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EXPERIMENTAL AND NUMERICAL ANALYSIS
ON THE COOLING PERFORMANCE OF SYNTHETIC JET
AT LOW REYNOLDS NUMBER
by
ROGER HO ZHEN YU
Thesis submitted in fulfillment of the
requirements for the degree
of Master of Science
May 2018
ii
ACKNOWLEDGEMENT
First, I would like to thank the Almighty God for giving me the grace to
undertake and successfully complete this study. Without His grace, I would not able
to overcome the struggles I faced during this challenging period of my life.
I would like to thank Dr. Mohd Azmi bin Ismail, my main academic supervisor
for his limitless patience and motivation. His support and guidance proved to be a
driving force for me to complete this thesis. I will always remember how he used his
expertise to help me in order to write a strong thesis.
I would also like to thank Dr. Istishah bin Ramdan, my co-supervisor who is
always willing to spend her time reading my countless drafts and providing
constructive feedback to help me in expressing the ideas and thoughts effectively.
Without him, I would not able to write this thesis confidently.
Finally, I must express my very profound gratitude to my parents, family and
friends for providing me with unfailing support and continuous encouragement
throughout the process of researching and writing up of the thesis. This
accomplishment would not have been possible without all of you. Thank you.
iii
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENT ii
TABLE OF CONTENTS iii
LIST OF TABLES v
LIST OF FIGURES vi
LIST OF ABBREVIATIONS ix
LIST OF SYMBOLS x
ABSTRAK xi
ABSTRACT xii
CHAPTER ONE: INTRODUCTION
1.0 Overview 1
1.1 Research background 1
1.2 Problem statement 4
1.3 Research objectives 5
1.4 Research scope 5
CHAPTER TWO: LITERATURE REVIEW
2.0 Overview 6
2.1 Flow characteristic of synthetic jet 6
2.2 Stroke length 14
2.3 Orifices 14
2.4 Distances of orifices and targeted surface 19
2.5 New types of synthetic jet 20
2.6 Chapter summary
23
CHAPTER THREE: RESEARCH METHODOLOGY
3.0 Overview 24
3.1 Methodology (Experimental study) 26
3.2 Assemble experimental rig 34
3.3 Numerical setup 35
iv
3.4 Formula 40
3.5 Chapter summary 41
CHAPTER FOUR: RESULTS AND DISCCUSION
4.0 Overview 42
4.1 Experiment study 42
4.2 Comparison of horizontal and vertical arrangement 52
4.3 Simulation analysis 57
4.4 Chapter summary 80
CHAPTER FIVE: RESEARCH CONCLUSION
5.0 Concluding remarks 82
5.1 Future work 83
REFERENCES 84
APPENDICES
Appendix A: Python coding
Appendix B: Sample calculation
LIST OF PUBLICATIONS
v
LIST OF TABLES
Page
Table 2.1 Tested jet orifice configurations (Lee, Woyciekoski, &
Copetti, 2016)
17
Table 3.1 Comparison of the mesh quality of the model
37
Table 3.2 Materials properties
38
Table 4.1 Random error of the experimental study at driven frequency
of 8 Hz
45
Table 4.2 Grid independent test
57
vi
LIST OF FIGURES
Page
Figure 1.1 Synthetic jet
3
Figure 2.1 Rapid cooling capability of synthetic jet ejector heat
(Mahalingam & Glezer, 2004)
8
Figure 2.2 Downstream evolution of the centerline velocity by (Van
Buren & Amitay, 2016)
9
Figure 2.3 Flow morphology of vortex formation and evolution at
various stroke (a)L0/D < 4, (b)4 ≤ L0/D < 8, (c)8 ≤L0/D < 16, (a)L0/D ≥ 16 (McGuinn,Farrelly,Persoons,&
Murray,2013)
10
Figure 2.4 Velocity vectors and pressure contours for different times in
a cycle of operation (Jain, Puranik, & Agrawal, 2011)
12
Figure 2.5 Continued operation (Jain, Puranik, & Agrawal, 2011)
13
Figure 2.6 Temperature contour on heated plate subjected three-holes
synthetic jets (Tan & Zhang, 2013)
16
Figure 2.7 Average jet centerline velocity for all configurations of jet
orifice. (Lee, Woyciekoski, & Copetti, 2016)
18
Figure 2.8 Comparison of average Nusselt number coefficient with
respect to heated. (Lee, Woyciekoski, & Copetti, 2016)
18
Figure 2.9 Exit velocity for an unconfined synthetic jet in a time period
of the cycle. (J.-Z. Zhang, Gao, & Tan, 2013)
19
Figure 2.10 (a) Principle of SynJet actuator; (i) passive state, (ii) blowing
state for generation of turbulent air flow for cooling and (iii)
suction state; (Buchberger et al., 2016)
22
Figure 3.1 Flow chart of the investigation
25
Figure 3.2 Pneumatic synthetic jet
26
Figure 3.3 Pressure control valve
27
Figure 3.4 Double acting pneumatic cylinder 27
Figure 3.5 Pneumatic valve with solenoid
28
Figure 3.6 The design of the air chamber with cylinder fitting and
orifices
29
vii
Figure 3.7 Dimension of the air chamber
29
Figure 3.8 Raspberry pie
30
Figure 3.9 Electric circuit
31
Figure 3.10 Airflow TA45 anemometer
32
Figure 3.11 Heat sink dimension
32
Figure 3.12 Locations of thermocouples at the heat sink
33
Figure 3.13 TC-08 thermocouple data logger
33
Figure 3.14 Experiment setup
35
Figure 3.15 3D model in SCDM
36
Figure 3.16 Meshed model
37
Figure 3.17 Boundary condition of the model (a) inlet, (b) heater, (c)
wall, (d) air inlet, (e) outlet
39
Figure 4.1 Graph of velocity against time at horizontal arrangement
44
Figure 4.2 Graph of average peak velocity against driven frequency at
two different arrangement
45
Figure 4.3 Graph of total thermal resistance against driven frequency at
D= 0 cm
47
Figure 4.4 Graph of total thermal resistance against driven frequency at
P= 9.0 W
48
Figure 4.5 Graph of heat transfer coefficient against driven frequency at
D= 0 cm
49
Figure 4.6 Graph of heat transfer coefficient against driven frequency at
P= 9.0 W
50
Figure 4.7 Graph of Nu against Re at D= 0 cm
51
Figure 4.8 Graph of Nu against Re of two experiment study
52
Figure 4.9 Graph of total thermal resistance against driven frequency at
D= 0 cm
53
Figure 4.10 Graph of total thermal resistance against driven frequency at
D= 0.5 cm
54
viii
Figure 4.11 Graph of total thermal resistance against driven frequency at
D= 1.0 cm
54
Figure 4.12 Graph of heat transfer coefficient against driven frequency at
D= 0 cm
55
Figure 4.13 Graph of Nu against Re at D= 0 cm
56
Figure 4.14 Temperature contour for grid independent test (a) coarse,
(b) medium, (c)fine
58
Figure 4.15 Comparison of the velocity of the time step of 0.1 s and
0.125 s at the driven frequency of 8Hz
59
Figure 4.16 Comparison of the velocity of the driven frequency of 2 Hz
and 8 Hz
59
Figure 4.17 Average velocity of driven frequency of 8 Hz on the
simulation and experimental analysis
60
Figure 4.18 Temperature contour and velocity vector at D= 0 cm and
horizontal arrangement (a) f = 2 Hz, (b) f = 4 Hz, (c) f = 6
Hz, (d) f = 8 Hz
62
Figure 4.19 Velocity vector at D=0cm and horizontal arrangement (a) f =
2 Hz, (b) f = 4 Hz, (c) f = 6 Hz, (d) f = 8 Hz
64
Figure 4.20 Temperature contour at D=0cm and vertical arrangement (a)
f = 2 Hz, (b) f = 4 Hz, (c) f = 6 Hz, (d) f = 8 Hz
65
Figure 4.21 Velocity vector at D=0cm and vertical arrangement (a) f = 2
Hz, (b) f = 4 Hz, (c) f = 6 Hz, (d) f = 8 Hz
67
Figure 4.22 Graph of temperature against driven frequency at D= 0 cm
68
Figure 4.23 Graph of heat transfer coefficient against driven frequency at
D= 0 cm
69
Figure 4.24 Graph of Nu against Re at D= 0 cm
70
Figure 4.25 Temperature contour at D=1.0cm and horizontal arrangement
(a) f = 2 Hz, (b) f = 4 Hz, (c) f = 6 Hz, (d) f = 8 Hz
72
Figure 4.26 Velocity contour at D=1.0cm and horizontal arrangement (a)
f = 2 Hz, (b) f = 4 Hz, (c) f = 6 Hz, (d) f = 8 Hz
74
Figure 4.27 Temperature contour at D=1.0cm and vertical arrangement
(a) f = 2 Hz, (b) f = 4 Hz, (c) f = 6 Hz, (d) f = 8 Hz
75
ix
Figure 4.28 Velocity vector at D=1.0cm and vertical arrangement (a) f =
2 Hz, (b) f = 4 Hz, (c) f = 6 Hz, (d) f = 8 Hz
77
Figure 4.29 Graph of temperature against driven frequency at D= 1.0 cm 78
Figure 4.30 Graph of heat transfer coefficient against driven frequency at
D= 1.0 cm
79
Figure 4.31 Graph of Nu against Re at D= 1.0 cm 80
LIST OF ABBREVIATIONS
CFD
Computational Fluid Dynamics
ANSYS
American Computer-aided engineering software
SCDM SpaceClaim Direct Modeler
x
LIST OF SYMBOLS
V Velocity
f Driven frequency
t
Time
Q Heat dissipation
h Heat transfer coefficient
AT Total surface area of heater
Ts Average temperature of the heat sink
T∞ Ambient temperature
RT
Th
D
Re
Dh
v
Nu
k
Total thermal resistance
Heater’s temperature
Distance between the orifices and the heat sink
Reynolds number
Hydraulic diameter of the orifice
Kinematic viscosity of the fluid
Nusselt number
Thermal conductivity of the fluid
xi
KAJIAN EKSPERIMEN DAN SIMULASI PRESTASI PENYEJUKAN JET
SINTETIK PADA NOMBOR REYNOLDS YANG RENDAH
ABSTRAK
Jet sintetik adalah satu kaedah yang berpotensi dalam penyejukan
mikroelektronik. Dengan ciri-ciri aliran bendalir aktif dan sifar aliran jisim bersih, jet
sintetik merupakan satu peranti yang cekap dalam penyejukan mikroelektronik. Dalam
kajian ini, prestasi penyejukan jet sintetik pneumatik disiasat dengan nombor Reynolds
yang rendah. Kedua-dua eksperimen dan simulasi berangka dijalankan. Siasatan
dijalankan dengan dua susunan yang berbeza. Udara yang dijana daripada rongga jet
sintetik dihantar secara mendatar. Seterusnya, aliran udara dihantar secara menegak.
Parameter-parameter kajian dilakukan berdasarkan lapan nombor Reynolds jet sintetik
yang berbeza, kuasa dibekalkan kepada sink haba, dan jarak antara orifis dan sinki
haba. Kajian ini menunjukkan bahawa peningkatan frekuensi jet sintetik
meningkatkan halaju udara dan membawa kepada prestasi penyejukan yang lebih baik.
Kajian ini juga menunjukkan bahawa pemalar pemindahan haba jet sintetik adalah dua
kali lebih tinggi daripada perolakan semulajadi. Keputusan juga menunjukkan bahawa
apabila kuasa yang dibekalkan meningkat, lebih tinggi nombor Nusselt dan secara
tidak langsung membekalkan prestasi yang lebih baik dalam penyejukan jet sintetik.
Walau bagaimanapun, sekiranya jarak antara orifis dan sinki haba ditingkatkan,
prestasi penyejukan akan merosot. Jarak 0 cm menghasilkan 20% permindahan haba
lebih baik daripada jarak 1 cm. Kajian ini juga menunjukkan susunan udara secara
mendatar mempunyai prestasi penyejukan 8% lebih tinggi berbanding dengan susunan
aliran udara menegak.
xii
EXPERIMENTAL AND NUMERICAL ANALYSIS ON THE COOLING
PERFORMANCE OF SYNTHETIC JET AT LOW REYNOLDS NUMBER
ABSTRACT
Synthetic jet is a potential alternative for microelectronic cooling. With the
active fluid flow characteristic and zero net mass flow, the synthetic jet has been
considered as the high-efficiency device in cooling microelectronic devices. In this
study, a pneumatic synthetic jet with low Reynolds number is investigated for the
cooling performance of an aluminium heat sink. Both experimental study and
numerical simulations are conducted. The investigations are carried out with two
different arrangements. The air is generated from the synthetic jet in the cavity and
delivers horizontally to the heated heat sink through the orifices. Another arrangement
is vertical air flow delivered upward from the synthetic jet to the heated heat sink
through the orifices. The characteristics study is performed at eight different Reynolds
number of the synthetic jet, power supplied to the heat sink, and the distance between
the orifices and the heat sink. The study shows that as the driven frequency of the
synthetic jet increases, the airflow velocity also increases, and it leads to an improved
cooling performance. The findings show that the heat transfer coefficient of the
synthetic jet is two times higher than that of the natural convection. The results also
show that as the power supplied increases, Nusselt number increased which lead to the
better cooling performance of the synthetic jet. However, as the distance of the orifices
to the heated heat sink increases, the cooling performance is eventually decreased. The
distance of 0 cm results in 20% higher heat transfer coefficient as compared to the
distance of 1 cm. The study also reveals that the horizontal air flow arrangement
provides 8% higher cooling performance as it is compared to the vertical air flow
arrangement.
1
CHAPTER ONE
INTRODUCTION
1.0 Overview
This chapter introduces the invention and general design of synthetic jet. The
first section provides the research background of the synthetic jet. The second section
highlights problem statement in the present study. The third section shows the present
research aims and objectives. The fourth section presents the scope of study. The final
section is an outline of the whole thesis.
1.1 Research background
Microelectronic devices are widely being used in the new modern era of the
human society. With the high demand of the microelectronic devices, the usage of the
devices increases with time. Thus, the reliability and optimum performance level of
the microelectronic devices are the two important aspects of the design of the devices.
Operating temperature such as ambient temperature, humidity, size of the devices and
others are undeniable affected the reliability and performance of the microelectronic
devices. But, the most common and usual factor that always decreases the reliability
of the devices is the thermal overstressing. According to law of the conversion of
energy, the energy cannot be created and destroyed. The energy can only be transferred
from one form to another form with the constant total net energy. Microelectronic
devices utilize the electrical energy and eventually releases the thermal energy. As the
thermal energy increases, the temperature of the microelectronic device also increases.
Microelectronic devices operate efficiently under the acceptable range of the operating
temperature. Based on the research (Lall et al., 1997), the reliability of the devices
2
decreases as microelectronic temperature exceed 100℃. The increasing temperature
will increase the thermal stress on the devices which eventually decreases the lifespan,
performance and the efficiency of the microelectronic devices.
Thermal control and appropriate cooling method of the microelectronic devices
plays an important role in maintaining its reliability and performance. According to
the Newton’s law of cooling, the thermal energy transfers from hot surface to the cold
surface. Heat is first generated from the integrated components inside the devices and
released to the devices’ surface. Lastly, heat is released from the heated surface of the
devices to the ambient. In this work, heat transfers from the heated surface to the
ambient is investigated with the suggested thermal management technique. Nowadays,
the commercial cooling method is required to mitigate the high heat flux. There are
three types of methods to dissipate the heat to the ambient which are the active cooling
method, the passive cooling method and the combination of the active and passive
methods. The active cooling method enhances the heat transfer with the help of
external energy consumption. The cooling devices increase the rate of the fluid flow
and ultimately remove the heat from the heated surface. Active cooling method is well
believed as the considerable approach in heat transfer enhancement. Fan, blower,
synthetic jet and others are the example of the active cooling technique. Passive
cooling method increases the heat transfer rate with no additional energy consumption.
Heat sink is an example of passive cooling method. This method increases the level of
the heat dissipation rate and natural convection. Hybrid cooling technique is known as
the combination of active and passive cooling method which is widely use in the
computer power unit (CPU).
3
As the demand of the microminiaturization of integrated microelectronic
devices increases, more effective and better cooling management is needed to remove
the high heat dissipation level of devices. Synthetic jet is an active cooling approach
which controls the operating temperature to an acceptable level. Synthetic jet has been
widely developed and explored in the field of the microelectronic cooling system with
its promising cooling technique in thermal management. This jet is called synthetic
jet because air vortex ring is synthesized by the diaphragm (Smith & Glezer, 1998).
Synthetic jet improves the fluid mixing and provides a better heat transfer in cooling
the microelectronic devices. Synthetic jet consists of few parts such as the air cavity,
diaphragm and orifice as shown in Figure 1.1. In general, a diaphragm is attached with
the air cavity and the orifice is located opposite the diaphragm. The location of the
diaphragm and orifice could be located at anywhere in the air cavity as it fits the
requirement of the synthetic jet design. The synthetic jet is usually driven by electric
system, pneumatic system, magnetic system, mechanical system, and others.
Figure 1.1 Synthetic jet
4
As shown in Figure 1.1, the diaphragm is forced to move upwards. This
movement causes the fluid to be expelled out of the air cavity through the orifices. A
vortex ring is generated at the end of the orifices and expelled out to the targeted
surface. The diaphragm is then forced to move downwards. Once the diaphragm
reaches a maximum displacement, fluid is then ingested from the surrounding area into
the air cavity. This process is continued until the operation is stopped. The periodic
movement of the diaphragm recycles the ambient air and provides a net mass of fluid
that entrains in and expels out from the air cavity, with a value which is equal to zero.
This attractive characteristic increases the application of synthetic jet in the
microelectronic thermal management system. Besides, synthetic jet does not require
any external air/fluid source to drive fluid flow to the targeted area. In this case,
synthetic jets are therefore regarded as an extremely promising alternative to the steady
cooling jet with high heat transfer capability.
1.2 Problem statement
Synthetic jet has been well believed to be a promising cooling scheme in the
thermal management system. This has drawn the attention of the researchers, in a quest
to investigate the cooling performance of the synthetic jet. Synthetic jet mostly is
powered by speaker which produced the high Reynolds number of fluid flow and
located in the impinging direction. Speaker produced high Reynolds number which
has shown the promising synthetic jet performance at the impinging direction. But as
the Reynolds number of the synthetic jet is increases, the power consumption by the
jet is increases. The power consumption can be reduced as the Reynold number
decreases. Reynolds number of fluid flow undoubtedly influence the cooling
5
performance of the synthetic jet. Thus, low Reynolds number and horizontal direction
has drawn the attention in the investigation.
1.3 Research objectives
The research aims to investigate the cooling performance of the synthetic jet. The main
objective is to develop a pneumatic synthetic jet model, then investigate the cooling
performance of the synthetic jet. The study objectives are:
1. To investigate the effect of different low Reynolds number from 0 to 163.2 of
the synthetic jet and power supplied to the heat sink to the cooling capability
of the synthetic jet.
2. To study the cooling performance of the synthetic jet at different distance
between the orifice and the heat sink.
3. To identify the cooling performance of the synthetic jet at two different
arrangement of the experiment (horizontal and vertical arrangement).
4. To validate the cooling performance of the synthetic jet simulation study to the
experimental study.
1.4 Research scope
Synthetic jet has emerged as a cooling scheme in controlling the operating
temperature of microelectronic devices to an acceptable level. In this research, a
pneumatic synthetic jet is developed and modelled the cooling performance of the
synthetic jet is studied at low Reynolds number (ranged from 0 to 163.2). There are
some limitations that have been set in this research. The heat loss is negligible. The
vibration of the synthetic jet system is neglected. The study also assumed ambient
velocity in the test section is 0 m/s and air humidity is constant during the experiment.
6
CHAPTER TWO
LITERATURE REVIEW
2.0 Overview
This chapter reviews the literature on design and the cooling performance of
the synthetic jet. It begins with a brief introduction of the design of the synthetic jet.
Several factors that influence the cooling performance are explained next in the section.
This includes the fluid flow characteristic of synthetic jet, orifice shape, stroke length
of the synthetic jet and distance between the orifice and the targeted surface. Finally,
a short summary about the characteristic of the synthetic jet is discussed.
2.1 Flow characteristic of synthetic jet
The high demands for high and better effective cooling of the microelectronic
devices and the integrated devices leads to the innovation of synthetic jet. Synthetic
jet is capable to manipulate the operating temperature to an optimum level and show
promising features in thermal management for microelectronic cooling. Research on
the synthetic jet as microelectronic cooling devices recently has been a popular topic.
Synthetic jet consists of a diaphragm which is the main actuator of the system. The
fluid flow is produced as the diaphragm move upward and downward periodically.
Thus, the fluid flow plays an important role in developing the synthetic jet.
Mahalingam & Glezer (2004) had reviewed on the design and thermal
characteristic of the synthetic jet. In their findings, the temperature of the high-
powered heat sink reduced by 35℃ (from 71.5 ℃ to 36℃) as shown in Figure 2.1.
This phenomenon eventually showed that synthetic jet is capable to act as the cooling
device. Besides, from the study synthetic jet also transferred more heat rate (around
7
20%-40%) from the heated heat sink to the surrounding as it is compared to
conventional fan. An experiment study on the heat transfer measurements of the
synthetic jet by using the interferometry was carried out by Bhapkar et al. (2014). Their
study focused on the effect of excitation frequency synthetic jet on the heat transfer
rate by using interferometry. The findings showed the good agreement on the result
between the interferometry measurement and the result measured from the
thermocouple. The results also showed that as the exciting frequency of the synthetic
jet increased, the average heat transfer coefficient also increased.
Qayoum et al (2010) had executed an experimental study on the laminar
boundary layer heat transfer enhancement of the synthetic jet on the flat surface. The
result revealed that the average heat transfer coefficient of the synthetic jet increased
during the excitation of the voltage. Besides, the heat transfer coefficient also increased
with the amplitude modulation at low frequencies. Liu et.al. (2015) had proposed an
investigation of the piezoelectric actuator driven synthetic jet. The investigation was
focused on the flow and heat transfer characteristics of the fluid which was produced
from the synthetic jet. The results showed that heat transfer coefficient was highly
depended on the driven frequency. The higher the driven frequency, the higher the heat
transfer coefficient. Besides, it also showed that the synthetic jet is highly
recommended cooling scheme which it was capable to provide 2-8 times heat transfer
rate as it compared to natural convection.
8
Figure 2.1 Rapid cooling capability of synthetic jet ejector heat
(Mahalingam & Glezer, 2004)
The vortex dynamic and heat transfer of the synthetic jet is investigated by
Silva-Llanca & Ortega (2017). It shows that the heat transfer is influenced by the
coalescence of the vortex that is produced by the synthetic jet. The collision between
the heated wall and the secondary vortices leads to the reducing in the heat transfer of
the jet. It also proves that the completion of vortex cycle is directly proportional to the
Reynolds number and inversely proportional to the frequency of the synthetic jet.
Convection instability is investigated by the previous researchers (Leigh et al. 2016).
It shows that the unsteadiness of the flow was due to the boundary layer disturbance
at the end wall plume. There are two modes of the excitation on convection flow which
are the high frequency mode (7500 Hz) and the low frequency mode (4000 Hz). The
study shows that higher frequency mode is achieved when small amplitudes is existed
while the lower frequency mode is achieved when large amplitudes.
9
C.S. Greco et al. (2016) also investigates on the flow field of the single and
twin circular synthetic jet. Twin circular synthetic jet provides higher turbulence level
and axial velocity of the flow with a lower axial velocity phase than the single circular
synthetic jet. A comparison between steady jet and synthetic jets with low aspect ratio
rectangular orifice has been proposed by Van Buren & Amitay (2016) as shown in
Figure 2.2. The findings show that synthetic jet provides a three times higher
momentum than steady jet. Thus, synthetic jet shows a better capability of cooling
performance in term of Reynolds number and Nusselt number.
Figure 2.2 Downstream evolution of the centerline velocity for the steady (white
symbol and synthetic (gray symbols) jets with aspect ratios AR = 6 (circle), 12
(square), and 18 (triangle). (a) velocity scales derieved from the entire actuation
cycle and (b) velocity scales derieved from the blowing portion of the cycle by
(Van Buren & Amitay, 2016)
10
Xia & Zhong (2014) studied an investigation on the enhancement of the
laminar flow mixing of the synthetic jet. The findings showed that the driven frequency
and the stroke length of the synthetic jet affected the performance of the synthetic jet.
The result also showed that at high driven frequency or large stroke length, the well
laminar flow mixing was achieved. An experimental study on the nearby flow field of
the single and twin circular shaped synthetic jet had been proposed by Carlo Salvatore
et al. (2013). Twin circular shaped synthetic jet generated a double vertex ring
structure and it also produced high jet centerline velocity as it is compared to the single
synthetic jet. McGuinn et al. (2013) also had investigated on the flow characteristics
of the synthetic jet. The focus was the effect of the stroke length on the fluid flow of
the synthetic jet. The findings showed a strong agreement that better heat transfer
enhancement can be achieved by varying the stroke length of the synthetic jet as shown
in Figure 2.3. As the stroke length of the synthetic jet increased, the amount of the
fluid drawn in and out of the orifice increased which leads to the increasing in the heat
transfer rate.
Figure 2.3 Flow morphology of vortex formation and evolution at various stroke
(𝑎)𝐿0
𝐷< 4, (𝑏)4 ≤
𝐿0
𝐷< 8, (𝑐)8 ≤
𝐿0
𝐷< 16,
(𝑎)𝐿0
𝐷≥ 16 (McGuinn et al. (2013))
11
Feero et al. (2015) proposed an investigation on the cavity shape of the
synthetic jet. In the investigation, three types of cavity shapes had been tested which
are the cylindrical shape, conical shape and contraction shape. The findings showed a
strong agreement that shapes of the cavity also affected the performance of the
synthetic jet. The cylindrical shape cavity showed a better synthetic jet’s performance
as it is compared to the other two cavities at the largest momentum flux. Besides, an
investigation of the effect of the cavity and orifice on the flow characteristic of the
synthetic jet was performed by Jain et al. (2011). The result showed that the orifices
played an important factor in the flow characteristic of the synthetic jet as shown in
Figures 2.4 and 2.5. As the parameter of the orifices changed, the maximum fluid
velocity can be achieved.
12
Figure 2.4 Velocity vectors (Right) and pressure contours (Left) for different times in
a cycle of operation. Colour map for velocity vector showing magnitude in m/s.
Pressure contour are for gage pressure and unit used is Pa (Jain et al. (2011))
