Particle lift force and particle behavior in suspension flows

Abstract

In this study, particle lift force and its influence on particle behavior are considered in suspension flows. Saffman force, a particle lift force derived in parallel laminar shear flow, is extended to a general particle lift force in turbulent flow, and it is shown to be dependent on particle slip velocity and flow vorticity. The particle lift force is characterized by a dimensionless parameter, the particle lift number (PI). The particle lift force and its influence on particle behavior are considered in turbulent flow through the use of a decomposition technique, and the effect of flow structure on particle behavior, which arise from fluctuation and mean vorticities, is analyzed. A lateral migration or an accelerated-flight motion induced from the particle lift force is derived, the lateral migration associated with a flow eddy is defined as particle length scale, and its relationship with the eddy length scale is established to consider the response of particle to the flow eddy. From the present analysis, the physical significance of the PI number is shown to be a balance of inertial and lift effects. When considering inlet deposition phenomenon in aerosol sampling, an analytical solution of aerosol particle motion is obtained in Poiseuille tube flow, and the physics of the particle deposition is demonstrated as a non-equilibrium motion resulting from the particle lift force and drag force. An inlet deposition length from the leading edge of sampling probes is developed to predict a region of intrinsic deposition of aerosol particles in the sampling probes. The effect of turbulence on particle lift force and particle behavior is exemplified in a turbulent suspension flow. A lift/drag force ratio, which is formulated as a function of particle diameter and flow vorticity, is introduced to confirm the influence of particle lift force on particle behavior, and a lift force ratio, which is formulated as a function of vorticity intensity, is introduced to present the turbulence effect on particle behavior. In modeling particle diffusion, the lift-induced motion of a particle has been identified as another form of particle diffusion, and the model developed in this study can be used not only to predict the particle diffusivity but also to explain the physics of particle diffusion.