Study On The Elastic Unstable Flow Of Viscoelastic Fluid In Microchannel And Mixing Enhancement

Viscoelastic fluid is a typical non-Newtonian fluid.Compared with Newtonian fluids,it has a unique elastic effect,in addition to its viscosity.This elasticity gives it unique properties such as non-zero positive stress difference and additional non-linear effects,where elastic non-linear effects are more pronounced in micro-scale flows and may induce elastic unstable flows.The flow of viscoelastic fluid in the porous channel is widely found in nature and industrial production,such as oil extraction engineering,bioengineering,chemical filtration and so on.In addition,viscoelastic fluids can flow unstably in porous channels.Previous studies have used theoretical,experimental and numerical simulation methods to study this flow.But limited by experimental methods and computational simulation methods,the study of the characteristics and mechanisms of viscoelastic flow was not perfect.At the same time,with this unstable flow,simple microchannels can be designed to achieve efficient flow mixing,which has great potential for application in biological detection,chemical synthesis,and quantum dot production.In order to investigate the unstable flow of viscoelastic fluids in microchannels,a numerical simulation method of viscoelastic fluid flow in the microchannels was established based on Open Foam,and a flow mixing visualization experiment system was established.Direct numerical simulation was applied for the viscoelastic fluid flow in pore channels of two-dimensionally homogeneous porous media.Statistical and transient flow characteristics,as well as the effect of elastic stress generated by polymers were analyzed based on the simulation results.The results showed that with the increase of Weisenberg number,the state of viscoelastic fluid flow changed from laminar flow to unstable flow,and the unstable flow is dominated by elasticity,which makes the behaviors of elastic unstable flow differs from the one of inertial unstable flow in pore channels.Based on the phenomenon of elastic unstable flow which was generated in the two-dimensional homogeneous porous media model,a simple microchannel was designed to induce viscoelastic fluids to generate elastic unstable flow which thereby,enhances flow mixing.After the designed channel was processed and produced,a visual experiment was performed.The experimental results show that compared with Newtonian fluids,viscoelastic fluids can generate unstable flow at low Reynolds number,and its mixing efficiency has a significant increase compared to Newtonian fluid which is as high as 86%.In order to give a deep sight of this unstable flow and the mechanism of mixing enhancement,numerical simulations of the flow in the channel were performed.The results are in good agreement with the experimental results.Through DNS,we discussed the flow patterns and the role of polymers playing in viscoelastic fluid flow to respectively figure out the underlying mechanism of the efficient mixing and the occurrence of the unstable flow motions.It showed that when the elasticity is strong enough,the viscoelastic fluid flow is irregularly twisting and swinging in the channel,and as a result enhances the mixing by increasing the intersecting frequency of fluids of different concentrations.Moreover,owing to the special designed geometry,the polymers act as the energy supplier of the unstable flow motions,which keeps the fluctuations from decaying towards the laminar regime.