Experimental Procedures

Prior to putting all the devices and components for the experimental system together, the boiling surface is polished by fine sand paper (Number 3000) and cleaned by alcohol. In each test, we need to remove the non-condensable gases in the empty test chamber by running a vacuum pump for about 15 minutes and then fill the FC-72 liquid into the test chamber. Next, the FC-72 liquid in the test chamber is heated to the saturation state by employing a digital temperature controller and cartridge heater. Moreover, the FC-72 liquid is boiled vigorously for 2 hours to remove the dissolved noncondensible gases in it. After the working fluid pressure and temperature stabilize to one atmosphere and at the saturation 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.

11

Table 2.1 Thermophysical properties of FC-72.

Properties at 25^οC 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 hfg 88 J/g (at normal boiling point)

Liquid Density ρ _ 1680 kg/m³

Absolute Viscosity µ 6.4× 10^-3 poises ; 6.4× 10^-4 kg/m∙s Kinematic Viscosity ν 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 β 0.00156 /°C

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

12

To Degassing Tank and Drain

Computer

Boiling Surface FC-72

Digital

To Degassing Tank and Drain

Computer

Boiling Surface FC-72

Digital

Fig. 2.1 Schematic diagram of the test apparatus.

13

Embedded Copper Block

Electric Film Heater

Copper Surface

Embedded Copper Block

Electric Film Heater

Copper Surface

Teflon Surface

Perspective view

57

Conductive Pastes

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

14

Copper Surface 5

Copper Block

Electric Film Heater Conductive Pastes

Copper Surface 5

Copper Block

Electric Film Heater Conductive Pastes

Fig. 2.3 Locations of three thermocouples in the copper block and one thermocouple below the heater (not to scale).

15

Teflon substrate Flush-mounted Copper Block

(unit:mm)

Strings

62

57 30

Teflon substrate Flush-mounted Copper Block

(unit:mm)

Strings

62

57 30

Fig. 2.4 Schematic diagram of placing strings on heating surface (not to scale).

CHAPTER 3

DATA REDUCTION

3.1 Boiling Heat Transfer Coefficient

The space-average boiling heat transfer coefficient over the upper surface of the heated square copper block at long time when the flow is at a statistical state is defined as

sat superheat defined as the difference between the average surface temperature and the saturated temperature of FC-72. The average heated surface temperature is estimated from the measured average temperature from the thermocouples installed at locations near the upper surface of the copper block according to the steady-state one-dimensional conduction heat transfer. Specifically,

(3.2)

where TCu

k

= the average measured temperature from the thermocouples (°C)

Cu

δ

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

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

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

V I

Q_t = ⋅ (3.3) where

Q_t

I = electric current passing through the film heater (Amp.)

= total power input to the upper surface of the copper block (W)

V = voltage drop across the film heater (Volts)

In fact, the Teflon insulator cannot completely prevent the heat loss from the surfaces of the copper block. Heat loss across the insulator does exist, mainly from the lateral sides of the copper block and heater and from the bottom of the heater. The heat loss is estimated by one-dimensional heat conduction in the Teflon insulator and convection from the insulator surface to the ambient based on a model schematically shown in Fig. 3.1. Thus, we have

: the ambient temperature (°C)

5 , T6 , T7

k

: the average measured temperatures at the measured locations inside the Teflon insulator, as schematically shown in Fig. 3.1

T

L

: thermal conductivity of the Teflon insulator (W/m·K)

5 , L_{6 ,} L₇

A

: shortest distances between locations #5, #6, #7 and the insulator surfaces (m)

T,5 , A_T,6 , A_T,7 : bottom and lateral surface areas of the Teflon block

h_i : estimated natural convection heat transfer coefficient from the Teflon block surfaces to the surroundings by correlations from Incropera et al. [24].

(W/m²

h₅: estimated from Nu_L =0.27Ra_L^1/4 for the bottom surface of the Teflon block.

: estimated from for the lateral surfaces of

the Teflon Block.

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

Cu

where A_Cu is the area of the upper surface of the copper block.

3.2 Uncertainty Analysis

An uncertainty analysis is carried out here to estimate the uncertainty levels in the experiment. Kline and McClintock [25] proposed a formula for evaluating the uncertainty in the result F as a function of independent variables, X1, X2,

X₃···X_n

F=F (X ,

1 ,X2, X3···Xn

The absolute uncertainty of F is expressed as

)

and the relative uncertainty of F is

2

If F =X₁^aX₂^bX₃^c... , then the relative uncertainty is

∂ and ∂ are, respectively, the sensitivity coefficient and uncertainty X_i

level associated with the variableX . The values of the uncertainty intervals_i ∂ are X_i obtained by a root-mean-square combination of the precision uncertainty of the instruments and the unsteadiness uncertainty, as recommended by Moffat [26]. 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, ∆Tsat=Tw-T

( ) (2) Uncertainty of total power input, Q

V

(3) Uncertainty of net wall heat flux, q

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

sat

(3.13) 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 (%)

± 0.5%

± 1.0%

Parameter measurement Temperature, T (°C)

Temperature difference (°C) System pressure, P (kPa)

± 0.2

± 0.4

± 0.5 Boiling heat transfer on the copper flat plate Power input, Qt

Imposed effective heat flux, q (%)

n

Heat transfer coefficient, h (%)

±

(%)

8.2%

± 14.8%

± 13.4%

Perspective view

Fig. 3.1 Locations of three thermocouples in the Teflon substrate in order to estimate heat loss (not to scale).

CHAPTER 4

SATURATED POOL BOILING HEAT TRANSFER ENHANCEMENT OF FC-72 OVER A SMALL HEATED HORIZONTAL COPPER SURFACE

The experimental results for the possible enhancement of saturated pool boiling heat transfer of FC-72 by placing flexible strings above the heating surface measured in the present study are examined in this chapter. The present experiments are carried out for the diameter of the nylon strings varied from 74 to 259 μm, the height of the strings from 0 to 2 mm, and the length of the strings from 10 to 12 mm for the pitch of the strings mainly fixed at 2.0 mm with the FC-72 liquid in the test chamber maintained at saturated liquid state corresponding to the atmospheric pressure. The measured data are presented in terms of the boiling curves and boiling heat transfer coefficients for various diameters, heights, length of the flexible strings and for a bare heating surface. Effects of the experimental parameters on the possible boiling heat transfer enhancement will be examined in detail. Selected results are presented in the following to illustrate the possible pool boiling heat transfer enhancement by the flexible strings.

4.1 Single-phase Natural Convection Heat Transfer

Before conducting the pool boiling experiment, we first measure steady natural convection heat transfer over the heated small copper surface without the installation of the strings 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 [27] 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<Ra_L<10⁸

(4.2) . 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

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 facility, the boiling curve for saturated pool boiling of FC-72 on a bare heated copper surface is also obtained. These data are compared with those from Chang and You [28] for pool boiling of FC-72 on a small square copper plate of 1cm² in surface area in Fig. 4.2.

The results indicate that the present data are in qualitative agreement with those of Chang and You [28] before and after the onset nucleate boiling.

4.3 Effects of String Diameter on Boiling Heat Transfer Enhancement

Attention is then turned to examining how the diameter of the flexible strings affects the boiling heat transfer of FC-72 on the heated plate. This is illustrated in Figs.

4.3-4.11 by showing the boiling curves and the variations of the boiling heat transfer coefficients with the wall superheat for various string diameters at different string heights and lengths. The results for the boiling curves clearly indicate that the presence of the flexible strings significantly reduces the temperature overshoot at the

k

Nu_L = hL

αν

β ³

L

)L Ra g (∆T^sat

=

onset of boiling and hence the boiling hysteresis. Besides, the inception of the boiling takes place at a much lower wall superheat with the strings installed. It is also noted that at the same wall superheat ΔTsat the boiling heat transfer coefficients are substantially higher for the surface covered by the strings for certain dw,wand hw

when compared with that for the bare heated surface, as clearly seen by checking with the data for the variations of the heat transfer coefficient with ΔT_sat. These data show that an increase in the boiling heat transfer coefficient of more than 100% over that for a bare heated surface by placing the flexible strings on the heating surface can be obtained. Note that at a smallΔTsat the boiling heat transfer coefficient is already relatively high for the plate covered with the strings but the flow over the bare surface is still in single-phase state. Moreover, the extent of the boiling heat transfer enhancement varies nonmonotonically with size, looseness and height of the strings.

This is attributed to the complex effects of these parameters on the bubble dynamics in the boiling flow. A close inspection of these data reveals that when the strings directly contact the heating surface ( hw

w

=0mm ) and the strings are tightly fixed at their ends ( =10mm), placing the strings with the largest diameter of 259μm results in the highest heat transfer enhancement ( Fig. 4.3 ). While for the strings with d_w

w

=74μm & 158μm the enhancements are somewhat smaller and nearly the same.

Note that for the strings being slightly loose at =11mm the heat transfer enhancement for the three different string size do not differ significantly with each other, as evident from the data given in Fig. 4.4. The boiling heat transfer coefficient is enhanced by about 60 to 80%. But for the strings loosened substantially at  = 12_w mm the data in Fig.4.5 show that the boiling heat transfer is noticeably better for the smaller strings. At d_w

Now when the strings are installed at a slightly higher position with h

=74μm a boiling heat transfer coefficient enhancement of more than 100% can be procured.

w=1mm the effects of the string diameter on the boiling heat transfer enhancement shown in

Figs. 4.6-4.8 are relatively different. For the tight strings with  =10_w mm the boiling heat transfer enhancement does not change significantly with the string size ( Fig.

4.6 ). The heat transfer enhancement is larger for smaller-diameter strings when

 is slightly increased to 11w mm ( Fig. 4.7 ). While for  is increased further to 12_w mm it is of interest to note that the enhancement is largest for the mid-size strings with d_w

For the string height increased further to 2mm the situation again is somewhat different. At

=158 μm and smallest for the large strings ( Fig. 4.8 ).

 =10w mm the string diameter exhibits a much smaller influence on the boiling heat enhancement and the enhancement is all less than 50% ( Fig. 4.9 ). But for the strings being loosened slightly at  =11 & 12_w mm the smallest string with dw

w

=74μm clearly gives the largest enhancement ( Figs. 4.10 & 4.11 ). And, the enhancement is more significant for =11mm .

4.4 Effects of String Height on Boiling Heat Transfer Enhancement

Next, how the boiling heat transfer is affected by the string height is illustrated

in Figs. 4.12-4.20. These results indicate that for the small-diameter strings with dw=74μm and tightly fixed at _w=10mm the heat transfer enhancement is largest for the strings directly contacting the heating surface, as seen in Fig. 4.12. Then, for the strings fixed slightly loosely at  =11_w mm the strings placed slightly away from the heating surface with h_w

w

=1mm give the best boiling heat transfer performance ( Fig. 4.13 ). But for the strings fixed loosely at =12mm, the case with the strings directly contacting the heating surface ( h_w=0mm ) gives the substantially higher boiling heat transfer enhancement ( Fig. 4.14 ). For the mid-size strings at dw=158μm the enhancement is most prominent at h_w

w

=0mm for the strings tightly fixed at

=10 mm, as noted from the data given in Fig. 4.15. But for  =11_w mm the enhancement is largest at h_w=1mm and lowest at h_w

w

=2mm ( Fig. 4.16 ). While for the strings loosely fixed at =12 mm the enhancement is rather large and

comparable for hw=0 & 1mm ( Fig. 4.17 ). Note that the heat transfer enhancement nearly vanishes when the strings are far away from the heating surface at h_w=2mm.

The data in Figs. 4.18-4.20 for the large-diameter strings at dw=259μm indicate that the enhancement is significant for the strings directly contacting the heating surface ( hw=0 mm ) and tightly fixed (_w=10 mm ). For the strings loosely fixed at  =12 _w mm only small heat transfer enhancement can be obtained by placing strings of large size ( Fig. 4.20).

4.5 Effects of String Length on Boiling Heat Transfer Enhancement

Then, the effects of the string length on the boiling heat transfer are shown in Figs. 4.21-4.29. The results in Figs 4.21-4.23 for the small-diameter strings at dw=74 μm indicate that for the strings directly contacting the heating surface hw

w

=0mm the longest strings with =12 mm have a substantially larger boiling heat transfer enhancement ( Fig. 4.21 ). However, for the strings fixed away from the heating surface at hw=1 & 2mm the slightly shorter strings of _w=11mm give the best heat transfer performance ( Figs. 4.22 & 4.23 ). For the mid-size strings ( d_w

w

=158μm ) the boiling heat transfer enhancement is largest for the longest strings ( =12mm ) at h_w=0mm ( Fig. 4.24 ). But at h_w

w

=1mm the boiling heat transfer enhancement are rather close for =11mm and  =12_w mm ( Fig. 4.25 ). While for the strings fixed at a higher position with hw=2mm the slightly loosened strings with w=11 mm show the best performance ( Fig. 4.26 ). Now for the large-diameter strings with d_w=259μm the enhancement is better for the shorter strings when they contact the boiling surface at hw=0mm, as evident from data given in Fig. 4.27. But at hw

w

=1 &

2mm the slightly loosely fixed strings at =11 mm show the best heat transfer performance ( Figs. 4.28 & 4.29 ).

4.6 Effects of String Pitch on Boiling Heat Transfer Enhancement

Finally, we examine the effects of the string pitch on the boiling heat transfer in Figs. 4.30-4.35. The results in Figs 4.30-4.32 for the small-diameter strings at d_w=74 μm and hw=1mm indicate that an reduction of the string pitch from 2mm to 1mm can enhance the boiling heat transfer to a larger degree by increase the string looseness.

But for the mid-size strings ( dw=158μm ) the reduction in the string pitch only slightly affects the boiling heat transfer, as evident from the data shown in Figs 4.33-4.35.

4.7 Concluding Remarks

In this chapter how the installation of the flexible nylon strings on the saturated pool boiling heat transfer of FC-72 over a small heated horizontal square plate have been investigated. The effects of the string diameter, length and height on the boiling heat transfer enhancement have been examined in detail. The major results from this investigation can be summarized at follows.

(1) The extent of the boiling heat transfer enhancement by the flexible strings depends strongly on all experimental parameters, namely, the string diameter, length and height.

(2) A boiling heat transfer coefficient enhancement of more than 100% over that for a bare heated plate can be obtained by a suitable choice of the experimental parameters.

(3) The bubble dynamics near the heated surface is conjectured to be affected substantially by the string size, looseness and position. But the details on how these parameters affect the near-wall bubbles requires further investigation.

Ra_L

10 20 30 40 50 60

N u

L

Radziemska and Lewandowski (2005) : Nu_L=(2.1e^-48W+1.2)Ra_L^0.2

present data at 1 atm

Singal-phase Natural Convection Heat Transfer at T_sat=56°C

104 ^{7 8} 9 10⁵ 2 3 4 5 6 7 8

Fig. 4.1 Comparison of the present single-phase natural convection data with the empirical correlation of Radziemska and Lewandowski (2005).

∆T

_sat

(°C) q ( W /c m

2

)



Chang and You (1996) for polished plate



present data

10^-1 10⁰

2 3 4 5 6 7 8 9

10¹

2 3 4 5 6 7 8 9 2 3 4

10^{0 2 3 4 5 6 7 8 9}10^{1 2 3 4 5}

Fig. 4.2 Comparison of the present nucleate boiling heat transfer data on smooth plate with Chang and You (1996).

Fig. 4.3 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at hw=0mm and w=10mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2 )

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=0mm, _w=10mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.4 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at hw=0mm and w=11mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2)

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=0mm, _w=11mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.5 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at hw=0mm and w=12mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2)

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=0mm, _w=12mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.6 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at h_w=1mm and _w=10mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2 )

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=1mm, _w=10mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.7 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at hw=1mm and w=11mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2 )

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=1mm, _w=11mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.8 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at hw=1mm and w=12mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2)

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=1mm, _w=12mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.9 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at hw=2mm and w=10mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2 )

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=2mm, _w=10mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.10 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at hw=2mm and w=11mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2)

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=2mm, _w=11mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.11 Effects of string diameter on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at hw=2mm and w=12mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



dw=74µm



dw=158µm



dw=259µm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2)

(a) ∆T_sub =0°C at 1atm , fixed parameter : h_w=2mm, _w=12mm



bare surface



dw=74µm



dw=158µm



dw=259µm

Fig. 4.12 Effects of string height on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at dw=74μm and w=10mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



^hw=0mm



^hw=1mm



^hw=2mm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2)

(a) ∆T_sub =0°C at 1atm , fixed parameter :d_w=74µm, _w=10mm



bare surface



^hw=0mm



^hw=1mm



^hw=2mm

Fig. 4.13 Effects of string height on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at d_w=74μm and _w=11mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



^hw=0mm



^hw=1mm



^hw=2mm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2)

(a) ∆T_sub =0°C at 1atm , fixed parameter :d_w=74µm, _w=11mm



bare surface



^hw=0mm



^hw=1mm



^hw=2mm

Fig. 4.14 Effects of string height on saturated pool boiling curves (a) and boiling heat transfer coefficients (b) at d_w=74μm and _w=12mm.

12 13 14 15 16 17 18 19

∆T_sat (°C)

1500 2000 2500 3000 3500 4000 4500 5000 5500

h (

W

/m2. K

)



bare surface (b)



^hw=0mm



^hw=1mm



^hw=2mm

0 10 20 30 40 50

∆T_sat (°C)

0 2 4 6 8 10

q (W/cm2)

(a) ∆T_sub =0°C at 1atm , fixed parameter :d_w=74µm, _w=12mm



bare surface



bare surface

在文檔中 加裝可移動的尼龍線在一小水平加熱銅板上對FC-72池沸騰熱傳增強及氣泡特性研究 (頁 29-0)