Numerical Study Of Inertial Migration Of Spherical Particles In Micropipe Flow

When suspended particles are transported in a Poiseuille flow,due to fluid inertial forces they might migrate and accumulate at certain radial positions,causing them to naturally separate or to be easily removed or processed.In the rapidly developing field of microfluidic control,this passive particle screening technology has a low cost and high efficiency,which has a broad application prospect.Suspended particles moving in a viscous flow is a topic of multiphase flow.When the flow Reynolds number is very small and the fluid inertia effect can be linearized,approximate analytical solution can be obtained by using the method of asymptotic expansion.However,for the particle migration problem in engineering applications at finite Reynolds numbers,theoretical solution is no longer achievable.Experimental observations of both particle migration and local flow field are also difficult due to instrument limitations.With the rapid developments of computer technology and numerical algorithms,numerical simulations can now be conducted to study particle inertial migration.In this thesis,the Lattice Boltzmann method is used as a direct simulation method to study the migration of neutrally buoyant,spherical particles in microscale pipes with different cross-sections.The unique feature of this work is to systematically simulate and compare particle transport processes involving one,two,and multiple particles in different pipes including a circular tube,groove tube,square and rectangular tube,in order to gain a deep understanding of the mechanisms governing the particle inertial migration.Firstly,the inertial migration of a single particle is simulated in pipes of different shapes,Reynolds numbers and particle sizes.Secondly,the interaction between two particles in pipe flow is studied,and it is found that in the lateral plane of the equilibrium position,the axial migration and alignment of the two particles occur due to the wake flow and the axial distance of the particles approaches a constant value.Finally,the transport process of multiple particles is simulated,and the alignment of multiple particles in a pipe and the formation of a particle chain with uniform inter-particle axial distance are examined.The simulations consider the effects of pipe Reynolds number,particle blocking ratio and particle volume fraction.It is found that the chain formation is enhanced as the flow Reynolds number is increased.An interesting observation is that,while for axisymmetric circular pipe the particle migration trajectory project on the cross section is a straight line,the migration trajectory in the rectangular pipe projected onto the cross section plane can be a curved path due to change of particle angular velocity in response to the subtler local shear rate distribution in the flow.The above results provide theoretical guidance for the design of passive particle screening technology in microfluidic devices.Another contribution of this thesis is the derivation of a theoretical solution for unsteady unidirectional laminar flow in a rectangular pipe,which provides a benchmark case for the validation of transient laminar flow in a rectangular duct.