31
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 · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

  • Upload
    others

  • View
    25

  • Download
    1

Embed Size (px)

Citation preview

Page 1: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Nucleate Boiling Heat Transfer Enhancement with Electrowetting

AritraSur,YiLu,CarmenPascentePaulRuchhoeftandDongLiu

UniversityofHouston,Houston,TX77204USA

Page 2: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Outline q  Introduction q  Electrowetting q  Experimental design and measurements q  Electrowetting modulated nucleate boiling q  Conclusions

2

Page 3: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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%

Page 4: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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)

Page 5: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 6: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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?

Page 7: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Current Enhancement Technology

7

q  Surfaces with hybrid wettability

q  Surfaces with hierarchical micro/nanoscale structures

Page 8: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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?

Page 9: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Outline q  Introduction q  Electrowetting q  Experimental design and measurements q  Electrowetting modulated nucleate boiling q  Conclusions

9

Page 10: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 11: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 12: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 13: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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!

Page 14: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Outline q  Introduction q  Electrowetting q  Experimental design and measurements q  Electrowetting modulated nucleate boiling q  Conclusions

14

Page 15: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Experimental Setup

15

Page 16: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

!

!!

!

Page 17: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 18: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Wall Temperature and Heat Flux

18

IR Image

Wall temperature Heat flux

Page 19: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 20: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Outline q  Introduction q  Electrowetting of droplets vs. bubbles q  Experimental design and measurements q  Electrowetting modulated nucleate boiling q  Conclusions

20

Page 21: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 22: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 23: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Fully-Developed Nucleate Boiling

23

Hydrophobic surface q” = 62.6 kW/m2

EW modulated q” = 62.6 kW/m2

Page 24: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 25: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

Film Boiling to Nucleate Boiling Transition

25

Applied heat flux = 82 kW/m2

EW waveform applied

Page 26: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 27: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 28: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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 )

Page 29: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 30: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

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

Page 31: Nucleate Boiling Heat Transfer Enhancement with Electrowetting · Boiling curve Boiling heat transfer coefficient q Delayed onset of film boiling q CHF is enhanced under the influence

31

Acknowledgements q  Financial support from

o  National Science Foundation (NSF) o  University of Houston