nucleate boiling heat transfer enhancement with electrowetting · boiling curve boiling heat...
Post on 14-Mar-2020
37 Views
Preview:
TRANSCRIPT
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
4
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
5
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
7
q Surfaces with hybrid wettability
q Surfaces with hierarchical micro/nanoscale structures
8
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
10
θ0 θa
θ0 èθa
Electrowetting Theory
cosθa = cosθ0 +
ε0εd
2dσ LV
V 2
11
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
12
P2 mode (V = 32 V, f = 28 Hz)
P4 mode (V = 32 V, f = 79 Hz)
Visualization Simulation
Electrowetting - Bubbles
13
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
14
Experimental Setup
15
Test Device
16
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
17
Synchronous
Wall Temperature and Heat Flux
18
IR Image
Wall temperature Heat flux
Electrowetting Signals
19
. . / 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
20
Onset of Nucleate Boiling
21
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
22
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
23
Hydrophobic surface q” = 62.6 kW/m2
EW modulated q” = 62.6 kW/m2
Fully-Developed Nucleate Boiling
24
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
25
Applied heat flux = 82 kW/m2
EW waveform applied
Wall Temperature Variation
26
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
27
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
28
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
29
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.
30
31
Acknowledgements q Financial support from
o National Science Foundation (NSF) o University of Houston
top related