The effective range of miniature pulsating heat pipe
1
Zi-Rong Lin, Eton Lee & L. Winston Zhang, Ph.D., P.E.
Novark Technology Inc
S. F. Wang, Ph.D.Key Laboratory of Enhanced Heat Transfer and Energy
Conservation of the Ministry of Education, South China University of Technology
Contact: [email protected]
Outline
Introduction Experiment setup Operation analysis of Miniature Pulsating
Heat Pipes (MPHP) Heat transport capability of MPHP Correlation prediction of MPHP Conclusions
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Be made of a long continuous capillary tube bent into many turns
Contains more working fluid
No wick structure
Ring circuit closed structure avoided the vapor-liquid convection
Simple designSmall size (meets the compact cooling requirement )Low cost
Excellent thermal performance
PHP is considered as the most promising heat transfer device 3Heat transfer mechanism of PHP
What is a pulsating Heat Pipe (PHP)?
Introduction
Review of previous studies and main conclusions
1、Studied the operation mechanism of an PHP through the visualization experiments
2、Effect of experimental parameters including liquid filling ratios , operating directions, working fluids and so on
3、Functional thermal fluids (such as nano-fluid and microcapsule fluid) be used to enhancethe heat transport capability of an PHP
(3) Working fluid had a great influence on the thermal performance of PHPs
(1) PHPs were hard to operate in horizontal mode; There were a certain critical number of turns to reduce the performance gap between horizontal and vertical mode.
(2) The best liquid filling ratio was around 50% and slightly varied according to gravity orientations.
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Compared to conventional heat pipes (one of the proven technologies), PHPs have few advantages in conventional sizes (outer diameter larger than 3mm). Conventional heat pipes have a more extensive range of applications due to wick structures to assist working fluid cycle. Therefore, the research range of PHPs should be focused on miniature sizes (outer diameter less than 3mm，miniature PHPs). which conventional heat pipes are hard to reach.
Advantage analysis
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(1) As a novel heat pipe applied in cooling system, the effective range of MPHP should be found out. The important parameters focused on inner diameter and heat transfer length
(2) A predicting correlation for the heat transport capability of MPHPs, considering the effect of heat transfer length, inner diameter, gravity, heat flux input and temperature difference
Research objective
Experiment setup
nickel electric wire is woundaround evaporation section
(20mm) to simulate the heating condition
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A
VInsulation area
water cooling system
Flowmeter
Transformer
condensation section
adiabatic section
evaporation section
Liquid injection port
T1 T2 T3 T4nickel chromeelectric wires
T5 T6 T7 T8
T9 T10 T11 T12
T13 T14 T15 T16
T17
T18
T19Air temperature
water-inlet temperature
water-outlet temperature
OMEGA K-type thermocouples are installed
to measure the wall temperature at different
positions, which reflect the internal working fluid oscillation indirectly.
Condensation section （20mm） is
cooled by water
Four-turn MPHP was selected as a typical
shape
Working fluid: pure waterFR:50%+/-5%
(3) Base on the application requirements, the operating temperature of electronic chip is usually not higher than 120 . therefore, when the average temperature of evaporation section was over 100 , the experiment would be stopped.
(1) Copper tubes with inner diameter of 0.4, 0.8, 1.3 and 1.8 mm were used as manufacturing material ;（wall thickness was 0.6mm）
Note:
(2) Heat transfer length of 100, 150 and 200 mm were adopted for comparative experiments
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Operation analysis of MPHP
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0 500 1000 1500 2000 2500 3000 3500 400020406080
100120140160180200220240
64W56W48W40W32W
24W16W
8W
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16
Tem
pera
ture（℃
）
Time（s）
(a)Di=0.8mm,Do=2mm,L= 100mm, horizontal mode
Temperature oscillation emerges at 40W
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
20
40
60
80
100
120
140
160
180
200
220
240
8W16W 24W 32W 40W 48W 56W 64W 72W 80W
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16
Tem
pera
ture（℃）
Time（s）
(b)Di=1.3mm,Do=2.5mm,L= 200mm, vertical mode
a MPHP starts up and stable temperature oscillation emerges at 16W, earlier than other cases. It is because that using vertical bottom heating mode assists the condensed working fluid to flow back to the evaporation section with the help of gravity.
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0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
1 4 0
1 6 0
1 8 0
2 0 0
2 2 0
2 4 0
4 0 W
3 2 W
2 4 W
1 6 W
8 W
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16
Temp
erat
ure（
℃）
T ime（ s）
(c)Di=0.4mm,Do=1.6mm,L= 100mm, vertical mode
When inner diameter decreases to 0.4mm, stable temperature oscillation does not emerge, even though the heating power up to 32W in vertical mode
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0 500 1000 1500 2000 2500 3000 3500 4000 45000
20
40
60
80
100
120
140
160
180
200
220
240
80W
72W64W56W48W
40W32W
24W
8W16W
T1T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16
Tem
pera
ture（
℃）
Time（s）
(d)Di=1.8mm,Do=3mm,L= 100mm, horizontal mode
a “dry-out” point emerges at 80W
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0 500 1000 1500 2000 2500 3000 3500 40000
20
40
60
80
100
120
140
160
180
200
220
240
48W
40W32W
24W16W
8W
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16
Tem
pera
ture（℃
）
Time（s）
(e)Di=1.3mm,Do=2.5mm,L= 150mm, horizontal mode
intermittent temperature oscillation emerges from 24W to 40W
dries out at 48W
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Effective range of MPHP
0.4 0.8 1.3 1.8
100 Fail to start up Start up at 40W Start up at 16W Start up at 16W
150 Fail to start up Start up at 64W Start up at 16W Start up at 16W
200 Fail to start up Fail to start up Start up at 16W Start up at 16W
MPHPs operation in vertical bottom heating mode
0.4 0.8 1.3 1.8
100 Fail to start up Start up at 40W Start up at 40W Start up at 40W
150 Fail to start up Fail to start upIntermittent oscillation Fail to start up
200 Fail to start up Fail to start up Fail to start up Fail to start up
MPHPs operation in horizontal heating mode
( ) / ( )Di mm L mm
( ) / ( )Di mm L mm
The recommended inner diameter of MPHPs should be bigger than 0.8mm in vertical bottom heating mode
The heat transfer length should be controlled less than about 100mm in horizontal heating mode 13
Heat transport capability of MPHP
0 20000 40000 60000 80000 100000 120000
0
20000
40000
60000
80000
100000
120000
q c hea
t flu
x ou
tput
/ Wm
-2
qh heat flux input/ Wm-2
0.4mm/200mm 0.8mm/200mm 1.3mm/200mm 1.8mm/200mm
vertical mode
0 20000 40000 60000 80000 100000 120000
0
20000
40000
60000
80000
100000
120000
vertical modeq c hea
t flu
x ou
tput
/ Wm
-2
qh heat flux input/ Wm-2
0.4mm/150mm 0.8mm/150mm 1.3mm/150mm 1.8mm/150mm
0 20000 40000 60000 80000 100000 120000
0
20000
40000
60000
80000
100000
120000
vertical mode
qh heat flux input/ Wm-2
q c hea
t flu
x ou
tput
/ Wm
-2
0.4mm/100mm 0.8mm/100mm 1.3mm/100mm 1.8mm/100mm
0 10000 20000 30000 40000 500000
10000
20000
30000
40000
50000
0.4mm/200mm 0.8mm/200mm 1.3mm/200mm 1.8mm/200mm
q c hea
t flu
x ou
tput
/ Wm
-2
qh heat flux input/ Wm-2
horizontal mode
0 10000 20000 30000 40000 500000
10000
20000
30000
40000
50000
q c hea
t flu
x ou
tput
/ Wm
-2
qh heat flux input/ Wm-2
horizontal mode
0.4mm/150mm 0.8mm/150mm 1.3mm/150mm 1.8mm/150mm
0 10000 20000 30000 40000 500000
10000
20000
30000
40000
50000
qh heat flux input/ Wm-2
q c hea
t flu
x ou
tput
/ Wm
-2 0.4mm/100mm 0.8mm/100mm 1.3mm/100mm 1.8mm/100mm
horizontal mode
(a) L=200mm, vertical mode (c) L=150mm, vertical mode (e) L=100mm, vertical mode
(b) L=200mm, horizontal mode (d) L=150mm, horizontal mode (f) L=100mm, horizontal mode
Thermal performance of vertical mode is obviously better than horizontal mode
With the decrease of inner diameter and heat transfer length, the performance gap between vertical and horizontal mode decreases
Inner diameter of 1.3mm is considered as the best size
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Correlation prediction of MPHP
0.19004 -0.06365 -0.17595 5.079930.67019 /c hq Di L q Ja Bo Wa
is the ratio of the inner diameter to that of the heat transfer length of the tube and it represents the geometry of the MPHPs/D i L
The jakob number is the ratio of sensible heat to that of the latent heat of the working fluid.
/p fgJa C T h
The bond number is the ratio of the buoyancy force to that of the surface tension force of the working fluid 0.5( ) /l vBo Di g
0.251 ( / )v lWa The Wallis number can be used to explain the flooding phenomenon that influences dry-out to occur at the evaporation section
Heat flux outputcq Heat flux inputhq Temperature differenceT
A predicting correlation for the heat transport capability of PHPs, considering the effect of heat transfer length, inner diameter, gravity, heat flux input and temperature difference
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0 200000 400000 600000 800000 100000012000000
200000
400000
600000
800000
1000000
1200000
vertical mode
Experimental data Dimensionless fitting formula
(R2=0.9853)
q c hea
t flu
x ou
tput
/ Wm
-2
qh heat flux input/ Wm-2
0 20000 40000 60000 80000 100000 1200000
20000
40000
60000
80000
100000
120000 Dimensionless fitting formula 0.8mm/125mm 1.3mm/125mm 0.8mm/175mm
q c hea
t flu
x ou
tput
/ Wm
-2
qh heat flux input/ Wm-2
vertical mode
L
(a) 122 sets of experimental data ( L=100/150/200mm) for prediction fitting formula
(b) Thermal performance predicted through fitting formula
Comparison heat flux measurement versus prediction
The correlation prediction agrees with the experimental results fairly well
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Conclusions(1) Increasing inner diameter or decreasing heat transfer length is
beneficial to MPHPs startup
(2) The recommended inner diameter of MPHPs should be bigger than 0.8mm in vertical mode, while the heat transfer length should be controlled less than about 100mm in horizontal mode.
(3) Inner diameter of 1.3mm is considered as the best size in different heating mode.
(4) The dominating dimensionless parameters are used to predict the heat transport capability of MPHPs. And the correlation prediction agrees with the experimental results fairly well.
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Thank you！
