Heat transfer enhancement by acoustic streaming in a closed cylindrical enclosure filled with water

International Journal of Heat and Mass Transfer 60 (2013) 230–235

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Heat transfer enhancement by acoustic streaming in a closed cylindrical enclosure filled with water B. Tajik, A. Abbassi ⇑, M. Saffar-Avval, A. Abdullah, H. Mohammad-Abadi Department of Mechanical Engineering, Amirkabir University of Technology, P.O. Box 15916-34311, Tehran, Iran

a r t i c l e

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Article history: Received 8 February 2012 Received in revised form 20 December 2012 Accepted 21 December 2012 Available online 31 January 2013 Keywords: Heat transfer enhancement Acoustic streaming Standing wave Cylindrical enclosure Ultrasonic

a b s t r a c t Heat transfer enhancement in presence of acoustic streaming in a closed cylindrical enclosure filled with water on a down-ward-facing horizontal heating surface was measured experimentally. Acoustic streaming is induced by the vibration of the lower plate by means of an ultrasonic Bolted Langevin transducer. Standing wave was generated between a large heating source as a reflector and the vibrating plate. The upper plate was heated with a constant heat flux and side-walls were kept at the constant temperature. Therefore, the gravitational effects on the flow fields were negligible in this study and the heat transfer enhancement was due to vibrations. The effects of the transducer power, the height of the heater above the vibrating plate and the heat flux of the heater were considered separately in these measurements. The results show that the enhancement of the heat transfer between the heat source and the bulk water near the vibrating plate can be up to 390% with the acoustic streaming generated by the ultrasonic vibrations. The increase in the transducer power and the decrease in the height of the heater cause the higher heat transfer coefficient in the enclosure. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Acoustic streaming is a steady circular flow occurring near the vibrating solid surfaces in a high-intensity sound field. This phenomenon is associated with the friction between a vibrating medium in contact with a solid wall. It is not important whether the source of relative motion arises from either acoustic oscillations in the fluid or vibrations of the solid [1]. There are two types of acoustic streaming: Eckart streaming and Rayleigh streaming. Rayleigh streaming is a large scale streaming formed around surfaces in a standing wave field and the size of this type of streaming is characterized by the acoustic wavelength. This type of streaming is driven by viscous forces existing close to a surface in an oscillatory flow field. Eckart streaming, applies to large scale phenomena and occurs in travelling wave fields. The size of the streaming patterns is characterized by the size of the physical domain. The radiation pressure gradient due to the absorption of the acoustic wave in the propagation process is the driving force of the flow. Streaming velocities become greater at higher frequencies due to high absorption, and because of this feature, noticeable Eckart streaming is generally observed at high frequencies (>1 MHz) [2].

⇑ Corresponding author. Tel.: +98 2166408544; fax: +98 2166419736. E-mail addresses: [email protected] (B. Tajik), [email protected] (A. Abbassi), [email protected] (M. Saffar-Avval), [email protected] (A. Abdullah), [email protected] (H. Mohammad-Abadi). 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.12.066

In addition, by applying an ultrasonic vibration to liquids, a fluctuation of pressure arises and cavitation bubbles are observed. The cavitation phenomenon by ultrasonic vibration occurs at frequencies ranging from 15 to 18 kHz. Both of the acoustic streaming and the cavitation cause high heat transfer enhancement in liquids. Heat transfer by acoustic streaming can be predicted by forced convection and enhancement by cavitation can be explained by the turbulence heat conductivity of microjets. The large scale motion induced by acoustic streaming is effective for cooling large heating surfaces, while very small scale motion induced by cavitation is more effective for cooling small heating surfaces [3]. On the other hand, the occurrence of cavitation bubbles requires considerably high-power ultrasonic vibrations [4] and this phenomenon can cause corrosion and damaging sensitive surfaces. The influence of acoustic streaming on heat transfer processes has been investigated in several previous works [3–15]. The experimental approaches that have been developed can be divided into three main types: the study of natural convection heat transfer in a gas [5–8] or liquid [3,4,10–12] media, the effect of ultrasound on phase-change heat transfer [13,14], the influence of ultrasonic vibration on forced convection heat transfer [3,15]. In these cases, effective experimental parameters like ultrasonic power, frequency or the gap between the heat source and the vibrating source has been investigated. All of previous works reported in the literature have shown that in the presence of an acoustic field, regardless of the heat transfer mode (e.g., natural convection, phase change heat transfer), the heat transfer process can be greatly enhanced.

B. Tajik et al. / International Journal of Heat and Mass Transfer 60 (2013) 230–235

231

Nomenclature A C0 D EF f g h H0 Kf NuD P Pr qn Q

heat source surface area sound speed diameter of the heater surface enhancement factor frequency of vibration gravitation acceleration heat transfer coefficient height of the heater thermal conductivity Nusselt number using a diameter of circular plate (hD/Kf) input ultrasonic power Prandtl number (m/a) heat flux heater power

For acoustic streaming generated by an standing wave field, temperature drop of the heat source due to the ultrasonic vibration, has strong dependence on the gap between the vibrating surface and stationary (heat source) surface and is maximized when the gap is equal to a multiple of half wavelength [8]. For natural convection, most of the studies focused on the heat transfer from a very small heating surface (e.g., a hotwire) or in a very small gap [3–10]. In addition, fluid media in most of researches is an ideal gas like air [5–8]. The purpose of the present study was then to compare heat transfer from a large horizontal heating source with and without acoustic streaming generated in a closed cylindrical enclosure filled with water. On the other hand, at previous works in water, heating source was located inside a standing wave field, which was generated between another stationary surface and vibrating surface [3,4,10–15]. However, in this study, heating source was the stationary surface itself and standing wave was generated between heating source as a reflector and vibrating source. As a result, it can be assumed that the generated streaming was a pure acoustic streaming without any turbulence by means of the other parts like heating source inside the flow field (cf. [16]). The height of the heater was adjusted equal to multiple of half wavelength in order to generating powerful acoustic streaming. In this study, the effective parameters like height of the heater, input power of ultrasonic vibration and input power of heater are investigated.

2. Experimental apparatus and methods A diagram of the test equipment is shown in Fig. 1. Experiments were carried out using a handmade ultrasonic bath, which was composed of two coaxial cylinders. The internal diameter and the height of the inner cylinder were 25 and 30 cm, respectively. The size of the inner cylinder was larger than the ultrasonic wave length in experiments. The side walls and bottom of the inner cylinder were made of Stainless Steel 316 with a thickness of 0.8 mm and this cylinder constituted the main heat transfer test section. This part of equipment was filled with distilled water which was boiled for 20 min before experiments in order to stabilize the nuclei and the dissolved gases content in the water to reduce cavitation phenomena. The ultrasonic vibration source was an 18 kHz Bolted Langevin ultrasonic transducer (BLT) with a tip diameter of 60 mm which was exactly stuck to the center of the steel bottom. The ultrasonic waves were supplied upward into the water from the upper surface of the steel bottom. The acoustic power

RaD t T

Rayleigh number using a diameter of circular plate (gb(TH  T1)D3/ma) time temperature

Greek symbols a thermal diffusivity b volumetric thermal expansion coefficient k wavelength of the sound waves m kinematic viscosity q fluid density Subscripts H heat source values 1 bulk values  averaged throughout heating surface

of ultrasonic vibrations was measured as the input power to the transducer. The water level has a strong influence on the resonance state in the water bath [10]. Therefore, a water level of 23 cm was fixed for all tests, which was optional to stabilize the resonance condition at the fundamental harmonic frequency of the transducer. The water surface was opened to the atmosphere by an opened tap above the main tank. The temperature of the side-walls was maintained constant by the water cooler that flows between coaxial cylinders from below. The outlet cooling water was sprinkled in an open storage tank to cooling again. Two K-type thermocouples were used to measure the inlet and outlet cooling water temperatures. The difference between these temperatures during all experiments was less than 0.5 °C. The accuracy of the thermocouples is ±1.5 °C; therefore, it can be assumed that the temperature of the side walls is constant during all experiments. A disk shape thin Mica heater with a diameter of 100 mm was used as a heating source. As, it is shown in Fig. 2, the heater was bolted on a circular copper plate with a diameter of 118 mm and a thickness of 5 mm to generate a constant heat flux. In order to reach the best contact between heater and copper plate, a thin layer of Silicon was used. Down side of the copper plate was contacted with water as a heating source and upper side was covered by insulting materials against the heat loss and sealed in a thick PTFE board. To do test in different heights of heater, the diameter of 124 mm was used for the PTFE board. Therefore, it was able to moving up and down in the main tank. In order to pass water from one side of the board to another side, at vertical moving of the heater unit, four small notches were cut at the outer edge of the board. The heater unit was connected to an adjustable screw device set over the main tank and was able to move vertically for experiments at different heights of the heater. The height of the heater unit was measured by a digital caliper that was fixed over the main enclosure. Four K-type thermocouples (Testo Co., Type 15) were installed at four small holes (as shown in Fig. 2) on copper plate and fixed between the copper plate and heater. The values measured by these thermocouples were regarded as the surface temperatures of the copper plate with an error of less than 0.1 °C. It was used the averaged value of these four temperatures as the mean temperature of the heating source in calculations. Heat flux was calculated from the measured input power of heater (Q) to the copper plate area (qn = Q/A). The bulk temperature of water was measured by three immersed K-type thermocouples (Testo Co., 0602 5792). These

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Fig. 1. Experimental apparatus.

Fig. 2. Heater unit.

wire-shape thermocouples were used to reduce any unwanted turbulence in the enclosure. The tips of these thermocouples were fixed at the height of 1.5 cm above vibrating plate. The mean value of these three temperatures was used as the mean bulk temperature. The temperature difference between these bulk thermocouples was less than 2 °C at most. Ten thermocouples were used for temperature measurements and real time values were recorded by data logger (Testo Co., Control unit 454), which connected to the computer. All of thermocouples are K-type and have an accuracy of ±1.5 °C (Testo Co., class1). The measured temperatures are:     

Temperatures of four different points of the heater Temperatures of three different points of the bulk water Temperature of the inlet cooling water Temperature of the outlet cooling water Ambient temperature

The exact speed of sound was needed to determine the wavelength of the ultrasonic vibrations (k = C0/f). Therefore, the speed of sound was measured for various temperatures in our experimental condition by means of two ultrasonic probes to

reassurance. Since, the heat source was down-ward facing, variation of the bulk temperature among vertical direction was small, hence, the bulk temperature was used to determine the wavelength of the vibrations.

3. Experimental procedure The main cylindrical enclosure was filled with distilled water that degassed by boiling. The corresponding height of water is kept constant during all experiments and is equal to 23 cm. To investigate the effect of height of the heater on the heat transfer enhancement, the height was set to be k (full wavelength), and then increased to be 3k/2 and 2k. The external part of the main tank was fed with cooling water in order to keep constant temperature of the side walls. After reaching a thermal equilibrium in the enclosure, a controlled voltage is used to heat the Mica heater as a constant heat source. Three inlet power values (40, 60 and 80 W) were supplied to the heating source to compare the effect of the heat flux. At this heat fluxes, heat transfer is at the natural heat transfer regime, without phase change. The temperature of the heat source reached a steady state temperature in 11 min. Ultrasonic vibration

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was exited after the second thermal equilibrium, when there were no changes in temperatures. The temperature of the heat source suddenly started to decrease with ultrasonic vibration. A maximum temperature drop of 28 °C was recorded in less than 1 min and heater reached another steady state condition after 3–5 min. The recording of the measurements was continued until the third thermal equilibrium was occurred. The uncertainty in heat flux comes from measured quantities of heater power and heater surface area. Another source of error is uncertainty in determination of temperatures. The error propagation analysis based on Taylor series has been used to estimate the uncertainty. Uncertainty analysis reveals that the maximum  is within ±5.7% for stationary condition. uncertainty in h

a h¯ (W/m2.K)

850 700 550 400

100 0

1000

1200

h¯ (W/m2.K)

400

Calculated without ultrasonic

100 0

200

400

600

800

1000

1200

c

P = 157.5 W P = 100.8 W P = 78.75 W P = 56.7 W

1000

h¯ (W/m2.K)

850 700 550 400

Calculated without ultrasonic

250 100 0

200

400

600

800

1000

1200

Fig. 4. The mean heat transfer coefficient versus time without and with standing wave for different ultrasonic powers, when Q = 80 W at (a) H0 = 2k (b) H0 = 3k/2 and (c) H0 = k. (Ultrasonic vibration was excited at t = 800 s).

T1-Temp. of the Heat Source T2-Temp. of the Heat Source T3-Temp. of the Heat Source T4-Temp. of the Heat Source Center T5-Temp. of the Bulk Water T6-Temp. of the Bulk Water T7-Temp. of the Bulk Water T8-Ambient Temp.

T4

T5, T6, T7

20 T8

10 400

600

1400

Time (s)

30

200

1400

Time (s)

40

0

1400

550

60

Temperature (C)

800

700

70

T3

600

250

ð1Þ

T2

400

P = 157.5 W P = 100.8 W P = 78.75 W P = 56.7 W

1000 850

Fig. 3 is a sample that shows temperature changes of the heat source and bulk of water, while the height of the heater is 3k/2. In this Figure, the ultrasonic input power and the heating power is 100.8 W and 80 W, respectively. As, it is shown, a sudden decrease in temperature of the heat source (27–28 °C) is occurred as soon as the ultrasonic power is exited at t = 800 s. Temperatures of the heat source drops during a short time almost less than 1 min. Because, when the lower plate starts to vibrate, acoustic streaming is created between the ultrasonic vibrator and the heat source, which causes the convective heat transfer enhancement and temperature drop in the heating source. After creating of flow field, bulk temperatures starts to increase as a result of water circulation loops generated by acoustic streaming. But, differences between heat source temperatures and bulk temperatures do not change when a steady state condition was reached. When the ultrasonic vibrations were induced, some fluctuations can be seen for recorded temperatures of the heat source. These oscillations are as a result of a faint cavitation phenomenon which creates randomly some small bubbles.  the meaTo determine the mean heat transfer coefficient (h), surements of four temperatures of the heat source and three bulk  can be calculated from temperatures were averaged. Therefore, h mean temperature of the heat source (T H ) and mean bulk temperature (T 1 ):

T1

200

Time (s)

b

50

Calculated without ultrasonic

250

4. Experimental results

 ¼ q =ðT H  T 1 Þ h n

P = 157.5 W P = 100.8 W P = 78.75 W P = 56.7 W

1000

800

1000

1200

1400

1600

Time (s) Fig. 3. Temperatures of the heat source and bulk water versus time without and with standing waves. (H0 = 3k/2, P = 100.8 W, Q = 80 W).

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a

a

1000

H0 = λ

1000

H0 = 3λ /2

850

h¯ (W/m2.K)

h¯ (W/m2.K)

850 700 550 400

Q = 40 W Q = 60 W Q = 80 W

250

H0 = 2λ

700 550 400 250 100

100 600

800

1000

1200

0

1400

200

400

600

b

b

1000

h¯ (W/m2.K)

h¯ (W/m2.K)

700 550 400

1200

1400

1000

1200

1400

1000

1200

1400

550 400 250 100

800

1000

1200

0

1400

200

400

600

c

1000

H0 = λ

1000

H0 = 3λ /2

h¯ (W/m2.K)

850

700 550 400

Q = 40 W Q = 60 W Q = 80 W

250

800

1000

1200

H0 = 2λ

700 550 400 250 100

1400

0

Time (s)

200

400

600

800

Time (s)

Fig. 5. The mean heat transfer coefficient versus time, when ultrasonic vibration was excited at t = 800 s for different heat fluxes, when P = 78.75 W at (a) H0 = 2k (b) H0 = 3k/2 and (c) H0 = k.

d

H0 = λ

1000

H0 = 3λ /2

Fig. 4 shows the heat transfer coefficient versus time before and after ultrasonic vibrations for different heights of the heating source. In these figures, continuous recording of temperatures repeated several times for each case. Since the results were very close together for each case, the average reported values were used to plot the heat transfer coefficients. In order to compare different cases on the graphs, the starting time of the vibrations was fitted at t = 800 s. It can be seen that the increase in the ultrasonic input power causes the increase in the heat transfer coefficient at all heights. On the other hand, a sudden increase in the heat transfer coefficient occurs as a result of acoustic streaming, when the ultrasonic transducer starts to vibrate. The mean heat transfer coefficient can be calculated based on the empirical relationship (cf. [17]) in stable condition before the ultrasonic vibration. For circular plates of diameter D in the stable horizontal configuration, the data of the Kadambi and Drake [17] suggest that:

ð2Þ

h¯ (W/m2.K)

850

NuD ¼ 0:82RaD Pr0:034

800

Time (s)

850

h¯ (W/m2.K)

1000

H0 = 2λ

Time (s)

1=5

1400

700

Q = 40 W Q = 60 W Q = 80 W

100 600

1200

H0 = 3λ /2

850

250

c

1000

H0 =λ

1000

850

100 600

800

Time (s)

Time (s)

H0 = 2λ

700 550 400 250 100 0

200

400

600

800

Time (s) Fig. 6. The mean heat transfer coefficient versus time without and with standing wave at different H0, when Q = 80 W for (a) P = 56.7 W (b) P = 78.75 W (c) P = 100.8 W and (d) P = 157.5 W. (Ultrasonic vibration was excited at t = 800 s).

 ¼ q =ðDTÞ, the unknown dependent variable Since h n DT ¼ T H  T 1 appears in the Nusselt number. Hence, to avoid iterating, we need to eliminate DT from the Rayleigh number. This can be done by introducing a modified Rayleigh number, RaD defined as:

B. Tajik et al. / International Journal of Heat and Mass Transfer 60 (2013) 230–235

6 H0 =λ H0 =3λ/2

Enhancement Factor

5

H0 =2λ

235

H0 = k, using 157.5 W ultrasonic power. It means that with a small ultrasonic power less than 160 W and inside a small volume of heat transfer water, the heat transfer coefficient increases about 392% without any pump.

4

5. Conclusions 3 2 1 0

0

20

40

60

80

100

120

140

160

180

Ultrasonic input power (W) Fig. 7. Influence of ultrasonic input power on the enhancement factor.

RaD ¼ RaD NuD ¼

gbDTD3 qn D

ma

K f DT

¼

gbqn D4 K f ma

ð3Þ

All properties are evaluated at T 1 . RaD is replaced with RaD =NuD in Eq. (2), therefore, DT is calculated from Eqs. (2) and (3): Now, we must return the calculation, reevaluating all properties except b at  are estimated for T f ¼ T 1 þ ðDT=2Þ. Thus, the corrected DT and h all cases. When the bulk temperature is 20 °C, for the heating power of 80, 60 and 40 W, the mean heat transfer coefficient is about 187, 175 and 160 (W/m2 K), respectively. It means that, if the heating power increases from 40 W to 60 W, the heat transfer coefficient increases about 9.3%. The calculated value is shown to compare with experimental results in Fig. 4. In this figure different ultrasonic powers used to create acoustic streaming. Fig. 5 shows the mean heat transfer coefficient, for different heating powers and different heights of heater, when the ultrasonic vibration was excited at t = 800 s. As it is shown, at the first  is proporsteady state condition without ultrasonic vibrations, h tional to the heating power similar to the above calculated values. It can be seen at the beginning of the vibration time, too. But, after the generation of the acoustic streaming, some fluctuations are occurred, therefore, there are not comparable differences between the heat transfer coefficients for various heating powers. Of course, the increase in heat transfer coefficient is very small and negligible as a result of heating power in comparison with the increase of that by means of acoustic streaming. Fig. 6 represents the variation of heat transfer coefficient versus time with and without acoustic streaming for different height of  decreases with the increase the heating surface. As expected, the h of H0, because the velocities of acoustic streaming with a large H0 (as it was shown in Ref. [16]) are lower than that those with a small height when the ultrasonic power is fixed. It can be seen for all different ultrasonic powers. To quantify the effect of standing wave, an enhancement factor  with ultrasonic divided by the value EF was defined as the ratio of h at rest. In this case:

 with ultrasonic h EF ¼  h without ultrasonic

ð4Þ

Values of this enhancement factor for different cases are shown in Fig. 7. As it is shown, the average enhancement factor ranges from 2.55 at H0 = 2k, using 56.7 W ultrasonic power up to 4.92 at

The enhancement of the heat transfer for a horizontal downward heat source by acoustic streaming inside a cylindrical enclosure is investigated experimentally. Heat source has a constant heat flux and lower plate vibrates vertically by means of an ultrasonic transducer. Acoustic streaming is generated as a result of the standing wave between the heating surface and the vibrating plate. The results show that the heat transfer coefficient can be increased up to 390% by ultrasonic vibrations. The enhancement of heat transfer increases with the increase in ultrasonic power and decrease in distance between heating source and vibrating plate. The variation of heat transfer coefficient as a result of different heating power is very small and negligible in comparison with the increase of that by means of acoustic streaming. References [1] B. Loh, S. Hyun, P.I. Ro, C. Kleinstreuer, Acoustic streaming induced by ultrasonic flexural vibrations and associated enhanced of convective heat transfer, J. Acoust. Soc. Am. 111 (2) (2002) 875–883. [2] M.K. Aktas, Thermoacoustically induced and acoustically driven flows and heat transfer in enclosures, PhD thesis, Drexel University, May 2004. [3] S. Nomura, M. Nakagawa, Heat transfer enhancement by ultrasonic vibration, Proc. ASME/JSME Therm. Eng. Joint Conf. 4 (1995) 275–282. [4] S. Nomura, K. Murakami, Y. Aoyama, J. Ochi, Effects of changes in frequency of ultrasonic vibration on heat transfer, Heat Transf. Asian Res. 29 (5) (2000) 358– 372. [5] P.I. Ro, B. Loh, Feasibility of using ultrasonic flexural waves as a cooling mechanism, IEEE Trans. Ind. Electron. 48 (1) (2001) 143–150. [6] T. Wu, P.I. Ro, Heat transfer performance of a cooling system using vibration piezoelectric beams, J. Micromech. Microeng. 15 (2005) 213–220. [7] S. Hyun, D. Lee, B. Loh, Investigation of convective heat transfer augmentation using acoustic streaming generated by ultrasonic vibrations, Int. J. Heat Mass Transfer 48 (2005) 703–718. [8] D. Lee, B. Loh, Smart cooling technology utilizing acoustic streaming, IEEE T. Compon. Pack. T. 30 (4) (2007) 691–699. [9] Y. Iida, K. Tsutsui, R. Ishii, Y. Yamada, Natural-convection heat transfer in a field of ultrasonic waves and sound pressure, J. Chem. Eng. Jpn. 24 (6) (1991) 794– 796. [10] S. Nomura, A. Yamamoto, K. Murakami, Ultrasonic heat transfer enhancement using a horn-type transducer, Jpn. J. Appl. Phys. Part 1 41 (5B) (2002) 3217– 3222. [11] H. Yukawa, T. Hoshino, H. Saito, The effect of ultrasonic vibrations on free convective heat transfer from heated wire to water, Heat Transf. Jpn. Res. 5 (1) (1976) 37–49. [12] H. Yukawa, T. Hoshino, H. Saito, Effect of ultrasonic vibration on free convective heat transfer from an inclined plate in water, Heat Transf. Jpn. Res. 5 (4) (1976) 1–16. [13] D.W. Zhou, D.Y. Liu, X.G. Hu, C.F. Ma, Effect of acoustic cavitation on boiling heat transfer, Exp. Therm. Fluid Sci. 26 (2002) 931–938. [14] H. Kim, Y.G. Kim, B.H. Kang, Enhancement of natural convection and pool boiling heat transfer via ultrasonic vibration, Int. J. Heat Mass Transfer 47 (2004) 2831–2840. [15] H.V. Fairbanks, Influence of ultrasound upon heat transfer systems, Ultrason. Symp. (1979) 384–387. [16] B. Tajik, A. Abbassi, M. Saffar-Avval, A. Abdullah, H. Mohammad-Abadi, Eng. Appl. Comput. Fluid Mech. 6 (3) (2012) 366–381. [17] J.H. Lienhard IV, J.H. Lienhard V, A Heat Transfer Textbook, third ed., Phlogiston Press, Cambridge, Massachusetts, USA, 2006.