Research On Control Method For Complex Flows In The Local Region Of Eccentric Cylinder Flow

Polymer fluids is in a complex flow with extensional and shear flow during processing.The effects of these two flow components on the micro-structure evolution of fluids are different.A rheometer that can generate a stable and controllable flow field is experiment fundamentals to investigate the effects of various flow components on the rheological behaviors of materials.Traditional rheometers generally produce a simple flow field with only one flow component,which is beneficial to characterize the rheological properties of materials.However,because of the limitation of objective conditions,some rheological experiments need to conduct in a complex flow with various flow components,which is disadvantageous to characterize the rheological properties of materials.Therefore,it is necessary to control the flow properties of complex flow fields.Eccentric cylinder flow can generate a complex flow in the entire flow field,but the research on the application of rheological experiments is less.The reason is that the research on the control method for the flow properties in the local region of eccentric cylinder flow is insufficient currently,so the quantitative control for strain rate or stress in the flow field cannot be achieved,which is necessary for rheological experiments.Therefore,this study proposed a quantitative control method for extensional and shear rates at the local region of eccentric cylinder flow.In this work,the theoretical model of eccentric cylinder flow was established,and the effects of controllable parameters,including angular velocity ratio between inner and outer cylinders,eccentric ratio,and radius ratio,on the ratio between extensional and shear flows were discussed.The results indicated that,compared with eccentric ratio and radius ratio,angular velocity ratio has a wider controllable range and a better control performance for the ratio between extensional and shear flows.A novel method for accurately controlling the ratio between extensional and shear flows at the local region of eccentric cylinder flow by using angular velocity ratio was first proposed.Based on this method,it is found that a local“pure shear”or“pure extensional”flow can be separately produced at the specific streamline or region when counter-or co-rotating inner and outer cylinders at the critical angular velocity ratio.An eccentric cylinder apparatus with an angular velocity precision of 10^-4 rad/s and a position precision of 0.02 mm was developed,achieving the generation and control of high precision eccentric cylinder flow.A Particle Image Velocimetry（PIV）device and a droplet deformation measurement platform were built.Strain rates of the flow field were validated by measuring the velocity distribution and droplet deformation.The critical angular velocity ratios to produce a local“pure shear”flow at the specific streamline in eccentric cylinder flow were calculated.This critical angular velocity ratio is inversely dependent on the radius ratio,and decreases with increasing eccentric ratio.The local complex flow at the specific streamline is a superposed shear flow that consists of steady and oscillatory flows,which was validated by experiments.The results show that the ratio between steady and oscillatory flows is related to both eccentric and radius ratios.A larger eccentric ratio is beneficial to produce a stronger oscillatory shear flow,whereas a higher radius ratio is beneficial to obtain a stronger steady shear flow.In addition,the critical angular velocity ratios to generate a local“pure extensional”flow with a cross-like shape at the specific region in eccentric cylinder flow were calculated.This critical angular velocity ratio is inversely dependent on the radius ratio,and increases with eccentric ratio.When inner and outer cylinders oscillate with this critical angular velocity ratio,an oscillatory extensional flow can be generated in this local region,which was validated by experiments.The results show that smaller eccentric and radius ratios are beneficial to obtain an oscillatory extensional flow with a lower frequency,whereas larger eccentric and radius ratios are beneficial to obtain an oscillatory extensional flow with a higher frequency.This is the 3rd method to generate oscillatory extensional flow,which is suitable for the low viscosity fluids.The novel method proposed in this research uses angular velocity ratio to achieve the accurate control for the ratio between extensional and shear flows at the local region of eccentric cylinder flow.Compared with the traditional control method using eccentric ratio,the novel control method has the advantages of a wider controllable range and a better control performance for the flow properties.By controlling the angular velocity ratio accurately,two special flow field can be generated in the specific streamline or region in eccentric cylinder flow,including superposed shear flow and oscillatory extensional flow.These two special flow fields are promising to be rheological tools for investigating the effects of shear and extensional flow on the dynamics of dispersed phases.This work is of great theoretical and practical significance for promoting the theory of flow property control at the local region in complex flows and enriching rheological experiment methods.