Computing Separated Flows in MEMS Devices

Abstract

Fluid flows in micro devices span the entire Knudsen (Kn) number regime. Depending on the Kn range, a full continuum or a full free-molecular analysis may be applicable. In the present study, flows in the Kn range of 10−3 to 10−1 are considered and they are modeled using a conventional Navier-Stokes solver. Its boundary conditions, however, have been modified to account for the slip velocity and the temperature jump conditions encountered in these micro-sized geometries. The computations have been performed for straight micro channels, a micro backward facing step, and a micro filter. The present results are then compared with analytical formulae and other computations available in the literature. The results indicate that the rarefaction and compressibility effects present in these micro devices have been accurately predicted. In the case of slip flow, the separation is found to occur at a higher Reynolds number compared to the corresponding no-slip flow case. As the next step of the study, micro synthetic jets will be computed and the optimal cavity actuator geometries will be sought for desired flow deflections.

Analysis of fluid flow through micro‐fluidic devices using characteristic‐based‐split procedure

“…Obtained slip-velocity distribution along the channel wall, normalized with the inlet centreline velocity, are presented and compared with references in Figure 4. The result obtained using CBS algorithm with second-order slip-velocity boundary condition is in good agreement with the References [5,17]. Maximum di erence in magnitude between the computed slip-velocity and Reference [5] is less than 1%.…”

“…Change in local Kn value due to adverse pressure gradient near the recirculation region can be also seen in the ÿgure. Variation of the normalized streamwise velocity at ÿve normal locations are plotted and compared with the results of Reference [5] in Figure 7. Pressure is normalized using dynamic head at the inlet and compared with the results of References [5,16] in Figure 8.…”

“…The grid, used for pressure calculations, is shown in Figure 5. As a veriÿcation study for this type of ow problem, ÿrst, a case with inlet to outlet pressure ratio = 2:32, nitrogen as the working uid and Kn at the outlet equal to 0.018 is analysed and the results are compared with Baysal and Aslan [5] and Beskok [16].…”

“…N-S and Burnett equations solver are used by Agarval and Yun [17] for the same geometry also. Baysal and Aslan [5] used N-S solver with ÿrst-and second-order slip-velocity boundary condition for the solution of the same problem. Obtained slip-velocity distribution along the channel wall, normalized with the inlet centreline velocity, are presented and compared with references in Figure 4.…”

“…Direct Simulation Monte Carlo Method (DSMC) is a good choice for analysis of high Kn ow problems. But, if Kn is smaller than 0.1, to use DSMC as a solver is relatively expensive compared to N-S solvers [2,5].…”