Hydrodynamic instabilities and flow modification caused by preferential concentration of inertial particles

Cornell University, Ithaca NY

Investigators

Abstract

1233793 Koch Experimental studies reveal that gas flows can be greatly altered by the presence of suspended particles with diameters of 10 microns to 1 mm even when the concentration of particles by volume is relatively small. Such particle-laden gas flows arise in many industrial processes such as circulating fluidized beds and processes that use gas to convey solid particles. Despite the importance of these flows and the documented influence of solid particles, there remains a lack of understanding of the physical mechanisms by which particles influence gas flows. This research project will elucidate one such mechanism. Centrifugal forces tend to expel particles from regions of a flow that are strongly rotating; the particles then accumulate in intermediate regions dominated by straining flows. The researchers postulate that the gravitational forces acting on the more concentrated regions can reinforce the original gas flow leading to a strongly fluctuating gas flow with an inhomogeneous particle distribution. The physical mechanism will be revealed through a linear stability analysis, a study of the growth of small perturbations in the particle concentration and gas velocity from a simple base state condition where the particle concentration is uniform and the gas is undergoing a shearing flow where the gas velocity is a linear function of spatial position. The analytical study will be complemented by a numerical simulation of the nonlinear dynamics of the gas velocity under the influence of many interacting solid particles. The simulations will provide a test of the analytical theory while also revealing the complex flows that develop when the perturbations grow beyond the scope of the linear stability analysis. Building upon the insight obtained these studies, a model will be developed for the effect of gravitational-inertial particles on turbulent flows. This model will be based on rapid distortion theory wherein a small eddy and its associated particle density fluctuations are influenced primarily by a local linear flow produced by larger eddies. The large scale flows will be modeled as consisting of alternating periods of straining and rotational flows. The resulting statistical theory will be tested and refined using full numerical simulations of a gas-particle mixture subject to random isotropic forces that produce the turbulence. The project will reveal a new physical mechanism by which gas phase turbulence is produced by the gravitational forces acting on particles that concentrate in non-rotating regions of a gas flow. This new qualitative understanding of particle-laden turbulent flows will aid engineers designing many industrial processes such as automotive, aircraft, and rocket engines; industrial furnaces; and fluidized beds used in fossil and biorenewable energy conversion and in chemical plants. To help students understand the complexities of particle-laden flows, the simulations developed in the project will be incorporated in an educational module teaching how the nature of gas-particle flows is influenced by particle size. This lesson will be disseminated using SimCafe, a Wiki-based online resource for teaching and simulation being developed at Cornell.