• 검색 결과가 없습니다.

The dropwise condensation heat transfer

N/A
N/A
Protected

Academic year: 2022

Share "The dropwise condensation heat transfer"

Copied!
30
0
0

로드 중.... (전체 텍스트 보기)

전체 글

(1)

The dropwise condensation heat transfer

characteristics of CNT/OTS layered surface associated with non-condensable gas effect

Taeseok Kim

Advisor professor: Sung Joong Kim

Advanced Thermal-Hydraulics Engineering for Nuclear Application lab.

Department of Nuclear Engineering at Hanyang University

2020. 12. 16 ~ 18

(2)

Introduction

Dropwise condensation

Dropwise Filmwise

Large thermal resistance

Easy to contact Steam on surface

Droplet sweeping and re-nucleation Hard to contact

Steam on surface

(3)

Introduction

Dropwise condensation (Constriction resistance)

3

• Constriction resistance is caused by the bending of the heat flux lines in substrate

• It causes non-uniform heat flux and surface temperature on dropwise condensation

B. B. MIKIC, “On mechanism of dropwise condensation,” International Journal of Heat and Mass Transfer 12 10, 1311 (1969);

https://doi.org/10.1016/0017-9310(69)90174-4.

Time

Heat flux line

Temperature fluctuation Droplet distribution

(4)

Introduction

Dropwise condensation (Constriction resistance)

4

• High thermal conductive material can reduce the constriction resistance

• The substrate material is often limited by its mechanical strength and integrity

• Changing the substrate material is impractical in many industrial applications

High thermal conductivity of surface

High heat transfer coefficient High thermal conductive material coatings

Can reduce the constriction resistance

without substrate change

(5)

Introduction

Surface modification technique

5

Substrate

Vacuum

Source (Target)

Coating

• Surface modification can change the surface material without altering substrate

• CVD and PVD are well known surface modification techniques

• However, it needs high temperature or vacuum condition

• It is difficult to apply the commercial heat transfer tubes such as heat exchanger

PVD CVD

(6)

Introduction

The objective of this study

• Useful surface modification technique(Layer-by-layer) was selected.

• High thermal conductive material(Multi-walled carbon nanotube) was deposited

• Condensation heat transfer experiment was conducted on modified surface

High thermal conductive coatings

Condensation heat transfer experiment

(7)

Surface modification

LbL assembled MWCNT coating

7

PEI

water

CNT

water

10 min

SS316L

Rinsing 10 min Rinsing

Substrate

(+) Polyethyleneimine(PEI) (-) Multi-walled CNT(MWCNT)

10 bi-layer

2 bi-layer LbL-assembled PEI/MWCNT

One bi-layer Tensile strength ↑

Thermal conductivity ↑

Easy to deposit Capable on complicated

or large area

MWCNT

Layer-by-Layer(LbL)

(8)

Surface modification

LbL assembled MWCNT coating

Coating material CNT

Bi-layer 10

Pore size ~ 64 nm

Thickness ~ 760 nm (ref) Contact angle ~ 20

o

Pore size distribution

Contact angle SEM image

~ 20 o

(9)

Surface modification

Hydrophobic coating

9

Octadecyltrichlorosilane(OTS)

Methyl group (Hydrophobic)

Head group (Silane)

~ 2 h dipping Annealing

Chemical adsorption

Substrate

Van der Waals force

Columnar structure OTS

Covalent bonding

Si

Cross linking

Si Si

Si

OTS

solution

(10)

Surface modification

Surface preparation

(b)

Bare CNT CNT+HPo

• Three surface were prepared for the condensation heat transfer experiment

• The substrate was selected to commercial stainless-steel (Bare)

• LbL-assembled MWCNT surface(CNT) cannot promote the dropwise condensation

• Hydrophobic coatings on the CNT surface was deposited (CNT+HPo)

CA ~ 85

o

CA ~ 20

o

CA ~ 105

o

(11)

Method

Experimental set up (Test Loop)

11

Air injection line

Coolant line Steam line

Test section

(12)

Method

Experimental set up (Test section)

Steam flow area 0.02 m

2

Test specimen Length

1.18 m

Test specimen diameter

19.05 mm

Steam pressure 1 bar

Steam mass flux 0.1 kg/m

2

s ( ~ 0.16 m/s) Coolant mass flux 1600 kg/m

2

s

( ~ 1.6 m/s)

1.18m

(13)

Result

Condensation heat transfer coefficient

13

CNT+HPo

Bare

CNT

(14)

Result

Droplet morphology

14

CNT+HPo

Bare Sliding

Flooding

Miljkovic et al. (2013)

(15)

Result

Droplet morphology (Regime map)

15

Cassie state

Wenzel state

• Droplet morphology regime map was developed by Enright et al.

• CNT+HPo surface was in Wenzel state region, and it caused flooding condensation

R. ENRIGHT et al., “Condensation on Superhydrophobic Surfaces: The Role of Local Energy Barriers and Structure Length Scale,” Lang muir 28 40, 14424 (2012); https://doi.org/10.1021/la302599n.

Enright et al. (2012)

(16)

Result

Droplet morphology (Condensation heat transfer)

16

• Flooding has no enhancement of condensation heat transfer than the sliding Wen et al. (2018) Miljkovic et al. (2013)

HTC enhancement of CNT+HPo

was not caused by nanostructure

(17)

Result

Effect of the constriction resistance

17

• The CNT layers of the CNT+HPo surface can reduce the constriction resistance

• Constriction resistance was inserted to the condensation HTC model

B. B. MIKIC, “On mechanism of dropwise condensation,” International Journal of Heat and Mass Transfer 12 10, 1311 (1969); https://

doi.org/10.1016/0017-9310(69)90174-4.

𝑅 𝑐𝑠 ≈ 𝛽 2 1 − 𝛽 2

𝜓 4𝑘 𝑠 𝑟 𝑅 = 𝑅 𝑙𝑣 + 𝑅 𝑑𝑟𝑜𝑝 + 𝑅 𝑐𝑜𝑎𝑡 + 𝑅 𝑙𝑠 + 𝑅 𝑐𝑠

(previous)

Coating effect (present)

𝑅𝑐𝑠of CNT+HPo

𝑅𝑐𝑠of Bare

Thermal resistance of the single droplet

(18)

Result

Effect of the constriction resistance

18

• The present model (with constriction) is in line with the experimental result

(19)

Result

Effect of the non-condensable gas

19

• The condensation HTC of each surface decreased as air mass fraction increased

A. DEHBI, “A generalized correlation for steam condensation rates in the presence of air under turbulent free convection,” Internation al Journal of Heat and Mass Transfer 86, 1, Elsevier Ltd (2015); https://doi.org/10.1016/j.ijheatmasstransfer.2015.02.034.

Bare CNT+HPo CNT

(20)

Result

Effect of the non-condensable gas

20

• Dropwise condensation HTC was similar with the filmwise as air fraction increased

W

air

~ 3% W

air

~ 6%

W

air

~ 9% W

air

~ 15%

(21)

Conclusion

21

Effect of CNT layers on the dropwise condensation

LbL assembled CNT coatings

Reduce the constriction resistance

Condensation HTC enhancement

(22)

Acknowledgement

This work was supported by the National Research Foundation of Korea

(NRF) funded by the Ministry of Science & ICT (MSIT, Korea) grant numbers

[NRF-2016R1A5A101391921]

(23)

Thank you

for your

attention

(24)

Appendix

Data reduction (LMTD method)

𝑈 = 𝑄

𝐴 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 ∆𝑇 𝐿𝑀𝑇𝐷

𝑄 = ሶ 𝑚𝐶

𝑝

𝑇

𝑜𝑢𝑡

− 𝑇

𝑖𝑛

∆𝑇

𝐿𝑀𝑇𝐷

= 𝑇

𝑜𝑢𝑡

− 𝑇

𝑖𝑛

𝑙𝑛 𝑇

𝑠𝑡𝑒𝑎𝑚

− 𝑇

𝑖𝑛

𝑇

𝑠𝑡𝑒𝑎𝑚

− 𝑇

𝑜𝑢𝑡

𝑁𝑢 = 0.023𝑅𝑒 0.8 𝑃𝑟 0.4

ℎ = 1

1

𝑈 − 𝑑 𝑜 𝑑 𝑖

1

𝑐𝑜𝑛𝑣 − 𝑑 𝑜

𝑘 𝑙𝑛 𝑑 𝑜 𝑑 𝑖 𝑅 𝑐𝑜𝑛𝑑 = 𝑙𝑛 𝑟 2 Τ 𝑟 1

2𝜋𝑘𝐿

𝑇 𝑠𝑢𝑏 = 𝑄

T

u

b

e

w

a

l

l

(25)

Appendix

25

Dropwise condensation HTC model

∆𝑇 = ∆𝑇 𝑐𝑢𝑟𝑣 + ∆𝑇 𝑙𝑣 + ∆𝑇 𝑑𝑟𝑜𝑝 + ∆𝑇 𝑙𝑠 + ∆𝑇 𝑐𝑜𝑎𝑡 + 𝑞 𝑑 × 𝑅 𝑐𝑠

∆𝑇 𝑐𝑢𝑟𝑣 = 2𝑇 𝑠𝑎𝑡 𝜎 ℎ 𝑓𝑔 𝜌 𝑙 𝑟

∆𝑇 𝑙𝑣 = 𝑞 𝑑

2𝜋𝑟 2 1 − cos 𝜃 ℎ 𝑙𝑣

∆𝑇 𝑑𝑟𝑜𝑝 = 𝜃𝑞 𝑑 4𝜋𝑟𝑘 𝑙 sin 𝜃

∆𝑇 𝑙𝑠 = 𝑞 𝑑

𝜋𝑟 2 sin 2 𝜃 ℎ 𝑙𝑠

∆𝑇 𝑐𝑜𝑎𝑡 = 𝛿𝑞 𝑑

𝜋𝑟 2 sin 2 𝜃 𝑘 𝑐𝑜𝑎𝑡

∴ 𝑞 𝑑 =

∆𝑇 − 2𝑇 𝑠𝑎𝑡 𝜎 ℎ 𝑓𝑔 𝜌 𝑙 𝑟 𝛿

𝑘 𝑐𝑜𝑎𝑡 sin 2 𝜃 + 𝑟𝜃

4𝑘 𝑙 sin 𝜃 + 1

2ℎ 𝑙𝑣 (1 − cos 𝜃) + 1 ℎ 𝑙𝑠 sin 2 𝜃

D. NIU et al., “Dropwise condensation heat transfer model considering the liquid-solid interfacial thermal resistance,” International Journal of Heat a nd Mass Transfer 112, 333 (2017); https://doi.org/10.1016/j.ijheatmasstransfer.2017.04.061.

(26)

Appendix

26

𝐴 1 = ∆𝑇 2ℎ 𝑓𝑔 𝜌 𝑙

𝐴 2 = 𝜋𝑟(1 − cos 𝜃)(𝑅 𝑑𝑟𝑜𝑝 + 𝑅 𝑐𝑠 )

𝐴 3 = 𝜋𝑟 2 (1 − cos 𝜃)(𝑅 𝑣𝑙 + 𝑅 𝑙𝑠 + 𝑅 𝑐𝑜𝑎𝑡 )

𝐵 1 = 𝐴 2 𝜏𝐴 1

𝑟 𝑒 2 − 𝑟 2

2 + 𝑟 𝑚𝑖𝑛 𝑟 𝑒 − 𝑟 − 𝑟 𝑚𝑖𝑛 2 ln 𝑟 − 𝑟 𝑚𝑖𝑛 𝑟 𝑒 − 𝑟 𝑚𝑖𝑛 𝐵 2 = 𝐴 3

𝜏𝐴 1 𝑟 𝑒 − 𝑟 − 𝑟 𝑚𝑖𝑛 ln 𝑟 − 𝑟 𝑚𝑖𝑛 𝑟 𝑒 − 𝑟 𝑚𝑖𝑛 𝜏 = 3𝑟 𝑒 2 𝐴 2 𝑟 𝑒 + 𝐴 3 2

𝐴 1 (11𝐴 2 𝑟 2 − 14𝐴 2 𝑟 𝑒 𝑟 𝑚𝑖𝑛 + 8𝐴 3 𝑟 𝑒 − 11𝐴 3 𝑟 𝑚𝑖𝑛 )

𝑛 𝑟 = 1

3𝜋𝑟 𝑒 3 𝑟 𝑚𝑎𝑥

𝑟 𝑒 𝑟 𝑚𝑎𝑥

− 2

3 𝑟(𝑟 𝑒 − 𝑟 𝑚𝑖𝑛 ) 𝑟 − 𝑟 𝑚𝑖𝑛

𝐴 2 𝑟 + 𝐴 3

𝐴 2 𝑟 𝑒 + 𝐴 3 exp(𝐵 1 + 𝐵 2 )

Dropwise condensation HTC model

𝑅 𝑐𝑠 ≈ 𝛽 2 1 − 𝛽 2

𝜓 4𝑘 𝑠 𝑟

𝛽 = 𝑟 2 𝑏 2 𝜓 = 32

3𝜋 2 1 − 𝑟 𝑏

1.5

(27)

Appendix

27

𝑁 𝑟 = 1

3𝜋𝑟 2 𝑟 𝑚𝑎𝑥

𝑟 𝑟 𝑚𝑎𝑥

− 2 3

𝑞 𝑑𝑟𝑜𝑝 ′′ = (න

𝑟

𝑚𝑖𝑛

𝑟

𝑒

𝑞 𝑑𝑟𝑜𝑝 𝑟 𝑛 𝑟 𝑑𝑟 + න

𝑟

𝑒

𝑟

𝑚𝑎𝑥

𝑞 𝑑𝑟𝑜𝑝 𝑟 𝑁 𝑟 𝑑𝑟) 𝑟 𝑚𝑎𝑥 = 6 cos 𝜃 𝑟 − cos 𝜃 𝑎 sin 𝜃

𝜋(2 − 3 cos 𝜃 + cos 3 𝜃) 𝜎 𝑙𝑣 𝜌 𝑙 𝑔

0.5

Dropwise condensation HTC model

𝑟 𝑒 = 4𝑁 𝑠 −1/2 𝑁 𝑠 = 0.037

𝑟 𝑚𝑖𝑛 2

S. KIM and K. J. KIM, “Dropwise Condensation Modeling Suitable for Superhydrophobic Surfaces,” Journal of Heat Transfer 133 8, 081502 (2011); htt ps://doi.org/10.1115/1.4003742.

(28)

Appendix

28

Dropwise condensation HTC model

∆𝑔 = ℎ 𝑓𝑔 𝑇 𝑙 − 𝑇 𝑠𝑎𝑡

𝑇 𝑠𝑎𝑡 + 𝜈 𝑙 𝑝 𝑙 − 𝑝 𝑠𝑎𝑡 𝑇 𝑠𝑎𝑡

∴ ∆Ψ = 𝜌 𝑙 𝜋𝑟 3 sin 3 𝜃 න

0 𝜃

𝑓𝑔 𝑇 𝑠𝑎𝑡

1 − cos 𝜙 2 sin 4 𝜙 ×

−∆𝑇 𝑠𝑢𝑏 + 𝑞 𝑑𝑟𝑜𝑝 𝑟 𝛿

𝜋𝑟 2 sin 2 𝜃 𝑘 𝑐𝑜𝑎𝑡 + 𝑞 𝑑𝑟𝑜𝑝 (𝑟)𝜙

4𝜋𝑟 sin 𝜃 𝑘 𝑙 + 𝑞 𝑑𝑟𝑜𝑝 𝑟

𝜋𝑟 2 sin 2 𝜃 ℎ 𝑙𝑠 𝑑𝜙 +𝜎 𝑙𝑣 2 − 3 cos 𝜃 + cos 3 𝜃 𝜋𝑟 2

𝝏∆𝜳 ቤ

𝝏𝒓 𝒓=𝒓

𝒎𝒊𝒏

= 𝟎

∆Ψ 𝑟 = න[∆𝑔 + 𝑝 𝑠𝑎𝑡 𝑇 𝑠𝑎𝑡 − 𝑝 𝑙 𝑣 𝑙 𝑑𝑚 𝑙 + 𝜎 𝑙𝑣 𝐴 𝑙𝑣 + 𝜎 𝑠𝑙 𝐴 𝑠𝑙 − 𝜎 𝑠𝑣 𝐴 𝑠𝑣

𝑑𝑣 = 𝐴 𝑠 𝑑 ҧ 𝜀 = 𝜋𝑟 3 sin 3 𝜃 1 − cos 𝜙 2

sin 4 𝜙 𝑑𝜙

(29)

Appendix

29

Dropwise condensation HTC model

𝐴 𝑑 = න

𝑟

𝑚𝑖𝑛

𝑟

𝑒

𝜋𝑟 2 sin 2 𝜃 𝑛 𝑟 𝑑𝑟 + න

𝑟

𝑒

𝑟

𝑚𝑎𝑥

𝜋𝑟 2 sin 2 𝜃 𝑁 𝑟 𝑑𝑟 𝐴 𝑁𝑢 = ℎ 𝐹 𝑙

𝑘 𝑣 = 0.664𝑅𝑒 1/2 𝑃𝑟 1/3 𝑅𝑒 < 5 × 10 5 𝑁𝑢 = 0.037𝑅𝑒 0.8 − 871 𝑃𝑟 1/3 , 𝑅𝑒 > 5 × 10 5

𝑞 𝑐𝑣 ′′ = 𝐴 𝑠𝑜 + 𝐴 𝑙𝑣𝐹 ∆𝑇/𝐴 𝑞 𝑐𝑣 ′′

𝐴 𝑠𝑜 = 𝐴 − 𝐴 𝑑 𝐴 𝑙𝑣 = න

𝑟

𝑚𝑖𝑛

𝑟

𝑒

2𝜋𝑟 2 (1 − cos 𝜃)𝑛 𝑟 𝑑𝑟 + න

𝑟

𝑒

𝑟

𝑚𝑎𝑥

2𝜋𝑟 2 (1 − cos 𝜃)𝑁 𝑟 𝑑𝑟 A 𝐴 𝑠𝑜

𝐴 𝑙𝑣

∴ 𝑞 ′′ = 𝑞 𝑐𝑣 ′′ + 𝑞 𝑑𝑟𝑜𝑝 ′′

𝐴 𝑑

J. WANG et al., “Improved modeling of heat transfer in dropwise condensation,” International Journal of Heat and Mass Transfer 155, Elsevier Ltd (2 020); https://doi.org/10.1016/j.ijheatmasstransfer.2020.119719.

(30)

Appendix

30

Droplet morphology regime map

< 𝐿 > = 2 × 𝑟 e

𝐸 = −1 𝑟 cos 𝜃 Τ 𝑎

참조

관련 문서

[r]

* 주위에서 실제 발생하는 복사 열전달

It reveals that remarkable improvement in the convective heat transfer coefficient of MWCNT/Fe 3 O 4 hybrid nanofluid was shown at a weight concentration

The rate of heat transfer depends upon the temperature gradient and the thermal conductivity of the material.. More fundamental questions arise when you examine the reasons

Also, the design of an optimal heat sink requires a complete knowledge of all heat transfer characteristics between the heat source and the ambient air

5.16 Overall heat transfer coefficient and convective heat transfer coefficient of hot side according to mass flow rate and inlet temperature (High temperature condition)

여기서 한 걸음 더 나아가 본다면, 겨울 바람 속의 몸을 움추린 사람은 강제 대류의 외부 유동에 설명되어 있는 실린더 주변에서의 유동과 다름이

▪ Local temperature of PMSM can be calculated through thermal resistance model and heat generation model, presenting the cooling performance thermal