| The measurement of flow velocity in microchannels is not only the basis for exploring the fluid transport mechanism at the microscale,but also is of great significance for the precise manipulation of microflows,the in-vitro construction of complex biomechanical microenvironments,and the control of biochemical reaction processes.Commonly invasive methods of flow velocity measurement can cause certain perturbation of the flow field,therefore,non-invasive measurement methods based on particle,scalar and point detection scanning have become the current research hotspots.However,the measurement method based on point detection scanning requires high-level optical instruments,which are usually expensive.The particle-based measurement method is affected by the followability of tracer particles,thus the measured results cannot necessarily be the real velocity of the flow field.The scalar-based measurement method obtains the velocity by monitoring conserved scalars,which eliminates the disturbance on flow field and the followability influence caused by the particle size.Nevertheless,this method is generally applied to high Reynolds number motion dominated by convection,ignoring the existence of the diffusion term.In the case of low Reynolds number laminar flow in microchannels,the spatiotemporal distribution of the scalar image is bound to be affected by the combined action of convection and diffusion,and their coupling effects become relatively complicated.Thus,further researches are needed to investigate how to determine the information of the flow field and related influencing factors through the spatiotemporal scalar profiles in mrochannels.In this thesis,based on theoretical analysis,numerical simulation and experimental verification,the coupling relationship between the spatiotemporal concentration profiles and the average fluid velocity in shallow microchannels is clarified,and an approach for determining average flow velocity based on spatiotemporal concentration profiles in shallow microchannels is proposed.Firstly,the flow governing equation and the Taylor-Aris dispersion equation in shallow microchannels under two conditions that the fluid flows from the inlet uniformly and from the small hole are theoretically derived,and the quantitative relationship between the spatiotemporal concentration profiles and the average fluid velocity is established.Comparing the analytical solution and the numerical solution of the concentration,the accuracy of the finite difference method for solving the spatiotemporal concentration profiles in microchannels is verified.Then two different calculation methods for the inversion velocity are proposed.Secondly,the simulation analysis of the spatiotemporal concentration profiles and the inversion velocity results in shallow microchannels are carried out.The spatial variation law of the average velocity and the spatiotemporal concentration profiles formed under different boundary conditions are studied.The calculation methods of the inversion velocity are compared to confirm the superiority of the optimization algorithm.Subsequent studies are carried out on the computational performance,robustness,initial value requirements and influencing factors under noise conditions of the optimization algorithm.Finally,the feasibility of the method is verified by experiments.A microfluidic chip is designed and manufactured which meets the experimental requirements.The working principle and specific implementation of the fluid loading module is described,thus a microfluidic experimental platform is built.Then the experimental verification work is carried out.The experimental results show that the error between the average flow velocity in microchannels obtained by the method in this thesis and the measurement calculation result of the flow sensor and the COMSOL simulation result is within 5%.In summary,the approach for determining average flow velocity based on spatiotemporal concentration profiles in shallow microchannels is easy to be implemented and high-precision,it can be an effective method for measuring the average flow velocity. |
