A Study Of The Influence Of The Vortices Regime On The Coagulation Effects

Particle image velocimetry(PIV)was applied to measure the Taylor-Couette flow and obtain the information about the vortex flow at different rotation speeds.Based on the results of PIV measurements,the Taylor-Couette vortex flow was simulated with computational fluid dynamics(CFD)by Fluent under the same conditions.The vortices regimes and their corresponding rotation speed ranges were verified to study the differences and interrelationships between the morphological characteristics of vortices and the vortices regimes(It was determined by the rotational Reynolds number Re or the inner cylinder's rotation speed n).The vortices regimes and their corresponding rotation speed ranges were validated.In order to quantitatively analyze the relationships between the morphological characteristics of vortices and the flocculating effects,the coagulation experiments were performed in a Taylor-Couette reactor and were simultaneously measured by PIV considering both the chemical and hydrodynamic conditions.On the basis of the rotational Reynolds number,the speed of the inner cylinder was calculated and used to control the vortex flow.Consequently,different types of vortices were created reproducibly.By studying the coagulation processes of various coagulants under different hydraulic conditions,the movement characteristics and the morphological characteristics of the flocs generated in the process of various coagulations in the same experimental space.The influence of the hydrodynamic conditions on the coagulation effects was investigated depending on the characteristic of the flow field,the turbidity removal rate,and the variation of the flocs morphology under different vortices regimes.The dynamics mechanism of flocculation was also studied to find a solution on the contradiction between the traditional vortex flocculation theory and high efficiency flocculation reaction system,making up for the deficiency of traditional vortex flocculation theory.Through the experiment of Taylor-Couette flow in combination with the PIV measurements,numerical simulation,and flocculation experiment,the following conclusions can be drawn:1.The vortices distribution can be observed and measured on the meridian plane of the Taylor-Couette flow,while no vortice can be observed on the annular cross section.This is because the plane vortices arrange along the axial section in the annular space and behave differently in regimes at different inner cylinder's rotation speeds.Therefore,the vortex characteristics were studied by the vortex distribution on the meridian plane.2.Through the comprehensive comparison of the results of PIV measurement and the numerical simulation which were obtained under the same conditions,it can be found that different rotation speeds corresponded to different vortices regimes.According to the various vortex forms and the quantitative analysis of their corresponding inner rotation speed,the rotation speeds can be divided into several intervals.In the same interval,the characteristics of each orientation velocity and the vortices regime were very similar.However,the characteristics and vortices regimes were quite different for the different intervals.In the present thesis,the rotational Reynolds numbers were calculated from the inner rotation speeds.According to the correlation analysisthe rotational Reynolds numbers,the rotation speeds of the vortex flow were divided into four intervals and the corresponding vortex regimes were as follows: laminar Taylor vortex flow(LTVF): Re is 100 ~ 350,n is 2 ~ 7 r/min;wavy vortex flow(WVF): Re is 350 ~ 2000,n is 7 ~40 r/min;modulated wavy vortex flow(MWVF): Re is 2000 ~ 3150,n is 40 ~ 63 r/min;turbulent Taylor vortex flow(TTVF): Re > 3150,n > 63 r/min.3.The coagulation experiments show that the coagulation process which can form appropriate flocs with uniform particle size distribution has good coagulation effect rather than form big flocs only,i.e.,the laminar Taylor vortex flow generate the largest flocs and its corresponding turbidity removal rates are lower;both the wavy vortex flow and the modulated wavy vortex flow can generate relatively large flocs with uniform particle size distribution,and their corresponding turbidity removal rates are higher;the turbulent Taylor vortex flow can not generate big flocs and its corresponding turbidity removal rates are the lowest.The results suggest that the hydraulic conditions are important factors in affecting the coagulation effect.Comparing the coagulation effects of the four different types of vortex,the WVF shows the best effect,the LTVF has low mixing efficiency,the MWVF show good coagulation effects only by adding enough coagulants dosage,the TTVF is not suitable for flocculation.4.The WVF provides a mechanism for multiple appearances of time-dependent states.It expands and contracts periodically with time,and the bulk fluid motion is observed,so that the WVF is beneficial for the inter-particle collision,which can generate large and uniform flocs,leading to the highest flocculation efficiency among the various coagulants.The WVF has the best flocculation effects among the four vortex flow regimes;therefore,it can provide the best hydraulic conditions for flocculation reaction.5.The disciplines mentioned above can be applied in the flocculation pool: at the beginning of the flocculation,it is better to use the hydraulic conditions with high Reynolds number which is beneficial to create WVF;in the middle stage of the coagulation process,it is better to use the hydraulic conditions with lower Reynolds number which could create initial WVF;at the late stage of the coagulation process,however,it is better to use LTVF to generate larger flocs.