Laminar and turbulent particle laden flows : a numerical and experimental study

Abstract: Particle laden flows are widespread in nature and in industrial applications. Dust storms, geophysical flows, pharmaceutical processes, fluidized beds and blood flow are only a few examples. Both global and local properties of a particle laden flow may alter significantly in comparison to the corresponding single phase flow. While the bulk behavior of such flows is subject to rheological investigations since many decades, it is just recently that we learn about the detailed dynamics of individual finite-size particles and their influence on the behavior of the carrier phase. The main goal of the present thesis is to employ latest numerical and experimental methods to gain a more fundamental understanding of the dynamics of finite-size particles and associated changes in laminar and turbulent suspension flows at relatively high particle concentrations. To this end, the current research project is conducted in three main steps. First, the inertial migration of finite-size neutrally buoyant spherical particles in a laminar square duct flow is investigated numerically. Next, using fully resolved Direct Numerical Simulations (DNSs), we study a turbulent square duct flow laden with finite-size neutrally buoyant spherical particles. Finally, an experimental framework is developed to explore the spatio-temporal dynamics of finite-size spherical particles in semidilute suspensions in micro-scale systems. Recently, inertial migration of particles in laminar pressure driven flows were successfully utilized to sort and separate particles and cells. While these studies are restricted to very dilute suspensions, little is known about particle separation based on inertial migration for suspensions at high solid volume fractions. In the present study, we investigate the inertial migration of particles by means of an Immersed Boundary Method (IBM), in a laminar square duct flow for suspensions with solid volume fractions between 0.4% to 20% and bulk Reynolds numbers ranging from 144 to 550. The bulk Reynolds number is found to be the key parameter in defining the final distribution of particles over the duct cross section in a dilute suspension (phi=0.4%). However, as solid volume fraction increases, we show that the behavior of particles depends on both the solid volume fraction and the bulk Reynolds number. We also found that the presence of solid particles induces secondary motions in a laminar duct flow regardless of the solid volume fraction. Indeed, we observed similarities to secondary motions of an unladen turbulent duct flow. Previous investigations of turbulent particulate flows have been mostly focused on the behavior of very small particles in canonical flows such as a plane channel flow and flow over a flat plate. However, industrial flow processes commonly take place in more complex geometries. Here, we also study turbulent duct flows laden with finite-size neutrally buoyant spherical particles. We consider a fixed bulk Reynolds number of Re=5600 and suspensions at three different solid volume fractions of 5%, 10% and 20%. We show that the turbulence intensity increases for suspensions up to 10% but then strongly decreases for 20%. This drop is observed to go along with a distribution of particles that mostly accumulate at the duct corners for solid volume fractions of 5% and 10% and in the duct core region for phi=20%. The interaction between particles and turbulent structures as well as particles and turbulent induced secondary flows are documented in detail in the present study. Despite recent advances in computer technology, computational cost of fully resolved numerical simulations of suspension flows are still very high. Hence, experimental studies are essential to access the overall dynamics of suspension flows. In the present study, a measurement framework is developed based on Astigmatism Particle Tracking Velocimetry (APTV) to study the three-dimensional dynamics of finite-size particles in micro-scale flows. The measurement technique is successfully applied to study the behavior of decelerating suspensions flowing over a backward-facing step at low bulk Reynolds number of Re=4. Suspensions of neutrally buoyant PMMA particles with 30 micro meter diameter at three different solid volume fractions of 0.1%, 2% and 10% are considered. We utilize Refractive Index Matching (RIM) to gain optical accessibility to suspensions also at high solid volume fractions. Moreover, a few number of particles are fluorescently labelled to act as suspension tracers. By resolving the 3D particle dynamics, an onset of reduced particle velocities could be evaluated with increasing solid volume fraction for a flow with strong out-of-plane particle displacements.