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By Vishal Singhal and Suresh Garimella, Purdue University, West Lafayette, IN
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Active liquid cooling with microchannels is an attractive solution for dissipating
the ever-increasing heat loads from microprocessors and other
electronic devices. The pump used to force liquids through microchannels
must be small, quiet, low-cost, and reliable [1]. For mobile computers,
weight and power consumption of the pump are also issues of concern. A
novel micropump designed with these considerations has been developed at
Purdue University and analyzed using FIDAP. It integrates two existing pumping
technologies. A vibrating diaphragm causes periodic inflow and outflow on
either side of the pump with net flow in one direction, and a periodic electric
field induces charges in the fluid that cause a uni-directional net force on the
fluid.
Flow rectification in a valveless micropump in (top image) expansion mode, and (bottom image) contraction mode; thicker arrows imply higher volume flow rates
The micropump consists of two nozzle-diffuser elements that are connected
through a pumping chamber with a piezoelectrically actuated diaphragm.
When activated, the diaphragm periodically increases and decreases the volume
of the pumping chamber. By orienting the elements so that the wide end
of one is attached to the chamber on one side and the narrow end of the other
is attached to the opposite side, a net pumping action across the chamber
occurs as the diaphragm vibrates up and down.
When the microelectronic component being cooled is operational, the fluid
present in the microchannel has a temperature gradient across the height of
the channel. This temperature gradient causes a gradient in the electrical conductivity
of the fluid. To take advantage of this unique condition, a number of
thin, closely spaced parallel electrodes are positioned (via deposition) in the
nozzle-diffuser elements. An alternating voltage applied to these electrodes
gives rise to a non-uniform charge distribution in the fluid that can be used as
a separate pumping mechanism. The flow rate due to the combined effect of
this induced electrohydrodynamic (EHD) force and the mechanical force of the
vibrating diaphragm is larger than that due to either force acting alone.
Net flow rate obtained from the combined action of the vibrating
diaphragm and EHD, as well as their action independent of each other
for a pump with a diaphragm diameter of 2 mm
Using FIDAP, the vibrating diaphragm was modeled by specifying its position
and speed as a function of time, through user-defined subroutines. A spines
approach was used for remeshing. The effect of fluid-structure interaction on
the diaphragm motion was neglected. The built-in EHD module with a force
density approach was used for modeling induction EHD. Additional species
equations were introduced to solve for the voltage and charge density distributions.
The vibrating diaphragm and induction EHD models were independently
validated by comparing to experimental results in the literature [2].
Velocity vectors in the mid-plane due to the combined action of the
vibrating diaphragm and EHD
A number of micropumps of varying dimensions were studied. In one case,
the overall dimensions were 1.5 mm x 1 mm, with a channel depth of 50 µm,
and diaphragm diameter of 1 mm. Using an electrode width and spacing of
10 µm each, a steady-state flow rate of approximately 3.7 x 10-10 m3/sec = 22.2
µl/min was predicted. The flow rate increased as the electrode width and spacing
were decreased, and as the diaphragm diameter was increased. Results also
indicated that for large diaphragms, the flow achieved from the combined
action of the vibrating diaphragm and EHD was larger than the sum of the
flows obtained from their individual actions; this effect was not observed in
pumps with smaller diaphragms.
References:
- V. Singhal, S.V. Garimella, and A. Raman, Applied Mechanics Reviews, 57, Issue 3,
pp. 191-221, 2004.
- V. Singhal and S.V. Garimella, “A Novel Valveless Micropump with Electrohydrodynamic
Enhancement for High Heat Flux Cooling,” IEEE Transactions on Advanced Packaging
(in press).
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