유체역학및열전달 Chapter 13. Heat transfer to fluids with Phase …contents.kocw.net/KOCW/document/2014/Pusan/hyeongyu/13.pdf · 2016-09-09 · Heat transfer to fluids with

부산대학교현규유체역학및열전달 1

유체역학및열전달

Chapter 13. Heat transfer to fluids with Phase

Change

부산대학교 화공생명공학부현 규 (Kyu Hyun)

부산대학교현규

Heat transfer with phase change

l Condense on a cold surface

유체역학및열전달 2


Heat transfer from Condensing Vapors



Heat transfer with phase change



Coefficient for film-type condensation (1)

• In film-type condensation, the condensate film starts to form at the top of the tube

• The thickness of the film increases rapidly near the top of the tube and then more and more slowly in the remaining length

• The heat is assumed to flow through the condensate film solely by conduction

• The local coefficient is given by (heat flux가 같음)

-Cold wall

df

x

kh =\

- Vertical tubes

d

x

diametertubeDD =< -- ,)~( 23 1010d

dTk

dAdqTh

dAdq

xD

=D= ,

• 응축으로 생성되는 막(film) 자체는 두껍지 않다.


y



• Layer flow with free surface (Gravity driven)• Film Thickness

• The local heat-transfer coefficient

31

2

3/

÷÷ø

öççè

æ=

gbm

rmd&

d

bm&

=G Mass rate per unit length (condensate loading)

31

2

3/

÷÷ø

öççè

æ G=\

gf

f

rm

d f 는 film을의미함

312

3

/

÷÷ø

öççè

æ

G=\

f

ffx

gkh

mr

(13.3)




• Vapor condense at outside of tubes • The local heat transfer coefficient (일반적인 정의)

d

dLDTmd

dATdqh

oooox p

lD

=D

=&

dL

condensateofrateflowlocalmonvaporizatiofheat

==

&

l

oDmp&

=G

dLTdho

x DG

=\l

-파이프를 타고 흘러내리므로 단위길이당 흘러내리는 질량유속은원주의 길이를 local flow rate로나누는 값이 된다.

(13.5)




• The average coefficient for the entire tube

oT

b

Too

T

oo

T

TLLDTm

TAqh

DG

=D

=D

=l

pl &

tubeofbottomatloadingcondensatelengthtubetotalL

oncondensatiofratetotalmtransferheatofratetotalq

b

T

T

T

=G===

&

dLkd

gT

ff

fo

G÷÷ø

öççè

æ G=D\

lrm

31

2

3/

-여기서 중요한 두 가지. 1. 단위길이당 흘러내리는 총 질량유속이 tube 마지막 부분(bottom

of tube)에서 흘러내리는 유속과 같다는 것이다.2. ΔTo 는 필름의 평균 온도와 바깥쪽 vapor의 평균 온도 차이를

나타내는 것으로서, local heat transfer에서와 같은 값을나타낸다. â 따라서 앞의 (13.3)과 (13.5)에서 ΔTo 를 계산할 수있다.

TL

이 식을 위의 식에 대입하면

(13.6)




• The average coefficient for the entire tube

• Reynolds number를이용해서정의하면

GG÷÷ø

öççè

æG=

ddLg

Lk

hf

f

T

fb31

312

3 /

/

mr

TL위 식을 0부터 LT까지 적분하면

òò ÷÷ø

öççè

æG=GG

G Tb L

f

f

T

fb dLg

Lk

dh0

312

0

31

3

/

/

mr

4123312

943033

4//

. ÷÷ø

öççè

æ

D=÷

÷ø

öççè

æ

G=\

fo

ff

fb

ff

LTgk

horgk

hmlr

mr

31

3131

23

2

4714471 /

//

Re.. -

-

=÷÷ø

öççè

æ G=÷

÷ø

öççè

æ

f

b

ff

f

gkh

mrm

f

b

mG

=4Re

(13.6)을이용




• For Laminar flow

• For the wavy laminar and turbulent regions : the empirical equation

31

31

2

2

471 /

/

Re. -=÷÷ø

öççè

æ=¢

gkhuN

f

f

f rm

-Characteristic length, but this length is not the same as film thickness

2131806440

31

2

2

10825 /...

/

)PrRe.(Re -- ´+=÷÷ø

öççè

æ=¢

gkhuN

f

f

f rm

-Small ripples cause eddies which improve the heat transfer near the liquid surface유체역학및열전달 10


Heat transfer to Boiling Liquids (비등 액체로의 열전달)


(포화액체의 풀 비등)


Heat transfer to Boiling Liquids

l Line A-B



Natural Convection to air from a hot, horizontal pipe (1)



Natural Convection to air from a hot, horizontal pipe (2)




-Natural Convection

l Line A-B

- Pure convection- Heat transferred by superheated

liquid rising to the liquid-vapor interface where evaporation takes place

-Onset of boiling (Point B)



Superheating

- Superheating is an exception to this simple rule; a liquid is sometimes observed not to boil even though its vapor pressure does exceed the ambient pressure. The cause is an additional force, the surface tension, which suppresses the growth of bubbles.

- or by heating it in a sealed vessel with a headspace, where the liquid water is in equilibrium with vapor at the saturated vapor pressure.

- Water is said to "boil" when bubbles of water vapor grow without bound, bursting at the surface.

- For a vapor bubble to expand, the temperature must be high enough that the vapor pressure exceeds the ambient pressure (the atmospheric pressure, primarily). Below that temperature, a water vapor bubble will shrink and vanish



-Natural Convection

l Line A-B

- Pure convection- Heat transferred by superheated

liquid rising to the liquid-vapor interface where evaporation takes place

-Onset of boiling (Point B)




-Individual Bubble regime

l Line B-C

- As the temperature drop is raised more sites become active,

- improving the agitation of the liquid - increasing the heat flux and the heat-

transfer coefficient

-Regime of slugs and Bubbles



Nucleate boiling – linear scale graph

-Individual Bubble regime -Regime of slugs and Bubbles

-핵비등에서 높은열전달 속도는 주로기포의 동적작용에의해 액체에서생기는 난류의결과이다.




-Transition film boiling

l Line C-D

- So many bubbles are present - They tend to coalesce and cover portions

of the heating surface- 절연성 증기층 (Layer of insulating

vapor)




-Stable film boiling

l Line D-E

- The hot surface becomes covered with a quiescent film of vapor, through which heat is transferred by conduction or radiation (after E point)










-열전달 면적이 크다

Documents

유체역학및열전달 Chapter 13. Heat transfer to fluids with Phase …contents.kocw.net/KOCW/document/2014/Pusan/hyeongyu/13.pdf · 2016-09-09 · Heat transfer to fluids with