Design And Characterization Of A Novel Gas-Liquid Dispersion Impeller

The gas-liquid dispersion impeller is wildly used in many reaction processes involving oxidation or hydrogenation.A typical example is that ventilation needs to be maintained to meet the needs of microorganisms for oxygen in the process of microbial fermentation.Radial flow impeller is the most popular gas-liquid dispersion type,which is the key device of the ventilated mechanical stirred reactor and the main energy consuming component of the agitating system.Under the circumstance with the expansion of industrial production and the gradual shortage of energy,the design and development of a low-power consuming and high efficiency gas-liquid dispersion impeller becomes more and more urgent.In this thesis,a novel radial flow impeller for gas-liquid dispersion was designed and developed by means of theoretical analysis,computational fluid dynamics(CFD)simulation and experimental test,and its performance was further characterized.The CFD simulations were firstly carried out to study the structure and performance characteristics of three traditional radial impellers—flat blade disc turbine(RT),semi-circular disc turbine(CD)and asymmetric parabolic disc turbine(BT),and the results that the evolution trend of the three impellers was lower power number,higher pumping efficiency and greater axial projection area were concluded.Also the influences of the outer edge structure and size of different blades on the performance of impellers was studied,and the results showed that increasing the vertex curvature of the blade outer edge would reduce the power number and improve the pumping efficiency.Reducing the axial height of the blade outer edge would also reduce the power number,but improve the pumping efficiency little.And then a novel impeller with annular sector paraboloid blades was designed and proposed.The fan-shaped turbine(FT)showed the best stirring performance.Compared with the best-performing BT(Bakker Turbine)impeller in the present applications of industries,the power number of the fan-shaped turbine was decreased by 30.8%,the pumping efficiency was increased by 22.6%,and the axial projection area ratio of impeller was increased by 21.5%.It was believed that the novel impeller with annular sector paraboloid blades would be promising for efficient mass transfer in gas-liquid dispersion operations.The novel impeller FT and two gas-liquid dispersion impellers which are used wildly in industry — flat blade disc turbine(RT)and asymmetric parabolic disc turbine(BT)were manufactured by 3D printing technology.The experiments of characteristics of the novel impeller(FT)and comparison with other two impellers were conducted in the stirred tank under gassed condition.It was found that the novel impeller FT showed the lower power number,only about 1.7,and higher relative power demand,closed to 1.0 at high agitation speed.The dispersion of critical rotational speed of impeller FT was slightly lower than impeller BT,but significantly higher than impeller RT.The gas holdup and oxygen transfer coefficient of impeller FT were closed to BT,both of which were higher than RT.Oxygen mass transfer efficiency of impeller FT was significantly higher than that of RT and BT,about40~70% higher than impeller RT and 17~40% higher than impeller BT respectively.The velocity of the flow field was measured by 2D particle image velocimetry(PIV)technology.And the reliability of the established CFD model was verified by power number,velocity and other experimental data.Also the micro fluid characteristics were analyzed by CFD method and compared with impeller RT and BT under same operation conditions.The results showed that the velocity distribution of the FT impeller in different positions was similar to that of BT impeller,and both of FT and BT were different from RT impeller.In most of the positions,the values of the flow field velocity,shear stress,turbulent kinetic energy and turbulent kinetic energy dissipation rate of the FT impeller were less than those of the RT impeller,but the distribution of these characteristic parameters of the impeller FT was more uniform.The characteristics described above of the impeller BT were between those of impeller RT and impeller FT.The results were concluded by simulation of gas-liquid flow as follows: the bubble in the flow field of impeller BT was more than impeller FT,and the impeller RT was the least.The small size bubbles in the flow field of impeller FT accounted for higher ratio than the other impellers.