Experimental study of the ultrasonic effect on heat transfer inside a horizontal mini-tube in the laminar region

Accepted Manuscript Research Paper Experimental Study of the Ultrasonic Effect on Heat Transfer inside a Horizontal Mini-Tube in the Laminar Region Hou Kuan Tam, Lap Mou Tam, Afshin J. Ghajar, I Ping Chen PII: DOI: Reference:

S1359-4311(16)32035-X http://dx.doi.org/10.1016/j.applthermaleng.2016.09.166 ATE 9191

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

29 February 2016 28 September 2016 30 September 2016

Please cite this article as: H. Kuan Tam, L. Mou Tam, A.J. Ghajar, I. Ping Chen, Experimental Study of the Ultrasonic Effect on Heat Transfer inside a Horizontal Mini-Tube in the Laminar Region, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.09.166

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Experimental Study of the Ultrasonic Effect on Heat Transfer inside a Horizontal Mini-Tube in the Laminar Region Hou Kuan Tama,*, Lap Mou Tama,b, Afshin J. Ghajarc, I Ping Chena a

Department of Electromechanical Engineering, Faculty of Science and Technology, University of Macau, Macau, China

b

Institute for the Development and Quality, Macau, China

c

School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, Oklahoma, USA

Abstract Ultrasound has been widely used in the drying, welding, cleaning, chemical, and heat transfer processes. Moreover, in recent years there has been an increased interest in the application of ultrasound for heat transfer enhancement in the field of heat transfer. However, only a few papers have investigated the effect of ultrasound on heat transfer inside the horizontal tube. Therefore, the objective of this experimental study is to investigate the influence of ultrasound on the heat transfer inside the horizontal tube in the laminar region. In this study, a stainless steel test tube with a diameter of 4 mm was used as the test section under the uniform wall heat flux boundary condition. The entrance and fully developed regions heat transfer coefficients were analyzed. The Reynolds number for the experiments ranged from 600 to 3000. A series of experiments with the different numbers of ultrasonic heads and the different locations of the heads placed on the tube were conducted. The results

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showed that a substantial enhancement in heat transfer by ultrasound was observed in the laminar region. Based on the combinations of different numbers and positions of ultrasonic heads, two ultrasonic heads were observed to give a better heat transfer enhancement in the entrance and fully developed regions. Keywords: Horizontal mini-tube, Ultrasound, Heat transfer enhancement, Laminar region

* Corresponding Author: Dr. Hou Kuan Tam, Department of Electromechanical Engineering, Faculty of Science and Technology, University of Macau, Avenida da Universidade, Taipa, Macau, China. E-mail: [email protected], Phone Number: (+853) 8822 4289, Fax Number: (+853) 8822 2426

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Nomenclature cp

specific heat of the test fluid evaluated at Tb, J/(kg⋅K)

Di

inside diameter of the test section (tube), mm

g

acceleration due to gravity, m/s2

Gr

Grashof number [= g·β·ρ2·Di3·(Tw-Tb)/µ b2], dimensionless

h

local peripheral heat transfer coefficient, W/(m2⋅K)

k

thermal conductivity evaluated at Tb, W/(m⋅K)

L

length of the test section (tube), m

Nu

local average or fully developed peripheral Nusselt number (= h·Di/k), dimensionless

Pr

local bulk Prandtl number (= cp·µb/k), dimensionless

Re

local bulk Reynolds number (= ρ·V·Di/µb), dimensionless

St

local average or fully developed peripheral Stanton number [= Nu/(Pr·Re)], dimensionless

Tb

local bulk temperature of the test fluid, ºC

Tw

local inside wall temperature, ºC

TC

thermocouple

V

average velocity in the test section, m/s

x

local axial distance along the test section from the inlet, m

Greek symbols β

coefficient of thermal expansion of the test fluid evaluated at Tb, K-1

µb

absolute viscosity of the test fluid evaluated at Tb, Pa⋅s

µw

absolute viscosity of the test fluid evaluated at Tw, Pa⋅s

ρ

density of the test fluid evaluated at Tb, kg/m3 3

Subscripts l

laminar flow

t

turbulent flow

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1. Introduction Heat transfer enhancement is an important topic in the area of thermal engineering. For the tube flow, the heat transfer can be enhanced by the passive or active methods [1]. Passive methods consist of roughing the tube inner surface, inserting the swirlflow devices into the tube, and adding the solid-particles into the fluid. Traditional active methods are the application of mechanical aids, vibration, and electrostatic fields on the tube. Tam and his coworkers [2] applied the passive method, i.e. the micro-fins, to increase the heat transfer inside the macro-tube of 14.8 mm diameter. Although the increase of heat transfer was obvious in the upper transition and turbulent regions, that method was not applicable for the mini- or even the microtubes because it was difficult to fabricate the small fins on the inner surface of the mini- or micro-tubes. Therefore, an alternative heat transfer enhancement method is required for the smaller diameter tubes. In the last decades, ultrasound has been gradually used in the chemical processes, cleaning, and thermal systems. Regarding to the heat transfer enhancement by ultrasound, Legay et al. [3] reviewed some ultrasonic papers in the studies of boiling heat transfer, convective heat transfer, and heat exchanger. However, only a few papers were related to the tube flow heat transfer with ultrasound. Monnot et al. [4] applied ultrasound to a tank filled with water and measured the heat transfer coefficient in the coil tube immersed into the tank. The maximum enhancement ratio of the heat transfer coefficient was 2.04. It was indicated that ultrasound could improve heat transfer especially in the higher flow rates. Legay and his coworkers [58] investigated the heat transfer enhancement by ultrasound with two types of heat exchangers, double-tube heat exchanger and shell-and-tube heat exchanger. For those types of heat exchangers, the heat transfer enhancement ratio ranged from 1.2 to 2.54.

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Dhanalakshmi et al. [9] used ultrasonic transducers to increase the heat transfer in the 50 mm furnace tube, which was partially immersed in the furnace. It was observed that ultrasound had about 30% heat transfer enhancement for Reynolds numbers of less than 500. Recently, Li et al. [10] and Zheng et al. [11] studied the effect of ultrasound on sub-cooled boiling heat transfer around the tubes with different surface characteristics immersed in different solutions. The results showed that the heat transfer enhancement induced by ultrasound for the tubes with surface structures was more effective than that for the smooth tube. Also, the poor heat transfer of LiBr solution in the smooth tube can be improved by the finned tube with the application of the ultrasound to the solution. Although the above-mentioned papers showed the enhancement of heat transfer for the tube by ultrasound, they did not investigate the influence of ultrasound on heat transfer inside the mini- and micro-tubes. Moreover, the effect of the arrangement of ultrasonic device on the heat transfer was not discussed in those studies. Therefore, the objective of this experimental study is to investigate the influence of ultrasound on the heat transfer inside the horizontal mini-tube in the laminar region and find out the better arrangement of the ultrasonic device for the tube.

2. Experimental setup The experimentation for this study was performed using a relatively simple but highly effective apparatus. The apparatus used was designed with the intention of conducting highly accurate heat transfer measurements. The apparatus consists of four major components. These are the fluid delivery system, the flow meter, the test section assembly, and the data acquisition system. An overall schematic for the experimental test apparatus is shown in Fig. 1. The fluid delivery system consists of a

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high pressure cylinder filled with ultra-high purity nitrogen in combination with a stainless steel pressure vessel. After the working fluid passes through the apparatus, it is collected into a container. The working fluid, distilled water is stored in the stainless steel pressure vessel. As the pressurized nitrogen is fed into the pressure vessel, the working fluid is forced up a stem extending to the bottom of the vessel, out of the pressure vessel, and through the flow meter and test section. Flow rate of the water is regulated using a metering valve. Flow meter was factory calibrated. The accuracy of the mass flow rate is within ±1.75%. After passing through the flow meter, fluid enters the test section assembly. The test section assembly contains the test section as well as the equipment necessary for measurement of inlet and outlet fluid temperature and generation of the ultrasound. The test section is mounted on the aluminium supports. The adjustable bolts and a level were used to keep the test section in a horizontal position. In this study, the test section was a horizontal stainless circular tube with 4 mm inside diameter and 6.18 mm outside diameter. The total length of the test section was 1350 mm, providing a maximum length-to-inside diameter ratio (L/Di) of 330. As shown in Fig. 1, electric copper wires were soldered on to both ends of the test tube. A DC power supply was used to provide the uniform wall heat flux boundary condition. The voltage was measured at the soldered positions of the tube and the current was measured from the electric wire connected from the soldered position to the DC power supply. The range of wall heat flux of this study was from 2.9 kW/m2 to 14.8 kW/m2. For the temperature measurements, the inlet and exit bulk temperatures were measured by means of thermocouple probes (Omega TMQSS125U-6) placed before and after the test section. Also, for the heat transfer experiments, self-adhesive thermocouples (Omega SA1XL-T-72) were placed along

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the test section. All the thermocouples and thermocouple probes were calibrated by a NIST-calibrated thermocouple probe (±0.22°C) and an Omega HCTB-3030 constant temperature circulating bath. Therefore, the temperature sensors were as accurate as ±0.22°C.

Figs. 2 (a & b) show the arrangement of the thermocouples (TC) and ultrasonic heads on the test section. From Fig. 2(a), it can be seen at each station that the thermocouples denoted as TC1, TC2, TC3 and TC4 are placed 90̊ apart around the periphery of the tube. After installation of the thermocouples, the tested tube was covered by self-adhesive elastomeric insulating material. From the local peripheral wall temperature measurements at each axial location, the inside wall temperatures and the local heat transfer coefficients were calculated by the method shown in [12]. In these calculations, the axial conduction was assumed negligible (RePr > 3000), but peripheral and radial conduction of heat in the tube wall were included. In addition, the bulk fluid temperature was assumed to increase linearly from the inlet to the outlet. Also, the dimensionless numbers, such as Reynolds, Prandtl, Grashof, and Nusselt numbers, were computed by the computer program developed by [12]. The range of Reynolds number for this study was from 600 to 3000. Heat balance errors were calculated for all experimental runs by taking a percent difference between two methods of calculating the heat addition. The product of the voltage drop across the test section and the current carried by the tube was the primary method, while the fluid enthalpy rise from inlet to exit was the secondary method. In all cases the heat balance error was less than ±10%. The primary method was the one used in the computer program [12] for all heat flux and heat transfer coefficient calculations. The maximum uncertainty of the heat transfer coefficient over the entire range of

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Reynolds numbers was 15.6%. The calculation method was based on Kline and McClintock [13]. The Kline and McClintock method was used to determine the uncertainty of a calculation given certain measurements and the tolerances on those measurements. In this study, those measurements such as the measurements of bulk and surface temperatures, voltage, current, tube diameter and heated length were involved in the calculation of the uncertainty of the heat transfer coefficient. Fig. 2(a) also shows the total of six ultrasonic heads placed on the test section (three at the top and three at the bottom). The ultrasonic heads are arranged at x/Di of 25, 105 and 185 and labeled as TOP#25, TOP#105, TOP#185, BOT#25, BOT#105, BOT#185. As shown in Fig. 3, each ultrasonic head is a piezoelectric transducer which has a base diameter of 45 mm. Accroding to Steinke and Kandlikar [14], vibration induced by piezoelectric materials being attached to the periphery of the conventional macro-channel and mini- and micro-channels is potentially an active technique of heat transfer enhancment. The frequency and the power of the head used in this study were listed as 40 kHz and 60 W, respectively. The electric wires were connected from the positive and negative electrodes of the ultrasonic head to the ultrasonic generator. At the bottom of each head, there is a 4mm wide V-slot cut and the tested tube passed through the V-slot. While the ultrasound was applied to the test section, the heat was produced from the ultrasonic head. To avoid the heat from each head entering the tested tube, a thin paper of 0.6 mm was placed at the V-slot and an external USB fan was installed nearby the ultrasonic head. The fan was turned on during the operation of the ultrasonic head. For data acquisition, a National Instruments SCXI-1000 data collecting system was used. All digital signals from the flow meter and thermocouples were acquired and recorded by the Windows-based PC with a self- developed LabVIEW program.

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3. Results and discussion To verify the new experimental setup, experiments without ultrasound were conducted first. Fig. 4 shows the comparison of the current 4 mm tube fully developed heat transfer data (at the x/Di = 250) with the data of Ghajar and Tam [15] (at the x/Di = 200) for a 15.8 mm stainless steel tube with a square-edged inlet. The square-edged inlet data [15] was used because the current inlet was also similar to a square-edged inlet. Basically, the present laminar and lower transition heat transfer data could follow the data trend of [12], the experimental setup and the heat transfer data were confirmed to be reliable. In the figure, it should be noted that the parallel shift from the classical fully developed value of Nu = 4.364 for the uniform wall heat flux boundary condition in the laminar region is due to the buoyancy effect. The current data was closer to the classical laminar line because the buoyancy effect of the current study was weaker than that in [12]. As shown in Table 1, the less buoyancy effect (i.e. the lower Grashof number) for the current study was due to the less uniform heat flux applied onto the test tube. Fig. 5 shows the repeatability of the experimental data tested at x/Di of 250 with and without the ultrasound. From the figure, it can be confirmed that the laminar data of this study is repeatable because the deviation of the no-ultrasound data is within ±3% and that of the ultrasound data with the ultrasonic head combination 18 (see Table 2) is within ±2%.

3.1 Fully developed flow results After the verifications of the experimental setup, the heat transfer measurements with the different ultrasound combinations for the 4 mm mini-tube were conducted.

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Table 2 lists the combinations of the ultrasonic heads activated at the top or bottom of the axial location x/Di of 25, 105 or 185. The number of heads used in each combination is also shown in Table 2. Fig. 6 shows the fully developed laminar region Nusselt numbers at x/Di of 250 with the single ultrasonic head activated at each test (i.e., combinations 1 to 6 shown in Table 2). As seen in the figure, an obvious increase of heat transfer was observed for all ultrasonic combinations when compared with no ultrasound applied onto the tested tube. Moreover, it could also be observed that the combinations 3 to 6 had slightly higher heat transfer enhancement than the combinations 1 and 2. However, a significant difference between the ultrasonic head placed at the top or bottom of the tube cannot be seen in the figure. Table 3 shows the average and the range of heat transfer enhancement in percent for combinations 1 to 6. In the table, combination 3 gave the best average heat transfer enhancement value of 9.9%. The average values for combinations 3 to 6 ranged from about 6% to 10%. However, the average heat transfer increase for combinations 1 and 2 was less than 6%. Since the ultrasonic head of combinations 1 and 2 was arranged at the location of x/Di of 25, the ultrasonic effect on heat transfer might have been interfered with due to the unstable entrance flow inside the tube. Fig. 7 shows the fully developed laminar region Nusselt numbers at x/Di of 250 with double ultrasonic heads activated at each test (i.e., combinations 7 to 15 shown in Table 2). As seen in the figure, an obvious increase of heat transfer was observed for all ultrasonic combinations when compared with no ultrasound applied onto the tested tube. However, it was seen that the combination 13 provided less heat transfer enhancement while both of the two heads (BOT#25 and TOP#25) were activated at the location of x/Di of 25. It could be explained that the ultrasonic effect on heat

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transfer may have been interfered with due to the unstable entrance flow inside the tube. For this reason we do not recommend any of the results for the x/Di of 25. Table 4 shows the average and the range of heat transfer enhancement in percent for combinations 7 to 15. Except combination 13, the average heat enhancement for all other combinations ranged from 8.1 to 12.9%. As compared with the single head (as seen in Table 3), the heat transfer enhancement of the double heads was more significant. Fig. 8 shows the fully developed laminar region Nusselt numbers at the x/Di of 250 with three and six ultrasonic heads activated at each test (i.e., combinations 16 to 18 shown in Table 2). As seen in the figure, an obvious increase of heat transfer was observed for all ultrasonic combinations when compared with no ultrasound applied onto the tested tube. It was also seen that the experimental data of three heads used (combinations 16 and 17) was slightly higher than that of the six heads used (combination 18). The reason may be due to the stronger counteraction of ultrasound generated from the six heads. The explanation for the results observed in this case should be further verified by other scientific methods such as observing the flow characteristics through the transparent tube or measuring the tube vibration. Table 5 shows the average and the range of heat transfer enhancement in percent for combinations 16 to 18. The average heat enhancement with using three heads and six heads were around 9% and 6%, respectively. Those values were not greater than those of the double heads (as shown in Table 4). It was denoted that the increase of heat transfer was not absolutely increased by the number of ultrasonic heads. Therefore, double heads are sufficient for heat transfer enhancement by ultrasound.

3.2 Entrance flow results

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In this study, the entrance region heat transfer experiments were also conducted. Fig. 9 shows entrance flow laminar region Nusselt numbers at x/Di of 50 with the single ultrasonic head at each test (i.e., combinations 1 to 6 shown in Table 2). As shown in the figure, an obvious increase of heat transfer was observed for all ultrasonic combinations when compared with no ultrasound applied onto the tested tube. Moreover, it could also be observed that the combinations 3 to 6 had greater heat transfer enhancement than the combinations 1 and 2. However, a significant difference between the ultrasonic head placed at the top or bottom of the tube cannot be seen in the figure. The observations of heat transfer enhancement in the entrance region was the same as those in the fully developed region when compared with Fig. 6. Table 6 shows the average and the range of heat transfer enhancement in percent for combinations 1 to 6. In the table, combination 3 gave the best average heat transfer enhancement value of 18.4%. The average values for combinations 3 to 6 ranged from about 14% to 18%. However, the average values for combinations 1 and 2 was less than 10%. Since the ultrasonic head of combinations 1 and 2 was arranged at the location of x/Di of 25, the ultrasonic effect on heat transfer might have been interfered with due to the unstable entrance flow inside the tube. Fig. 10 shows the entrance region Nusselt numbers at x/Di of 50 with double ultrasonic heads activated at each test (i.e., combinations 7 to 15 shown in Table 2). As seen in the figure, an obvious increase in heat transfer was observed for all ultrasonic combinations when compared with no ultrasound applied onto the tested tube. However, it was seen that the combination 13 provided less heat transfer enhancement while both of the two heads (BOT#25 and TOP#25) were activated at the location of x/Di of 25. It could be explained that the ultrasonic effect on heat transfer may have been interfered with due to the unstable entrance flow inside the

13

tube. The same results were also observed in the fully developed region (see Fig. 7). Therefore we do not recommend any of the results for the x/Di of 25. Table 7 shows the average and the range of heat transfer enhancement for combinations 7 to 15. Except the combination 13, the average heat transfer enhancement for all other combinations ranged from 10.8 to 19.2%. As compared with the single head (as seen in Table 6), basically in the entrance region, the heat transfer enhancement of the double heads was also more significant. Fig. 11 shows the entrance region Nusselt numbers at x/Di of 50 with three and six ultrasonic heads activated at each test (i.e., combinations 16 to 18 shown in Table 2). As seen in the figure, an obvious increase of heat transfer was observed for all ultrasonic combinations when compared with no ultrasound applied onto the tested tube. It was also seen that the experimental data of three heads used (combinations 16 and 17) was almost the same as that of the six heads used (combination 18). Table 8 shows the average and the range of heat transfer enhancement for the combinations 16 to 18. The average heat transfer enhancement with using three heads and six heads were approximately 10%. Those values were not greater than those of the double heads (as shown in Table 7). It was denoted that the increase of heat transfer in the entrance region was not absolutely increased by the number of ultrasonic heads. Therefore, double heads were sufficient for heat transfer enhancement ultrasound based on the entrance and fully developed region results. Fig. 12 compares the average heat transfer enhancement of the entrance region and fully developed region by ultrasound. From the figure, it can be seen that the heat transfer enhancement by ultrasound in the entrance region (x/Di = 50) is much more pronounced than that in the fully developed region (x/Di = 250). Especially, the two ultrasonic heads (combinations 7 to 12) show a better heat transfer enhancement for both of the fully developed and entrance regions. The reason why the effect of

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ultrasound on entrance region heat transfer is more pronounced is due to the thinner thermal boundary layer, which is easier to be disturbed by the ultrasonic vibration. Therefore, more heat could be transferred through the boundary layer from the wall to the tube-center free stream region. In the future, it is recommended to verify the disturbance of thermal boundary layer by ultrasound with the more sophisticated methods such as laser-induced fluorescence (LIF) and particle image velocimetry (PIV).

4. Conclusions In this study, laminar heat transfer experiments for a horizontal mini-tube were conducted with ultrasonic device under uniform wall heat flux boundary condition. The current experimental setup was verified with the experimental data published in the open literature. From the experimental results, it can be summarized that: •

To increase the fully developed region heat transfer, the installation of ultrasonic head at the earlier entrance location (i.e., x/Di = 25) is not recommended.

•

For either the fully developed or entrance region, double ultrasonic heads basically provide a better heat transfer enhancement than other number of heads.

•

Heat transfer enhancement by ultrasound in the entrance region is more significant than that in the fully developed region.

•

In the future, it is proposed to use more sophisticated methods such as PIV and LIF to investigate the mechanism of ultrasonic effect on heat transfer inside mini-tubes.

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Acknowledgments This research is supported by the Institute for the Development and Quality, Macau.

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References [1]

R. L. Webb and N. H. Kim, Principles of Enhanced Heat Transfer, 2nd Edition, Taylor & Francis, New York, NY, 2005.

[2]

H. K. Tam, L. M. Tam, A. J. Ghajar, S. C. Tam, T. Zhang, “Experimental Investigation of Heat Transfer, Friction Factor, and Optimal Fin Geometries for the Internally Micro-fin Tubes in the Transition and Turbulent Regions,” Journal of Enhanced Heat Transfer, 2012, 19(5), pp. 457-476.

[3]

M. Legay, N. Gondrexon, S. L. Person, P. Boldo and A. Bontemps, “Enhancement of Heat Transfer by Ultrasound: Review and Recent Adances,” International Journal of Chemical Engineering, 2011, Article ID 760108, 17 pages.

[4]

A. Monnot, P. Boldo, N. Gondrexon, and A. Bontemps, “Enhancement of Cooling Rate by Means of High Frequency Ultrasound,” Heat Transfer Engineering, 2007, 28(1), pp. 3-8.

[5]

N. Gondrexon, Y. Rousselet, M. Legay, P Boldo, S. L. Person and A. Bontemps, “Intensification of Heat Transfer Process: Improvement of Shelland-Tube Heat Exchanger Performances by Means of Ultrasound,” Chemical Engineering and Processing: Process Intensification, 2010, 49(9), pp. 936-942.

[6]

M. Legay, B. Simony, P. Boldo, N.Gondrexon, S. L. Person and A. Bontemps, “Improvement of Heat Transfer by Means of Ultrasound: Application to a Double-Tube Heat Exchanger,” Ultrasonics Sonochemistry, 2012, 19(16), pp. 1194-1200.

[7]

M. Legay, S. L. Person, N. Gondrexon, P. Boldo and A. Bontemps, “Performances of Two Heat Exchangers Assisted by Ultrasound,” Applied Thermal Engineering, 2012, 37, pp. 60-66.

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[8]

N. Gondrexon, L. Cheze, Y. Jin, M. Legay, Q. Tissot, N. Hengl, S. Baup, P. Boldo, F. Pignon, and E. Talansier, “Intensification of Heat and Mass Transfer by Ultrasound: Application to Heat Exchangers and Membrane Separation Processes,” Ultrasonics Sonochemistry, 2015, 25, pp. 40-50.

[9]

N. P. Dhanalakshmi, R. Nagarajan, N. Sivgaminathan and B. V. S. S. S. Prasad, “Acoustic Enhancement of Heat Transfer in Furnace Tubes,” Chemical Engineering and Processing: Process Intensification, 2012, 59, pp. 36-42.

[10] B. Li, X. Han, Z. Wan, X. Wang, Y. Tang, “Influence of Ultrasound on Heat Transfer of Copper Tubes with Different Surface Characteristics in Sub-cooled Boiling,” Applied Thermal Engineering, 2016, 92, pp. 93-103. [11] M. Zheng, B. Li, Z. Wan, B. Wu, Y. Tang, J. Li, “Ultrasonic Heat Transfer Enhancement on Different Structural Tubes in LiBr Solution,” Applied Thermal Engineering, 2016, 106, pp. 625-633. [12] A. J. Ghajar and J. Kim, Calculation of Local Inside-Wall Convective Heat Transfer Parameters from Measurements of the Local Outside-Wall Temperatures along an Electrically Heated Circular Tube, in Heat Transfer Calculations (edited by Myer Kutz), McGraw-Hill, New York, NY, 2006, pp. 23.3-23.27. [13] S. J. Kline and F. A. McClintock, “Describing Uncertainties in Single Sample Experiments,” Mech. Eng., 1953, 75(1), pp. 3-8. [14] M. E. Steinke and S. G. Kandlikar, “Review of Single-Phase Heat Transfer Enhancement Techniques for Application in Microchannels, Minichannels and Microdevices”, International Journal of Heat and Technology, 2004, 22(2), pp. 3-11.

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[15] A. J. Ghajar and L. M. Tam, “Heat Transfer Measurements and Correlations in the Transition Region for a Circular Tube with Three Different Inlet Configurations”, Experimental Thermal and Fluid Science, 1994, 8(1), pp. 7990.

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List of Figures Fig. 1 Experimental setup Fig. 2 (a) The arrangement of the thermocouples and the ultrasonic heads; (b) Photograph of the test section showing the arrangment of the ultrasonic heads on the top and bottom of the test tube Fig. 3 Ultrasonic head Fig. 4 Comparison of the present fully developed heat transfer data with experimental data of [15] at x/Di = 200 Fig. 5 Repeatability of the test data Fig. 6 Fully-developed laminar region Nusselt numbers for combinations 1 to 6 at x/Di = 250 Fig. 7 Fully-developed laminar region Nusselt numbers for combinations 7 to 15 at x/Di = 250 Fig. 8 Fully-developed laminar region Nusselt numbers for combinations 16 to 18 at x/Di = 250 Fig. 9 Entrance flow laminar region Nusselt numbers for combinations 1 to 6 at x/Di = 50 Fig.10 Entrance flow laminar region Nusselt numbers for combinations 7 to 15 at x/Di = 50 Fig.11 Entrance flow laminar region Nusselt numbers for combinations 16 to 18 at x/Di = 50 Fig.12 Comparison of the heat transfer enhancement by ultrasound for the entrance and fully developed regions

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PC Based Data Acquisition System with LabVIEW Programs DC Power Source

Test Section

Ultrasound Generator

Surface Thermocouples

Metering Valve Outlet Thermocouple

Inlet Thermocouple

Container National Instruments Data Acquisition System

Filter Coriolis Liquid Flow Meter High Pressure Nitrogen Tank

Pressure Vessel Filled with Distilled Water

Fig. 1 Experimental setup

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(a)

(b) Fig. 2 (a) The arrangement of the thermocouples and the ultrasonic heads; (b) Photograph of the test section showing the arrangment of the ultrasonic heads on the top and bottom of the test tube

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10mm

40mm The negative electrode

Piezoelectric ceramic

45mm

Fig. 3 Ultrasonic head

24

49mm

3mm

25.5mm 4mm

The positive electrode

0.67

StlPr

0.01

SttPr

-1

-0.33

=4.364Re Pr

0.67

=0.023Re

-0.2

.

(Pr=5)

(µb/µw)

0.14

StPr

0.67

Ghajar and Tam [15]: 15.8mm tube. Square-edged Current data, 4mm tube, No-ultrasound

0.001 600

1000

2500 3000

Re

Fig. 4 Comparison of the present fully developed heat transfer data with experimental data of [15] at x/Di = 200

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7.0 No-ultrasound (deviation of 3%) Combination 18 (deviation of 2%) 6.5

Nu

6.0

5.5

5.0

4.5

4.0 400

600

800

1000

1200

1400

1600

Re

Fig. 5 Repeatability of the test data

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1800

2000

6.4 No-ultrasound Combination 1 Combination 2 Combination 3 Combination 4 Combination 5 Combination 6

6.2 6.0

Nu

5.8 5.6 5.4 5.2 5.0 4.8 400

600

800

1000

1200

1400

1600

1800

2000

Re

Fig. 6 Fully-developed laminar region Nusselt numbers for combinations 1 to 6 at x/Di = 250

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7.0 No-ultrasound Combination 7 Combination 8 Combination 9 Combination 10 Combination 11 Combination 12 Combination 13 Combination 14 Combination 15

Nu

6.5

6.0

5.5

5.0

400

600

800

1000

1200

1400

1600

1800

2000

Re

Fig. 7 Fully-developed laminar region Nusselt numbers for combinations 7 to 15 at x/Di = 250

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6.4 No-ultrasound Combination 16 Combination 17 Combination 18

6.2 6.0

Nu

5.8 5.6 5.4 5.2 5.0 4.8 400

600

800

1000

1200

1400

1600

1800

2000

Re

Fig. 8 Fully-developed laminar region Nusselt numbers for combinations 16 to 18 at x/Di =250

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12 No-ultrasound Combination 1 Combination 2 Combination 3 Combination 4 Combination 5 Combination 6

11 10

Nu

9 8 7 6 5 4 400

600

800

1000

1200

1400

1600

1800

Re

Fig. 9 Entrance flow laminar region Nusselt numbers for combinations 1 to 6 at x/Di = 50

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12 No-ultrasound Combination 7 Combination 8 Combination 9 Combination 10 Combination 11 Combination 12 Combination 13 Combination 14 Combination 15

11 10

Nu

9 8 7 6 5 4 400

600

800

1000

1200

1400

1600

1800

Re

Fig. 10 Entrance flow laminar region Nusselt numbers for combinations 7 to 15 at x/Di = 50

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12 No-ultrasound Combination 16 Combination 17 Combination 18

11 10

Nu

9 8 7 6 5 4 400

600

800

1000

1200

1400

1600

1800

Re

Fig. 11 Entrance flow laminar region Nusselt numbers for combinations 16 to 18 at x/Di = 50

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25

Average % of increase of heat transfer

Entrance region (x/Di = 50) Fully developed region (x/Di = 250) 20

15

10

5

0 0

2

4

6

8

10

12

14

16

18

Combinations

Fig. 12 Comparison of the heat transfer enhancement by ultrasound for the entrance and fully developed regions

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List of Tables Table 1 Comparison of the heating condition of the present study with [15] Table 2 The combinations of ultrasonic heads Table 3 Percentage of the range and average heat transfer enhancement for combinations 1 to 6 at x/Di = 250 Table 4 Percentage of the range and average heat transfer enhancement for combinations 7 to 15 at x/Di = 250 Table 5 Percentage of the range and average heat transfer enhancement for combinations 16 to 18 at x/Di = 250 Table 6 Percentage of the range and average heat transfer enhancement for combinations 1 to 6 at x/Di = 50 Table 7 Percentage of the range and average heat transfer enhancement for combinations 7 to 15 at x/Di = 50 Table 8 Percentage of the range and average heat transfer enhancement for combinations 16 to 18 at x/Di = 50

34

Table 1 Comparison of the heating condition of the present study with [15]

Range of Uniform Heat Flux Grashof Number (Gr)

Current Experiments

Ghajar and Tam [15] Experiments

2.97-14.7 kW/m2

4-670 kW/m2

400-2200

1000-250000

35

Table 2 The combinations of ultrasonic heads Location of the Ultrasonic Heads Activated Label

No. of Heads TOP#25 BOT#25 TOP#105 BOT#105 TOP#185 BOT#185

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 3 3 6

● ● ● ● ● ● ● ●

● ● ●

● ● ●

● ● ●

● ●

● ●

● ●

● ● ●

● ● ●

●

36

● ●

●

● ● ●

Table 3 Percentage of the range and average heat transfer enhancement for combinations 1 to 6 at x/ Di = 250 Combinations

Label of Heads

1 2 3 4 5 6

TOP#25 BOT#25 TOP#105 BOT#105 TOP#185 BOT#185

37

Heat Transfer Enhancement (%) Range Average 3.2-8.1 5.8 4.2-5.9 5.4 7.4-14.5 9.9 1.8-8.6 6.3 6.3-10.6 8.3 7.7-9.5 8.5

Table 4 Percentage of the range and average heat transfer enhancement for combinations 7 to 15 at x/ Di = 250 Combinations

Label of Heads

7 8 9 10 11 12 13 14 15

TOP#25, TOP#105 TOP#25, TOP#185 TOP#105, TOP#185 BOT#25, BOT#105 BOT#25, BOT#185 BOT#105, BOT#185 TOP#25, BOT#25 TOP#105, BOT#105 TOP#185, BOT#185

Heat Transfer Enhancement (%) Range Average 8.8-14.1 11.5 9.3-15.5 11.2 7.5-10.2 9.0 9.7-16.7 12.9 8.3-11.5 9.7 9.3-14.1 11.0 3.2-5.2 4.5 3.4-13.9 8.1 6.8-13.2 9.9

38

Table 5 Percentage of the range and average heat transfer enhancement for combinations 16 to 18 at x/ Di = 250

Combinations 16 17 18

Label of Heads TOP#25, TOP#85, TOP#105 BOT#25, BOT#85, BOT#105 TOP#25, BOT#25, TOP#85, BOT#85, TOP#105, BOT#105

39

Heat Transfer Enhancement (%) Range Average 7.8-9.8

9.1

7.9-10.0

8.9

5.3-7.1

6.6

Table 6 Percentage of the range and average heat transfer enhancement for combinations 1 to 6 at x/Di = 50 Heat Transfer Enhancement (%) Combinations

Label of Heads Range

Average

1

TOP#25

4.4-13.3

9.6

2

BOT#25

1.3-9.2

6.2

3

TOP#105

12.0-19.3

18.4

4

BOT#105

6.3-17.2

14.2

5

TOP#185

11.1-19.7

15.3

6

BOT#185

9.9-18.5

14.5

40

Table 7 Percentage of the range and average heat transfer enhancement for combinations 7 to 15 at x/Di = 50 Heat Transfer Enhancement (%) Range Average 11.4-19.2 15.8

Combinations

Label of Heads

7

TOP#25, TOP#105

8

TOP#25, TOP#185

13.6-23.7

19.2

9

TOP#105, TOP#185

6.5-12.9

10.8

10

BOT#25, BOT#105

13.7-23.5

19.1

11

BOT#25, BOT#185

6.9-13.5

11.7

12

BOT#105, BOT#185

14.4-20.5

18.1

13

TOP#25, BOT#25

4.0-10.5

7.8

14

TOP#105, BOT#105

8.2-13.3

10.9

15

TOP#185, BOT#185

10.1-17.2

12.2

41

Table 8 Percentage of the range and average heat transfer enhancement for combinations 16 to 18 at x/Di = 50 Combinations 16 17 18

Heat Transfer Enhancement (%)

Label of Heads TOP#25, TOP#85, TOP#105 BOT#25, BOT#85, BOT#105 TOP#25, BOT#25, TOP#85, BOT#85, TOP#105, BOT#105

42

Range

Average

6.1-13.1

10.8

6.8-11.1

9.8

6.0-15.7

10.4

Highlights: •

For a mini-tube, a substantial heat transfer enhancement by ultrasonic head was observed in the laminar region.

•

Based on the combinations of different numbers and positions of ultrasonic heads, two ultrasonic heads were observed to give a better heat transfer enhancement.

•

Heat transfer enhancement by ultrasound in the entrance region is more significant than that in the fully developed region.

43