Nucleate Boiling Heat Transfer Enhancement with Electrowetting Aritra Sur, Yi Lu, Carmen Pascente Paul Ruchhoeft and Dong Liu University of Houston, Houston, TX 77204 USA
Nucleate Boiling Heat Transfer Enhancement with Electrowetting
AritraSur,YiLu,CarmenPascentePaulRuchhoeftandDongLiu
UniversityofHouston,Houston,TX77204USA
Outline q Introduction q Electrowetting q Experimental design and measurements q Electrowetting modulated nucleate boiling q Conclusions
2
Introduction
3
q Nucleate boiling – liquid-vapor phase change
o One of the most efficient modes of heat transfer
ü Transfers enormous amount of heat with small driving temperature difference
o Widely applied in energy conversion, power generation and thermal management
o Current limitations ü Low boiling heat transfer coefficient (BHTC) ü Highest critical heat flux (CHF) is only around 10% of
theoretical maximum ü An increase in CHF by 30% alone will increase the power
density of pressurized water reactors by 20%
Boiling 101
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Fig. 1 A typical boiling curve.Wall superheat ΔTsat = (Tw -Tsat)
Heat flux q’’
ΔTONB
CHF Film boiling
Single-phase
Transition boiling
ExcursionONB W
all h
eat f
lux
q”
q Goals for boiling heat transfer enhancement o Onset of nucleate boiling (ONB) at low wall superheat o Steeper boiling curve – high HTC o Extremely high CHF
Wall superheat ΔTsat = (Tw -Tsat)
Hea
t flu
x q’
’
ΔTONB
CHF Film boiling
Single-phase
Transition boiling
Excursion
Wall superheat ΔTsat = (Tw -Tsat)
Hea
t flu
x q’
’
ΔTONB
Single-phase
Nuc
leat
e bo
iling
Fig. 1 Boiling curves: (a) a typical boiling curve; and (b) an ideal boilingcurve.
(a) (b)
Effects of Surface Wettability q Surface wettability plays a critical role in nucleate boiling
o Hydrophobic surfaces have lower energy barrier for nucleation and promotes ONB
o High HTC depends on: nucleation site density, bubble departure size/frequency, and contact line motion, etc.
o Higher CHF can be obtained if the surface remains wetted by liquid and the vapor phase boundary is restricted
q A dilemma: On one hand, hydrophobicity promotes
ONB; on the other hand, hydrophilicity enhances CHF
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6
Effects of Surface Wettability
Better boiling heat transfer!
CHF ONB ONB
CHF
Jo H. et al. 2011. IJHMT. ‘ A study of nucleate boiling on hydrophilic, hydrophobic and heterogeneous wetting surfaces
How can we harness the benefits of
both hydrophobicity and
hydrophilicity?
Current Enhancement Technology
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q Surfaces with hybrid wettability
q Surfaces with hierarchical micro/nanoscale structures
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Hybrid surface
Current Enhancement Technology
q It is working! q But …
o Complicated to fabricate o Multiple parameters
affecting boiling process o Difficult to optimize o Wettability
distribution fixed once fabricated
Can we actively control the
spatiotemporally dynamic boiling
process?
Outline q Introduction q Electrowetting q Experimental design and measurements q Electrowetting modulated nucleate boiling q Conclusions
9
q Electrowetting (EW): Modification of surface wettability with an applied electric field, also termed “electrowetting on dielectric” (EWOD)
q EW is represented by the change of contact angle θ
o Hydrophilic surface: θ < 90°
o Hydrophobic surface: θ > 90° (Superhydrophobic: θ > 120°)
Electrowetting
Electrode Dielectric layer Teflon layer
Silicon substrate
EW of water droplet on Teflon-coated surface
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θ0 θa
θ0 èθa
Electrowetting Theory
cosθa = cosθ0 +
ε0εd
2dσ LV
V 2
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Contact angle saturation
Water on Teflon θ0 = 120° d = 1 µm d = 500 nm
θ0: inherent contact angle;
θa: apparent contact angle
ε0: permittivity in vacuum;
εd: relative permittivity
d: dielectric layer thickness;
V: applied voltage
σLV: liquid surface tension
where
q EW theory was first developed by Gabriel Lippmann in 1875, known as the Young-Lippmann equation
q DC electrowetting
q AC electrowetting
Electrowetting - Droplets
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P2 mode (V = 32 V, f = 28 Hz)
P4 mode (V = 32 V, f = 79 Hz)
Visualization Simulation
Electrowetting - Bubbles
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Zhao Y. and Cho S.K., 2006, Lab Chip, “Micro air bubble manipulation by electrowetting on dielectric”
q Contact angle variation is significant enough to reverse the surface wettability
q Under AC EW, interfacial oscillation generates strong streaming flow around the bubble
Ko et al., 2009, App Phy Lett, “A synthetic jet produced by electrowetting-driven bubble oscillation in aqueous solutions”
It is possible to modulate/enhance
nucleate boiling with EW!
Outline q Introduction q Electrowetting q Experimental design and measurements q Electrowetting modulated nucleate boiling q Conclusions
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Experimental Setup
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Test Device
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n Coating material: DuPont™ Teflon® (AF1600) dissolved in FC40 (2%)
n Adhesion promoter: Fluorosyl™ FCL-52 in fluorosolvent FSM660-4 (0.4%)
n Fabrication method o Dip coat Fluorosyl solution
o Spin coat Teflon
!
!!
!
Measurement Parameters q Optical imaging
o Nucleate bubble dynamics o Boiling regime identification
q IR thermography o Boiling surface wall temperature o Heat flux
q Data acquisition o Power supply to heater o Liquid pool temperature
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Synchronous
Wall Temperature and Heat Flux
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IR Image
Wall temperature Heat flux
Electrowetting Signals
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. . / 2r m sV a=
. .r m sV a=
. . / 3r m sV a=
2 2. .r m s DCV a V= +
a
a
a
0
0
0
0
with DC offset
a
VDC
a
- a
a
- a
a
- a
Time
Outline q Introduction q Electrowetting of droplets vs. bubbles q Experimental design and measurements q Electrowetting modulated nucleate boiling q Conclusions
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Onset of Nucleate Boiling
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Hydrophobic Surface q” = 3.7 kW/m2
EW modulated (Vr.m.s = 78 V, f = 10 Hz)
q” = 6.8 kW/m2
Onset of Nucleate Boiling
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q Change in bubble geometry o Absence of vapor patch upon departure
q Delayed ONB o Hydrophobic surface = 3.7 kW/m2
o EW modulated = 6.8 kW/m2
q Shorter bubble departure time o Hydrophobic surface = 1.5 sec o EW modulated = 430.25 ms
q Decreased bubble footprint size o Hydrophobic surface, radius = 3 mm o EW modulated, radius = 2.75 mm
q Reduced contact angle o Hydrophobic surface ~ 104 – 118 ° o EW modulated ~ 80 °
Hydrophobic surface
EW modulation
0 200 400 600 800 1000 1200 1400 16000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Rad
iusofbubblefootp
rint(m
m)
T ime(ms )
Hydrophobic s urfac e
0 200 400 600 800 1000 1200 1400 1600100
102
104
106
108
110
112
114
116
118
120Hydrophobic s urfac e
Contactangle(Deg
rees
)
T ime(ms )
0 50 100 150 200 250 300 350 400 45040
50
60
70
80
90
100
110
120
Contactangle(Deg
rees
)
T ime(ms )
E Wmodulated(Vr.m.s .= 78V,f= 10Hz )
0 50 100 150 200 250 300 350 400 450 5000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5E Wmodulated (V
r.m.s .= 78V ,f= 10Hz )
Rad
iusofbubblefootp
rint(m
m)
T ime(ms )
100 ms
Fully-Developed Nucleate Boiling
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Hydrophobic surface q” = 62.6 kW/m2
EW modulated q” = 62.6 kW/m2
Fully-Developed Nucleate Boiling
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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0104.5
105.0
105.5
106.0
106.5
107.0
107.5
108.0
Walltem
per
ature
(°C
)
T ime(s ec )
Hydrophobic s urfac eE Wmodulated(V
r.m.s= 78V,f= 10Hz )
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
398
400
402
404
406
408
410
412
414
416
Wallh
eatflux(kW
/m2 )
T ime(s ec )
Hydrophobic s urfac eE Wmodulated(V
r.m.s= 78V,f= 10Hz )
Average wall temperature Average boiling heat flux
q Effect of AC EW
o Decrease in average wall temperature by 1.5ºC
o Increase in boiling heat flux by 10 kW/m2
Film Boiling to Nucleate Boiling Transition
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Applied heat flux = 82 kW/m2
EW waveform applied
Wall Temperature Variation
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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4105
110
115
120
125
130
135
140
145
Tem
per
ature
(°C
)
T ime(s )
S patiallyaverag edwalltemperature
E W s ig na lapplied
0.74s ec
CHF Enhancement q Wall heat flux = 86.9 kW/m2
q EW effect on boiling regime o Absence of vapor film o Surface exposed to bulk fluid o Delayed onset of film boiling o Higher departure frequency o Smaller departure size
q EW effect on wall temperature o Lower wall temperature
o Hydrophobic surface > 200 °C o EW modulated ~ 110 °C
o Enhanced HTC o Enhanced wall heat flux
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Hydrophobic surface
EW modulation
EW-Enhanced Boiling Heat Transfer
Boiling curve Boiling heat transfer coefficient
q Delayed onset of film boiling q CHF is enhanced under the influence of EW q Overall enhancement of boiling HTC
0 10 20 30 40 50 60 70 800
20
40
60
80
100
120
140
160
Wallh
eatflux(kW
/m2 )
Walls uperheat(°C )
Hydrophobic s urfac e
E W-modulated(Vr.m.s = 78V,f= 10Hz )
Onset of film boiling
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0 20 40 60 80 100 120 140 160 180 2000
1
2
3
4
5
6
7
8
9
10
Boilinghea
ttran
sfer
coefficien
t(kW
/m2 -K)
Wallheatflux (kW/m2)
Hydrophobic s urfac eE W-modulated(V
r.m.s= 78V,f= 10Hz )
Enhancement Mechanisms
q Nucleate boiling regime o Altering the bubble dynamics o Enhancing microcovection in the liquid o Re-creating the microlayer o Augmenting the quenching heat transfer
q Filmwise transition boiling o Destabilizing the liquid-vapor interface
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Conclusions q We have demonstrated that the bubble dynamics can be
effectively controlled by EW and nucleate boiling heat transfer can be favorably improved over the entire range of boiling regimes.
q We have developed experiments and are developing theoretical/numerical models to understand the physical mechanisms of EW-enhancement of nucleate boiling.
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Acknowledgements q Financial support from
o National Science Foundation (NSF) o University of Houston