Study On Vortex Evolution Of Developing Flow And The Application In Microcavities Of Microchannels

The study on fluid flow characteristics and vortex evolution of developing flow in microchannel with microcavities is one of the focuses and difficulties in microfluidics.Microchannels with microcavities depending on its special structure and microscale effect,which is different from macroscale flow,are widely used in engineering practice,such as micro mixing equipment,highly efficient heat exchangers,coolers,and cell/particle separation.The flow characteristics inside the microcavity have an important influence on the working performances of the microchannels with microcavities,as well as significantly affect the vortex evolution of developing flow inside the microcavity,which need systemic and exhaustive research in the further.The fluid flow characteristics and vortex evolution of developing flow in microchannel with microcavity were studied by micro-Particle Image Velocimetry and High-Speed Imaging Technique.The main contents and conclusions of this paper are as follows:(1)Flow evolution characteristics in long rectangular microcavity of microchannel.For the first time,flow visualization experiments were carried out to study the vortex evolution of developing flow in long rectangular microcavities,and the effects of Reynolds number(Re)and microcavity aspect ratios(e)on the vortex evolution of developing flow were analyzed.The vortex structure in the microcavity varies with time in the development stage.Under certain circumstances(Re=50,e=3),the flow form in the microcavity flow has three patterns:attached flow,transitional flow and separated flow.Re has a significant influence on the evolution of vortex.With the increase of Re,the development time of vortex evolution in the microcavity has a trend of decreasing as a whole,but the migration phenomenon of separatrix I leads to a sudden increase in time.The e has a significant influence on the evolution of vortex.When e=1,due to the small volume of the microcavity,the vortex occupies the whole microcavity.When e=3and 5,the flow form in the microcavity flow has three patterns:attached flow,transitional flow and separated flow.But at e=5,mass transfer is very difficult in the deeper part of the microcavity.(2)Flow evolution characteristics in circular microcavity of microchannel.The effects of Reynolds number(Re)and microcavity aspect ratios(e)on the vortex evolution of developing flow were analyzed.The vortex structure in the microcavity varies with time in the development stage.The flow form in the circular microcavity flow has three patterns:attached flow,transitional flow and separated flow.Re has a significant influence on the evolution of vortex.With the increase of Re,the development time of vortex evolution in the microcavity has a trend of decreasing as a whole,but the migration phenomenon of separatrix I leads to a sudden increase in time.The e_chas a significant influence on the evolution of vortex.When the e_c is small,the circular microcavity is filled with the vortex structure.When e_c is large,there are three stages of attached flow,transitional flow and separated flow in the circular microcavity.This is caused by the increase of microcavity space,the increase of fluid flow area and the decrease of microcavity wall restriction.(3)The application of long rectangular microcavity and circular microcavity in biomechanics was studied.The effect of Reynolds number on droplet cell culture and vortex characteristics,shear stress distribution,culture flow and transport characteristics in the flow field of aneurysm was analyzed.The long rectangular microcavities allow for easy formation and fixation of droplets.Under the shear action of continuous phase flow,the vortex structure appears in the flow field of the droplet.The vortex develops from nothing and subsequently vanishes with the increase of Re.Re has no significant effect on the internal shear stresses of the droplet,and the average value of the shear stresses is extremely low(<1.5×10^-4pa).The circular microcavity structure is very similar to an aneurysm.With the increase of Re,lateral vortex are generated in the circular microcavity and vortex are gradually formed,which are gradually transferred from the upstream to the downstream.Re has a great influence on the shear stress in the circular microcavity,especially when Re reaches 177,it produces the maximum shear stress on the downstream lateral wall of the circular microcavity.Therefore,the downstream lateral wall of aneurysm is more likely to grow and rupture.