Numerical Simulation And PIV Experimental Study Of Fluid Flow Field In A Stirred Tank With Double-layer Inclined Blade Combined Impellers

Biogas anaerobic fermentation technology can realize the recycling of organic waste and improve the utilization rate of energy,but the fermentation process is complex and has many influencing factors.In this paper,with the support of Jiangsu distinguished professor project(1711440002),CFD numerical simulation and experimental research are used to investigate the influence of double-layer inclined blade combined impellers on the flow field distribution and change rule in the stirred tank when it is mechanically stirred.CMC solution,a non-Newtonian fluid with high transparency and similar fluidity to the fermentation liquid,is taken as the research object.The main contents and achievements of this paper are as follows:1.Determine the rheological parameters of CMC solution,and use fluent software to activate the non-Newtonian fluid model.The Reynolds time averaged k-? turbulence model was used to study the effects of four kinds of double-layer structure 4PBT+4PBT combined impellers,4PBT+6PBT combined impellers,6PBT+4PBT combined impellers and 6PBT+6PBT combined impellers on the mixing flow field.The results show that the mixing effect of 4PBT + 4PBT combined impellers are the best.When the combined impellers are used for mixing,the main circulation area in the tank is the largest,the reverse circulation area is the smallest,the convection between the fluids is intense,the average velocity and the proportion of high speed area are the largest,the mechanical energy consumption is the smallest,and there is no dead water area.2.PIV test device is used to study the internal flow of four different combined impeller schemes in the stirred tank.It is found that the numerical simulation results based on the realizable k-? model are similar to the test results,which verifies that the realizable k-? model can be used to study the flow field distribution in the stirred tank.In the case of 4PBT + 4PBT combined impellers,the realizable k-? model is used to study the flow field distribution in the stirred tank The numerical simulation results of the mixing flow field obtained by k-? are in good agreement with the streamline distribution and the time average radial velocity distribution.3.Based on 4PBT + 4PBT combined impellers,CMC solution was used as non-Newtonian fluid,and realized k-? turbulence model was used to simulate the effect of impeller speed,pitch between two blades and rheological characteristics of non-Newtonian fluid on the internal flow field of the stirred tank.The results are as follows:(1)When the stirring speed N is 100r/min,200r/min,300r/min and 400r/min respectively,with the increase of the rotating speed,the main circulation area expands,but the reverse circulation area decreases,the convection between the fluids intensifies,which affects the average velocity of the flow field,increases the mechanical energy consumption,and reduces the proportion of the dead water area and the low-speed area in the internal flow field of the stirring tank.Therefore,increasing the rotating speed is helpful to the fluid mixing in the stirred tank,but at the same time,it will increase the mechanical energy consumption.(2)When the pitch C of the double-layer paddles is 75 mm,100mm,125 mm and 150 mm respectively,increasing the pitch of the paddles will significantly expand the main circulation area and the reverse circulation area,and the flow velocity in the upper area of the agitation tank will increase,which will affect the flow of the larger range of fluid in the tank.When the distance between the two blades is C=125mm,the average velocity of the flow field is the largest and there is no dead water area,and the proportion of the high-speed area of the flow field in the mixing tank is the largest,so it is most suitable for the mixing of the fluid in the mixing tank.(3)When the mass fraction ? of CMC solution is 0.1%,0.2%,0.3% and 0.4%,as the viscosity of the solution increases,the rheological characteristics of the non-Newtonian fluid become more obvious,the main circulation flow area shrinks and reverses The area of the circulating flow is enlarged,the convection between the fluids is weakened,the average velocity of the flow field is reduced,the energy consumption of mechanical stirring is increased,and the proportion of the dead water area and the low speed area of the flow field inside the stirring tank is increased.High-viscosity CMC solution is not conducive to the mixing of non-Newtonian fluids,but can increase the speed of rotation to promote the mixing of non-Newtonian fluids in the stirring tank.