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Experimental study of cooling performance of pneumatic synthetic jet with singular
slot rectangular orifice
Roger Ho Zhen Yu, Mohd Azmi bin Ismail, Muhammad Iftishah Ramdan, and Nur Musfirah binti Mustaffa
Citation: AIP Conference Proceedings 1818, 020072 (2017);
View online: https://doi.org/10.1063/1.4976936
View Table of Contents: http://aip.scitation.org/toc/apc/1818/1
Published by the American Institute of Physics
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Experimental Study of Cooling Performance of Pneumatic
Synthetic Jet with Singular Slot Rectangular Orifice
Roger Ho Zhen Yu1, a), Mohd Azmi bin Ismail1*, b), Muhammad
Iftishah Ramdan1*,c), and Nur Musfirah binti Mustaffa1
1
Advanced Packing and SMT group, School of Mechanical Engineering, University Sains Malaysia, Penang,
Malaysia
a)
Corresponding author: [email protected]
b)
[email protected]
c) [email protected]
Abstract. Synthetic Jet generates turbulence flow in cooling the microelectronic devices. In this paper, the experiment
investigation of the cooling performance of pneumatic synthetic jet with single slot rectangular orifices at low
frequency motion is presented. The velocity profile at the end of the orifice was measured and used as characteristic
performance of synthetic jet in the present study. Frequencies of synthetic jet and the compressed air pressure supplied
to the pneumatic cylinder (1bar to 5bar) were the parameters of the flow measurement. The air velocity of the synthetic
jet was measured by using anemometer air flow meter. The maximum air velocity was 0.5 m/s and it occurred at
frequency motion of 8 Hz. The optimum compressed air supplied pressure of the synthetic jet study was 4 bar. The
cooling performance of synthetic jet at several driven frequencies from 0 Hz to 8 Hz and heat dissipation between
2.5W and 9W were also investigate in the present study. The results showed that the Nusselt number increased and
thermal resistance decreased with both frequency and Reynolds number. The lowest thermal resistance was 5.25ԨȀܹ
and the highest Nusselt number was 13.39 at heat dissipation of 9W and driven frequency of 8Hz.
INTRODUCTION
A microelectronic equipment performance is limited by its heat transfer effectiveness [1]. Synthetic jet is a
sophisticated microelectronic cooling and capable to reduce microelectronic temperature efficiently. It is also
known as active flow control techniques. The synthetic jet system has the setup of a closed cavity with an
oscillating diaphragm as shown in Fig. 1 [2]. The two stroke cycle (suction and ejection) are utilized at the present
synthetic jet. During the suction period, the diaphragm is forced downward to suck the air into the air cavity. This
stroke increases the air volume and lowers the pressure inside the cavity. Once the diaphragm located at the
maximum displacement, the ejection stroke started. During ejecting phase, the diaphragm is forced upward and
pushes the air flows through the orifice as shown in Fig.2. The process is continuous until the operation is stopped.
Synthetic Jet cooling scheme offers zero net mass flux and no external fluid source is required.
FIGURE 1. Synthetic Jet Schematic Geometry
The shape of the ducting system of the orifice influences synthetic jet. Investigation on the square,
circular and rectangular shaped orifice has been conducted by previous researches [3]. Rectangular shaped orifice
Engineering International Conference (EIC) 2016
AIP Conf. Proc. 1818, 020072-1–020072-11; doi: 10.1063/1.4976936
Published by AIP Publishing. 978-0-7354-1486-0/$30.00
020072-1
had been proven to be different in terms of the fluctuating velocity measurement with the time as compares to the
circular shaped orifice. Besides, the rectangular orifice has a better performance at lower axial distance for given
flow parameter and geometry [3].
FIGURE 2. Schematic diagram of the synthetic jet operation
FIGURE 3. Schematic of the evolution of a rectangular synthetic jet [4]
Geometric conditions such as orifice and actuator shapes are the factors of the evolution of mean velocity
profile in axial direction of synthetic jet as shown in Fig. 3 [5]. In addition, the stroke length and the Reynolds
number have been classified as the main parameters of synthetic jet performance. The stroke length is defined as
the distance between the jet orifice and the initial target surface. The Reynold number is represented as the
dimensionless flow characteristic produced by the synthetic jet [6].
It is proven that synthetic jet can increases the cooling performance up to 11 times than natural convection
cooling [3, 7]. Driven frequency of the synthetic is also an undeniable factor. Higher driven frequency increases
the cooling performance of the synthetic jet [8]. It is also well believed that the rectangular orifice with the largest
hydraulic diameter at the smallest aspect ratio produces the best cooling performance of the synthetic jet [7].
This paper covers the effect of different synthetic jet driven frequency and compressed air pressure
supplied to the pneumatic control system. Besides, cooling performance of synthetic jet with single rectangular
slot orifice at different driven frequency and heat dissipation rate is also investigated.
020072-2
EXPERIMENT SETUP
Pneumatic Synthetic Jet
The pneumatic synthetic jet consists of a double acting pneumatic cylinder, diaphragm and a steel cylinder
(Fig. 4). The synthetic jet is placed at the fitting of the air chamber by epoxy.
Pneumatic
Cylinder
Steel
Cylinder
Diaphragm
Air
Chamber
FIGURE 4. Pneumatic Synthetic Jet
Air Chamber with orifices
The air chamber is made of Perspex material with single slot rectangular orifice. The air chamber is designed
with dimension of 90mm X 62mm X 65mm while the rectangular shaped orifice with the dimension of 40mm X
9mm as shown in Fig. 5. The air chamber is clamped on a piece of wood in order to reduce vibration. Fig. 5 also
shows the design of the air chamber with cylinder fitting and orifice.
40mm
FIGURE 5. Air Chamber with Single Rectangular Slot Orifice
Flow measurement
The raspberry pie control system is employed to control the frequencies of the pneumatic cylinder. The
electrical connection is set up as shown in Fig. 6.
020072-3
FIGURE 6. Raspberry Pie Control System
Pressure control valve is used to control and vary the compressed air pressure supplied to the pneumatic
cylinder. The compressed air pressure supply is varied in the experiment between 1 bar and 5 bar.
FIGURE 7. Pressure Control Valve
The 5/3 way pneumatic valve with solenoid is used in this investigation to deliver the high pressure compressed
air to the pneumatic cylinder and to control the flow speed produced by the synthetic jet. The respond time for the
solenoid is 3 second.
FIGURE 8. Pressure Valve with Solenoid
An airflow anemometer is placed at the exit of the air chamber and is used to measure the velocity of the flow.
Different frequencies in the range between 1Hz and 8Hz are utilized to the synthetic jet and various compressed
air pressure from 1 bar to 5 bar are supplied to the pneumatic cylinder
020072-4
Temperature Measurement of Heat sink
13.6mm
43mm
47mm
FIGURE 9. Microelectronic heat sink
Microelectronic heat sink with dimension of 47mm X 43mm X 13.6mm as shown in Fig. 9 is used in the
investigation. A heater of 50mm X 50mm was placed at the bottom of the heat sink. A raspberry pie controller
was used to manipulate the driven frequencies of the synthetic jet. Type K thermocouples was used and placed at
different locations along the experiment. One of the thermocouples were placed at the heater while 5 other
thermocouples were placed at different locations at the heat sink as shown in Fig. 10. Another thermocouple was
used to measure the ambient temperature. All temperatures were recorded and stored in the TC-08 Thermocouple
Data Logger.
(b)
(a)
FIGURE 10. (a) Locations of thermocouples at the heat sink. (b) TC-08 Thermocouple Data Logger
Experiment Procedure
A power supply is connected to the heater. Another power supply is connected to the raspberry pie and solenoid
pressure valve. A pressure control valve is connected to the solenoid pressure valve. The synthetic jet is connected
to the solenoid pressure valve. A python code is built in raspberry pie in order to control the driven frequencies
of the synthetic jet. The experiment started with a fixed frequency of 5 Hz and the pressure supply is varied from
1 bar to 5 bar. An anemometer is used to measure and record the velocity of the airflow. The experiment is
conducted with fixed pressure supply of 4 bar and the driven frequency ranged between 1Hz to 8Hz. A
thermocouple Date Logger is used to measure and record the temperature of the heat sink.
020072-5
FIGURE 11. Experiment setup of synthetic jet study
DATA DEDUCTION
Reynolds number {Re} is defined as the ratio of inertial forces to viscous forces. As the driven frequency of
the synthetic jet increases, Re increases as well. Re of a fluid is calculated based on the procedure proposed by
[9]:
ܴ݁ ൌ ୙ή஽೓
௩
(1)
where ‫ܦ‬௛ is the hydraulic diameter of the orifice and ‫ ݒ‬is the kinematic viscosity of the fluid. is the flow velocity
at the exit of the orifice during the ejection phase.
Thermal Resistance
Thermal resistanceሼ்ܴ } is one of the factors that affect the cooling performance of the synthetic jet and
it can be expressed by the following equation (2):
்ܴ ൌ
where ܶ௛ = the heater temperature,
ܶஶ = the ambient temperature,
and Q = the heat dissipation from heater.
்೓ ି்ಮ
୕
020072-6
(2)
Heat transfer measurement
The heat transfer coefficient (݄) of the heat sink was calculated based on the Newton’s Law of Cooling [3].
(3)
ܳ ൌ ݄‫ ் ܣ‬ሺܶ௦ െ ܶஶ ሻ
where is ‫ ் ܣ‬the total surface area of the heat sink (without the contact surface),
ܶ௦ = the average heat sink temperature,
The Nusselt number is used to characterize the heat transfer from a solid surface to the surroundings.
ܰ‫ݑ‬௔௩௘ ൌ
௛ೌೡ೐ ஽೓
(4)
௞
Where k = the air thermal conductivity.
The heat loss in the study is negligible.
RESULT AND DISCUSSION
0.40
0.35
0.30
V(m/s)
0.25
P=1bar
P=2 bar
0.20
P=3bar
P=4bar
0.15
P=5bar
0.10
0.05
0.00
0
5
10
15
20
25
Time(s)
FIGURE 12. Air Velocity against Time at 5Hz
Figure 12 shows the measurement of the air velocity at given time with different compressed air pressure
supplied to the pneumatic cylinder a constant frequency of 5 Hz. The air velocities of compressed air for all
pressure supplied increase with time until 8 seconds where the diaphragm of the synthetic jet starts to move and
reaches its maximum velocity. After 8 seconds, the air velocity fluctuates along the time. This process is
020072-7
continuous and creates a repeated pattern of velocity profile. The potential reason of the occurrence is the air
being pushed in and out through the synthetic jet. This periodic motion causes the formation of the same velocity
pattern for all the different pressures supplied to the pneumatic cylinder. From Fig. 12, compressed air of 1 bar
shows the lowest flow velocity. This shows that the 1 bar compressed air does not provide the sufficient pressure
to the pneumatic cylinder to perform normally. That happens due to the surface friction between the diaphragm
and the steel cylinder surfaces. This phenomenon eventually slows down the movement of the diaphragm
especially at compressed air pressure of 1 bar. Fig. 12 also shows that 4 bar and 5 bar of the compressed air have
similar air velocity. This means the compressed air pressure of 4 bar is enough to overcome friction between
diaphragm and steel cylinder surfaces. This proves that the pneumatic cylinder requires compressed air pressure
of 4 to 5 bar for the optimum performance of the synthetic jet.
0.60
0.50
V(m/s)
0.40
2 Hz
0.30
4Hz
6Hz
0.20
8Hz
0.10
0.00
0
2
4
6
8
10
12
14
16
Time(s)
FIGURE 13. Velocity against Time at 4bar
Figure 13 illustrates the air velocity versus time for various frequencies with a constant supply of the
compressed air pressure of 4 bar. The frequency of the synthetic jet is adjusted by using python code of the
raspberry pi during the experiment. The higher the frequency of the synthetic jet, the higher the air velocity.
Frequency of 2 Hz produces the lowest air velocity. Thus, there is one diaphragm revolution every half second to
deliver the air to the orifice. Meanwhile, the frequency of 8 Hz produces the highest air velocity. The revolution
period of diaphragm of 8 Hz frequency is calculated as 0.125 second which is considered fast. At first, the flow
is produced in the cavity by the movement of the diaphragm. Then, the air is pressed downward and created a
forward flow to the exit of the orifice. Later on, the air is sucked back to the air chamber and creates a backflow.
During the process of exchanging direction of the flow, not all the flow is being full exchanged in and out at higher
frequency.
Figure 13 shows that there is harmonic motion produced at each frequency. Frequency of 2 Hz takes the
longest time to complete one revolution of harmonic motion while frequency of 8 Hz takes the shortest time to
complete one revolution of harmonic motion. Therefore, the higher the frequency of the synthetic jet, the shorter
time the synthetic jet needs to complete one revolution of harmonic motion.
020072-8
14.00
12.00
RT (Ԩ/ܹ)
10.00
8.00
2.5W
6.00
5.6W
9.0W
4.00
2.00
0.00
0
1
2
3
4
5
6
7
8
f (Hz)
FIGURE 14. Graph of ்ܴ௢௧௔௟ against ݂
The pressure supplied to the synthetic jet is set to 4 bar which is the optimum pressure for the experiment. Fig.
14 plots against frequency (ˆሻ at three heat dissipation rates (Q). At the driven frequency at 0Hz (natural
convection), the thermal resistance is at its highest with heat dissipation rate of 2.5W while the lowest thermal
resistance is at 9W. As the heat dissipation increases, more heat is transferred to the surrounding area which lead
to the decrease of the thermal resistance. The figure also shows that the thermal resistance decreases linearly as
the driven frequency of the synthetic jet increases. As the driven frequency increases, the air velocity also increases
which leads to higher convection effect. Heat dissipation of 9W shows the lowest thermal resistance which
indicates high rate of heat transfer from the heat sink to the surroundings. According Fig. 14, the lowest of thermal
resistance is 5.25 ԨȀܹ at ݂= 8 Hz at Q = 9W.
25.00
h (W/(m2/K))
20.00
15.00
2.5W
5.6W
10.00
9.0W
5.00
0.00
0
1
2
3
4
5
6
7
8
f (Hz)
FIGURE 15. Graph of ݄against ݂
Figure 15 shows the higher the driven frequency of the synthetic jet, the higher the heat transfer coefficient.
The higher the heat dissipation rate and lower temperature difference between the heat sink and the ambient
induces higher heat transfer coefficient as shown in equation 2. The heat transfer coefficient increases from
7.79ܹȀሺ݉ଶ ‫ܭ‬ሻ to 22.77 ܹȀሺ݉ଶ ‫ܭ‬ሻof driven frequencies of 2Hz to 8Hz at heat dissipation of 9W.
020072-9
16.00
14.00
12.00
Nu
10.00
2.5W
8.00
5.6W
6.00
9.0W
4.00
2.00
0.00
0
50
100
150
200
250
300
350
400
450
500
Re
FIGURE 16. Graph of Nu against Re
As shown in Fig. 16, the Nu increases as the Re increases. Nu for the heat dissipation of 9W is higher than the
heat dissipation of 5.6W and 2.5W. As the driven frequency of the synthetic jet increases, the flow velocity
increase as well as shown in Fig. 13 which increase the Reynold number (Re) of the flow. This phenomenon leads
the increases in the heat transfer coefficient (h) and Nusselt number (Nu). The highest Nu is found to be 13.39 at
driven frequency of 8Hz and heat dissipation of 9W. Besides, the Nusselt number of 8Hz 3 times higher than the
0Hz (natural convection) at the heat dissipation of 9W.
CONCLUSION
The experimental study on the cooling performance of the pneumatic synthetic jet had been successfully
conducted. The pneumatic synthetic jet produces unique air velocity profile at various compressed air pressure
and diaphragm frequencies. The maximum air velocity obtained in this experimental study is 0.5 m/s at the
frequency of 8 Hz and a constant air pressure of 4 bar. Heat transfer coefficient increases as the driven frequency
increases because flow velocity gradient within boundary layer increase with the driven frequency. The highest
Nusselt number of 13.39, the highest heat transfer coefficient of 22.77 ܹȀሺ݉ଶ ‫ܭ‬ሻ and the lowest thermal
resistance of 5.25 ԨȀܹare discovered at the driven frequency of 8Hz and heat dissipation of 9W. This
investigation shows that synthetic jet enhances cooling performance of microelectronic device at low Reynold
number.
ACKNOWLEDGMENTS
Many thanks to Grant code 304/Pmekanik/60313021 and Conference Fund 2016 from Universiti Sains
Malaysia due to sponsored this paper.
REFERENCES
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3.
P. Lall, M. P. (1997). Influence of temperature on Microelectronics and System reliability. CRC Press, New
York.
B. Smith, A. G. The formation and evolution of synthetic jets. Phys. Fluids 31, 2281-2297 (1998).
Chaudhari, M., Puranik, B., & Agrawal, A. Exp. Therm Fluid Sci., 34 (2), 246-256 (2010).
020072-10
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Chaudhari, M., Puranik, B., & Agrawal, A. Int. J. Heat Mass Transfer, 53 (5–6), 1057-1069 (2010).
Krishnan, G. and K. Mohseni. J. fluids Eng. 131 (12), 121101-121101 (2009).
Pavlova, A., and Amitay, M., J. Heat Transfer 128 (9), 897–907 (2006).
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