 0.9%, and the relationship could be described as k/Utip 2∝Cv-0.073 . By contrast, turbulent kinetic energy dissipation rate was much enhanced with the particle concentration increasing, which can described asε/(D2N3)∝Cv1.113.