Experimental Study And Numerical Simulation Of Single-phase Mixing And Gas-liquid Two-phase Dispersion Intensification In A Stirred Tank With An Improved Lsb Agitator

Stirred tank reactor is an important equipment in process industries.It is widely used in chemical engineering,biological,pharmaceutical,food,and metallurgy,etc.Agitator is the key component in the stirred tank,which provides the required energy to the fluid in the stirred tank.The specific flow pattern affects the"transmission process and reaction"efficiency in the stirred tank reactor.In this paper,experimental research and computational fluid dynamics（CFD）are used to study the single-phase and multiphase hydrodynamic characteristics in a stirred tank with long-short blades impeller in detail.The research results can provide reference and guidance for the optimal design of large-scale stirred tank.Firstly,the work focuses on the design improvement of the long-short blades（LSB）impeller by using pitched short blades（SBs）to regulate the flow field in the stirred vessel.The effects of pitched short blades（30°,45°,60°and 90°）and pumping mode（up-pumping mode and down-pumping mode,respectively）on fluid flow,power characteristics,turbulence characteristics and pumping capacity are investigated.The numerical simulation uses the Kinetic Energy Transport（KTE）model,and the experimental results obtained by particle image velocimetry（PIV）verify the prediction accuracy of large eddy simulation（LES）coupled sliding mesh method.The simulation results show that the power number of LSB agitator with pitched short blades is relatively small,the turbulent vortex under the pitched short blades is limited to a smaller range,the turbulence intensity at the lower part of the agitator increases,while the turbulence intensity at the upper part decreases,in the up-pumping mode.In the up-pumping mode,the power number is significantly increased.Compared with the vertical short blades,the axial flow in the tank is significantly enhanced,which effectively strengthens the mass exchange between the axial flow and radial flow in the whole stirred tank.The turbulent kinetic energy in the tank is significantly improved and the turbulent kinetic energy distribution is more uniform.Secondly,the fluid flow and mixing characteristics of LSB impeller completely immersed in liquid phase are studied.PIV coupled planar laser induced fluorescence（PLIF）are used in the experimental study,and LES based on SGS model is used in the numerical simulation.The velocity field distribution,tracer concentration distribution and power number obtained by experiment are in good agreement with the LES prediction results.The influence of different tracer feeding positions on the local mixing process at different detecting positions is investigated.The empirical correlation between mixing time and pumping number is obtained,that is,Nθ_min=7.1N_q^-1/3（T/D）².Compared with Rushton turbine（RT）and dual RTs,when the LSB impeller is submerged,the low-speed flow area（or stagnation area）in the tank is significantly reduced.Due to the radial flow of the long blades,part of the axial flow is transformed into radial flow,which effectively strengthens the fluid flow and turbulent kinetic energy distribution in the whole stirred tank.Compared with the single RT stirred tank and the dual RTs stirred tank system,the TKE in the LSB stirred tank is significantly improved,and the TED distribution is more uniform,and the mixing time required to achieve the same mixing effect is shorter.Then,the gas-phase hydraulic characteristics of LSB agitator are studied by double conductivity probe technology.The effects of pitched short blades,operation modes（down-pumping mode and up-pumping mode）,and operation conditions（stirring speed,power consumption,etc.）on the total gas holdup,local gas holdup and local bubble size distribution in the stirred tank are investigated.The experimental results show that the bubble size in the upper part of the stirred tank is large,about 4～6 mm,while the bubble size in the lower part of the tank is relatively small,about 1～2 mm.Efficient fluid exchange between radial flow generated by long blades（LBs）and axial flow generated by short blades enhances the redistribution of bubbles in the whole stirred tank.With the gradual increase of power consumption per unit volume（P/V）,the total gas holdup and local gas holdup in the stirred tank increase significantly,and the bubble distribution in the whole stirred tank is more uniform.In all research conditions,the 45°pitched short blades has the best gas-liquid dispersion effect in the tank in the up-pumping mode.Using the experimental results,the correlation of total gas holdup under different power inputs and operating conditions is obtained,namely:ε_G=α·（P/V）^β·N^γ,The range ofαis 0.134～0.161,the range ofβis-1.65～1.88,and the range ofγis 7.68～8.91.On the basis of experimental research,more detailed flow field information can be obtained by using numerical simulation,and the regulation mechanism of gas-liquid flow can be deeply understood from the mechanism.For the prediction of gas-liquid flow,bubble breakage and coalescence are the key factors affecting the bubble size distribution in the tank.Therefore,it is necessary to select an appropriate mathematical model of breakage and coalescence.In this paper,the high-speed photography method is used to study the breaking mechanism of bubbles in LSB stirred tank,which provides an experimental basis for the selection of computational fluid dynamics（CFD）model of gas-liquid flow.The bubble breakage of mother bubbles with different sizes under different operating conditions are investigated,such as the effects of Reynolds number（Re）and mother bubble size（b_d）on the breaking probability,breaking time and the number of daughter bubbles.The experimental results show that the mother bubble experiences different deformation before breaking,such as necking,stretching,elongation,pulling,etc.,forming bubbles with different shapes.The deformation form is the key factor affecting the number of daughter bubbles.With the increase of power inputs and mother bubble size,the breaking probability increases gradually,while the binary breaking probability decreases gradually.Most of the breaking phenomena occur near the blades.The initial breaking time increases with the increase of power input.When the mother bubble size is large,the bubble breaking time changes in a wide range.After comparison and screening,the experimental results of bubble breakage obtained are in good agreement with the prediction results of Luo&Svendsen breakage model.Finally,the self-inducing process of LSB impeller and the characteristics of gas-liquid dispersion and mass transfer in stirred tank are simulated by Euler-Euler model.The multi reference frame（MRF）method is used to deal with the rotation of the impeller,and the turbulence model uses the standard k-εmodel,the population balance model（PBM）is used to predict the size and distribution of bubbles in the stirred tank.In the PBM model,the gas phase is divided into 21 sub size groups.Combined with the bubble breakage model and bubble coalescence model,the effects of flow field distribution,turbulent kinetic energy and dissipation on the self-inducing process under different power inputs in the LSB stirred tank and the gas-phase hydraulic parameters in the stirred tank are deeply studied,such as local gas holdup,BSD,etc.The predicted local gas holdup and bubble size distribution are consistent with the results measured by double conductivity probe technology.The numerical simulation results show that due to the radial flow generated by the long blades,a pressure depression area is formed in the inner central part of the long blades sweep,and there is a pressure gradient distribution along the axial direction.The pressure gradient is the main driving force of gas suction.The bubbles sucked in through the pressure depression area are entrained into the tank by the fluid flowing downward from the tank center,and the fluid flow brings the bubbles to the bottom of the stirring drum,redistribute bubbles throughout the stirred tank.When the liquid level in the tank is low（e.g.H/T=1.125）and the rotating speed N is high,large bubbles will be brought to the bottom of the tank,resulting in high local gas holdup in the whole stirred tank.However,when the liquid level is high（e.g.H/T=1.5 or 2.0）,only small bubbles can be brought to the bottom of the tank by drag.Therefore,the gas holdup in the upper part of LSB agitator is high and the bubble size is relatively large,while the gas holdup in the lower part of agitator is low,but the bubble size is small.When N increases with the increase of liquid level height,the power number P_o decreases and the power consumption P/V increases.When n is fixed and the liquid level height H/T increases from 1.125 to 2.0,the load energy of LSB impeller increases significantly with the increase of power input,and the P/V value is almost unchanged.When the height diameter ratio of liquid level height to stirred tank diameter is 1.1～2.0,the predicted volumetric gas-liquid mass transfer coefficient is in good agreement with the previous experimental results（AICh E Journal,2016;62（4）:1322-1330）,and the volumetric gas-liquid mass transfer coefficient only depends on the power consumption per unit volume.