2007 NSTI Nanotechnology Conference and Trade Show - Nanotech 2007 - 10th Annual

The Effect of Micro-Channel Geometries on the Performance of Micro-Screw-Pump

Haifa El-Sadi* and Nabil Esmail
concordia university, CA

Keywords:
eccentrically, microchannel, micro screw, MEMS, load, Reynolds number

Abstract:
Abstract: Industrial applications today are challenging the performance of microelectromechanical systems (MEMS) into their products. These are mechanical devices that are able to perform work, and yet have characteristic lengths less than 1 mm. When a micro screw placed eccentrically inside a micro channel, a net force is transferred to the fluid due to the force that is transferred to the fluid due to the differential pressure on the depth of the thread and pressure gradient along the screw axis. Various shapes of micro-channel were investigated using the polyflow CFD software. More particularly, the cylinder and frustum micro-channel were investigated, and the results obtained in the study were compared to previous experimental studies. The effect of micro-channel geometry, screw eccentricity, Reynolds number, and pump load on the performance of the screw-pump has been studied in detail. The screw eccentricity is a very crucial parameter in determining the performance of the micro-screw-pump. Ninth designed micro-pump with varying the channel, the eccentricity and the screw length were used in this study. In the present investigation, the screw geometry was modeled by Pro-E software. The cad package Pro-E (PTC COMPANY) is used in different modules such as mechanical design and stress analysis. The CFD is used to solve Navier-Stokes equations. This CFD package uses the finite element method. It enables the use of different discretization schemes and solution algorithms, together with various types of boundary conditions. As part of the same package, (a preprocessor) Gambit is used to draw the geometry and generate the required grid for the solver. In fact, it is the screw eccentricity that also provides the driving force to the fluid inside the micro-screw-channel, by introducing unequal shear stresses on the upper and lower surfaces of the screw. the present study on the micro-screw pump showed different flow patterns when the channel geometry was changed and these differences in the flow pattern affected all the flow parameters. The Reynolds number for the micro-screw-pump is based on the screw angular velocity, and not on the average fluid velocity in the micro-screw-pump. The results showed that the values for the velocity were smaller for the frustum channel than for cylinder. In this case, since the power input equals the resisting viscous torque multiplied by the screw angular velocity, the Reynolds number determines the amount of power input to micro-screw-pump. The pump load is modeled by increasing the pressure on the outlet boundary of the pump to simulate the pressure rise needed from the pump to overcome the pressure losses in the external fluid circuit. The pressure at the pump inlet is held at zero, while the pressure at the pump outlet is varied to simulate different loads. The results for different pressure loads showed that the average velocity of the flow decreases as the pressure load increases, and this is typical behavior for any conventional pump, because the problem configuration is in essence the same as that of a micro-screw pump. Since the energy that the pump adds to the flow is in the form of flow energy as pressure rise, the backpressure is expected to affect the pump efficiency significantly. In the case of high backpressure, the energy addition to a fluid particle is high. The average velocity at the outlet becomes negative when the pressure exceeds the maximum load capability of the pump. The flow behaves in this manner for all micro-channel geometries studied in this investigation. The steady-state results were compared with existing experimental results, and the comparisons showed very good agreement. Other geometries that may possess superior viscous driving characteristics need to be suggested and studied.

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