Experimental Investigations of Different Loudspeakers Applied as Synthetic Jet Actuators

actuators
Communication
Experimental Investigations of Different Loudspeakers Applied
as Synthetic Jet Actuators
Paweł Gil * and Joanna Wilk

 Department of Thermodynamics, Faculty of Mechanical Engineering and Aeronautics, Rzeszow University of
 Technology, 35-959 Rzeszów, Poland; joanwilk@prz.edu.pl
 * Correspondence: gilpawel@prz.edu.pl

 Abstract: The paper presents the preliminary results of the experimental investigation of four various
 loudspeakers used for driving the synthetic jet actuator. The parameters, characteristic synthetic
 jet velocity, pressure inside the cavity, device sound pressure level (SPL), and the heat sink thermal
 resistance, were presented for various input power and driving frequency. The resonance frequency
 was determined based on electrical impedance. The highest synthetic jet momentum velocity was
 achieved at diaphragm resonance frequency. The maximum sound pressure level was observed,
 also at resonant frequency. For the same real power delivered to the actuator and for its resonance
 frequency, the heat sink thermal resistance had the lowest value for the specific loudspeaker. In
 turn, the synthetic jet velocity reached maximum for this actuator. For all actuators tested, the sound
 pressure level was dependent on momentum velocity.

 Keywords: synthetic jet; synthetic jet actuator; heat sink; SPL

 



 


Citation: Gil, P.; Wilk, J. 1. Introduction
Experimental Investigations of Synthetic jet (SJ) actuators are a novel, interesting kind of generators of the fluid
Different Loudspeakers Applied as flow. They have been widely researched since 1990 [1]. SJ actuators generate synthetic
Synthetic Jet Actuators. Actuators
 jets from the fluid, which is moved by loudspeakers [2–4], piezoelectric transducers [5–8],
2021, 10, 224. https://doi.org/
 pistons [9], or plasma [10]. SJ may be useful in heat transfer enhancement processes [11–15],
10.3390/act10090224
 mixing [16], flow control [17,18] or propulsions [19]. Recently, the synthetic jets were
 extensively investigated. A number of literature positions on SJ properties and applica-
Academic Editor: Luigi de Luca
 tions can be found. As exemplary references, the review articles [20,21] are cited. The
 first position [20] includes a brief review of heat transfer enhancement using SJ character-
Received: 25 June 2021
Accepted: 2 September 2021
 ized by different parameters. Similarly, the second article [21] presents the review of SJ
Published: 5 September 2021
 performances and parameters that can impact the fluid flow and heat transfer processes.
 The SJ actuator is a simple apparatus, also known as a zero-net-mass-flux device due
Publisher’s Note: MDPI stays neutral
 to lack of fluid mass addition to the working system. It consists of three basic parts: a
with regard to jurisdictional claims in
 cavity with an orifice, diaphragm, and the element causing the diaphragm deflection. In
published maps and institutional affil- the case of applying a loudspeaker, the synthetic jet is formed when the working fluid is
iations. periodically sucked into and ejected from an orifice in a cavity by the diaphragm vibrations,
 which are caused by a loudspeaker. Thus, the loudspeaker parameter, as well as orifice and
 cavity geometry, will affect the synthetic jet actuator characteristics.
 Heat transfer enhancement with the use of synthetic jet obtains maximum at a specific
Copyright: © 2021 by the authors.
 axial distance between the cooled object and orifice [12–14]. This distance causes the
Licensee MDPI, Basel, Switzerland.
 increase of the total space occupied by the cooling device. A possible solution was presented
This article is an open access article
 in [22], where the heat sink was located in the synthetic jet actuator cavity, which resulted
distributed under the terms and in a great reduction of the device’s space requirement.
conditions of the Creative Commons In the present paper, the authors report the results of the preliminary investigations of
Attribution (CC BY) license (https:// four loudspeakers used as synthetic jet actuators for the heat sink heat transfer enhance-
creativecommons.org/licenses/by/ ment. It is a continuation of the research presented in [22–24], where only one loudspeaker
4.0/). was tested. The synthetic jet velocity, pressure inside the cavity, device sound pressure

Actuators 2021, 10, 224. https://doi.org/10.3390/act10090224 https://www.mdpi.com/journal/actuators

Actuators 2021, 10, 224 2 of 10

 level, and the heat sink thermal resistance were tested. Various input powers and driving
 frequencies of SJA were applied. The basic thermal, flow, and acoustics performances for
 Actuators 2021, 10, x FOR PEER REVIEW
 the SJA with various types of loudspeakers were obtained. 2 of 10

 2. Materials and Methods
 wasIntested.
 the present work, four
 The synthetic types of tested
 jet velocity, loudspeakers
 pressure were applied
 inside the cavity, device as SJ actuators
 sound pressure
 and are depicted in Table 1.
 level, and the heat sink thermal resistance were tested. Various input powers and driving
 frequencies of1.SJA
 Table were applied.
 Specification The basic tested.
 of loudspeakers thermal, flow, and acoustics performances for
 the SJA with various types of loudspeakers were obtained.
 Impedance, BL, SPL, Mass of the
 Case and Type of Loudspeaker
 2. Materials andRMethods
 [Ω] [T.m] [dB] Diaphragm Mms [g]
 L1 W.18.200.8.FGXIn the present 6.1work, four types7.5 89
 of tested loudspeakers were applied 16.5
 as SJ actuators
 L2 W.18.180.8.FCX_v2 5.5 7.1 90 14.6
 and are depicted in Table 1.
 L3 M.18.200.8.MCX 5.7 7.4 92 13.5
 L4 M.18.150.8.MC 5.6 3.9 90 10.8
 Table 1. Specification of loudspeakers tested.

 The photo of the heat sink integratedImpedance,
 with the SJA isBL, presented Mass1ofand
 in Figure the the
 Dia-
 SPL,
 Case and Type of Loudspeaker
 cross section is presented in Figure 2. The enclosure phragm
 R [Ω] of the heat
 [T.m]sink was
 [dB]based on a cylinder
 Mms [g]
 with outer and inner diameter equal to 180 mm and 170 mm, respectively. The height of the
 L1 W.18.200.8.FGX 6.1 7.5 89 16.5
 enclosure was 50 mm. Made of milled aluminum, the enclosure featured 32 fins of 2-mm
 L2 W.18.180.8.FCX_v2 5.5 7.1 90 14.6
 thickness, 16 with a length of 42 mm and 16 with a length 57 mm. A total of 16 orifices
 L3 M.18.200.8.MCX 5.7 7.4 92 13.5
 with the diameter of 10 mm and length of 5 mm were arranged at equal, angular intervals
 L4 M.18.150.8.MC 5.6 3.9 90 10.8
 on the cylindrical surface of the outer ring. The abovementioned, four different models of
 loudspeakers were used to set the air of the room temperature and atmospheric pressure
 The photo of the heat sink integrated with the SJA is presented in Figure 1 and the
 as the working fluid in motion.
 cross section is presented in Figure 2. The enclosure of the heat sink was based on a cylin-
 SJ actuators were supplied with a sinusoidal signal from RIGOL DGG4162 function
 der with outer
 generator andJingdian
 (Puyuan inner diameter equal Co.,
 Technology to 180 mm
 Ltd, and 170China)
 Suzhou, mm, respectively.
 and amplified Thebyheight
 the
 of the enclosure was 50 mm. Made of milled aluminum, the enclosure featured
 AUNA CD-708 audio amplifier. Electric parameters of loudspeakers tested were measured 32 fins of
 2-mm
 with thethickness, 16 with
 use of Keithley 2700a length of 42(22-bit)
 multimeter mm and 16 with Beaverton,
 (Tektronix, a length 57USA).
 mm. AnA total of 16
 accuracy
 orifices with the diameter of 10 mm and length of 5 mm were arranged at
 of measurements of the effective voltage was better than 0.2% of the value measured. The equal, angular
 intervalscurrent
 effective on the cylindrical surfaceindirectly
 was determined of the outer ring.measure
 by the The abovementioned,
 of the voltage four
 dropdifferent
 across
 models of loudspeakers were used to set the air of the room temperature
 the standard resistor 1 Ω (±0.01%). In turn, measurements of the apparent power were and atmospheric
 pressure as
 performed the an
 with working fluid
 accuracy in motion.
 ±0.25% of the set value.

 Figure 1. Heat sink integrated with the enclosure of SJA.
 Figure 1. Heat sink integrated with the enclosure of SJA.

Actuators 2021, 10, 224 3 of 10
 Actuators 2021, 10, x FOR PEER REVIEW 3 of 10

 Figure 2. The cross section of SJA with integrated heat sink, 1, loudspeaker; 2, heat sink base; 3, fin;
 Figure 2. The cross section of SJA with integrated heat sink, 1, loudspeaker; 2, heat sink base; 3, fin;
 4, orifice; 5, cavity.
 4, orifice; 5, cavity.

 SJ actuators
 The were supplied
 ambient temperature with
 in the a sinusoidal
 laboratory roomsignal from RIGOL
 was controlled withDGG4162 function
 an air condition-
 generator
 ing system. (Puyuan
 The atmosphericJingdianpressure
 Technology Co., Ltd, with
 was measured Suzhou, China) andHPB200W2DA-B
 the Honeywell amplified by the
 AUNA CD-708 audio amplifier. Electric parameters of loudspeakers
 (Plymouth MN, USA) barometer with an accuracy of ±40 Pa while the ambient temperature tested were meas-
 ured with the use of Keithley 2700 multimeter (22-bit)
 measurement was performed with the use of the ATU 08 constant current thermometer (Tektronix, Beaverton, USA). An
 with an accuracy of ±0.5 K. During measurements, the ambient temperature was inmeas-
 accuracy of measurements of the effective voltage was better than 0.2% of the value the
 ured. of
 range The effective
 20–23 ◦ C andcurrent was determined
 the barometric pressureindirectly
 was inby thetherange
 measure of the voltage
 of 990–1002 hPa. drop
 For
 across thethermal
 obtaining standard resistor 1 Ω (±0.01%).
 characteristics of the SJAIntested,
 turn, measurements
 the measuringofsystem the apparent
 described power
 in
 were in
 detail performed
 [23,24] was with an accuracy ±0.25% of the set value.
 applied.
 Theheat
 The ambient
 sink basetemperature
 temperaturein theandlaboratory
 ambientroom was controlled
 temperature with an with
 were measured air condi-
 the
 tioning system. The atmospheric pressure was
 K-type thermocouples (AHLBORN, Holzkirchen, Germany) with the reference junctionmeasured with the Honeywell
 HPB200W2DA-B
 temperature (Plymouth
 stabilized by theMN, KayeUSA)170 barometer with an accuracy
 Ice Point Reference. The voltageof ±40 signal
 Pa whilefromthe
 ambient temperature
 thermocouple was measured measurementwith awas performed
 Keithley 2700. withThe the use of of
 accuracy thetheATU 08 constant
 temperature
 current thermometer
 measurement with an to
 was estimated accuracy
 be better of ±0.5
 thanK.±During
 0.07 K measurements,
 in the considered thetemperature
 ambient tem-
 perature
 range [13].wasThe in the range was
 temperature of 20–23 °C and
 measured afterthethe
 barometric pressuresteady
 heat sink reached was in the and
 state, range theof
 990–1002
 number of hPa. For obtaining
 measurements wasthermal
 equal tocharacteristics
 100. of the SJA tested, the measuring sys-
 temThe
 described
 synthetic in detail in [23,24]
 jet velocity waswas applied. with the use of a constant temperature
 measured
 The heat
 anemometry sink base
 method. Thetemperature
 ATU 08 bridge and ambient
 (Kraków, temperature
 Poland), which were measured with the
 features separate
 channels for constant temperature
 K-type thermocouples (AHLBORN, anemometry
 Holzkirchen, measurements
 Germany) with and the channels
 referencefor tem-
 junction
 perature fluctuations
 temperature stabilized measurements
 by the Kaye 170 withIcecold-wire method,
 Point Reference. Thewas applied.
 voltage signalThefrom probe
 ther-
 equipped
 mocouplewith wastwo wires—for
 measured withthe fluid velocity
 a Keithley 2700. The andaccuracy
 fluid temperature measuring—was
 of the temperature measure-
 used.
 mentThewasmeasuring
 estimatedsystem was calibrated
 to be better than ±0.07inKthe in velocity range oftemperature
 the considered 0.3–49 m/s. range The accu-[13].
 racy of measurements was ± 0.1 m/s in the range up to 2.6 m/s
 The temperature was measured after the heat sink reached steady state, and the number and ± 2% of the measured
 value in the range of
 of measurements was 2.6–49
 equalm/s. The velocity was measured in the isothermal condition
 to 100.
 (the heater was switched
 The synthetic off). The
 jet velocity wasNational
 measured Instruments
 with the use NI-USB-6211
 of a constant card was used ane-
 temperature for
 the acquisition
 mometry method.of the Theconstant
 ATU 08temperature
 bridge (Kraków, anemometry
 Poland), bridge output signal.
 which features separate The more
 channels
 detailed description
 for constant of the SJ
 temperature velocity measurements
 anemometry measurements is included
 and the in [23]. for temperature
 channels
 fluctuations measurements with cold-wire method, was applied. The816-1
 The sound pressure level (SPL) was measured with the use of Testo probe (Lenzkirch,
 equipped
 Germany) sound level meter. The Testo 816-1 complies
 with two wires—for the fluid velocity and fluid temperature measuring—was used. with the requirements of IECThe
 61672-1
 measuring system was calibrated in the velocity range of 0.3–49 m/s. The accuracyofof
 Class 2 standard. Its measuring range is 30–130 dB in a frequency range
 20–8000 Hz. Before
 measurements wasmeasurements,
 ±0.1 m/s in the the range sound
 up tolevel meter
 2.6 m/s and was±2%calibrated with thevalue
 of the measured use ofin
 athe
 Testo 0554
 range of 0452
 2.6–49calibrator. The National
 m/s. The velocity Instruments
 was measured in theNI-USB-6211 card was(the
 isothermal condition used to
 heater
 record the output signal of the meter. The sound level meter
 was switched off). The National Instruments NI-USB-6211 card was used for the acquisi-was located at a distance of
 1 tion
 m from
 of thetheconstant
 synthetic jet actuator.
 temperature The measurements
 anemometry were performed
 bridge output signal. Theaccording
 more detailed to ISOde-
 3746:2010 standard.
 scription of the SJ velocity measurements is included in [23].
 The
 Thechanges of the pressure
 sound pressure insidewas
 level (SPL) themeasured
 SJA cavitywith werethe recorded using816-1
 use of Testo the GRAS 40PP
 (Lenzkirch,
 (Holte, Denmark) microphone with the National Instruments NI 9250 card. Its measuring
 Germany) sound level meter. The Testo 816-1 complies with the requirements of IEC
 range is 10–20,000 Hz and 33–128 dB. The typical uncertainty is presented in the Table 2.
 61672-1 Class 2 standard. Its measuring range is 30–130 dB in a frequency range of 20–
 8000 Hz. Before measurements, the sound level meter was calibrated with the use of a

Testo 0554 0452 calibrator. The National Instruments NI-USB-6211 card was used to rec-
 ord the output signal of the meter. The sound level meter was located at a distance of 1 m
 from the synthetic jet actuator. The measurements were performed according to ISO
 3746:2010 standard.
Actuators 2021, 10, 224 4 of 10
 The changes of the pressure inside the SJA cavity were recorded using the GRAS
 40PP (Holte, Denmark) microphone with the National Instruments NI 9250 card. Its meas-
 uring range is 10–20,000 Hz and 33–128 dB. The typical uncertainty is presented in the
 Table 2. Table 2. Typical uncertainty.
 Table 2. Typical uncertainty.
 Name Relative Uncertainty Absolute Uncertainty
 P (apparent power) ±0.55% -
 Name Relative Uncertainty Absolute Uncertainty
 U0 (momentum
 P (apparent power) velocity) ±0.55% ±5.0% - -
 ∆T (temperature
 U0 (momentum velocity) difference) ±5.0% - - ±0.25 K
 ΔT (temperature difference)
 R(thermal resistance) - ±3.4% ±0.25 K -
 R(thermal resistance) ±3.4% -
 Q(thermal power) ±1.5% -
 Q(thermal power) ±1.5% -

 The schemeThe scheme
 of the of the experimental
 experimental set-up isin
 set-up is presented presented
 Figure 3.in Figure 3.

 Figure 3. The schemeFigure
 of the 3.
 experimental
 The schemeset-up.
 of the experimental set-up.

 3. Results 3. Results
 3.1. Data Reduction
 3.1. Data Reduction
 The characteristic
 The characteristic parameterparameter of the synthetic
 of the synthetic jet, the synthetic
 jet, the synthetic jet velocity,
 jet velocity, was ob- was obtained
 as the centerline momentum velocity, according to the formula
 tained as the centerline momentum velocity, according to the formula [25] [25]
 s
 1 1 t
 Z
 = U0 = d (uC )2 dτ (1) (1)
 t 0

 where t is anwhere
 oscillation
 t is anperiod of theperiod
 oscillation loudspeaker
 of the diaphragm,
 loudspeakerτ diaphragm,
 is time, and uτc is time,
 the cen-
 and uc is the
 terline velocity at the orifice
 centerline exit.
 velocity at the orifice exit.
 Resonance Resonance
 frequenciesfrequencies
 were received
 were from thefrom
 received impedance vs. frequency
 the impedance depend-
 vs. frequency dependences,
 ences, where the electrical
 where impedance
 the electrical is defined
 impedance as as
 is defined
 E
 = Z= (2) (2)
 I
 where E and I are the effective voltage and effective current, respectively.
 where E and I are the effective voltage and effective current, respectively.
 In turn, the apparent power supplied the SJA is given by
 In turn, the apparent power supplied the SJA is given by

 P = E· I (3)

 The heat sink thermal resistance was calculated according to the equation

 ∆T
 R= (4)
 Q

 = ∙ (3)
 The heat sink thermal resistance was calculated according to the equation
Actuators 2021, 10, 224 5 of 10
 ∆ 
 = (4)
 
 where ∆T is a difference of the heat sink base temperature and the ambient temperature
 whereQ∆T
 while is a thermal
 is the difference of the
 power heat sinkby
 dissipated base
 thetemperature and the
 heat sink. It was ambient
 obtained temperature
 according to the
 while Q is the thermal power dissipated by the heat sink. It was obtained according
 energy balance of the SJA with integrated heat sink detail, discussed in [23,24]. to the
 energy balance of the SJA with integrated heat sink detail, discussed in [23,24].
 3.2. Results of Measurements
 3.2. Results of Measurements
 As the first result of the measurements of the parameters characterizing the operation
 As the first result of the measurements of the parameters characterizing the operation
 of the SJA with different loudspeakers, the dependence of the impedance vs. the frequency
 of the SJA with different loudspeakers, the dependence of the impedance vs. the frequency
 was obtained.
 was obtained. TheThesynthetic
 syntheticjetjetactuator impedance
 actuator as as
 impedance a function of the
 a function frequency
 of the is pre-
 frequency is
 sented in Figure 4.
 presented in Figure 4.

 28
 Case L1
 24 Case L2
 Case L3
 20 Case L4
 16
 Z [Ω]

 12

 8

 4

 0
 0 20 40 60 80 100 120 140 160
 f [Hz]

 Figure 4.
 Figure 4. SJA impedance as aa function
 function of
 of frequency,
 frequency,PP== 4.0
 4.0 W.
 W.

 From
 From the
 thecurves
 curvespresented
 presented above, thethe
 above, resonance
 resonance frequencies characterizing
 frequencies loudspeak-
 characterizing loud-
 ers tested tested
 speakers were obtained. The results
 were obtained. The included in Figurein4 Figure
 results included concern4 the case of
 concern thecase
 the apparent
 of the
 power
 apparent equal to 4 equal
 power W. Asto concluded in [25], the in
 4 W. As concluded resonance
 [25], the frequency
 resonancedid not depend
 frequency on the
 did not de-
 actuator supply voltage, and the same may be determined at one value
 pend on the actuator supply voltage, and the same may be determined at one value of of apparent power.
 The resonance
 apparent frequencies
 power. obtained
 The resonance were: 27obtained
 frequencies Hz for loudspeaker
 were: 27 HzL1, for43 Hz for L2, 63
 loudspeaker L1,Hz43
 for L3, and 42 Hz for L4.
 Hz for L2, 63 Hz for L3, and 42 Hz for L4.
 The
 The changes
 changes ofof the
 the pressure
 pressure inside
 inside the
 the SJA cavity were
 SJA cavity were the
 the next
 next measurement
 measurement results.
 results.
 Figure
 Figure 55 presents
Actuators 2021, 10, x FOR PEER REVIEW presents effective
 effective values
 values ofof the
 the pressure
 pressure in in function
 function of
 of the
 the signal
 signal frequency
 frequency at at the
 6 ofthe
 10
 constant apparent power equal to 1 W. The local maximum of the pressure
 constant apparent power equal to 1 W. The local maximum of the pressure tested occurred tested occurred
 at
 at the
 theresonance
 resonancefrequencies.
 frequencies.TheThehighest pressure
 highest pressure value characterized
 value characterized the the
 loudspeaker
 loudspeaker L2.
 L2.
 80
 The next measurements performed determined
 Case L1 the synthetic jet centerline momen-
 tum70velocity. The results are presented Case L2
 in Figure 6 in the form of dependence of SJ center-
 line60
 momentum velocity on the frequency Casefor
 L3 the loudspeakers considered at the constant
 apparent Case L4
 power 4 W. As can be seen, the maximum of the velocity occurred at the reso-
 50
 nance frequencies and took values equal to about 8.8 m/s for loudspeakers L1, L3, and L4,
 prms [Pa]

 40
 and about 12.3 m/s for the L2. The synthetic jet characteristic velocity for the synthetic jet
 30
 actuator with integrated heat sink and for various numbers of orifices and orifice diame-
 ters20
 was presented in [24].
 10
 0
 0 20 40 60 80 100 120 140 160
 f [Hz]

 Figure 5. Effective
 Figure 5. Effective pressure in the
 pressure in the SJA
 SJA cavity
 cavity as
 as aa function
 function of
 of frequency,
 frequency,PP== 1.0
 1.0 W.
 W.

 14The next measurements performed determined the synthetic jet centerline momentum
 Case L16 in the form of dependence of SJ centerline
 velocity. The results are presented in Figure
 12
 momentum velocity on the frequency Case L2 loudspeakers considered at the constant
 for the
 Case L3
 10 Case L4
 8
 [m/s]

 6
 0

60 Case L3
 Case L4
 50

 prms [Pa]
 40
Actuators 2021, 10, 224 30 6 of 10

 20
 10
 apparent
 0 power 4 W. As can be seen, the maximum of the velocity occurred at the resonance
 frequencies
 0 and
 20 took
 40 values
 60 80equal 100to 120
 about 8.8 160
 140 m/s for loudspeakers L1, L3, and L4, and
 about 12.3 m/s for the L2.f [Hz] The synthetic jet characteristic velocity for the synthetic jet
 actuator with integrated heat sink and for various numbers of orifices and orifice diameters
 Figure 5. Effective pressure in the SJA cavity as a function of frequency, P = 1.0 W.
 was presented in [24].

 14
 Case L1
 12 Case L2
 Case L3
 10 Case L4
 8
 U0 [m/s]

 6

 4

 2

 0
 0 20 40 60 80 100 120 140 160
 f [Hz]

 Figure 6.
 Figure 6. SJ
 SJ jet
 jet centerline
 centerline momentum
 momentum velocity
 velocity as
 as aa function
 function of
 of frequency,
 frequency,PP==4.0
 4.0W.
 W.

 The
 Theobtained
 obtainedresults
 resultspresented
 presentedininFigures
 Figures 4–6 show
 4–6 showthat thethe
 that bestbest
 SJ actuator parameters,
 SJ actuator parame-
 impedance, effective
 ters, impedance, pressure
 effective in theincavity,
 pressure and SJand
 the cavity, centerline momentum
 SJ centerline velocity
 momentum were
 velocity
 achieved for the resonance frequency in the case of L2 loudspeaker
 were achieved for the resonance frequency in the case of L2 loudspeaker tested. tested.
 The
 The next parameter measured
 next parameter measuredduring
 duringthe theinvestigations
 investigationsofofdifferent
 different loudspeakers,
 loudspeakers, SJ
 SJ actuators, was the sound pressure level. Figure 7 includes the
 actuators, was the sound pressure level. Figure 7 includes the dependence of SPL dependence of SPL
 on on
 the
 the frequency
 frequency at the
 at the constant
 constant apparent
 apparent power power
 4.0 W4.0for
 W the
 for four
 the four loudspeakers
 loudspeakers tested.
 tested. The
 The obtained
 obtained results
 results showed
 showed thatSPL
 that the thegenerated
 SPL generated
 by theby SJ the SJ actuators
 actuators dependeddepended on
 on the fre-
 Actuators 2021, 10, x FOR PEER REVIEW
 the frequency [26], and the maximum of SPL occurred for the resonance frequency.7 ofA
 10
 quency [26], and the maximum of SPL occurred for the resonance frequency. A further
 further
 increaseincrease in frequency
 in frequency causedcaused a slight
 a slight decrease
 decrease in SPL.
 in SPL. For fFor f >Hz,
 > 60 60 Hz,
 the the
 SPLSPL values
 values re-
 remained in the range of 55–65 dB for all loudspeakers
 mained in the range of 55–65 dB for all loudspeakers considered. considered.
 80
 70
 60
 50
 SPL [dB]

 40
 30
 Case L1
 20 Case L2
 Case L3
 10
 Case L4
 0
 0 20 40 60 80 100 120 140 160
 f [Hz]

 Figure7.7.Sound
 Figure Soundpressure
 pressurelevel
 levelgenerated
 generatedatatPP==4.0
 4.0W.
 W.

 4.4.Discussion
 Discussion
 The
 Theexperimental
 experimental results presented in in the
 the above
 abovesection
 sectionwere
 weredeveloped
 developedininterms
 termsof
 of their
 their maximum
 maximum values
 values received
 received at resonance
 at resonance frequencies.
 frequencies. Asfirst
 As the theresult,
 first result, the SJ
 the SJ center-
 centerline momentum
 line momentum velocity
 velocity in function
 in function of apparent
 of the the apparent power
 power waswas obtained.
 obtained. Figure
 Figure 8
 8 in-
 includes thisdependence.
 cludes this dependence.As Ascan
 can be
 be seen,
 seen, characteristics
 characteristics of
 of L1,
 L1, L3,
 L3, and
 andL4L4loudspeakers
 loudspeakers
 coincided,
 coincided,while,
 while,for
 forthe
 thecase
 caseof
 ofL2,
 L2,the
 theSJSJcenterline
 centerlinemomentum
 momentumvelocity
 velocitywas
 wasmuch
 muchhigher
 higher
 than for the other cases.
 than for the other cases.

 16
 14
 12

The experimental results presented in the above section were developed in terms of
 their maximum values received at resonance frequencies. As the first result, the SJ center-
 line momentum velocity in function of the apparent power was obtained. Figure 8 in-
 cludes this dependence. As can be seen, characteristics of L1, L3, and L4 loudspeakers
Actuators 2021, 10, 224 7 of 10
 coincided, while, for the case of L2, the SJ centerline momentum velocity was much higher
 than for the other cases.

 16
 14
 12
 10

 U0 [m/s]
 8
 6
 Case L1
 4 Case L2
 Case L3
 2
 Case L4
 0
 0 1 2 3 4 5 6 7 8
 P [W]

 Figure 8. SJSJcenterline
 centerlinemomentum
 momentumvelocities
 velocitiesat at
 individual resonance
 individual frequency
 resonance in function
 frequency of ap-
 in function of
 parent power.
 apparent power.

 Actuators 2021, 10, x FOR PEER REVIEW The next stage in the investigations was to find the dependence pressure
 dependence of the sound pressure 8 of 10
 level on
 on the
 theapparent
 apparentpower
 powerandandthe centerline
 the momentum
 centerline momentum velocity under
 velocity resonance
 under resonancefre-
 frequency condition
 quency condition forfor
 thethe actuators
 actuators tested.
 tested. TheThe results
 results areare presented
 presented in in Figure
 Figure 9. 9.

 80 80
 75 75
 70 70
 65 65
 60 60
 SPL [dB]

 SPL [dB]

 55 55
 50 50
 45 Case L1 45 Case L1
 Case L2 Case L2
 40 40
 Case L3 Case L3
 35 Case L4 35 Case L4
 30 30
 0 1 2 3 4 5 6 7 8 0 2 4 6 8 10 12 14 16 18
 P [W] U0 [m/s]
 (a) (b)
 Figure9.9.Sound
 Figure Soundpressure
 pressurelevel
 levelof
 ofthe
 theactuators
 actuatorsunder
 underresonance
 resonancefrequency
 frequencycondition
 conditionin
 infunction
 functionof:
 of:(a)
 (a)apparent
 apparentpower;
 power;
 (b) centerline momentum velocity.
 (b) centerline momentum velocity.

 Measurement data
 Measurement data included
 included in in Figure
 Figure 9a 9a represent
 represent thethe same
 same trend
 trend asas in
 inthe
 thecases
 cases
 investigatedinin[2,27]—the
 investigated [2,27]—the increase
 increase in SPL
 in SPL with with
 the Pthe P increasing.
 increasing. AboveAbove
 P = 3.0PW = 3.0
 the W the
 value
 value
 of SPL offorSPL
 L1, for
 L2, L1,
 andL2,
 L4and L4 actuators
 actuators fluctuated
 fluctuated around around
 the valuethe65value
 dB. In65the
 dB.case
 In the case
 of L3, theof
 L3, the SPL was lower. In turn, the graph in Figure 9b clearly shows the
 SPL was lower. In turn, the graph in Figure 9b clearly shows the increase in sound pressure increase in sound
 pressure
 level withlevel with the characteristic
 the characteristic velocity increasing.
 velocity increasing.
 As the
 As the last
 lastresult
 resultofofthe
 theconsiderations,
 considerations,the thethermal
 thermalcharacteristics
 characteristics ofofthe
 theactuators
 actuators
 integratedwith
 integrated withthe theheat
 heatsink
 sink were
 were determined.
 determined. TheThe
 heat heat performances
 performances are presented
 are presented in thein
 the form
 form ofheat
 of the the heat
 sink sink thermal
 thermal resistance
 resistance in function
 in function of theofapparent
 the apparent
 powerpower
 at theat the reso-
 resonance
 nance frequency
 frequency (Figure (Figure
 10) and the10) heat
 and sink
 the heat sink temperature
 temperature difference difference
 in functionin offunction of the
 the dissipated
 thermal
 dissipatedpower (Figure
 thermal power11).(Figure
 The heat11). sink was sink
 The heat characterized by the thermal
 was characterized resistance
 by the thermal re-
 equal to 1.4
 sistance K/W
 equal [23].
 to 1.4 K/WThus,[23].inThus,
 the case of the
 in the caseloudspeaker L2, the thermal
 of the loudspeaker resistance
 L2, the thermal re-
 was reduced
 sistance was to 0.17 K/W
 reduced at the
 to 0.17 K/W at theP power
 power = 7 W, Pwhich
 = 7 W, gave
 whichthe gave
 8.2 times better
 the 8.2 times cooling
 better
 compared to free convection
 cooling compared characterizing
 to free convection the heat sink
 characterizing operation
 the heat without the
 sink operation SJ actuator.
 without the SJ
 The L1, L3,The
 actuator. andL1,
 L4L3,loudspeakers achieved the
 and L4 loudspeakers comparable
 achieved value of thermal
 the comparable value of resistance
 thermal of re-
 about 0.2 K/W at P = 7 W. In summary, the actuator L2 allowed,
 sistance of about 0.2 K/W at P = 7 W. In summary, the actuator L2 allowed, at the same at the same apparent
 power,
 apparent thepower,
 reduction in thermalinresistance
 the reduction of about of
 thermal resistance 18%.
 aboutPresented in Figure
 18%. Presented 11 is the
 in Figure 11
 is the dependence of ∆T on thermal power dissipated, showing the similar tendency,
 which was an obvious consequence of the previous relationship. Characteristics of actua-
 tors L1, L3, and L4 were similar, while for L2 there was a significant difference. Under
 maximum ∆T equal to 60 K, the thermal power dissipated by the heat sink cooled with
 synthetic jets was about 275 W for L1, L3, and L4 actuators applied. The value of Q

dissipated thermal power (Figure 11). The heat sink was characterized by the thermal re-
 sistance equal to 1.4 K/W [23]. Thus, in the case of the loudspeaker L2, the thermal re-
 sistance was reduced to 0.17 K/W at the power P = 7 W, which gave the 8.2 times better
 cooling compared to free convection characterizing the heat sink operation without the SJ
Actuators 2021, 10, 224 actuator. The L1, L3, and L4 loudspeakers achieved the comparable value of thermal 8 of re-
 10
 sistance of about 0.2 K/W at P = 7 W. In summary, the actuator L2 allowed, at the same
 apparent power, the reduction in thermal resistance of about 18%. Presented in Figure 11
 is the dependence of ∆T on thermal power dissipated, showing the similar tendency,
 dependence
 which was an ∆T on thermal
 of obvious power of
 consequence dissipated, showing
 the previous the similar
 relationship. tendency, which
 Characteristics was
 of actua-
 an obvious consequence of the previous relationship. Characteristics of actuators
 tors L1, L3, and L4 were similar, while for L2 there was a significant difference. Under L1, L3,
 and L4 were similar, while for L2 there was a significant difference. Under maximum
 maximum ∆T equal to 60 K, the thermal power dissipated by the heat sink cooled with ∆T
 equal to 60 K, the thermal power dissipated by the heat sink cooled with synthetic jets was
 synthetic jets was about 275 W for L1, L3, and L4 actuators applied. The value of Q
 about 275 W for L1, L3, and L4 actuators applied. The value of Q achieved 330 W in the
 achieved 330 W in the case of the L2 loudspeaker tested.
 case of the L2 loudspeaker tested.
 0.6
 Case L1
 0.5 Case L2
 Case L3
 0.4 Case L4
 R [K/W]

 0.3

 0.2

 0.1
Actuators 2021, 10, x FOR PEER REVIEW 9 of 10
 0
 0 1 2 3 4 5 6 7 8
 P [W]
 Figure 10. Heat sink thermal resistances as a function of power delivered to the actuator at individ-
 Figure 10. Heat sink thermal resistances as a function of power delivered to the actuator at individual
 ual resonance frequency.
 resonance frequency.

 70

 60

 50
 ΔT [K]

 40

 30
 Case L1
 20
 Case L2
 10 Case L3
 Case L4
 0
 0 50 100 150 200 250 300 350
 Q [W]

 Figure 11. Heat sink temperature difference as a function of dissipated thermal power,
 power, PP == 7.0 W, at
 individual resonance frequency.
 frequency.

 The performed preliminary investigations of four loudspeakers used as synthetic jet
 actuators showed the impact of the loudspeaker type on the basic SJ actuators’ character-
 The flow and acoustic performances,
 istics. The performances, synthetic
 synthetic jetjet centerline
 centerline momentum
 momentum velocity,
 pressure inside the cavity, and device sound pressure level, were investigated.
 pressure inside the cavity, and device sound pressure level, were investigated. Additionally,
 Addition-
 thermal characteristics of the heat sink integrated with the SJ actuators tested were
 ally, thermal characteristics of the heat sink integrated with the SJ actuators tested obtained.
 were
 The presented results will be useful for synthetic jet actuator optimization, especially
 obtained. The presented results will be useful for synthetic jet actuator optimization, from
 es-
 the pointfrom
 pecially of energy conversion.
 the point of energy conversion.

 5. Conclusions
 5. Conclusions
 The different loudspeakers resulted in the different synthetic jet actuator performances.
 The different loudspeakers resulted in the different synthetic jet actuator perfor-
 From the energetic point of view, the best actuator is characterized by high energy conver-
 mances. From the energetic point of view, the best actuator is characterized by high energy
 sion; thus, the same input power generated high cavity pressure and high characteristic
 conversion; thus, the same input power generated high cavity pressure and high charac-
 velocity. High synthetic jet velocities corresponded with enhanced heat transfer process
 teristic velocity. High synthetic jet velocities corresponded with enhanced heat transfer
 from the heat sink.
 process from the heat sink.

 Author Contributions: Conceptualization, P.G. and J.W.; methodology, P.G. and J.W.; software,
 validation, formal analysis, P.G. and J.W.; investigation, P.G.; resources, J.W. and P.G.; writing—
 original draft preparation, J.W.; writing—review and editing, J.W.; visualization, P.G. and J.W.; su-
 pervision J.W.; project administration, funding acquisition, P.G. All authors have read and agreed
 to the published version of the manuscript.

Actuators 2021, 10, 224 9 of 10

 Author Contributions: Conceptualization, P.G. and J.W.; methodology, P.G. and J.W.; software,
 validation, formal analysis, P.G. and J.W.; investigation, P.G.; resources, J.W. and P.G.; writing—
 original draft preparation, J.W.; writing—review and editing, J.W.; visualization, P.G. and J.W.;
 supervision J.W.; project administration, funding acquisition, P.G. Both authors have read and agreed
 to the published version of the manuscript.
 Funding: This work was supported by the National Center for Research and Development, Poland
 Grant No.: LIDER/6/0024/L-10/18/NCBR/2019.
 Institutional Review Board Statement: Not applicable.
 Informed Consent Statement: Not applicable.
 Data Availability Statement: Some or all data generated or used during the study are available from
 the corresponding author by request.
 Conflicts of Interest: The authors declare no conflict of interest.

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