Objective of Present Study

Literature review conducted above clearly discloses that considerable works have been carried out in the past to investigate the enhancement in the pool boiling heat transfer over a surface by passive methods through fabricating surface microstructures such as roughness and micro-pin-fins and by covering the surface with mesh screens and particle coating. All these microstructures are fixed firmly onto the boiling surface. Besides, some effective active heat transfer augmentation methods such as vibrating or rotating heating surface and/or fluid and applying electric field to vibrate a heating surface have been suggested. Following the previous work on the enhancement of saturated pool boiling heat transfer [2], an experiment is conducted here to explore the possible enhancement in the subcooled FC-72 pool boiling heat transfer by placing movable copper particles on the boiling surface. The vigorous motion of the bubbles in the boiling flow can cause violent movement of the particles. Meanwhile, this violently moving particles can significantly affect the bubble dynamics near the surface. Thus we expect the boiling heat transfer from the surface can be enhanced. The method proposed here is passive in nature. However, it behaves like an active heat transfer enhancement method.

10

CHAPTER 2

EXPERIMENTAL APPARATUS AND PROCEDURES

A schematic arrangement of the experimental apparatus for the present investigation of the subcooled pool boiling heat transfer enhancement by movable metallic particles driven by the boiling flow is similar to that employed in the previous study [2] and is shown in Fig. 2.1. The experimental system includes a main test chamber, a test heater assembly, and other auxiliary parts such as a D.C. power supply, a data acquisition unit and a high-speed photographic unit. The working fluid, FC-72, is a highly wetting dielectric fluorocarbon liquid produced by 3M Industrial Chemical Products Division, which has been considered as a good candidate fluid for liquid immersion cooling applications. It is chemically stable, dielectric, and has a relatively low boiling point (Tsat=56C at atmospheric pressure). Some thermophysical properties of FC-72 are given in Table 2.1.

2.1 Main Test Chamber

The main test chamber is a hermetic stainless steel pressure vessel of 205 mm in height and 216 mm in diameter. An internal water condenser is installed inside the chamber and connects with a thermostat (LAUDA RK20) to maintain the bulk temperature of the working fluid in the chamber at the preset level. The maximum cooling power of the thermostat is 180W (at 20C). We further use an external temperature controller (FENWAL MYSPEC Digital Temperature Controller) to control the bulk temperature of FC-72 in the test chamber with an accuracy of  0.1C. Besides, a cartridge heater is located near the bottom of the test chamber to provide additional heating during the degassing process. In order to prevent the heat

11

loss from the vessel to the ambient, a superlon layer of 10-mm thick is wrapped around the chamber. Moreover, a pressure transducer with an operating range of 0-200 kPa is located at the gate valve to measure the pressure of the work fluid.

Meanwhile, the working fluid temperature is measured by two resistance temperature detectors (RTDs) located at the gate valve and at a selected location 5 cm above the bottom surface of the chamber with a calibrated accuracy of 0.1C. An auxiliary tank of 10-liter liquid FC-72 is placed right above the test vessel and it is only used for the subcooled pool boiling experiment to prevent regassing of the working fluid after degassing. A pressure transducer and a RTD are placed in the auxiliary tank to measure the internal gas pressure and liquid temperature. In addition, a test heater assembly is mounted to a stainless steel shelf to fix the PEEK (Polyether Ether Ketone) substrate. The working fluid is maintained at approximately 80 mm above the heated surface in the experiment.

2.2 Test Heater Assembly

A schematic of the test heater assembly is shown in Fig. 2.2. The assembly consists mainly of a film heater and it is adhered to the lower surface of a square copper plate by epoxy Omegabond 200. The plate is 10-mm thick with 30x30 𝑚𝑚² in surface area. The heater supplies the required power input to the copper plate. The copper plate is flush mounted onto a much larger PEEK block. Liquid FC-72 boils on the upper surface of the copper plate. More specifically, the copper plate is heated by D.C. current delivered from a D.C. power supply to the film heater. Besides, three calibrated copper-constantan thermocouples (T-type) with a calibrated accuracy of  0.2C are installed at selected locations in the copper plate right below the boiling surface. They are used for the control and determination of the boiling surface

12

temperature. The detailed locations of the thermocouples in the copper plate are shown in Fig. 2.3. Note that the whole copper plate is inserted into a PEEK block which serves as a heat insulator (k_T 0.25W/mK), intending to reduce the heat loss from the lateral and bottom surfaces of the plate to the ambient. Besides, the locations of thermocouples in the PEEK block which installed for the estimation of heat loss are shown in Fig. 2.4.

2.3 Confinement of Particles and Experimental Parameters

Solid particles of the same material and uniform size are placed freely on the upper surface of the copper plate, as schematically shown in Fig. 2.5. In order to insure that the particles would not be blown away by the vigorous motion of the bubbles, we install an acryl fence of 2-cm high and 1-cm thick around the edges of the heating copper plate. In the present study tests will be conducted for copper particles.

The density of copper is measured before the experiment with 𝜌_𝑐𝑢= 8990 𝑘𝑔 𝑚⁄ ³. The copper particle is chosen here because the copper have much higher density than liquid FC-72 and is used widely in the industry for its excellent conductivity which may enhance the thermal conduction from the plate. Besides, the chosen particles should not be too small so that they float in the liquid above the plate and do not contact the heating surface. Moreover, they should not be too large and cannot be moved by the boiling flow. Here, the particle diameter is selected to be 1.0 and 1.5 mm. On the other hand, the liquid FC-72 subcooling is varied from 5℃ to 20℃.

More specifically, four different liquid subcoolings at ∆𝑇_𝑠𝑢𝑏=5℃, 10℃, 15℃ and 20℃ are tested. The chosen particle size and number for the cases covered here are summarized in Table 2.2. The measured data expressed in terms of boiling curves and boiling heat transfer coefficients will be compared with that of a bare heating surface.

13

2.4 DC Power Supply

The power generated in the film heater in the test heater assembly is provided by a programmable D.C. power supply (Gpc 3030D). It offers a maximum D.C.

power of 180W for an output voltage of 60V and an output current of 3A. The power input to the copper block is transmitted through a GPIB interface to a personal computer. In order to measure the D.C. current, a precision ammeter (KYORITSU A.C./D.C. DIGITAL CLAMP METER) is arranged in series connection with the electric circuit. Besides, a YOKOGAWA data recorder is used to measure the voltage drop across the test heater assembly. All the voltage, current and power measurement devices are calibrated by a YOKOGAWA WT200 power meter according to the Center of Measurement Standards in Industrial Technology Research Institute of Taiwan.

2.5 Data Acquisition

A 20-channel YOKOGAWA data recorder (MX-100) combined with a personal computer is used to acquire and process the data from various transducers. All signals detected from the T-type thermocouples, RTDs, pressure transducer, ammeter, data recorders and power meter are all collected and converted by the internal calibration equations in the computer during the data acquisition.

2.6 Optical Measurement Technique

A high-speed camera along with a microscope is installed in front of the observation window to observe the boiling activity in the flow. The photographic apparatus consists mainly of a high speed digital video camera (IDT High-speed

14

CMOS Camera), a micro-lens (Optem Zoom 16), and a three-dimensional positioning mechanism. The high-speed motion analyzer can take photographs up to 143,307 frames/sec. In the present experiment the recording rate is 1000 frames/sec. After the experimental system reaches a statistical state, we start capturing the images of the particles and bubble in the boiling flow. Besides, we store and display the images in the personal computer through an image-capturing software.

2.7 Experimental Procedures

Before the experiment,

t

he boiling surface is polished by fine sand paper (Number 3000, 2000 and 1000) and cleaned by ethyl alcohol. In each test, we need to remove non-condensable gases existing in the empty test chamber by running a vacuum pump for about 15 minutes and then fill the FC-72 liquid into the chamber until the liquid level is higher than the heating plate for about 80 mm. Next, the FC-72 liquid in the test chamber is heated to 65℃ and boiled vigorously for about 1 hour to further remove the dissolved non-condensable gases in the dielectric liquid. During the process, the temperature in the chamber is detected and maintained by a digital temperature controller. After the working fluid pressure and temperature stabilize to one atmosphere and at the preset subcooled state, we turn on the test heater. The imposed heat flux on the boiling surface is adjusted by controlling the electric current delivered to the heater from the D.C. power supply. Upon reaching the statistical state, we begin collecting the required heat transfer data and visualizing the boiling activity.

After the experiment for the case of bare surface is finished, we put the chosen number and size of the copper particles on the copper plate and repeat the procedure described above. In order to keep the particles on the copper plate, we also place an acrylic rectangular enclosure onto the PEEK block around the edge of copper plate.

15

Effects of the particle size, number and subcooling of the working fluid on the possible boiling heat transfer enhancement will be investigated in the experiment.

16

Table 2.1 Thermophysical properties of FC-72.

Properties at 25^C FC-72

Appearance Clear, colorless

Average Molecular Weight 338

Boiling Point (1atm) 56C

Pour Point (1atm) -90C

Estimated Critical Temperature 449K

Estimated Critical Pressure 1.83 × 10⁶ Pa

Vapor Pressure 3.09 × 10⁴ Pa

Latent Heat of Vaporization h_fg (at normal boiling point)

88 J/g

Liquid Density _ 1680 kg/m³

Absolute Viscosity  6.4× 10^-3 poises ; 6.4× 10^-4 kg/m∙s Kinematic Viscosity  3.8 × 10^-3 stokes ; 3.8 × 10^-7 m²/ s

Liquid Specific Heat cp 1100 J/kg∙C

Liquid Thermal Conductivity k 0.057 W/m∙C

Coefficient of Expansion  0.00156 /C

Surface Tension  10 dynes/cm ; 10^-2 N/m

Material Subcooling Degrees 𝑁_𝑝

Copper

(1mm)

5℃

100 1000

200 1100

300 1200

400 1300

500 1400

600 1500

700 1600

800 1700

900 1800

10℃

^{200 1200}

400 1400

600 1600

800 1800

1000

15℃

^{200 800}

400 1200

600

20℃

^{800 1200}

Material Subcooling Degrees 𝑁_𝑝

Copper

(1.5mm)

5℃

^{200 600}

400 800

10℃

^{200 600}

400 800

15℃

^{200 600}

400 800

20℃

^{200 600}

400 800

Table 2.2 Cases covered in present study for 1.0 mm copper particle

Table 2.3 Cases covered in present study for 1.5 mm copper particle

Fig. 2.1 Schematic diagram of the test apparatus.

To Degassing Tank and Drain

Computer

To Degassing Tank and Drain

Computer

Fig. 2.2 Schematic diagram of the test heater assembly (not to scale).

30

30 7

100

Embedded Copper Plate

Electric Film Heater

Perspective view

Front view

Top view

PEEK substrate

PEEK substrate

(unit:mm)

Fig. 2.3 Locations of three thermocouples in the copper plate and one thermocouple below the heater

Fig. 2.4 Locations of two thermocouples in the PEEK (not to scale).

(unit:mm)

Perspective view

PEEK substrate





#7

#6

Front view

 ^3.5

 #7 #6

12

Top view

Copper Surface

 

#7 #6

Copper Plate

Copper Plate

31

Fig. 2.5 Schematic diagram of placing movable particles on heating surface with acryl rectangular enclosure (not to scale).

Perspective view

Top view

PEEK substrate

PEEK Surface

(unit:mm)

Right view

CHAPTER 3

DATA REDUCTION

3.1 Boiling Heat Transfer Coefficient

The space-average natural convection and boiling heat transfer coefficients over the upper surface of the heated square copper plate when the flow is at a statistical state are both defined as

h = 𝑞^𝑛⁄∆𝑇_𝑤 (3.1) where qn is the net heat flux imposed on the upper surface and ∆Tw is the wall superheat defined as the difference between the average surface temperature and the bulk liquid temperature of FC-72 which can be written as:

∆𝑇_𝑤 = 𝑇_𝑤 − 𝑇_𝑏 (3.2) The average heated surface temperature 𝑇_𝑤 is estimated from the measured average temperature from the thermocouples installed at different locations near the upper surface of the copper plate according to the steady-state one-dimensional conduction heat transfer. Specifically,



 



 





Cu n Cu

w k

q δ T

T (3.3)

where

T_Cu = the average measured temperature from the thermocouples installed in the copper plate (C)

k_Cu = the thermal conductivity of copper (W/m∙K)

= the vertical distance between the thermocouple tips and the upper surface of the copper plate (m)

The total power input Qt to the copper plate can be obtained from the measured voltage drop across the film heater in the test heater assembly and the current passing through it,

V I

Q_t   (3.4) where

Qt = total power input to the upper surface of the copper plate (W) I = electric current passing through the film heater (Amp.)

V = voltage drop across the film heater (Volts)

The substrate of the test section is made from PEEK, which have a much lower thermal conductivity (k_T 0.25W/mK) than the copper (k 386W/mK). In evaluating total heat loss from the heater assembly, we focus on heat transfer from the heater and copper plate surface to the PEEK. Figures 2.3 and 2.4 are the schematic diagrams of the thermocouples buried in the copper plate and PEEK. The heat dissipation model used to estimate the heat loss is shown in Fig. 3.1, and the total heat loss can be estimated as follows:

Q_loss = k_p ∙^(T8−T7)

L1 ∙ A₁+ 4 ∙ k_p ∙^(Tcu−T6)

L2 ∙ A₂+^{2π∙kp∙L3∙(Tcu−T6)}

ln( ^r6

rcu,2) +

2π∙k_p∙L₄∙(T_cu−T₆) ln( ^r6

rcu,3) +k_p ∙^{(Tcu−T′5)}

L5 ∙ A₅+k_p ∙^{(Tcu−T′6)}

L6 ∙ A₆

where

T₆ , T₇ , T₈ : the average measured temperatures from the thermocouples inside the PEEK insulator, as schematically shown in Figs. 2.3 & 2.4

𝐿 _1, 𝐿_{2 ,} 𝐿 _{3 ,} 𝐿 ₄ , 𝐿 ₅ , 𝐿 ₆: shortest distances between locations No.1~No.6 and the (3.5)

film heater or copper plate (m)

A1 , A2, A3, A4, A5, A6 : bottom and lateral surface areas of the copper plate

T'₅ , T'₆ : these two temperatures are calculated by using interpolation method based on 𝑇₆, as schematically shown in Fig. 3.2

Finally, the net imposed input heat flux to the upper surface of copper plate can be evaluated from the relation

Cu

where ACu is the area of the upper surface of the copper plate.

3.2 Uncertainty Analysis

An uncertainty analysis is carried out here to estimate the uncertainty levels in the experiment. Kline and McClintock [41] proposed a formula for evaluating the uncertainty in the result F as a function of independent variables, X₁, X_2, X₃∙∙∙∙∙∙∙∙∙∙∙∙Xn,

F=F (X1 ,X2, X3∙∙∙∙∙∙∙∙∙∙∙∙Xn) (3.7) The absolute uncertainty of F is expressed as

2

and the relative uncertainty of F is

2

2

level associated with the variableX . The values of the uncertainty intervals_i X_i are obtained by a root-mean-square combination of the precision uncertainty of the instruments and the unsteadiness uncertainty, as recommended by Moffat [42]. The choice of the variableX to be included in the calculation of the total uncertainty _i level of the result F depends on the purpose of the analysis.

The uncertainties of the parameters in the present study are calculated as follows:

(1) Uncertainty of temperature difference, Tw=Tw-Tb

 

(2) Uncertainty of total power input, Q_t

V

Cu (4) Uncertainty of space-average heat transfer coefficient, h

w

A summary of the results from the present uncertaintly analysis is given in Table 3.1.

Table 3.1 Summary of the results from the uncertainty analysis.

Parameter Uncertainty

Geometry Length & thickness (%)

Area (%) Particle diameter (%)

Particle density(%)

0.167%

0.334%

10.0%

13.4%

Parameter measurement Temperature, T (C)

Temperature difference, ∆𝑇_𝑤 (C) System pressure, P (kPa)

0.05

0.36

0.5 Boiling heat transfer on the copper flat plate Total power input, Qt (%)

Imposed net heat flux, 𝑞_𝑛(%) Heat transfer coefficient, h(%)

5.9%

8.3%

11.0%

Fig. 3.1 Schematic diagram of six main directions of the heat loss.

2

3

4 5

6

1

Copper block

Fig. 3.2 Schematic diagram of T'5 and T'6

Film heater

Copper block

T′₅ T′₆

T₆

CHAPTER 4

POSSIBLE SUBCOOLED POOL BOILING HEAT TRANSFER ENHANCEMENT OF FC-72 OVER HEATED COPPER SURFACE

The experimental results to illustrate possible enhancement of subcooled pool boiling heat transfer of FC-72 by placing movable particles on the heating surface obtained in the present study are examined in this chapter. The present experiments are carried out for the liquid subcooled temperature set at ∆𝑇_𝑠𝑢𝑏=5℃, 10℃, 15℃ and 20℃. The diameter of the particles d_𝑝 is fixed at 1.0 and 1.5 mm, and the total number of particles 𝑁_𝑝 varied from 100 to 1800 for the particles with the diameter 1.0 mm or from 100 to 800 for the particles with the diameter 1.5 mm. The FC-72 liquid in the test chamber is maintained at subcooled liquid state corresponding to the atmospheric pressure. Note that the maximum numbers of particles forming a single closely packed particle layer over the boiling surface 𝑁_𝑝𝑓 are 900 and 400 respectively for the particles with the diameter of 1.0 mm and 1.5 mm when each particle contact directly with neighboring particles. In the experiment tests are also conducted for the particle number well exceeds 𝑁_𝑝𝑓 and many particles are on top of the other particles. The measured data are presented in terms of the boiling curves and boiling heat transfer coefficients for various diameters and numbers of the copper particles. Effects of the experimental parameters on the possible subcooled boiling heat transfer enhancement will be examined in detail. Selected data are presented in the following to illustrate the possible pool boiling heat transfer enhancement by the boiling flow driven metallic particles.

4.1 Single-phase Natural Convection Heat Transfer

Before conducting the pool boiling experiment, we first measure steady natural convection heat transfer of FC-72 liquid over the heated copper surface without the presence of any particles which prevails at low imposed heat flux, intending to verify the present experimental setup. The measured data for the natural convection heat transfer coefficient are compared with the empirical correlation of Radziemska and Lewandowski [43] in Fig. 4.1. Their correlation is

NuL=(2.1e^-48W+1.2)RaL0.2

(4.1) where w is the width of the heating plate (m). The correlation given in Eq.(4.1) is based on the data for a small horizontal plate heated from below for 10⁵<RaL<10⁸. Note that the characteristic length L used in defining the dimensionless groups in the above equation is chosen to be the ratio of the heated surface area and its perimeter, and the Nusselt and Rayleigh numbers are respectively defined as

(4.2) and

(4.3) The results in Fig. 4.1 indicate that our natural convection data are in good agreement with that calculated from Eq. (4.1). Thus the experimental system established here is considered to be suitable for the present study.

4.2 Saturated Pool Boiling on Bare Copper Surface

To further verify the suitability of the present experimental system, we first measured boiling curve for saturated pool boiling of liquid FC-72 on the bare heated copper plate (𝑁_𝑝 = 0). These data are compared with that from Rainey and You [22]

in Fig. 4.2 for pool boiling of FC-72 on a square copper plate of 5 × 5 cm² in surface k

Nu_L  hL



 ³

L

)L Ra g (T^sat



area. Note that the present saturated pool boiling data are in good agreement with theirs.

4.3 Effect of Surface Aging on Boiling over Bare Copper Plate

It is well known that the change of the boiling surface properties with time, the so called “aging effect”, can be significant in affecting the boiling heat transfer from a surface after the surface has been used over certain period of time. Obviously, the measured boiling heat transfer data for the cases with and without the presence of the particles on the surface can be meaningfully compared only when the surface aging effect is small. Thus tests are carried out here to investigate the aging effect of subcooled FC-72 liquid boiling on the present heated surface. The results show that the boiling curves and heat transfer coefficients measured over a time interval of 6 hours do not differ to a noticeable degree, as seen from Fig. 4.3. But a significant aging effect is found for an interval of 24 hours. Thus in the present experiment the boiling heat transfer data for the corresponding cases with and without the presence of particles on the surface are obtained within 6 hours.

4.4 Effects of Moving Copper Particles on Boiling Heat Transfer

Possible FC-72 boiling heat transfer enhancement by the copper particles moving on the heated surface is then examined. Results are presented first for the small liquid subcooling with ∆𝑇_𝑠𝑢𝑏=5℃ by showing the boiling heat transfer data for the bare surface and for the surface with copper particles on it for d_𝑝=1 mm and various 𝑁_𝑝 in Figs. 4.4-4.18. The results in Figs. 4.4-4.6 for d_𝑝=1.0 mm and 𝑁_𝑝=100 to 300 indicate that at a small particle number the boiling heat transfer can be slightly enhanced by the copper particles only at low wall superheat near the onset of

nucleate boiling. The enhancement gets smaller at increasing wall superheat. Besides, the moving copper particles does not affect the boiling heat transfer to a significant

nucleate boiling. The enhancement gets smaller at increasing wall superheat. Besides, the moving copper particles does not affect the boiling heat transfer to a significant

在文檔中 放置可移動擾動銅粒子在一水平加熱銅板上對FC-72池沸騰熱傳增強研究 (頁 29-0)