(380d) The Roles of Eddy Size and Eddy Lifetime in Suspending Solid Particles
AIChE Annual Meeting
Wednesday, November 18, 2020 - 8:45am to 9:00am
The general consensus is that eddies play a key role in the process of suspending solid particles under turbulent-flow conditions. Such eddies should carry sufficient turbulent kinetic energy to lift a particle against gravity. The usual procedure is to estimate the typical velocity scale of such an eddy. This approach ignores that any turbulent (eddying) fluid parcel also contains smaller eddies which contribute to its turbulent kinetic energy. The total kinetic energy of an eddying parcel can be estimated from the local turbulent spectrum at the position of the particle in the flow field - which can be obtained by means of a Large Eddy Simulation (Giacomelli and Van den Akker, Flow, Turbulence & Combustion, 2020). In addition, one may wonder about the required size and lifetime of an eddying parcel. Its lifetime should be longer than the response time a particle needs to start moving or to adapt its velocity substantially. This particle response time depends on both particle size and particle density. In this presentation, all these aspects are addressed and combined. We found that for the purpose of suspending a particle a suitable eddying parcel should be some 20 to 30% larger than the particle to be suspended while its lifetime should be larger than 2.5 times the particle response time.