SM Firdaus1, a), MZ
Abdullah2, MK Khalil3 and Wan Amri2

1Faculty of Mechanical Engineering,
Universiti Teknologi MARA Cawangan Pulau Pinang, 13500 Permatang Pauh, Pulau
Pinang, Malaysia

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2School of Mechanical Engineering,

Universiti Sains Malaysia, 13000
Nibong Tebal, Pulau Pinang

 

3School of Material Science and Mineral Resources Engineering,

Universiti Sains Malaysia, 13000
Nibong Tebal, Pulau Pinang

 

a) Corresponding author: [email protected]

Abstract. The
miniaturization of electronic devices with high processing components is
promoting the rapid increase in heat generation of devices/systems. Synthetic
jet cooling flexible thin device has major advantages in order to suit the
device miniaturization and compactness. Diaphragm motion is the main component
in the device that generate fluid motion for cooling application. In this
paper, the synthetic jet diaphragm has been characterized using Laser Doppler
to record the motion to determine the motion type. Based on the recorded
motion, a User Define Function coding has been develop to imitate the motion in
Computational Fluid Dynamic (CFD) software in order to simulate the fluid
motion based from the actual diaphragm motion. The diaphragm motion showed a
dome shape motion as the laser tracking result were plotted at three points
across the diaphragm diameter. Results show that the diaphragm has sinusoidal
motion which produce maximum amplitude at 500Hz driving frequency. The UDF
program for diaphragm motion has been able to mimic the experimental
result. 

Keywords: electronic cooling; equation of motion; synthetic jet

introduction

Cooling solutions are critical for current
electronic devices to prevent over-heating resulting to further the lifespan of
electronic component. The power required is increasing as the device size
miniaturized that lead to low space available for cooling in electronic device.
The processing power is proportional with heat generation in electronic devices
with multi-processor 1, 2. Conventional fans and natural flow (passive cooling) has come into
its limit due to the minimum space available inside the device assembled
package or system 3.

 

Synthetic jet has become a promising future
cooling system with its zero-net mass input capabilities. An induced fluid flow
due to an upward diaphragm movement has entrained fluid motion into the cavity.
Then, as the diaphragm move downward, the fluid motion is expelled through one
or more opening slot, called nozzle 4 as shown in Fig. 1.

 

FIGURE 1 Synthetic
Jet working Principal; a. suction phase b. ejection phase 5

 

Most previous researcher claim that maximum
amplitude was obtain during driving the synthetic jet at resonance frequency.
However as far as authors’ concern, no literature was reported on diaphragm
equation of motion with experiment verification. In this work the diaphragm has
been applied at various driving frequency to measure the deflection. A User
Define Function (UDF) written subroutine has been develop to mimic the
diaphragm motion obtained in the experiment.

 governing equation

ANSYS FLUENT®
is a CFD based software has been used in this research to simulate the fluid
motion based on the conservation of mass, momentum and energy equation. The
governing equations employed in ANSYS FLUENT® for describing the transient
fluid flow are as follows:

 

Conservation of
mass is:

 .

(1)

Equation 1 is in
the general form, which is valid for incompressible and compressible flow.

Momentum
(non-accelerating reference frame):

 

 ,

 (2)                                                                                    

 

where ? is the fluid density; P is the pressure in the fluid; ?ij is the viscous stress
tensor; and gi and Fi are the gravitational
acceleration and external body force in the i-direction, respectively.

 

In the present
study, UDF is used to simulate the movement of diaphragm using a Structured
Dynamic Meshing. The UDF contains the periodic diaphragm movement by sinusoidal
equation as shown:

 

,

(3)

where A is the diaphragm amplitude; ? is the
angular frequency; and t is time dynamic meshes can be used to model flow
where the shape of the domain changes due to motion. The integral form of the
transport equation for a general scalar (F), on an arbitrary control volume (V), on a
moving mesh.

 

 

 

methodologies

 

In this study, the driving frequency
applied was varied from 300 Hz to 700 Hz with 100 Hz increment in numerical and
experimental. Fig. 2 shows the
detailed dimension of the synthetic jet model used in ANSYS FLUENT®
numerical analysis with a 5 mm volume cavity with 2 mm nozzle as illustrated in
Figure 2.

 

FIGURE 2 Synthetic jet
model details dimension

 

The computational domain was built by using
pre-processing software according to the dimension of the synthetic jet. Structured
dynamic meshing in Fig. 3 has been
used for the computational domain diaphragm motion and it save the computing
time compared to unstructured dynamic meshing6. The simulation was carried out for 1000 cycles to obtain a steady diaphragm
output. The internal iteration continued until the residue of mass and
momentum, and energy were reduced to below 10-3 and 10-6,
respectively, which is the convergence criterion for the computation. Data were
extracted from the compiled UDF motion at the diaphragm surface monitoring.

 

FIGURE 3 ANSYS FLUENT®
Computational Domain

 

 

 

 

Keyence® Laser Doppler experiment was used
to record the diaphragm motion as shown in Fig.
4(a). Three laser has been used in this experiment at three points on the
diaphragm. Middle laser was used to measure the middle amplitude during motion
while the left and right side laser to measure point further away from middle
point as shown in Fig. 4(b). It is
to draw the exact motion shape of the diaphragm when frequency applied. 

 

 
 
 

1    2  
     3

Laser
Point

(a)

(b)

 

FIGURE 4 Keyence® Laser Doppler Experimental Setup

 

Results and discussions

Keyence® Laser Doppler

 

Fig.
5 show the motion tracking for diaphragm to study
the motion shape at three laser points. Result shows that the diaphragm has a
dome shape motion when sinusoidal driving frequency applied. Point 2 has the
highest amplitude compared to the other points. Point 1 and 3 has the symmetry
value of amplitude of deformation. The different between point 1 and point 2
was below than 5% as shown in Table 1 which mean the diaphragm deformation can
be assume has the same value with point 2 at any point of the diaphragm
surface. 500 Hz driving frequency has the maximum amplitude compared to other
frequencies applied. Bhapkar, Srivastava, & Agrawal
(2014) and Kimber, Garimella, & Raman (2007) stated that resonance frequency has maximum amplitude which able
to dissipate maximum amount heat for electronic device cooling.

 

Figure 5 Diaphragm sinusoidal
motion laser tracking

 

 

TABLE 1 Percentage
different Point 1 and Point 2 amplitude

 

Frequency
(Hz)

Point
1 (m)

Point
2 (m)

Point
3 (m)

Percentage
Different (%)

300

0.000098

0.00010

0.000098

2%

400

0.00024

0.00025

0.00024

1%

500

0.000392

0.0004

0.000392

2%

600

0.000241

0.00025

0.000241

1%

700

0.000098

0.00010

0.00009

1%

 

 

Numerical Analysis

 

Fig.
6 illustrates the motion generated from the developed
UDF. The diaphragm has successfully simulate in sinusoidal motion. The compile
UDF ables to mimic similar sinusoidal motion as in the laser tracking
amplitude. This is important for the future fluid simulation that the diaphragm
motion will affect the fluid ejected characteristic such as vortex flow and
fluid velocity.

 

FIGURE 6 Surface Monitoring
UDF diaphragm motion

conclusion

 

Synthetic jet diaphragm has different
amplitude value when driving at various frequency. Resonance frequency produce
the maximum amplitude of the diaphragm. The diaphragm motion has small dome
shape motion but the different between the tracking points were less than 1%
which is not significant to be considered in UDF program. The UDF program has
been able to simulate sinusoidal motion of the diaphragm mimic to the experimental
results.

 

References

1        S. C. Lin and K. Banerjee, “Cool chips:
Opportunities and implications for power and thermal management,” IEEE
Trans. Electron Devices, vol. 55, no. 1, pp. 245–255, 2008.

2        K. Kota, P. Hidalgo, Y. Joshi, and  a. Glezer, “Thermal management of a 3D chip
stack using a liquid interface to a synthetic jet cooled spreader,” 2009
15th Int. Work. Therm. Investig. ICs Syst., no. October, 2009.

3        H. Wang, N. E. Jewell-larsen, and A. V
Mamishev, “Thermal management of microelectronics with electrostatic fl uid
accelerators,” ATE, vol. 51, no. 1–2, pp. 190–211, 2015.

4        A. Pavlova and M. Amitay, “Electronic
Cooling Using Synthetic Jet Impingement,” J. Heat Transfer, vol. 128,
no. 9, p. 897, 2006.

5        G. Krishnan and K. Mohseni, “An
experimental study of a radial wall jet formed by the normal impingement of a
round synthetic jet,” Eur. J. Mech. B/Fluids, vol. 29, no. 4, pp.
269–277, 2010.

6        X. Ma, Z. Fan, and X. Da, “Dynamic mesh
method for two-dimensional synthetic jet,” Procedia Eng., vol. 31, pp.
422–427, 2012.

7        M. Kimber, S. V. Garimella, and A.
Raman, “Local Heat Transfer Coefficients Induced by Piezoelectrically Actuated
Vibrating Cantilevers,” J. Heat Transfer, vol. 129, no. 9, p. 1168,
2007.

8        U. S. Bhapkar, A. Srivastava, and A.
Agrawal, “Acoustic and heat transfer characteristics of an impinging elliptical
synthetic jet generated by acoustic actuator,” Int. J. Heat Mass Transf.,
vol. 79, pp. 12–23, 2014.

 

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