Sep 27, 2002 - Jong-Soo Kim, Ngoc Hung Bui, Hyun-Seok Jung and Wook-Hyun Lee. 1. Introduction. Heat pipe is a very effective device Jor transmit- ting heat ...
KSME International Journal, VoL 17 No. 10, pp. 1533~ 1542, 2003
1533
The Study on Pressure Oscillation and Heat Transfer Characteristics of Oscillating Capillary Tube Heat Pipe Jong-Soo Kim* School of Mechanical Engineerhlg, Pukyong National University, San 100, Yongdang dong, Nam gu, Pusan 608-737, Korea
Ngoc Hung Bui Graduate student, School of Mechanical Engineering, Pukyong National Universitj', San 100, Yongdang dong, Nam-gu, Pusan 608-737, Korea
Hyun-Seok Jung Samsung Electronics Co. Ltd, 416, Maetan-3Dong, Paldal-Gu, Suwon City, Kyungki-Do, 442-742, Korea Wook-Hyun
Lee
Korea Institute of Energy Research., 71-2, Jang-Dong, Yuseong-Gu, Daejeon 305-343, Korea
In the present study, the characteristics of pressure oscillation and heat transfer perlbrmance in an oscillating capillary tube heat pipe were expe~'imentally investigated with respect to the heat flux, the charging ratio of working fluid, and the inclination angle to the horizontal orientation. The experimental results showed that the frequency of pressure oscillation was between 0.1 Hz and 1.5 Hz at the charging ratio of 40 vol.%. The saturation pressure of working fluid in the oscillating capillary tube heat pipe increased as the heat flux was increased. Also, as the charging ratio of working fluid was increased, the amplitude of pressure oscillation increased. When the pressure waves were symmetric sinusoidal waves at the charging ratios of 40 vol.% and 60 vol.~o, the heat transfer performance was improved. At the charging ratios of 20 v o l . % and 80 vol.%, the wavetbrms of pressure oscillation were more complicated, and the heat transfer performance reduced. At the charging ratio of 40 v o l . ~ , the heat transfer performance of the O C H P was at the best when the inclination angle was 90 °. the pressure wave was a sinusoidal wavelbrm, the pressure difference was at the least, the oscillation amplitude was at the least, and the frequency of pressure oscillation was the highest.
Key W o r d s : O s c i l l a t i n g Capillary T u b e Heat Pipe ( O C H P ) , Pressure Oscillation, Frequency, Power Spectrum
Nomenclature f
: Frequency, IHz]
p q
: Pressure, [kPa] : Heat flux, [ W / c m "~]
* Corresponding Author. E-mail : jskim @pknu.ac.kr TEL: +82-51 620 1502;FAX:--82 51 611 6368 School of Mechanical Engineering, Pukyong National University, San 100. Yongdang dong, Nam-gu. Pusan 608-737, Korea. (Manuscript Received March 2. 2001: Revised September 27, 2002)
Greeks ~y 8 A
Charging ratio, [vol.%] Inclination angle, [o] Thermal conductivity, [ W / m K ]
Subscripts elf
: Effective eva : Evaporating part c o n f f : Condensing part
1534
Jong-Soo Kim, Ngoc Hung Bui, Hyun-Seok Jung and Wook-Hyun Lee part and the condensing part. The heat transfer
1. Introduction
continuously occurs. As a result, thermal energy is rapidly transferred from the evaporating part to
Heat pipe is a very effective device Jor transmit-
the condensing part as well as the oscillation and
ting heat with high rates over considerable dis-
the circulation of vapor plugs and liquid slugs
tances with extremely small temperature drops,
occurring in the O C H P (lzumi et al., 1998 ; Kim et al., 1999 : Lee et al., 1999a, 1999b).
exceptional flexibility, simple construction, and easy control with no external pumping power. There are various parameters that limit the rate of
Both experimental and numerical investigations
heat transport through heat pipes. The capillary
on the O C H P have been carried out by some researchers to determine the pressure mechanism
limit is the most commonly encountered limita-
governing the flow of working fluid in the OCHP.
tion in the operation of heat pipes. It occurs when
Miyazaki and Akachi (1996) studied the pressure
the capillary pumping rate is not enough to pro-
oscillation characteristics of the O C H P with re-
vide liquid to the evaporating part. This is due
spect to the variations of heat flux and charging
to the fact that the sum of the liquid and vapor
ratio. They observed the movement phenomena
pressure drops exceeds the maximum capillary pressure that the wick can sustain. The entrain-
of oscillation wave of long liquid slugs and vapor plugs and proposed an analytical model
ment limit is due to the influence of the shear force which exists at the liquid-vapor interlace
based on these phenomena. They concluded that the heat transfer performance was improved when
(Faghri, 1995). The capillary and entrainment
the amplitude of pressure oscillation decreased
limits are overcome by using the oscillating capil-
and the frequency of pressure oscillation increas-
lary tube heat pipe (OCHP).
ed. Suzuki (2000)
also studied the temperature
The O C H P is a very promising heat transfer
oscillation characteristics of a copper-water oscil-
device (Akachi, 1994). In addition to its excellent heat transfer performance, it has a simple struc-
lating capillary heat pipe. The frequency of temperature oscillation was found and a proportional relation between effective thermal conductivity
ture: in contrast with conventional heat pipes, there is no wick structure to return the condensed working fluid back to the evaporating part. The O C H P is made from a long continuous capillary tube bent into many turns of serpentine structure. The working fluid is charged into the OCHP. The diameter of the O C H P must be sufficiently small
and frequency was proposed. Although some experimental evidence suggested the correlation between the pressure oscillation and heat transfer characteristics in the OCHP, studies related to the characteristics of pressure
so that vapor plugs can be formed by capillary
oscillation with respect to the variation of heat flux, the charging ratio of working fluid, and the
forces. The O C H P is operated within a 0.1--5 mm
inclination angle were not found. It was the aim
inner diameter range. The O C H P can operate successfully for all heating modes. The heat input,
of the present work to make clear the effect of pressure oscillation characteristics on the heat
which is the driving force, increases the pressure of the vapor plugs in the evaporating part. [n
transfer performance of the OCHP.
turn, this pressure increase will push the neighboring vapor plugs and liquid slugs toward the condensing part, which is at a lower pressure. However, due to the continuous heating, bubbles are endlessly formed by nucleate boiling in the evaporating part. The bubbles grow and coalesce to become vapor plugs. The flow of vapor plugs and liquid slugs moves to the condensing part by the pressure difference between the evaporating
2. Experimental Apparatus and Methods The schematic diagram of the experimental apparatus was shown in Fig 1. It consists of a test section, data acquisition system, heating system, and circulation system of cooling water. The test section was manufactured to the serpentine structure with fine flow channels on the surface
The S t u d y on Pressure Oscillation and Heat Transfer Characteristics o f Oscillating Capillary Tube ...
1535
of a brass plate. The length of the channel was
the looped type. The detailed specifications of
220 rhm. Each channel cross-section was rectan-
the test section were represented in Fig. 2. The
gular with a width of 1.5 mm and a height of 1.5 mm. For visualization o f the internal flow of the O C H P , the test section was covered with trans-
data acquisition system was divided into a temperature and pressure measuring system. Each measuring system was composed of a data logger
parent acryl and tightened with bolts. The test
and personal computer. The heating system was
section was formed by 10 turns (20 channels) of
an electrical heater plate of which the heat input was adjusted by using a voltage regulator and
Data logger (DR-230) ~
Volumetric Flowmeter ~
.... I V.,'////A! Computer r~----~l~---1
I
L . . ( ~ ......
IIII1[
i" ' ' ~ ...... ~
Data logger Computer (DAS-1800)
Fig. l
~
,101
Constant Temperature Bath
Section,:;=
Voltage
m
.eg°lator
(!:::::::::::~
Digital power meter
The schematic diagram of the experimental apparatus
digital power meter. The circulation system of cooling water was composed of a constant temperature bath and volumetric flow meter. The cooling water circulated through the test section, the volumetric flow meter, and the constant temperature bath. The locations of thermocouples for measuring the surface temperature of the test section were shown in Fig. 3. Three holes of a 1.1 mm diameter were perforated into the center of the test section across and under l mm from the bottom of the channels. Three holes were located at the eva-
24 - M6
_I~
•
c~l,/
~ ~
porating, the adiabatic, and the condensing part, respectively. A sheath type thermocouple of a 1.0 mm diameter was inserted and sealed into each hole. To minimize the contact resistance between
Evaporating section
g Adiabatic section
]9 7
301
30
Condensing section
191_
Thermocouple
8
15 rnm --, 1.5 mm 1. Test section 2. Transparent acryl
Fig. 2
3. Flap
Specifications o f the test section
I
I ~ =lmm)
Fig. 3
]
Locations of thermocouples
Jong-Soo Kim, Ngoc Hung BuL //yun Seok Jung and Wook-Hyun Lee
1536
the perforated hole a n d the t h e r m o c o u p l e , the
by using a high v a c u u m system, w h i c h consists of
silicon c o m p o u n d was injected into the gap of the
a rotary a n d diffusion p u m p . R - 1 4 2 b was used as
holes. T h e distance o f t e m p e r a t u r e sensors t h a t
the w o r k i n g fluid. T h e c h a r g i n g c y l i n d e r ( H P G -
were installed at the e v a p o r a t i n g part and con-
10, 96, T a i a t s u Co.) of 10 ml was used to c h a r g e
d e n s i n g part was 146 mm, a n d the sensor installed
the w o r k i n g fluid exactly.
at the a d i a b a t i c part was in the middle. Also,
ratio of the w o r k i n g fluid was c h a n g e d in t u r n
Also, the c h a r g i n g
sheath type t h e r m o c o u p l e s were installed at the
to 20 vol.%, 40 vol.%, 60 v o l . ~ , a n d 80 vol.9/o:
inlet and the outlet of the cooling water system
the heat flux to the e v a p o r a t i n g part was c h a n g e d in turn to 0.3 W / c m 2, 0.6 W / c m 2, 0.9 W / c m 2, a n d
to m e a s u r e the t e m p e r a t u r e s of the c i r c u l a t i n g c o o l i n g water. T e m p e r a t u r e m e a s u r e m e n t s were
1.2 W / c m 2 ; and the i n c l i n a t i o n angle to the hori-
c o n d u c t e d using the data logger ( D R - 2 3 0 , Y o k o -
zontal o r i e n t a t i o n was c h a n g e d in turn to 30 °,
gawa Co.). All data were processed using the
60 ° , and 90 ° .
personal c o m p u t e r in real time. T h e data were m e a s u r e d at a steady state a n d their average was
Uncertainty analysis
used for data reduction.
T e m p e r a t u r e s and pressure were m o n i t o r e d in
As s h o w n in Fig. 4, to m e a s u r e the s a t u r a t i o n
the test l o o p using the c o m p u t e r and d a t a acqui-
pressure in the test section, two pressure taps o f
sition system. E a c h sensor was c a l i b r a t e d to re-
a 1 . 0 m m d i a m e t e r were perforated into a flow
duce e x p e r i m e n t a l u n c e r t a i n t i e s before c o n n e c t e d
c h a n n e l at the e v a p o r a t i n g part a n d c o n d e n s i n g
to the data logger. T h e c o p p e r c o n s t a n t a n ther-
part. T h e pressure sensors
961, D r u c k
m o c o u p l e s with an e x p e r i m e n t a l u n c e r t a i n t y of
Co.) were installed. Pressure data were m e a s u r e d
+_0.2°C were used for t e m p e r a t u r e measurements.
using the D A S - 1 8 0 0 (Keithley Co.) at a steady
Pressure t r a n s d u c e r s were used for m e a s u r i n g the
state. T h e test section was e v a c u a t e d to 1 X 10 -a t o r t
pressure o f the w o r k i n g fluid with an estimated
(PDCR
accuracy o f _+0.2% of the full scale (980 k P a ) . T h e pressure r e a d - o u t u n c e r t a i n t y was 0.3%, and the r a n d o m u n c e r t a i n t y o f pressure m e a s u r e m e n t was expected to be less t h a n 59/00. Hence, the maxi-
Evaporating
m u m u n c e r t a i n t y for the pressure m e a s u r e m e n t s
L
at the e v a p o r a t i n g part a n d c o n d e n s i n g part was
seo,,on
5.5%.
3. Experimental Results and Consideration
Adiabatic section
3.1
Pressure oscillation characteristics
W h e n the i n c l i n a t i o n angle to the h o r i z o n t a l o r i e n t a t i o n ( i n c l i n a t i o n angle, in brief) was 90 °
Condensing: section
and the c h a r g i n g ratio o f w o r k i n g fluid ( c h a r g i n g ratio, in brief) was c h a n g e d in turn to 20 vol.%,
15mm
40 vol.%, 60 vol.%, a n d 80 vol.%, the v a r i a t i o n s
1.0
mm
1.5
mm
of s a t u r a t i o n pressure with respect to time at the given heat fluxes a n d c h a r g i n g ratios were s h o w n in Fig. 5. G e n e r a l l y , as the heat flux to the evap o r a t i n g part (heat flux, in brief') a n d the charging ratio was increased, the s a t u r a t i o n pressure
Fig. 4
Locations of pressure sensors
increased.
The Stuc(v on Pressure Oscillation a n d H e a t Transfer Characteristics o f Oscillating Capillary Tube ...
1537
At the c h a r g i n g ratio o f 20 vol.,%, the pres-
that at the heat flux o f 0.3 W / c m 2, the genera-
sure oscillations ( a r o u n d the average pressure)
tion and growth o f bubbles in the O C H P were
had c o m p l i c a t e d forms at the given heat fluxes.
anintermittent nucleate boiling process at only
T h e r e were not a b r u p t changes in the a m p l i t u d e
some turns a m o n g the total flow channels. At
o f pressure oscillation (oscillation amplitude, in
the r e m a i n i n g turns o f the flow channels, b u b b l e s
brief) at the heat fluxes o f 0.3 W / c r n 2 and 0.6 W /
were almost not generated, and the oscillation
cm='. However, at the heat f'lux o f 0.9 W / c m 2, the
p h e n o m e n o n was not clearly confirmed. As the
oscillation a m p l i t u d e a b r u p t l y c h a n g e d and the
heat flux was increased to 0.6 W / c m 2, small bub-
pressure wave became art irregular, n o n - p r e d i c -
bles generated on the walls (of the channels) in
table and unsteady state. This was due to the tact
the e v a p o r a t i n g
R-142b,
~=20vo1.%,
0=
90°
part.
R-142b,
The
o~=40vo1.%,
bubbles
0=
grew
and
90°
150
-ff
~ . ,,0 t~, ~':'~"'~.'"x.'~"-~"~. "''-- ~ .... ~'" ..... I " q=l.2W/cm' ~3,~ q=O.9W/cm~
140
~30
q=O.9%V/cm-'
12.
q=0.6V¢/cmz
t-
!
t.
q=0.6W/cm*
~-
¢,
q=0.3W/cm z
~,~
q=0.3W/cm"
s.
8. o
2
4
~
Time
8
IO
12
*)
14
4
t
(sec)
o
8
Time
10
12
14
(sec)
(a) R142b,
cz= 60vo1.%,
0= 90°
RI42b,
1.2W/cmz
ct= 60vo1.%,
0= 90 °
,.u,, [
i.2W/cm ~
/.
/
e~ ~,~
150
'i ~ ,I / ~
!/
, //'~
I,
, ,
~ ,'1~,
L.
0.6W/cnV
u.',
0.6W/cm z
/
*, ~~ , .
¢d L.
L
0.3W/cm z
0.9W/cm
"'----
e~_
0.3W/era: 50
o
2
4
6
Time ic)
8
I0
t~
2
14
4
(sec)
6
Time
8
IO
IZ
14
(sec)
(c) R-142b, ot =80voL%, 0 = 9 0 ° -~" :~s0
q=l'2W/cmZ
= ~ ,~.
q=O.9~rlcm2
q=~L3W/cmz . 2
4
6
Time
q=0.6W/cm"
. ~
.
. IO
IZ
14
(see)
,iel Fig. 5
T h e , , a r i a t i o n s o f s a t u r a t i o n pressurein the evaporating part at the given heat fluxes and charging ratios
1538
Jong Soo Kim, Ngoc Hung Bui, Hyun-Seok Jung and Wook-Hyun Lee
coalesced to become vapor plugs and the oscil-
and the saturation pressure of working fluid in-
lation phenomenon occurred. When the heat flux was increased to 0.9 W/cm 2. the oscillation of
creased. It led to an increase in the temperature difference and pressure difference between the
vapor plugs and liquid slugs occurred very actively. The movement direction of vapor plugs
evaporating part and condensing part. It was observed that bubbles formed at the evaporating
and liquid slugs changed irregularly in each flow channel. However, as the heat flux was increased,
part by nucleate boiling were dissipated in the
the working fluid supplied to the evaporating
condensing part. The space vacated by the bub-
part was not sufficient and the dry-out pheno-
bles was filled by liquid, and the process was
mena occurred in some flow channels. Because of
repeated. Hence, the bottom part of the pressure
the inter-connection of the channels, the working
waves was sharp and the operating state of the O C H P changed to an unsteady state (Lee et al., 1999b).
fluid from adjacent flow channels flowed into these channels and the explosion of evaporation occurred on the superheated walls in the evaporating part. Owing to this phenomenon, the satura-
long liquid slugs shortly before they came to the
In order to analyze the frequency characteris-
tion pressure abruptly increased and the pressure
tics of pressure oscillation, the signal processing tool using a discrete Fast Fourier Transform
wave represented an irregular, non-predictable and unsteady state (Kim et al., 1999, 2001 ; Lee et al., 1999a, 1999b).
of pressure oscillation. The experimentally obtained power spectra of the sinusoidal pressure
( F F T) was applied to obtain the power spectrum
At the charging ratios of 40 vol.% and 60 vol.
waveforms at the inclination angle of 90 ° and
%, the pressure oscillation had closed symmetric structures at the given heat fluxes, except the heat
the charging ratios of 40 vol.% (heat flux 0.6 W / c m 2 and 0.9 W / c m 2) and 60 vol.% (heat flux
flux of 0.3 W / c m 2. The symmetric structure of
0.9 W / c m 2 and
1.2 W / c m 2) were presented in
pressure wave was sinusoidal at the charging ratio of 40 vol.,% and the heat flux of 0.6 W / c m 2. 0.020
When the charging ratio was 60 vol.~o and the heat fluxes were 0.6 W / c m 2, 0.9 W / c m 2 and 1.2
a' --40v01.% 0.015
0=90 ~
W / c m 2, the pressure waves were also sinusoidal.
~.
For clarity, an enlarged figure of pressure oscillation at the heat flux of 0.6 W / c m 2 was present-
~ - 0.010
ed in Fig. 5(d). These states of the O C H P were
0.005
observed through the flow visualization experi-
0
ments obtained by Kim et a1.(1999), (2001). and Lee et al. (1999a), (1999b). They reported that
1
2
3
4
Frequency (Hz)
/a) At the charging ratio of 40 vo].°/oo
the steady state operation of the O C H P was observed at the charging ratios of 40 vol.% and 60 vol.~o. The active oscillations of working fluid occurred as the symmetric structure of pressure wave was obtained. The oscillation of working fluid was most active when the pressure wave was sinusoidal. At the charging ratio of 80 vol.%, the pressure waves were also almost symmetric at the given heat fluxes, except the heat flux of 0.3 W / c m 2. Detailed observations showed that the bottom part of the pressure wave has a sharp shape. This was due to the excessive charge of working fluid
q=O 6 W / c m z
- -
0.20 o~ = 6 0 v o 1 . %
~=
o,10
o 9o0 ¢4
~0.10
~
- -
q=0.9W/crn 2
....
q- 1.2Wion 2
0.05
.... 2
45, 3
"::-4
Frequency (Hz)
(b) At the charging ratio of 60 vol.% Fig. 6
The power spectra of the sinusoidal pressure oscillations by the fast Fourier transform
1539
The S t u d y on Pressure Oscillation a n d H e a t Transfer Characteristics o f Oscillating Capillary Tube ...
Fig. 6. The d o m i n a n t frequencies were determined
500'30 - -
to be 1.2207 Hz and 1.85547 Hz (at the charging
4~XD
ratio of 40 vol.%, Fig. 6(a)) and 1.70898 Hz and
=F..~ 40000
2.00195 Hz (at the charging ratio of 60 vol.%, Fig. 6 ( b ) ) . As shown in Fig. 6, the value of the
300(~
d o m i n a n t frequencies increased with increasing
25008
90" ]
$.
R-142b, ~I =40~01.%
3o" i
./ 1 ........... ~
.............
02
the heat flux and the charging ratio (from 40 vol.
04
~
06
C8
.......
10
J
12
14
q,.,,. (Wlcm z)
% to 60 vol.%).
(a) The effective thermal conductivity on heat flux 3.2
Correlation
of pressure oscillation
and
~c~ R-142b, 0.:40vo1.%
heat transfer characteristics
Figure 7 showed the variation of the effective thermal conductivity with heat flux at the given charging ratios. The effective thermal conducti-
~
~5o
= ~ ,o0
vol.%.
The
experimental
results
obtained
60 ° - ~ 3O" _~p-- 0°
e~
vity was maximal at the charging ratio of 40
5O 02
by
04
06
Kim et a1.(1999), (2001), and Lee et al.(1999a),
lo
~2
(b) The saturation pressure on heat flux
(1999b) also confirmed that the oscillation of working fluid was active at the charging ratio
3 R-142b,
of 40 v o l . % With this charging ratio, the circulation velocity of working fluid was at m a x i m u m value, which resulted in the best heat transfer
08
q,.,,,,(W/cm")
2
6~ =4(}vol,'¼~
-tl-- 60"
.
-.__~: 1
performance. Figure 8 described the dependence of the effec-
'~ c 02
tive thermal conductivity, the saturation pressure,
04
06
C8
10
12
q,,,,. (W/cmz)
the pressure difference, the oscillation amplitude,
(c) The pressure difference on heat flux
and the frequency on the given heat fluxes, respectively. Here, the charging ratio was 40 vol.%
~:, R-142b, cz =40vo1.%
and the inclination angle was changed in turn to
/
~
~
.
30 ° , 60 °, and 90 ° . As shown in Fig. 8(a), when the inclination E
60 30
65004 2
1
~
4ovo1.%
~
60v01.%
q~,,,, (VV/cm")
(d) The oscillation amplitude on heat flux
+
3j i[
R-142b,~=40voI.V,
2 /
~
]
t ~'~-'
/
~
~°
~
30 °
~ g
o
0.3
o,s
Heat
Fig. 7
os
12
i5
O2
06
08
q.,,.
flux (W/on1-')
The variation of the effective thermal conductivity with heat flux at the given charging ratios
O4
10
12
4
(W/eraz)
(e) The frequency on heat flux Fig. 8
The dependence of the various factors on the given heat fluxes [R-142b. a'=40 vol..9/o]
1540
Jong-Soo Kim, Ngoc Hung BuL ltvun-Seok Ju#lg and H/ook-Hyun Lee
angle was increased, tile effective thermal conductivity increased. The effective thermal conductivity was maximal at the inclination angle of 90 ° and the heat flux of 0.6 W / c m ~. This was due to tile fact that when the oscillation of working fluid occurred actively in each flow channel in the O C H P , the pressure oscillation was sinusoidal at the same conditions of the heat flux and charging ratio as mentioned above (see Fig. 5 ( b ) ) . The effective thermal conductivity was a representative index to show the performance of the O C H P . The effective thermal conductivity varied according to the working fluid, the charging
the given heat fluxes was described in Fig. 8(c). When the inclination angle was decreased, the pressure difference increased. Tile pressure difli~rence decreased as the heat flux was increased from / 9 0.3 W / c m z to 0.6 W~cm-. However, as the heat flux was increased to more than 0.6 W/cnfl. the pressure difference increased. The pressure difference ,*'as lit the least when the heat flux was 0.6 W / c m 2 and the inclination angle was 90 °. This was due to the symmetric structure of pressure wave which was sinusoidal at the heat flux of 0.6 W / c m z. When the oscillation of vapor plugs and liquid slugs occurred very actively between
ratio, the class and shape of the O C H P , the length ratio of the evaporating part and the condensing
tile evaporating part and the condensing part, it
part, and tile inclination angle of the O C H P (Maezawa, 1995).
on tile walls of tile channels. The thin liquid film
The effective thermal conductivity ,'~N is defined as f o l l o w s :
from the tail of liquid slugs during oscillation along the flow channel. By the very rapid oscillation of vapor plugs and liquid slugs along the
LO
was generated by the shedding of liquid mass
(1)
flow channels, a pseudo slug flow pattern near
is the length fYom the centre o1" the
tile annular flow pattern was confirmed shortly after each liquid slug passed. The heat transfer
A~ss= A ( T ~ , where L
was observed that a thin liquid film was formed
T~o"~)
evaporating part to that of the condensing part, Q is the heat transfer rate from the evaporating
coefficient increased because the thin liquid film
part to the condensing part, A is the total cross
decreased the thermal resistance between the surface of the walls and the working lluid. Besides,
sectional area of the channels in the O C H P , and
tile bubbles formed within the thin liquid film in
Te~ and Tcona a r e the mean surface temperatures
the ew~porating part separated shortly after they
of the evaporating part and the condensing part, respectively f l e e et al, 1999b). Figure 8(b) described the dependence of the saturation pressure on the given heat fluxes. As
generated f'rom the heated walls. The space vacated by the bubbles was filled by the liquid in the liquid film and the nucleate boiling process con-
the heat flux was increased, tile saturation pres-
tinuously occurred. Consequently. the heat transfer coefficient of nucleate boiling and the con-
sure linearly increased. At the inclination angle
densing temperature increased. The condensing
of 0 °. because the O C H P almost did not operate
pressure increased lind the pressure difference was tit tile least. As the heat flux was continuously
at every charging ratio and heat flux condition, when the heat flux was increased, only the saturation pressure ascended. As the inclination angle was increased, the oscillations of working fluid occurred and the saturation pressure decreased. The variation of the saturation pressure was small when the inclination angle was continuously increased. This was due to the influence of surface tension force which predominated over tile gravity force in the flow channels of the O C H P (Lee et al, 1999a). The dependence of the pressure difference on
increased, the d r y - o u t p h e n o m e n a occurred and the oscillation of vapor plugs and liquid slugs became slow. Because some of flow channels in the evaporating part started to be filled with vapor, the supply of working fluid to the evaporating part reduced. This led to tin increase in tile surface temperature of the channels in tile evaporating part. The evaporating pressure and the pressure difference increased. Hence, the heat transfer performance decreased. F r o m these results, one can conclude that tile pressure difference
The Study on Pressure Oscillation and Heat Transfer Characteristics of Oscillating CapillaO' Tube ... is related to the heat transfer pertbrmance of the OCHP. Figure 8(d)
1541
(1) The frequency of pressure oscillation was between 0. I Hz and 1.5 Hz at the charging ratio of
oscillation amplitude (in the evaporating part) on the given heat fluxes. As the inclination angle
40 vol.%. (2) At the charging ratios of 40 vol.% and 60 vol.,0/o, the d o m i n a n t frequencies of the sinusoidal
was decreased, the oscillation amplitude increased. The oscillation amplitude increased when the
pressure oscillations were determined. (3) The saturation pressure of working fluid
heat flux was increased. However, when the heat flux was increased to more than 0.9 W / c m 2, the
in the O C H P increased as the heat flux was in-
showed the dependence of the
oscillation amplitude decreased. As mentioned above, the oscillation p h e n o m e n o n was not clearly confirmed at the heat flux of 0.3 W / c m 2. Therefore, the oscillation amplitude was the least at the heat flux of 0.6 W / c m < Also, the pressure oscillation had a sinusoidal wavetbrm at the heat flux of 0 . 6 W / c m 2 as shown in Fig. 5(b). This
creased. (4) As the charging ratio of working fluid was increased, the amplitude of pressure oscillation increased. When the pressure waves were symmetric sinusoidal waves at the charging ratios of 40 vol.% and 60 vol.~o, the heat transfer pertbrmance was improved. At the charging ratios of 20 vol.,% and 80 vol.°/0o, the waveforms of pressure
implied that the oscillation amplitude was also
oscillation were more complicated, and the heat
related to the heat transfer performance of the
transfer performance reduced. (5) At the charging ratio of 40 vol.~o, the
OCHP. The influence of the given heat flux and the
heat transfer performance of the O C H P was at
inclination angles on the frequency of pressure
the best when the inclination angle was 90 °, the
oscillation was shown in Fig. 8 (e). When the in-
pressure wave was a sinusoidal waveform, the pressure difference was at the least, the oscillation
clination angle was decreased and the heat flux was increased to more than 0.6 W / c m 2, the frequency decreased. The frequency changed from
amplitude was at the least, and the frequency of pressure oscillation was the highest.
0.1 Hz to 1.5 Hz. The frequency was the highest at the heat flux of 0.6 W / c m 2 and the inclination
Acknowledgment
angle of 90 ° . The above experimental results showed that the
The authors wish to acknowledge the financial
heat transfer pertbrmance of the O C H P closely
support of the Korea Research F o u n d a t i o n made
correlated
in the program of year of 1998.
with the characteristics
of pressure
oscillation occurred in the operating process of
References
the O C H P . At the charging ratio of 40 vol.%, the heat transfer performance was at the best when the inclination angle was 90 °, the pressure wave was a sinusoidal w a v e | b r m , the pressure difference (between the evaporating part and the condensing part) was at the least, the oscillation amplitude was at the least, and the frequency of pressure oscillation was the highest.
Akachi,
H.,
1994, "'Looped Capillary T u b e
Heat Pipe," Proceedings o f 71th General Meeting conference of J S M E , Vol. 3, No. 940--10, pp. 606-- 61 I. Faghri, A., 1995, Heat Pipe Science And Tech-
4. Conclusions
nology, T a y l o r and Francis, Washington, pp. 1-34. izumi, T., Gi, K. and Maezawa, S., 1998, " H e a t
The following conclusions were obtained through a basic experimental study on the pressure oscillation and heat transfer characteristics of the O C H P .
Transfer Characteristics of D o u b l e - E n d e d Closed Oscillating Capillary T u b e Heat Pipe," Journal o f J A H P , Vol. 17, No. 1, pp. 7--1 l.(in Japanese) Kim, J.S., Jung, H.S.. Kim, J. H. and Kim, J . W . , 2001, "'Flow Visualization of Oscillation
1542
Jong-Soo Kim, Ngoc Hung Bui, H.vun-Seok Jung and Wook-Hvun Lee
Characteristics of Liquid and Vapor in Oscillating Capillary Tube Heat Pipe," The 6th Asian Symposium on Visualization, Pusan, Korea, pp. 398--401. Kim, ,l. S., Lee, W. H., Lee, ,l. H., Jung, H. S., Kim, J.H. and ,fang, J. 1., 1999, "Flow Visualization of Oscillating Capillary Tube Heat Pipe," Proceeding of Thermal EngineerhTg Conference of KSME, pp. 65--70. (in Korean) Lee. W. H., ,lung, H.S., Kim, J. H. and Kim, ,l. S., 1999a, " F l o w Visualization of Oscillating Capillary Tube Heat Pipe," 11th International Heat Pipe Conference, Tokyo, Japan, Vol. 2, pp. 131-- 136. Lee, W . H . , Kim, J.H., Kim, .l.S. and ,fang, I.S., 1999b, "The Heat Transfer Characteristics
of Oscillating Capillary Tube Heat Pipe," 2nd Two Phase Flow Modelling and Experimentation, Pisa, Italy, Vol. 3, pp. 1713--1718. Maezawa, S., 1995, "Working Principle and Thermal Characteristics of Heat Pipe and Thermosyphon," Journal of JAHP, Vol. 14, No. 2, pp. I - - I 1 . Miyazaki, Y. and Akachi, H., 1996, "Heat Transfer Characteristics of Looped Capillary Heat Pipe," Proceeding of the 5th International Heat Pipe Symposium, Melbourne, pp. 378--383. Suzuki, O., 2000-5, °'Heat transfer characteristics of a Bubble-Driven Capillary Heat Pipe," 37th National Heat Transfer Symposium of Japan, pp. 35--36.(in Japanese)
