Chaotic Mixing Performance Of Multiphase-fluid Intensified By Double Rigid-flexible Combination Impeller

As one of the core equipment of fluid mixing,stirred reactor is widely applied in many fields such as metallurgy,chemical industry,petroleum,whichdirectly influences the efficiency of the whole production process of production and corporate profits.Therefore,stirred reactor is one of major research hotspotsfor fluid mixing.High efficientand energy saving stirred reactor contributes to promote chemical process intensification,energy conservation and emission reduction,while the breakthrough lies in the reasonable impeller design,strengthen development of chaotic mixing theory.Traditional rigid impeller depend mainly on shearing action to the fluid transmission energy,and it is easy to form the exclusion zone near the blade tip region,as a result,it harms to high efficiency and energy saving.This thesis is based on the rigid-flexible combination impeller for mixing system including air,water,solid particles-.Combined the experiment and simulation to analysis the mixing characteristics in mixing system.With the help of ANSYS Workbench platform,the multiphase fluid mixing mechanism of the mixing system are studied.By using torque sensor to collect the torque signal in the process of multiphase fluid mixing,combining wavelet analysis and Matlab software,and computing macro-instability frequency,maximum Lyapunov index and K entropy wereused to studythe chaoticcharacteristics of fluid in agitation tank.Using pH indicator reaction experiments,chaotic characteristicswere verified.In view of the gas liquid two phase fluid mixing,the conductivity probe method was adopted to measure bubble size and local gas holdup.In view of the liquid solid two phase fluid mixing,the experiment using flow field visualization technology was employed to observe olid holdup distribution in fluid mixingsystem.Experiments provided a certain basis for design and application of new type impeller,and the main conclusions were:For gas-liquid mixing system:With the help of ANSYS Workbench platform,the performance of fluid-solid interaction was analyzed,and it foundthat the rigid flexible impeller contributed to the spread of uniform stress and energy.Because of its elastic-plastic,the rigid-flexible combination impeller(RFDT)system applied the fluid-structure coupling reaction force,the minimum stress of impeller was increased by 34.84%,while the maximum stress was reduced by 1.49%.Under the same power consumption,comparedwith the rigid impeller(RDT)system,flow velocity was increased and turbulence were intensfied.As a result,the maximum flow rate in RFDT system was increased by 15.79%,and the average turbulent move can also corresponding increased,thus overall gas holdup increased.Due to the lower level mixing regional in the area between two blades,the traditional double impeller was improved into double rigid impeller(DR-RDT)for gas-liquid two phase mixture,with the increasing of turbulent intensity in the middle region,for the strong shear force accelerates the bubble breakage and increases the bubble's radial distribution within the system.Compared with the traditional RDT system,DR-RDT system contributedto turbulent intensity and holdup also increasedto a certain extent,and a large deformation was caused by a wide range of turbulent and multi-body movement while DRF-RDT was applied in gas-liquid two phase fluid mixing,for the average speed of fluid,turbulent intensity and the overall gas holdup further increased.Visualization technologywas experimentally adapted,with the conductivity probe and chaotic parameter analysis method to analyze the gas-liquid two phases mixing.It found compared with RDT system,critical flooding points of distribution power consumption forRFDT,DR-RDT,DRF-RDT system were decreased by 10.20%,6.73%,14.49%,respectively.,with the ventilation 1000 m3/h.The bubble sizeswere decreasedwith the increasedpower consumption,but it increasedwith the increase of ventilation,at the same power consumption and ventilation,the local gas holdup in RFDT system was higher than that of RDT system,while comparing with the traditional impeller system,the gas holdup was high between the area of two blades in DR-RDT system,and it was more obvious in DRF-RDT system.For the analysis of chaotic parameters in agitation system,it found that the fluid inside the chaotic degree showedthe trend of firstly increase after decrease with the increase of power consumption/ventilation during the gas liquid two phase mixture,the chaotic parameters was successively increasing in RDT,RFDT,DR-RDT,DRF-RDT system.The chaotic degree wasalso successively increased.By using the method of acid-base indicator to verify the fluid internal chaos condition,the mixing time was shortened by 6.25%,21.25% and 31.25%,respectively in RFDT,DR-RDT,DRF RDT system,Comparedwith the traditional RDT system.Rigid-flexible combination impeller adaptedits firm-soft multi-body movement in the process of fluid-structure interaction,to a certain extent,and it disrupted the fluid flow field in quasi sequence structure,by highlighting the fluid multi-scale characteristic of the structure,as well as enhancing the fluid chaotic mixing.While DR-RDT system madethe regional turbulent intensity uniformity,at the same time,in the gas-liquid biphase mixing system,it acceleratedthe break and scatter of gas between the area of two blades,to make the energy dissipation more uniformly.Due to the instability of the flexible body in motion and the characteristics of large deformation,in the process of movement in DRF-RDT system,the adjustment of turbulent area,turbulent scope change constantly,madethe increase of gas holdup,and the enhancement of fluid chaos state.For liquid-solid mixing,(1)Double rigid-flexible combination impeller improved the flow velocity in the vicinity of the impeller,and reduced the "dead zone" between the upper and lower layer of impeller.Because of the fluctuations and disturbance caused by double rigid-flexible combination impeller,the fluid velocity in the reactor was increased,and the fluid flow was enhanced.The fluid mixing performance was improved.The double rigid-flexible combination impeller increases the interaction forces between the blade and the fluid which brought by coupling movement of rigid-flexible-flow,and the turbulent kinetic energy was strengthened,for the motion state of fluid in groove was more intense.Eventually,it contributed to the further implement the flow of energy and mass transfer.For solid-liquid two-phase fluid of double impeller mixing system,the solid-liquid mixed ability and disperse uniformity in double combination impeller system were higher than double impeller,while theparticle dispersion mixed ability of double rigid-flexible combination impeller system was superior to the double combination impeller;(2)DRF-RDTcan greatly shorten the mixing time of the floating particles system and sinking particle system in the same particle-holding and Pv,compared with RDT?RFDTand DR-RDT.At the same time,lnPvand lntmix showed good linear relationship.For the floating particle system,compared with the RDT and RFDT and DR-RDT,the mixing time of DRF-RDT was reduced by 50.27%,43.90% and 21.37% in the condition of Pv2000w/m3,Cv3%.For the sinking particle system,compared with the RDT and RFDT and DR-RDT,the mixing time of DRF-RDT was reduced by 47.00%,39.73% and 8.33% in the condition of Pv2000w/m3,Cv3%.Themixing time of the system was greatly reduced in the different particle-holding with DR-RDT,compared with RDT,RFDT and DRF-RDT.For the floating particle system,when Pv=2000w/m3,the mixing time of DRF-RDT was reduced respectively by 35.32%?40.49% and 50.27%in different Cvsystems.While for the sinking particle system,atPv2000w/m3,the mixing time of DRF-RDT was reduced,respectively by 21.14%,34.91% and 47%in different Cv systems.(3)LEmaxis positivein different Cv,which shows that the systems are in chaos.And with the increased of LEmax,the change of Pvshows a trend of increasing first and then decreasing.Under the same Cv,there is a big difference in the LEmax of different blade typessystem.LEmax is increased in the order of RDT,RFDT,DR-RDT and DRF-RDT,which means the chaotic state is enhanced.The RDT system can achieve a good mixingstate with the Pv4000w/m3,but RFDT,DR-RDT,DRF-RDT system can achieve the best with the Pv3500w/m3,and the LEmax in the DRF-RDT system is increased 11.08%,11.02% and 7.18%,respectively,compared with RDT in Cv1%,Cv2% and Cv3% for the floating particle system.While,the LEmax in the DRF-RDT system is increased by 19.67%,10.26% and 12.20%,respectively,compared with RDT in Cv1%,Cv2% and Cv3% for the sinking particle system.To sum up,rigid-flexible combination impeller can enhance the interaction between the impeller and fluid through the rigid-flexible-flow coupling movement behavior.As a result,the turbulent kinetic energy of mixing system was increased,and the transmission process of energy and momentum was intensied,which enhanced the chaotic mixing degree of the fluid correspondly.Especially double rigid-flexible combination impeller can disturb the flow field structure through the random jitter of the flexible connection pieces between the upper and lower impeller.At the same time,the spiral vortex can be formed between the upper and lower impeller,which can enhance the axial flow of fluid,and reduce the regular area.Thereby more fluid was induced into chaotic mixing.