Study On Flow And Heat Transfer Characteristics Of Oscillatory Flow Crystallizer

The crystallization process is widely used in chemical and pharmaceutical fields and in recent years,higher requirements have been put forward for the crystallization process regulation and product quality control,including specific crystal polymorphs,crystal habits,and product particle size.The high shear zone in the traditional stirred tank crystallizer(STC)limits the control of the secondary nucleation process,also accompanied with the disadvantages of low mixing and low heat transfer efficiency.Oscillatory flow,with excellent heat and mass transfer performance driven by non-linear flow,has received more and more attention in the crystallization process.So,in this paper the laws of hydraulics and heat transfer characteristics of this new type crystallizer with oscillatory flow are respectively analyzed by computational fluid dynamic(CFD)model in terms of cooling crystallization process.And detailed researches are as follows:Firstly,a three-dimensional unsteady state model for oscillatory flow inside the crystallizer is established.And subsequently the single-phase flow mixing characteristic is numerically investigated and also verified by the PIV experiment.The results show that a cyclic vortex structure can be generated by combining the tubular structure with smooth periodic contraction(SPC)and oscillatory flow,which can promote radial mixing of the flow field.And the vortex intensity is characterized by the vorticity value,indicating that the vorticity gradually increases with the increase of the oscillation intensity,resulting in higher shear rate simultaneously.Besides,the amplitude mainly affects the degree of vortex propagation and higher amplitude attend to cause stronger axial diffusion.When the amplitude is 4 mm(approximately 26.7% of the length of the crystallizer unit cavity structure),the vortex can propagate to the entire cell structure cavity.Secondly,the heat transfer characteristics of the SPC oscillatory flow crystallizer(OFC)are investigated and the distribution of local instantaneous heat transfer characteristics is obtained,indicating that the heat transfer performance increases with increasing Reo.It is attributed to the fact that the vortex structure generated by the nonlinear oscillatory flow can not only destroy the flow boundary layer,but also enhance the mixing performance of hot and cold fluids.And also,higher convective heat transfer coefficient is obtained when the velocity increase.The Nusselt number(Nu)is as 6.1 times large as that of the steady-state flow field;A critical amplitude is produced with gradually increasing the amplitude and when the instantaneous amplitude beyond it,the heat transfer performance will grow slowly.In addition,the spatial distributions of temperature and relative supersaturation in the oscillatory flow system are numerically obtained,indicating that the oscillatory flow can lead to a lower radial temperature gradient and more uniform temperature field in the system as well as higher relative supersaturation.Thirdly,the interaction among crystal particles,solution and wall is analyzed,and the slip velocity and collision probability of particles are investigated,showing that the crystal particles can be uniformly mixed and fully suspended in the oscillatory flow field.The slip velocity is positively correlated with the Reo and is not greatly affected by the particle concentration.The collision result between the particles and the wall shows that the ratio of particles colliding with the wall increases with increasing the oscillation intensity,but is not obviously affected by the amplitude.The slip velocity result of different particles shows that higher slip velocity is produced by larger particle,which will help to increase the diffusion rate of solute molecules around the crystal and increase the growth rate of the crystal.In summary,the typical transport characteristics and regulation laws inside the tube crystallizer with the SPC structure under oscillatory flow are investigated in detailed for the cooling crystallization process,which is helpful for the further design and application of SPCOFC in order to achieve fine control of crystalline products.