method is chosen in the CFD-ACE+ software settings to solve the Navier-Stokes equation for the
flow and the Poisson's equation for the electric field in the channel. Algebraic Multigrid Solver
(AMG) is elected in solving the linear equations for its advantage in solving unstructured
meshing. A maximum of 1000 iteration was carried out to ensure the convergence of solution,
unless a 10-4 order of reduction in the residue is met or the absolute value of the residue reaches
10-1.
5.2.3 Poiseuille Flow in Ridged Channels
The experiment in Chapter 3 has shown that a segment of ridged channels can achieve a
comparable mixing performance to the staggered herringbone mixer. With the model illustrated
in Figure 5-4, CFD-ACE+ is employed to quantitatively evaluate the performance of the ridged
channel mixer.
Two distinct streams, deionized water and a solution containing 0.1 mM specimen, are
introduced into the ridged channel (Figure 5-5a) at the same flow rate. Each fills half of the
cross section of the entrance. The diffusivity of specimen is set to be 3.3x10-10 m2/s. Without
the ridge structures, specimen molecules gradually diffuse into water stream from their interface
as the two fluids move through a regular channel (Figure 5-5b). Complete mixing is achieved
when specimen diffusion reaches from the interface of the two streams to the other side of water
stream and a uniform concentration (0.05 mM) is reached throughout of the channel cross
section. The process, however, takes considerable amount of time as the molecular diffusion is
generally very slow, especially for large molecules.
The ridged channel is able to increase the diffusion process by stretching and folding flows
so that the distance for diffusion to reach becomes shorter and the interfacial area becomes
larger. The mixing process in the channel is verified by inspecting the specimen distribution at
the cross sections in CFD-VIEW (Figure 5-5c). In the cross sectional views, blue stream