NIST Researchers Explain a Unique Multiphase Flow Phenomenon
January 1999
The transport of small particles through gaseous or fluid wakes is a common
phenomenon found in many technological and natural processes in such areas
as power generation and pollution control. Approximately a decade ago it
was discovered that the conventional wisdom that turbulence in these wakes
always results in effective particle mixing is not always true. It was
determined that particles in certain size ranges sometimes "demix" by
concentrating along the peripheries of the wake vortices, a phenomenon
referred to as "focusing." No convincing explanation could be given for
this non-intuitive particle behavior, although chaos was sometimes invoked
as a controlling factor. Recently, however, a joint CSTL/ITL investigation
has elucidated the underlying physical mechanism behind this focusing
phenomenon. Utilizing a combination of theoretical and computational
techniques, the NIST researchers have conclusively demonstrated that a
particle attractor exists in these types of flows and have mapped its
behavior as a function of particle size. They have also convincingly
demonstrated the important role that centrifugal force effects play in
flinging the particles toward the vortex peripheries. Finally, they have
proven mathematically that chaos plays no role in this type of particle
behavior. The results of this research will be appearing in a paper in the
Journal of Fluid Mechanics. The results could ultimately lead to improved
designs for such devices as combustors where focusing is clearly a
phenomenon to be avoided.
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