International Journal of Heat and Mass Transfer 54 (2011) 2056–2065

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International Journal of Heat and Mass Transfer

journal homepage: www.elsevier .com/locate / i jhmt

Multiple orifice synthetic jet for improvement in impingement heat transfer

Mangesh Chaudhari 1, Bhalchandra Puranik, Amit Agrawal ⇑Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400 076, India

a r t i c l e i n f o

Article history:Received 3 February 2010Received in revised form 13 November 2010Accepted 27 November 2010Available online 27 January 2011

Keywords:Electronic coolingTurbulent synthetic jetNusselt numberHot-wire anemometry

0017-9310/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.ijheatmasstransfer.2010.12.023

⇑ Corresponding author. Tel.: +91 22 2576 7516; faE-mail address: amit.agrawal@iitb.ac.in (A. Agraw

1 Present address: Department of Mechanical Enginof Technology, Pune.

a b s t r a c t

Synthetic jet is potentially useful for cooling of electronic components and its utility has been investi-gated in previous studies. Synthetic jet will become further attractive if additional cooling can beobtained without a corresponding increase in the input power. In this context, we explore the use of mul-tiple orifice single-cavity synthetic jet employed in direct impingement mode of cooling. Experiments areconducted for different configurations with a center orifice surrounded by multiple satellite orifices. TheReynolds number is in the range of 1000–2600 while the normalized axial distance is varied in the rangeof 1–30 in this study. The maximum heat transfer coefficient with multiple orifice synthetic jet is approx-imately 12 times that of the natural heat transfer coefficient and up to 30% more as compared to thatobtained with a conventional single orifice jet. Interestingly, the average Nusselt number gets maximizedat two axial distances-the two peaks can be of comparable magnitude. The appearance, location and mag-nitude of the two peaks depend on the number of satellite orifices and the pitch circle radius on which thesatellite holes lie. It is proposed that a transition in flow behavior from multiple-jet to a combined-jetoccurs, which leads to the appearance of this additional peak. The additional peak (at the smaller axialdistance) can be utilized in the design of cooling solutions for compact devices. The input power reducesslightly in the multi-orifice case with respect to the conventional design. The average velocity at the sur-face is also obtained with the help of hot-wire anemometry. The use of multiple orifice synthetic jet doesnot appear to have been explored earlier and the results are expected to be useful in several practicalapplications.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Due to high level of miniaturization, the power density is largeand increasing for a good number of electronics devices. Thermaloverstressing is one of the major causes for failure of electronic de-vices. A major challenge for thermal engineers is to provide inno-vative cooling solutions for such high heat flux components.Generally heat sinks with different fin geometries and fan arraysare used for heat removal with air as the working fluid. The heatsink provides a large surface area, and air has the advantages ofeasy availability and low cost. Use of ‘synthetic jet’ to drive theair is a relatively new approach for electronics cooling, and its use-fulness has been explored in previous studies [1–13]. A syntheticjet is formed when fluid is alternately sucked into and ejected froma small cavity by the motion of a diaphragm bounding the cavity,so that there is no net mass addition to the system. The jet is there-fore synthesized directly from the fluid in which the system isembedded. Due to the pulsating nature of the flow, the entrain-ment of ambient fluid into the main jet is higher in synthetic jet,

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x: +91 22 2572 6875.al).eering, Vishwakarma Institute

as compared to that in a continuous jet, which leads to effectivecooling.

Although the potential of synthetic jet for cooling applicationwas realized in the last decade itself, only a few studies have per-formed detailed heat transfer measurements involving syntheticjet. Pavlova and Amitay [1] experimentally investigated the effi-ciency and mechanism of cooling a constant heat flux surface byan impinging synthetic jet. In their measurements, high frequency(1200 Hz) jet was found to be more effective at smaller axial dis-tances and the low frequency (420 Hz) jet at larger axial distances.A comparison with continuous jet was also presented. Garg et al.[2] designed a meso-scale synthetic jet to provide a maximumvelocity of 90 m/s from a 0.85 mm hydraulic diameter rectangularorifice. Microscopic infrared thermal imaging technique was usedfor temperature measurements on a foil heater. A maximum heattransfer enhancement of approximately 10 times the natural con-vection was measured for 90 Vrms. Mahalingam and Glezer [3] dis-cussed the design and thermal performance of a synthetic air-jetbased heat sink for high power dissipation electronics. Approxi-mately 40% more heat dissipation occurred with synthetic jetbased heat sinks as compared to steady flow from a ducted fan.The average heat transfer coefficient in the channel flow betweenthe fins was 2.5 times that of steady flow in the duct at the sameReynolds number. Utturkar et al. [4] also reported the thermal

M. Chaudhari et al. / International Journal of Heat and Mass Transfer 54 (2011) 2056–2065 2057

characteristics of synthetic jet integrated with a heat sink. Gillespieet al. [5] studied the effect of rectangular synthetic jet on local con-vective heat transfer from a flat plate. Substantial enhancement inheat transfer was observed due to the strong mixing characteristicsof synthetic jet. Particle image velocimetry (PIV) was employed forvelocity measurements. Zhang and Tan [6] experimentally studiedthe flow and heat transfer characteristics of synthetic jet, using pie-zoelectric actuator with rectangular shape of the orifice. The flowmeasurements were performed using hot-wire anemometry andPIV techniques while an infrared camera was used for temperaturemeasurements. It was noticed that the synthetic jet spreads rapidlyalong the minor axis of the orifice, while along the major axis thejet initially contracts before spreading slowly. The cooling regionwas observed to be wider with a synthetic jet than with a contin-uous steady jet, as deduced from the local temperature distribu-tion. Arik [7] studied the heat transfer and acoustic aspects of arelatively small size synthetic jet. The heat transfer rates were pro-vided as a function of jet location, heater power, and driving volt-age and frequency. It was noted that the jet noise can be as large at73 dB, but can be reduced to less than 30 dB with certain noiseabatement techniques. Arik also studied [8] both the local and glo-bal heat transfer coefficients on a flat surface, with high-frequencyimpinging synthetic jet. The experiments were conducted at a res-onance frequency of 4500 Hz, and different voltage inputs. Heattransfer enhancement was found to be 4–10 times of natural con-vection. Mongia et al. [9] and Chaudhari et al. [10] studied syn-thetic jet placed in a duct. Through a detailed PIV study, theformer authors found that the flow velocity is reduced in certainregions and gets enhanced in some other region. The heat transfercoefficient, respectively decreases and increases in these regions.They also found that the cross flow without direct impinging flowhas a smaller heat transfer enhancement, as the fluid is flowingparallel to the surface of the heated copper plate. Chaudhariet al. [14] experimentally and Jain et al. [15] numerically investi-gated different shapes of the cavity in an effort to maximize theflow from the orifice. Optimum cavity size and shape can lead toenhanced heat transfer.

Our experience has also revealed that a substantial amount ofheat removal is attainable with synthetic jet, especially with thejet impinging directly on the heated surface [10–13]. For exampleFig. 1 suggests that the forced convection heat transfer coefficient

z

h avg

0 50 100 150 200 250 3000

20

40

60

80

100

120

140

160

d = 14d = 8d = 5

(W/m

2 -K)

(mm)

Fig. 1. Variation of average heat transfer coefficient with axial distance for differentorifice diameter (in mm) [12].

is about 11 times higher than the natural heat transfer coefficientat 4 Vrms [12]. Due to the use of acoustic speaker as the actuator in-stead of a piezoelectric actuator commonly employed in previousstudies, a high performance has been achieved at a comparativelylow input voltage in these experiments [14]. Chaudhari et al. [12]performed a direct comparison of synthetic and continuous jets,at the same Reynolds number (=4000) and boundary conditions,and found comparable performance for z/d P 5. The synthetic jetis expected to out-perform the continuous jet for Reynolds numbergreater than 4000 because of a higher exponent of Nusselt numberwith respect to the Reynolds number for the former jet. The valueof this exponent is 1.25 for synthetic jet as compared to 0.638 for acontinuous jet [16]; Katti and Prabhu [17] reported a value of 0.5and 0.663 in stagnation and transition regimes, respectively, forcontinuous jet.

It is however noticed from Fig. 1 that the maximum amount ofheat transfer takes place when the synthetic jet is approximately50 mm away from the heated surface [12]. The shape of the orificehas only a moderate effect on changing this distance [13]. This dis-tance is particularly relevant in the context of designing coolingsolutions for compact devices, where space is at a premium. Alter-nate configurations therefore have to be found, with the flexibilityof changing the design according to the requirement. The specificobjectives of the present study are to design a system such that:

(a) The heat transfer coefficient increases beyond that obtainedfrom the conventional design (such as the one presented inFig. 1).

(b) The optimal distance (distance corresponding to the maxi-mum value of heat transfer coefficient) reduces.

(c) The above objectives should be met without a correspondingincrease in the input power.

In the present study, an attractive alternative of using animpinging synthetic jet formed from a single cavity but with multi-ple orifices is explored. The case of multiple orifices has been inad-equately considered in the literature; our approach is thereforesufficiently original and intuitively capable of meeting objectives(a) and (c) given earlier. Depending upon the size and location ofthe orifices, the relative amount of fluid sucked into and ejectedfrom the orifices would vary. The dynamics of the flow is thereforeexpected to be substantially different between single and multipleorifice cases; a detailed study of the heat transfer characteristics ofimpinging synthetic jet from multiple orifices forms the subject ofthe current investigation. The heat transfer measurements for dif-ferent configurations of center orifice and satellite orifices havebeen performed in the present work. The effect of satellite orificeswith and without center orifice on the average Nusselt number isalso studied. The heat transfer results are supplemented by veloc-ity measurements close to the orifice and near the wall. The inputpower is another relevant parameter and has been measured aspart of this work.

2. Experimental set-up

Fig. 2 shows the schematic of the set-up used for the presentexperiments. The Reynolds number is calculated using the proce-dure given by Smith and Glezer [18]. The constructional detailsof the experimental set-up, the measurement techniques em-ployed, the heat loss estimation and the validation of the set-up,and the data reduction procedure are described in detail elsewhere[10–14]. The primary difference with respect to our earlier set-upis that the top plate has been replaced with that having a largernumber of orifices; see Fig. 2 and Table 1. The present experimentsare conducted for multiple synthetic jets that are formed using asingle cavity and a single actuator. The input voltage to the

Copper Plate Heater

Bakellite

Glass-wool

Speaker with cavity

2-d Traverse Stand

Perpex

line 0o

Satellite orifice

z

line 45o

Pitch circlediameter

d

H

t

L

R

Center orifice

Fig. 2. Schematic of the experimental setup (see Table 1 for values of the various parameters). Note that d is the orifice diameter and z is the axial distance.

Table 1Value of cavity parameters employed in this study. SeeFig. 2 for definition of the parameters.

Parameter Value (mm)

d 8, 5, 3t 2.4L 110R 20H 6.3

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actuator is maintained constant and the frequency of excitation iscontrolled by a signal generator. The maximum uncertainty in themeasurement of the average Nusselt number is ±7.5%, and for thevelocity measurements it is ±3%.

We have used the diameter of the center orifice as the charac-teristic length scale (i.e., in the calculation of Reynolds and Nus-selt numbers as well as for normalizing the axial distance z).This length scale is more convenient to employ than the equiva-lent diameter (based on total area of the openings). Note that theequivalent diameter would vary with the number of satelliteholes, making comparison for different cases cumbersome. Simi-larly, the velocity at the centerline of the center orifice and1 mm away from the orifice plate, has been employed as thecharacteristic velocity in the calculations. For cases with no centerorifice, the diameter of the satellite orifice and the velocity at thecenterline of the satellite orifice and 1 mm away from it has beenemployed as the characteristic length and velocity scales,respectively.

M. Chaudhari et al. / International Journal of Heat and Mass Transfer 54 (2011) 2056–2065 2059

3. Measurement of heat transfer coefficient

This section presents the behavior of the heat transfer coeffi-cient h (or Nu) as a function of z (or z/d), for different parameters.

3.1. Effect of satellite orifices

The effect of the number of satellite orifices on the heat transfercoefficient is discussed in this subsection. Fig. 3 presents the aver-age heat transfer coefficient for a center orifice of 5 mm with satel-lite orifices as a function of axial distance. The number of satelliteorifices (each of 3 mm diameter) is varied while maintaining thepitch circle radius on which the orifice center lies at 6 mm (see alsoTable 2). The excitation frequency is held constant at 200 Hz forthese experiments, which corresponds to the resonance frequencyof the cavity [14]. It is noticed that the heat transfer coefficient forthe configuration of center orifice with satellite orifices is higher ascompared to that for the no satellite orifice case. The maximum va-lue of heat transfer coefficient is 159 W/m2 K with satellite orifices,which is 30% higher as compared to that with a single center orifice[12]. The heat transfer coefficient increases with an increase in thenumber of satellite orifices at larger distances, but not at smallerspacings (z < 15 mm). An interesting observation is the presenceof a second maxima at small z which appears with 1, 2 and 4 satel-lite orifices but not for 8 satellite orifices. The magnitude of thissecond maxima increases with the number of satellite orifices. No-tice that there is a sharp increase at very small spacing, followed bya sharp decrease in the average heat transfer coefficient near the

h avg

0 25 50 75 100 125 1500

25

50

75

100

125

150

175

200

5 X 3 X 15 X 3 X 25 X 3 X 45 X 3 X 88 mm5 mm

(W/m

2 -K)

z(m)

Fig. 3. Average heat transfer coefficient for 5 mm center orifice and differentnumbers of satellite orifices, for an excitation frequency of 200 Hz. The baselinecases of 5 mm and 8 mm center orifice alone are also shown for comparison.

Table 2Nomenclature employed and configurations tested in the study.

Configuration Center diameter(mm)

Satellite diameter(mm)

Number of SatelliteOrifices

5 � 3 � 2 5 3 25 � 3 � 4 5 3 45 � 3 � 8 5 3 8N � 3 � 2 – 3 2N � 3 � 4 – 3 4N � 3 � 8 – 3 8

first maximum; on either side of the second maxima the decreaseis more gentle. Overall the effect of the number of satellite orificeson the average heat transfer coefficient is non-monotonic. A dis-cussion on the variation of heat transfer coefficient is provided la-ter in Section 6.

The baseline results for 5 mm and 8 mm center orifice (only) arealso included in Fig. 3 for comparison. Note that the use of multipleorifices increases the total area of the synthetic jet. One can attri-bute the increase in the average heat transfer to this increase inarea. However, as evident from Fig. 1, for a single orifice, fromRef. [12], that the heat transfer coefficient does not increase mono-tonically with orifice diameter. Further comparison between themultiple orifice synthetic jet and the single orifice synthetic jetcan be made by computing the equivalent area of the multiple ori-fices. For 5 mm center orifice and four 3 mm diameter satellite ori-fices, the equivalent diameter is 7.81 mm, which is close to the8 mm single-orifice case presented in Fig. 3. The value of the heattransfer coefficient with the single orifice is less than that for mul-tiple orifices with approximately the same area. More, importantlythe first maxima for the multiple orifice case is not exhibited at allwith the single orifice.

Fig. 4 shows the results of Fig. 3 in a non-dimensional mannerwhere the diameter of the center orifice is used to obtain the Nus-selt number and non-dimensional spacing. The Reynolds numbercorresponding to this excitation frequency is 2600. The maximumNusselt number is found to be 30 for the configuration of 5 mmcenter orifice with 8 satellite orifices.

The results in this section show, for the first time, the presenceof two peaks in the Nusselt number versus axial distance. The addi-tional peak at smaller spacing is however present with a certainnumber of orifices arranged at a certain pitch circle radius only(this latter result will be demonstrated later in Section 3.4). Thesecondary peak is however not observed for the two extreme casesof no satellite orifice and with 8 satellite orifices.

3.2. Effect of Reynolds number

The Reynolds number is varied by changing the excitation fre-quency of the actuator. The effect of reducing the Reynolds numberon the average Nusselt number is discussed in this subsection.

z/d

Nu av

g

0 5 10 15 20 25 300

5

10

15

20

25

30

35

40

5 X 3 X 15 X 3 X 25 X 3 X 45 X 3 X 88 mm5 mm

Fig. 4. Average Nusselt number for 5 mm center orifice and different numbers ofsatellite orifices, for Reynolds number of 2600. The baseline cases of 5 mm and8 mm center orifice alone are also shown for comparison.

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Fig. 5 shows the variation of the average Nusselt number with thenormalized axial distance for the 5 � 3 � 2 configuration (seeTable 2). The average Nusselt number increases with an increasein Reynolds number, as expected, and two peaks are again ob-served. The average Nusselt number values corresponding to thetwo local maxima are 13.6 and 21 for Re = 1400, and 26 and 30for Re = 2600. Therefore, for the first peak the increase in the aver-age Nusselt number is about 91% and for second peak it is about43%, with a 85% increase in Reynolds number.

Fig. 6 shows the variation of the average Nusselt number withthe normalized axial distance for different Reynolds numbers forthe 5 � 3 � 4 configuration. The variation of average Nusseltnumber is qualitatively similar for all Reynolds numbers, and theNusselt number increases with an increase in the Reynolds num-ber. It is also observed that there are two peaks for all Reynoldsnumbers investigated. While the location of the first peak at z/d = 2 is almost independent of Re, the location of the second peak

z/d

Nu av

g

0 5 10 15 20 25 300

5

10

15

20

25

30

35

Re = 1400Re = 2600

Fig. 5. Variation of average Nusselt number with normalized axial distance fordifferent Reynolds numbers, for 5 � 3 � 2 configuration.

z/d

Nu av

g

0 5 10 15 20 25 30-5

0

5

10

15

20

25

30

35Re = 1000Re = 1830Re = 2100Re = 2440Re = 2600

Fig. 6. Variation of average Nusselt number with normalized axial distance fordifferent Reynolds number, for 5 � 3 � 4 configuration.

is weakly dependent on the Reynolds number (at z/d � 9). As notedearlier, the increase in the average Nusselt number with an in-crease in the Reynolds number is more at the first peak as com-pared to that at the second peak.

Fig. 7 shows the variation of the average Nusselt number withthe normalized axial distance for different Reynolds numbers forthe 5 � 3 � 8 configuration. It is observed that the average Nusseltnumber increases monotonically up to z/d = 10 and then decreasesfor all the three Reynolds numbers investigated. That is, the sec-ondary maxima is absent with 8 satellite orifices, as also noted ear-lier. Furthermore, it is again observed that the average Nusseltnumber increases with an increase in Reynolds number.

The results in this section show beyond doubt that the Nusseltnumber can get maximized at up to two axial locations; these opti-mal distances can be utilized while selecting the spacing betweenthe heated surface and the impinging jet. The percentage increase

z/d

Nu av

g

0 10 20 300

5

10

15

20

25

30

35

Re = 1450Re = 2100Re = 2570

Fig. 7. Variation of average Nusselt number with normalized axial distance fordifferent Reynolds number, for 5 � 3 � 8 configuration.

h avg

0 20 40 60 80 1000

20

40

60

80

100

120

140

160

5 mm centerN X 3 X 2N X 3 X 4

z(mm)

(W/m

2 -K)

Fig. 8. Variation of heat transfer coefficient with axial distance for differentnumbers of satellite orifices and no center orifice. The baseline case of 5 mm centerorifice alone is also plotted for comparison.

M. Chaudhari et al. / International Journal of Heat and Mass Transfer 54 (2011) 2056–2065 2061

in the Nusselt number with respect to the Reynolds number isfound to be larger at the first maxima than at the second maxima.However, the magnitude of the second maxima is larger than thefirst one in all the cases investigated herein.

z/d

Nu av

g

0 5 10 15 20 25 300

2

4

6

8

10

12

14

16

Re = 1000Re = 1550

Fig. 10. Variation of average Nusselt number with normalized axial distance fortwo different Reynolds numbers, for N � 3 � 2 configuration.

3.3. Multiple orifices without center orifice

It is clear from the above results that the satellite orifices sub-stantially affect the heat transfer coefficient-qualitatively andquantitatively with the plate close to the orifice or away from it.In order to understand the effect of satellite orifices, the center ori-fice is blocked and the average heat transfer coefficient is againmeasured as a function of the axial distance. The results with a sin-gle orifice at the center, of diameter 5 mm, serve as the baselinecase against which the results are compared. The excitation fre-quency corresponding to a resonance frequency of 200 Hz is main-tained in these measurements.

Fig. 8 presents the comparison of the average heat transfer coef-ficient for different numbers of satellite orifices alone with that ob-tained from a single center orifice alone of 3 mm diameter, as afunction of the axial distance. The heat transfer coefficient in-creases and decreases gradually for the lone center orifice, butfor the satellite orifices without a center orifice, the heat transfercoefficient rapidly increases and gradually decreases with an in-crease in the axial distance. The average heat transfer coefficientwith only satellite orifices is high for z < 42 mm as compared tothat with the single center orifice of the same diameter. At largeraxial distances, a single center orifice gives approximately thesame or better heat transfer coefficient as obtained with multiplesatellite orifices. It is noticed that the average heat transfer coeffi-cient increases by 22% with an increase in the number of satelliteorifices from 2 to 4. The maximum heat transfer coefficient in-creases by 40% with satellite orifices as compared to that withthe center orifice at the same excitation frequency (Reynolds num-ber). Also, it is observed that the maximum heat transfer coeffi-cient shifts towards lower axial distance for the satellite orifices.The corresponding Nusselt number calculated by taking the satel-lite orifice diameter as the characteristic length scale is shown inFig. 9.

These results are further confirmed by changing the Reynoldsnumber of the flow. Figs. 10 and 11 show the variation of the aver-

z/d

Nu av

g

0 5 10 15 20 25 30 350

4

8

12

16

20

5 mm centerN X 3 X 2N X 3 X 4

Fig. 9. Variation of average Nusselt number with normalized axial distance fordifferent numbers of satellite orifices and no center orifice. The baseline case of5 mm center orifice alone is also plotted for comparison.

age Nusselt number for 2 and 4 satellite orifices, respectively, attwo different Reynolds numbers. The maximum Nusselt numberis found to be at z/d = 8 for both cases shown in Fig. 10 while thez/d corresponding to the maximum Nusselt number decreases withRe in Fig. 11. It is seen that 4 satellite orifices perform better thanthe 2 satellite orifices in terms of the average value of the Nusseltnumber. The maximum Nusselt number is found to be 13.5 and 16for 2 and 4 satellite orifices, respectively. The results presented inthis section show that use of orifices away from the center can helpreduce the distance corresponding to maximum heat transfercoefficient.

3.4. Effect of pitch of satellite orifices

The results discussed so far are with the centers of the satelliteorifices on a pitch circle radius (PCR) of 6 mm. The following mea-surements are for other values of the pitch circle radius. Fig. 12

z/d

Nu av

g

5 10 15 20 25 30-2

0

2

4

6

8

10

12

14

16

18

20

Re = 1000Re = 1550

Fig. 11. Variation of average Nusselt number with normalized axial distance fortwo different Reynolds numbers, for N � 3 � 4 configuration.

z

h avg

0 20 40 60 80 100 120 1400

20

40

60

80

100

120

140

160

180

PCR = 6 mmPCR = 8 mmPCR = 10 mm

(mm)

(W/m

2 -K)

Fig. 12. Variation of average heat transfer coefficient with axial distance fordifferent values of pitch circle radius for 5 � 3 � 4 configuration, at excitationfrequency of 200 Hz.

U

0

5

10

15

20

25

30

35

40Original Signal, 3 mm ODOriginal Signal, 5 mm ODReverted Signal, 3 mm ODReverted Signal, 5 mm OD

(m/s

)

2062 M. Chaudhari et al. / International Journal of Heat and Mass Transfer 54 (2011) 2056–2065

shows that for the pitch circle radii of 6 and 8 mm two peaks in theaverage heat transfer coefficient are observed but not for PCR of10 mm. The two peaks move towards each other and appear tomerge for PCR = 10 mm leading to a single peak. The magnitudeof the first peak increases with an increase in the pitch circle ra-dius, but the second peak is having nearly the same value for allthe three cases. The heat transfer coefficient is highest (160 W/m2 K) for the pitch circle radius of 10 mm. At higher axial dis-tances, the performance of all three cases is nearly the same. Thecorresponding Nusselt numbers for the different pitch circle radiiconfigurations are provided in Fig. 13. The maximum Nusselt num-ber is 31 for a pitch circle radius of 10 mm, at a Reynolds number2600.

The results in this section show that the presence/absence oftwo maxima is strongly dependent on the pitch circle radius. A dis-cussion on the effect of pitch circle radius on the heat transfer coef-ficient is provided later in Section 6.

z/d

Nu av

g

0 5 10 15 20 25 300

4

8

12

16

20

24

28

32

36

40

PCR = 6 mmPCR = 8 mmPCR = 10 mm

Fig. 13. Variation of average Nusselt number with normalized axial distance fordifferent values of pitch circle radius for 5 � 3 � 4 configuration, at Re = 2600.

4. Velocity measurements

In a typical synthetic jet, the flow is sucked into the cavity andejected out of it through a single orifice. With multiple orifices, therelative strength of flow at different orifices varies. Simultaneousvelocity measurements at the center and satellite orifices areundertaken in order to obtain the magnitude and phase differencein velocity at these locations. Velocity measurements are also per-formed so as to obtain some idea of the radial variation of the meanand r.m.s. velocities on the impinging surface. Accordingly, thevelocity measurements are divided into two subsections. Thesemeasurements should help provide a better understanding of thevariation of the heat transfer coefficient for different configurationsreported in the previous sections.

4.1. Velocity at orifice exit

The exit velocity, 1 mm away from the orifice and at the center-line, for both the center orifice and satellite orifice is measuredusing two separate single hot-wire probes. The data presented inFig. 14 is recorded at the excitation frequency of 200 Hz. The suc-tion and ejection parts of the cycle can be distinguished from thetime series. The hot-wire is however insensitive to the directionof the flow; the data corresponding to the suction stroke is accord-ingly reverted, as discussed in the earlier works of Pavlova and

τ0.01 0.02 0.03 0.04

-15

-10

-5

(ms)

Fig. 14. Hot-wire velocity signals obtained 1 mm away from the orifice plate and atthe centerline of center orifice and satellite orifice, for 5 � 3 � 4 configuration withPCR = 6 mm. The signal with the larger suction and ejection velocities correspondsto the center orifice.

Table 3Maximum ejection and suction velocities, 1 mm away from the orifice and at thecenterline of the center orifice and satellite orifice. The measurements are fordifferent distances between the jet and the plate.

Sr. No. Distance(mm)

Center orifice Satellite hole

Ejectionvelocity(m/s)

Suctionvelocity(m/s)

Ejectionvelocity(m/s)

Suctionvelocity(m/s)

1 5 25.10 11.67 18.20 4.662 10 25.00 10.81 17.10 5.373 15 25.20 10.24 17.94 5.334 20 25.10 10.56 17.93 5.245 100 26.20 11.10 18.10 5.30

M. Chaudhari et al. / International Journal of Heat and Mass Transfer 54 (2011) 2056–2065 2063

Amitay [1] and Chaudhari et al. [14]. The magnitude of velocity atthe center orifice is larger than that for the satellite orifice. Further-more, almost no phase lag exists in the exit velocity between thecenter orifice and the satellite orifice, i.e., all orifices suck and ejectfluid at the same time.

The above results are for an unconfined jet. In addition, velocitymeasurements in the presence of an impinging plate are alsoundertaken. The maximum suction and ejection velocities with achange in the distance between the jet and plate are provided in Ta-ble 3. It is observed that confinement has a negligible effect on theejection and suction velocities, at least in the range investigated.

4.2. Near-wall velocity measurements

Another set of velocity measurements is undertaken to getsome idea of the flow velocity distribution over the heated surface.These measurements are along the radial direction at a fixed dis-tance of 2.5 mm (y/d = 0.5 where y is the coordinate normal tothe plate) from the heated surface.

Fig. 15 shows the variation of the normalized mean radial com-ponent of velocity (Umean/Uexit) along the normalized radial dis-tance (r/d) for the 5 � 3 � 4 configuration, with a PCR = 6 mm.Note that Uexit is the mean velocity measured at the centerline ofthe center orifice and 1 mm away from the orifice plate. Thesemeasurements are along the 45� line (see Fig. 2) for various axialdistances between the jet and the impinging surface. The oscilla-tory behavior (of the type shown in Fig. 14) is not observed in ra-dial velocity, although these velocities vary with time, leading to afinite r.m.s. For the smaller axial distance (z/d = 1) two peaks areobserved: The first peak is at the origin while the other one is atthe inner edge of the satellite orifice. The mean velocity decreasesrapidly after the second peak. The maximum value of the normal-ized mean velocity is 0.67. At the axial distance of z/d = 2, twopeaks are again observed: one at center and the other at the outeredge of the satellite orifice (r/d = 1.5). Notice that the magnitude ofvelocity is larger for most radial locations (r/d > 1). The high mag-nitude of mean velocity helps to extract more heat, and leads to alocal maxima in heat transfer coefficient at z/d = 2, as seen earlierin Fig. 4.

At z/d = 3, two peaks of mean velocity are again observed; how-ever both of them have shifted away from the centerline of the

r/d

Um

ean

/Uex

it

0 1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

z/d = 1z/d = 2z/d = 3z/d = 10

Hal

fce

nter

orif

ce

Fig. 15. Variation of normalized mean radial component of velocity with normal-ized radial distance, for 5 � 3 � 4 configuration at Re = 2600. The vertical linesindicate the location of center and satellite orifices.

center orifice. The two peaks are at r/d = 1.2 and 2.5. Flow visuali-zation suggests recirculation of fluid at these radial locations (notshown). Note that recirculation may be the reason for the reduc-tion in the heat transfer coefficient at z/d = 3, as observed earlier.Additional discussion on this point is provided in Section 6. An in-crease in axial distance between the jet and the heated surface re-duces the mean velocity owing to entrainment of ambient fluid[19]. At z/d = 10, only a single peak is observed and there is norecirculation of fluid. The velocity over the surface is nearly uni-form, which suggests good flow and cooling of the surface, and thismay be the reason for the second maximum observed in Fig. 4.

Fig. 16 shows the radial distribution of the normalized rmsvelocity (radial component), for different normalized axial dis-tances. The normalization has been done by the mean exit velocityat the centerline of the orifice. The r.m.s. velocity is maximum atthe origin and decreases sharply for r/d > 0.8 for the lowest axialdistance (z/d = 1). At z/d = 2, the rms velocity is high and decreasessomewhat gradually along the radial distance. At z/d = 3 two peaksin the r.m.s. velocity is observed away from the centerline of theorifice. At the highest axial distance (z/d = 10) the rms velocitiesare nearly constant along the radial distance. The large value ofr.m.s. velocity is noteworthy; a substantially larger r.m.s. valuefor synthetic jet vis-a-vis a continuous jet has already been notedby Chaudhari et al. [14].

The velocity measurements are across the entire surface of thecopper block. Because the average value (over the heater surface)of the heat transfer coefficient has been measured, the radial aver-age of velocity is computed in an effort to correlate the heat trans-fer behavior to the average velocity over the surface. The radiallyaveraged velocity is plotted as a function of axial distance inFig. 17. It is encouraging to see that both radially averaged meanand r.m.s. velocities exhibit their maximum value at z/d = 2, whichcorrelates well with the first peak in the heat transfer coefficient.The mean velocity at z/d = 10 is larger than that at z/d = 3 whichis again in qualitative agreement with the result in Fig. 4. Ther.m.s. velocity however exhibits a decreasing trend for z/d > 2. Thatis, the mean velocity correlates with the heat transfer data betterthan the r.m.s. velocity; a similar observation has been reportedearlier by Chaudhari et al. [12] in the context of a single orifice syn-thetic jet. It is however noted that while the heat transfer coeffi-cient at z/d = 10 is 50% greater than the heat transfer coefficient

r/d

Urm

s/U

exit

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

0.1

0.2

0.3

0.4

0.5

0.6

z/d = 1z/d = 2z/d = 3z/d = 10

Fig. 16. Variation of normalized r.m.s. velocity (radial component) with normalizedradial distance, for 5 � 3 � 4 configuration at Re = 2600. The vertical lines indicatethe location of center and satellite orifices.

z/d

(U

mea

n)av

g/(

Uex

it),(

Urm

s)av

g/(

Uex

it)

0 2 4 6 8 10 120

0.1

0.2

0.3

0.4

0.5

Along 45o lineAlong 45 o line

Umean

Urms

Fig. 17. Variation of radially averaged mean and r.m.s. velocities with normalizedaxial distance, for 5 � 3 � 4 configuration at Re = 2600.

2064 M. Chaudhari et al. / International Journal of Heat and Mass Transfer 54 (2011) 2056–2065

for z/d = 2, the normalized mean velocity is greater for z/d = 2. Thiscan possibly be due to the difference in behavior between multi-ple-jets impinging on the surface versus a combined central jet,as discussed in Section 6. More detailed velocity measurementsare required to fully resolve this issue.

5. Power measurement

Fig. 18 presents the consumption of input electrical power formulti-orifice synthetic jet as a function of the excitation frequency.The power consumed for only the center orifice and for only the sa-tellite orifices are also included for comparison. The input powercurve exhibits a local minima at the resonance frequency of200 Hz for all cases investigated. The power also tends to reduceat higher frequencies. As already mentioned, most of the measure-ments in this study are at the resonance frequency. The configura-

+

+

++

++++++++++ +

+

+

+

++

+

f

PSJ

0 200 400 600 800 1000 1200 1400 16001.45

1.475

1.5

1.525

1.55

1.575

1.6

1.625

1.655 mm OD5 mm OD + 2 S holes5 mm OD + 4 S holes5 mm OD + 8 S holes4 S holes+

(W)

(Hz)

Fig. 18. Variation of power with excitation frequency for multi-orifice synthetic jet.The power consumption for a single orifice synthetic jet is also plotted forcomparison.

tion of 4 satellite orifices (3 mm OD each) without the centerorifice consumes nearly same amount of power as a single 5 mmcenter orifice at the resonance frequency.

It is interesting to notice that the input power reduces when thesatellite orifices are present. The increase in the number of satelliteorifices with the presence of the center orifice reduces the con-sumption of power at resonance frequency. Although the reductionin power is small, the measurements are consistent and repeatable,and therefore believed to be true. The reduction in power con-sumption is however considered to be insignificant from practicalviewpoint. Apparently the resistance to air flow reduces slightly inthe case of satellite orifices which leads to a slight reduction in in-put power.

6. Discussion

The satellite orifices with the center orifice increase the heattransfer coefficient as compared to that with a single center orifice.The appearance of the first peak in the heat transfer coefficient(Fig. 4) for the case of 1, 2 and 4 satellite orifices at small axial dis-tances (z/d = 2) is attributed to the presence of satellite orifices. Thesatellite orifices entrain fresh fluid thereby helping the center ori-fice in increasing the heat transfer coefficient.

It is also apparent from Fig. 4 that the effect of satellite orifice ismore pronounced at lower spacings. At lower spacings, there aremultiple jets, i.e., there exist individual jets from the satellite ori-fices as well as from the center orifice. These jets merge, to forma combined central jet with an increase in the axial distance, lead-ing to a change in flow and heat transfer behavior. The transitiondistance from individual jet-like behavior to combined-jet behav-ior can be estimated from the spread angle of the jet and the valueof the pitch circle radius. The estimated value (of 8–11.5 mm forPCR = 6 mm with 5 � 3 � 4 configuration) compares well withthe experimentally obtained data in Fig. 3. Similar estimates canbe obtained at other values of PCR. The proposed transition in fluidflow and heat transfer characteristics therefore appears consistentwith the data presented in Figs. 3 and 12. In addition, it is notedthat an increase in the pitch circle radius of the satellite orificeswith the center orifice covers a larger surface area of the heatedcopper block, which helps to extract more heat, and hence the heattransfer coefficient is high. It however appears that this argumentcan not be extended to very large values of the pitch circle radius.

The behavior of the heat transfer coefficient is non-monotonicwith respect to the number of satellite orifices (Fig. 4). It is believedthat 2 and 4 satellite orifices have a relatively large amount ofentrainment of the fresh fluid, leading to superior performance inthese cases as compared to the other configurations. The schematicof the flow in Fig. 19 shows that with 4 satellite orifices fresh fluidis entrained by jets emanating from all orifices. With an increase inthe number of satellite orifices to 8, a ring forms around the centerorifice which does not allow the fresh ambient fluid to reach thecenter orifice. Consequently, while the satellite orifices supplyfresh fluid, the center orifice recirculates the same fluid, leadingto a reduction in the heat transfer coefficient. Therefore, at smallaxial distances 2 and 4 satellite orifices give better performanceas compared to 8 satellite orifices.

The combined jet from 8 satellite orifices has a larger flow ratethan the combined jet from 1, 2 and 4 satellite orifices, whichleads to a slightly superior performance at large axial distances.In addition, at larger axial distances due to increase in the jetwidth, 8 satellite orifices with the center orifice cover the entiresurface area of heated block upon impingement, leading to a high-er heat transfer coefficient as compared to that with 1, 2 and 4satellite orifices. Due to decrease in velocity of the impinging jetthere is decrease in heat transfer coefficient at large axial

Satellite orifice

Fresh air

Center orifice

Fig. 19. Schematic of the flow for 5 � 3 � 4 configuration.

M. Chaudhari et al. / International Journal of Heat and Mass Transfer 54 (2011) 2056–2065 2065

distances for all configurations of multi-orifices with a single cen-ter orifice, as well as for satellite orifices without a center orifice.This behavior is similar to that observed for an impinging contin-uous jet.

In order to put these results in a better perspective, the heattransfer studies related to multiple steady jets are briefly recalled.Among others, Garimella and Schroeder [20] studied the thermalperformance of a heat sink using multiple impinging confined con-tinuous jets. It was reported that when multiple continuous jetswere employed instead of a single continuous jet, the heat transfercoefficient increased when the orifice-to-target plate distance re-duced. A similar observation has been reported by Katti and Prabhu[21]. In the above context multiple synthetic jets can be used to re-duce the spacing between the orifice and heated plate for enhance-ment in heat transfer.

7. Conclusions

This study explores the performance of a synthetic jet withaddition of orifices around the main orifice. The heat transfer coef-ficient as a function of distance between the orifice and heated sur-face, for different configurations of satellite orifices with andwithout the center orifice is measured and presented in this work.It is observed that the number of satellite orifices affects the heattransfer coefficient at lower axial distances (z/d < 3). In particular,two peaks in the heat transfer coefficient are observed with 1, 2,and 4 satellite orifices with variation in axial distance. At lower ax-ial distance (z/d = 2), 2 orifices give the best performance. On theother hand, only a single peak is observed for 8 satellite orifices.Furthermore, satellite orifices without the center orifice give betterperformance as compared to a singe center orifice (the conven-tional case). Note that a better performance is achieved withouta corresponding increase in the input electrical power. The near-orifice velocity measurements indicate that both suction and ejec-tion are in-phase between the orifices. The radial velocity over thesurface is found to correlate reasonably well with the heat transfermeasurements.

The satellite orifices with the center orifice give a relatively highheat transfer coefficient at lower axial distances which can be use-ful for cooling electronic components where space is a major con-straint. Also for cooling of hot spots on a heated surface thesatellite orifices without the center orifice can be used. Our results

suggest that according to the specific requirement, it is possible toselect a proper configuration with different number of satellite ori-fices, and with and without the center orifice. The proposed config-uration meets the posed objectives of higher heat removal, at ashorter distance, and without an increase in input power. The re-sults of multiple synthetic jet with a single cavity can thereforebe used for heat removal in compact electronic devices.

Acknowledgements

The first author is grateful to Vishwakarma Institute of Technol-ogy, Pune for sponsoring him during the course of this work. Thiswork is funded by Department of Information Technology, NewDelhi.

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