subsequent fully mixing by the recirculation. The time scale for the development of the flow
recirculation is approximated by L/v To meet the criteria, the time period r must satisfy
r > L2/v, or Re St < 1. Hence the temporal term can be removed from the momentum equation
in Equation 5-2. Physically, this implies the transition from one phase to another is so quick that
the flow is treated as a quasi-static flow, and the time only enters as a parameter to determine the
phase.
No overfeeding. The amount of fluid pumped in feeding phase has to be carefully
controlled that there is no overfed of fluid into the mixer. In the feeding phase of each cycle,
unmixed fluids are fed into the mixer and an equal amount of mixed fluid are pushed out of
mixer during the same period of time. It's desired to have the traveling distance of the incoming
fluid in the feeding phase less than the length of the mixer. Given the hydraulic diameter of the
ridged channel, D, and the average pressure gradient, P,/L, the average velocity in the channel
is approximated by PD /32pvL. Hence the time period has to satisfy r < 32pvLl/PD2,
where / is streamwise length of the ridged channel where recirculation takes place.
5.4 Simulation of Pulsed Recirculation Mixer
Pulsed recirculation mixer utilizing recirculation in ridged channel is investigated
numerically in this section. The micro mixer, called pulsed recirculation mixer, consists of a
segment of ridged channel as shown in Figure 2-10. CFD simulations is carried out to
demonstrate the mixing mechanism by flow recirculation and to evaluate its mixing
performance. A mixer model, containing 10 consecutive ridge structures, is created using CFD-
MICROMESH (Figure 5-11). The ridges have same geometry as the one used in the
investigation of flow recirculation. The ridged channel has spatial period of 108 micron, and the