Species Separation Via Boundary-Induced Pulsatile Flow in Annular Geometries

The ability of pulsatile fluid flow to enhance solute mass transfer and to separate different species has been demonstrated in the past for both open and annular pipe geometries. For the open pipe case, the flow profiles are dependent primarily on the Womersley number W, a dimensionless group related to the pipe's oscillation frequency; for the annular pipe case, though, the presence of an inner and an outer pipe allow for the introduction of an additional frequency that changes the behavior of the system markedly. Among the notable resulting changes is a positive shift in location of the maximum on the separation curve. Expressions were derived for the specific scenario in which the outer pipe is stationary and the inner pipe oscillates, and it was calculated, for example, that the optimal value of W shifts from roughly 0.01 in the open tube to 0.03 for an annular system with an inner-to-outer pipe radius ratio of 1/2. Reasons for this are advanced. The case of two nonzero frequencies will also be discussed.