Three types of one-sided actuating piezoelectric micropumps are studied in this paper. In the first type, one-sided actuating micropump with two check valves can enhance the flow rate and prevent the back flow in suction mode to keep the flow in one direction. Furthermore, the frequency modulator is applied in the micropump to adjust and promote the maximum flow rate higher than 5.0 mL/s. In the second type, valveless micropump with secondary chamber shows that the secondary chamber plays a key role in the application of the valveless micropump. It not only keeps the flow in one direction but also makes the flow rate of the pump reach 0.989 mL/s. In addition, when a nozzle/diffuser element is used in valveless micropump, the flow rate can be further improved to 1.183 mL/s at a frequency of 150 Hz. In the third type, piezoelectric actuating pump is regarded as an air pump in the application of a microfuel cell system, which can increase more air inlet to improve the fuel/air reaction and further increase the performance of fuel cell.
In recent years, micropump plays a significant role in many fields, specifically in chemical, medical, and thermal managements. Since there are distinct requirements in each field, several types of micropumps have been designed to meet those requirements. Micropumps, which have the advantage of small size and configurable dimension, are gradually becoming one of the solutions for electronic heat dissipation. There are several types of actuation approaches in developing, such as electromagnetic [
Micropumps can be categorized into two major types: displacement pumps [
Ma et al. [
A traditional piezoelectric micropump driven by central-actuating, which is shown in Figure
Specification of each design.
Experiment |
Design of valve | Design of pump chamber | Design of fins | |||||
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Thickness |
Length |
Shape | Chamber size (mm3) | Inlet/outlet |
Thickness |
Height |
Number of fin | |
1 | 0.5 | 5 | O |
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— | — | — |
2 | 1 | 5 | O |
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— | — | — |
3 | 1.5 | 5 | O |
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— | — | — |
4 | 0.5 | 5 | N |
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— | — | — |
5 | 0.5 | 5 | NC |
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— | — | — |
6 | 0.5 | 5 | NC |
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— | — | — |
7 | 0.5 | 5 | OC |
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— | — | — |
8 | 0.2 | 5 | OC |
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— | — | — |
9 | 0.2 | 5 | NCW |
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— | — | — |
10 | 0.2 | 5 | NCW |
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1 | 1.25 | 6 |
11 | 0.2 | 5 | NCW |
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1 | 2.5 | 6 |
12 | 0.2 | 5 | NCW |
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0.5 | 1.25 | 12 |
13 | 0.2 | 3 | NCW |
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— | — | — |
14 | 0.2 | 3.5 | NCW |
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— | — | — |
O: original valve; N: narrow valve; C: valve with cone; W: with combined design of cold plate and pump chamber.
Comparison of the actuating mechanism: (a) traditional micropump and (b) one-sided actuating diaphragm micropump.
Mechanism of actuating micropump: (a) outlet and (b) inlet.
The different geometry type of valve.
The liquid is actuated by the piezoelectric device in the pump chamber. The governing equation of the actuating part on the pump chamber can be expressed as
In the left-hand term,
Free body diagram: (a) one-sided motion and (b) valve motion.
The diaphragm fixed on the chamber is bent by piezoelectric device. Therefore, it can be seen as a fix-end beam and supports a uniform load of intensity
Therefore, the spring constant of the diaphragm is given by
The passive check valve, which is made of PDMS, is an important device in the design of the one-sided actuating diaphragm micropump. The passive check valve decides the performance of the pump. PDMS is an elastic structural element that can be expressed as a spring motion to store and release energy. The governing equation of oscillating motion for valves, which is shown in Figure
Substituting (
In this study, the shape of the check valve can be considered as a cantilever beam. The bending moment is a function of position along the valve, and the deformation of the valve is small. If there is a concentrated load at the valve’s free end as shown in Figure
The curvature change of actuating diaphragm which determines the behavior of oscillating flow is a great effect on the flow rate. Flow rate is proportional to the volumetric change of pump chamber. To obtain the maximum volumetric change and deflection of the pump diaphragm, the uncovered length of actuator diaphragm should be decided. The effect of uncovered length on flow rate is measured, which is shown in Figure
The effect of uncovered length on flow rate (chamber size: 5 * 45 * 28 mm3; valve thickness: 0.5 mm).
Compared to No. 5 and No. 6, which is 5 mm and 4 mm in depth separately, Figure
Effect of the chamber depth with different frequency.
In general, heat dissipation performance is determined by the flow rates and surface area of the fins. According to (
Effect of fin height and number of fin in chamber.
Because the valves are influenced by the drag force, the pump performance will be better if the motion of the actuator and valves match well. Under a fixed inertia force, the thicker check valve will cause the smaller response acceleration because of the larger spring constant and mass. Therefore, a large delay of valve motion will occur and reduce the oscillating flow rate. On the effect of valve's width, narrow valves are able to respond quickly due to the relatively low damping force. The comparison of valve's width and thickness are shown in Figure
The comparison of flow rates in different valve design.
The thickness of the valve will cause different mass and spring constant, as shown in (
The length valve can influence the valve spring constant. For example, the spring constant is larger when the valve's length is shorter. The larger spring constant makes the valves respond faster. The difference of valve length is shown in Figure
Measured flow rates of the pump with different valve lengths (without fin).
For studying the performance of micropump with frequency modulator, the schematic of experimental micropump is shown in Figure
(a) Schematic view of the micropump and a frequency modulator. (b) The design of a frequency modulator.
The resonant frequency of a vibrating plate with one fixed-end is related to density, Poisson’s ratio, Young’s modulus, length and thickness of the vibrating plate respectively [
The characteristics of a frequency modulator can be found by the relation between the resonant frequency and variable force,
Position of the screw.
Figure
Influence of the applied force
Figure
Influence of a frequency modulator on the amplitude of the PZT plate.
Influence of a frequency modulator on the pump flow rate.
Comparing with central actuating valveless pump [
In order to improve the reliability of micropump, Ma et al. [
The secondary chamber is located between the primary chamber and the outlet. The design of chamber and the operation mechanism of valveless pump are shown in Figures
(a) Design of chamber; (b) operation mechanism of valveless pump.
Theoretical analyses of valveless are also applied in (
Comparing with (
In terms of the actuator, the amplitude of the diaphragm,
The vertical displacement,
After differentiating (
In order to calculate
The similarity between the secondary chamber and a nozzle/diffuser element: (a) pump mode and (b) suction mode.
If the time-dependent pressure loss coefficient is similar to a nozzle/diffuser element, then
According to (
Because the secondary chamber has a significant effect on the flow rate and flow direction, the dimensions of the pump have been investigated to determine optimal performance. The effect of secondary chamber is shown in Figure
Experimental result for different types of pump.
The comparison of flow rate with different frequency.
The influence of secondary chamber width and height are simulated, as shown in Figure
The geometric dimension of pump design.
Case no. | Design of primary chamber | Design of secondary chamber | ||||||
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Length |
Width |
Height |
Diaphragm thickness (mm) | Length |
Width |
Height |
Diaphragm thickness (mm) | |
Basic | 45 | 28 | 4 | 0.5 | 7 | 20 | 4 | 0.5 |
1 | 45 | 28 | 4 | 0.5 | 7 | 28 | 4 | 0.5 |
2 | 45 | 28 | 4 | 0.5 | 7 | 12 | 4 | 0.5 |
3 | 45 | 28 | 4 | 0.5 | 7 | 20 | 2 | 0.5 |
4 | 43 | 28 | 4 | 0.5 | 7 | 20 | 4 | 0.5 |
5 | 41 | 28 | 4 | 0.5 | 7 | 20 | 4 | 0.5 |
6 | 45 | 28 | 3 | 0.5 | 7 | 20 | 4 | 0.5 |
7 | 45 | 28 | 2 | 0.5 | 7 | 20 | 4 | 0.5 |
8 | 45 | 28 | 1 | 0.5 | 7 | 20 | 4 | 0.5 |
9 | 45 | 28 | 4 | 0.3 | 7 | 20 | 4 | 0.5 |
10 | 45 | 28 | 4 | 0.7 | 7 | 20 | 4 | 0.5 |
The simulated flow rate of different secondary pumps.
The effects of chamber size are investigated in Case 4 through Case 7. From Figure
The measured flow rate with different primary length.
However,
This result decreases the volumetric displacement and lowers the maximum flow rate from 0.989 to 0.367 mL/s when the height of the primary chamber is decreased from 4 to 1 mm. As shown in Figure
The measured flow rate with different primary height.
The performance of 0.5 mm-thick diaphragm is the optimal choice in terms of flow rate, as shown in Figure
The measured flow rate with different diaphragm thickness in primary chamber.
A nozzle/diffuser element was installed to the pump inlet, as shown in Figure
Schematic view of the valveless pump with nozzle/diffuser element.
A novel design for proton exchange membrane fuel cells with a piezoelectric air pump (PZT-PEMFC) [
Operation modes of the PZT-PEMFC bicell: (a) pump mode for the upper cell and suction mode for lower cell and (b) suction mode for the upper cell and pump mode for lower cell.
An exploded drawing of the PZT-PEMFC bicell.
In a PZT-PEMFC bicell, two diffuser elements are applied to induce a larger air flow rate, as shown in Figure
The cathode chamber design of the PZT-PEMFC bicell.
As shown in Figure
The flow modes during the actuating process (a) suction mode (b) transition mode (c) pump mode.
Between the pump mode and the suction mode, a transition mode occurred when the outlet pressure was higher than the chamber and inlet pressures,
Thus, by using the Reynolds Transport Theorem and the continuity equation, the air flow rate can be written as
The inlet flow from the nozzle and diffuser could be found by using the diffuser element theory and the continuity equation as follows:
Thus, the inlet flow rate is given as follows:
The aspect ratio (AR) is defined as the cathode channel path divided by the channel opening width. The performances under different aspect ratios at a smaller diffuser angle
Different diffuser designs of the PZT-PEMFC.
Figure
While the actuating piezoelectric air pump works at the resonant frequency, the vibration amplitude of PZT can reach the maximum to intake more air into the chamber to enhance the performance of fuel cell. In this study, a diffuser
The
The piezoelectric actuating pump is used to compress the air into cathode chamber inside the bicell, which is different from the other designs that expose cathode to the ambient air. The active cathode with PZT device produces a forced convection in the small cathode chamber. In addition, the active cathode pumps out water vapor and solve the flooding problems in cathode. The design of piezoelectric air pump can improve the performance of fuel cell effectively.
The piezoelectric PEMFC with a nozzle and diffuser design further improved the fuel cell performance by adjusting an appropriate aspect ratio (AR) of the diffuser elements. Also, the nozzle/diffuser design can direct the air flow to prevent the backflow which may affect the performance of fuel cell.
Three types of one-sided actuating piezoelectric micropumps have been studied and applied in liquid or gas flow. The major conclusions from this study are summarized below. In the first type, the one-sided actuating micropump with two check valves can enhance the flow rate to prevent the back flow in suction mode and keeping flow in one direction. Moreover, the frequency modulator may be applied in the micropump to reach the maximum flow rate higher than 5.0 mL/s. However, the resonant frequencies of pump and valve may lose their harmonic motion and drop the actuating force in a higher frequency and a longer time period. In the second type, for the one-sided actuating valveless micropump with secondary chamber, the secondary chamber plays a key role in the application of the valveless micropump. It not only keeps the flow in one direction but also make the flow rate of the pump to reach 0.989 mL/s. Moreover, a nozzle/diffuser element is applied to the valveless micropump to avoid the fatigue of valve and increases its reliability, the flow rate can be further improved to 1.183 mL/s at a frequency of 150 Hz. The amplitude of the piezoelectric device can be changed and enlarged to improve the performance of micropump by using a frequency modulator. The maximum flow rate can be adjusted from 14.6 mL/min to 26 mL/min at 45 Hz and ±100 V. In the third type, the piezoelectric pump is regarded as an air pump in the application of a microfuel cell system, which can pump more inlet air to increase the fuel/air reaction and further increase the performance of fuel cell. Furthermore, the design of nozzle/diffuser in outlet/inlet can prevent the backflow to make the fuel cell have a better performance.