Ch. 2 Review of Thermodynamics, Fluid Mechanics, and Heat...
Transcript of Ch. 2 Review of Thermodynamics, Fluid Mechanics, and Heat...
Refrigeration and Air Conditioning
Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
Ch. 2 Review of Thermodynamics, Fluid Mechanics, and Heat Transfer
Refrigeration and Air Conditioning
-2- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.1 Introduction
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2.2 Thermodynamic properties
- 열역학적 상태량
(thermodynamic property): 관찰, 측정, 계산이 가능한 물질의 특성으로 물질의 상태(state)에 의해서만 결정되며, 도달한 경로에는 상관이 없음.
- intensive property: T, p, x
- extensive property: V, U, H, S,
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2.3 System of units
- English system: pound-second-foot - SI: kilogram-second-meter - 중력가속도 (standard acceleration of gravity):
)/()(17.32
1:
)/()(1:
2
2
sftalbmmlbfEnglish
smakgmNSI
kmaF
22 /17.32/807.9 sftsmg
- 힘
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- weight (=force of gravity upon the body)
)/(17.32)(17.32
1:
)/(807.9)(807.9
1*
)/(807.9)(1:
2
2
2
sftlbmmlbfEnglish
smkgmkgf
smkgmNSI
kmgF
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2.4 Specific volume, density, specific weight, and specific gravity
- 비체적 (specific volume, v)
lbmftEnglish
kgmSI
/:
/:
3
3
- specific property: 단위 질량당의 물성치 - density ( )
v
1
3
3
/:
/:
ftlbmEnglish
mkgSI
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2.5 Pressure
- - - - 절대압력(absolute pressure): 완전 진공상태 기준 - 상대압력(gauge pressure): 대기압 기준
A
Fp
psfftlbfinlbfpsiEnglish
mNPaSI
144/144/:
/:
22
2
Pacmkg f
42 108067.9/ 1
zgp
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2.6 Temperature
- 섭씨와 화씨 (Celsius scale and Fahrenheit scale) - 절대온도(absolute temperature)
325
9
)32(9
5
CF
FC
67.459
15.273
FR
CK
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-9- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.7 Heat and power units
- 열(heat, Q)과 일(work, W) - - Btu(British thermal unit): 1 pound의 물을 화씨 1도 올리는데 필요한 열량.
BtuEnglish
mNJSI
:
:
Jkcal 3101868.4 1
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- 현열(sensible heat) - 잠열(latent heat: heat of fusion, heat of vaporization) - 열전달률(heat transfer rate)과 일률(power)
)( 12 ttmcQ
min/000,33:
/:
lbffthpEnglish
sJWSI
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-11- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.8 First and Second Laws of Thermodynamics
dE
dtQ m h
VgZ m h
VgZ W
c v
c v i i
i
i e e
e
e c v
. .
. . . .
2 2
2 2
dS
dt
Q
Tm s m s Sc v c v
i i e e irr. . . .
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-12- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.9 Internal energy, enthalpy, and entropy
- 내부에너지(U): 분자의 운동에너지 - 비내부에너지 (specific internal energy): - 비엔탈피 (specific enthalpy): - 비열(specific heat):
정적비열: 정압비열:
m
Uu
Pvuh
vvv
vT
u
T
U
mT
Q
mc
11
ppp
pT
h
T
H
mT
Q
mc
11
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-13- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
-온도에 따른 이상기체의 비열
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단위 환산 (conversion of units)
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2.10 The perfect gas
- p-v-T 관계가 이상기체 상태방정식( )을 따르는 기체. - 많은 기체의 경우 압력이 낮아지면 이상기체의 특성을 나타냄. - 완전기체(perfect gas): 비열이 일정한 이상기체 - 상태방정식(Equation of State, EOS): p-v-T의 관계식. - 이상기체 상태방정식(Ideal gas EOS):
RTpv
RTpv
KkmolkJR /314.8 KkgkJM
RR /
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- 이상기체의 내부에너지 및 엔탈피: 온도만의 함수 - 비열이 일정한 경우 - 정압비열과 정적비열의 관계 (isentropic exponent)
2
1 12
2
1 12
dTchh
dTcuu
po
vo
)(
)(
1212
1212
TTchh
TTcuu
po
vo
Rcc vopo
kc
c
vo
po
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-17- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.11 Mixture of perfect gases
- 두종류의 이상기체 x 와 y의 경우 - 분압(partial pressure)
yx
yx
ppp
mmm
TV
,
m
RmRmR
yyxx
n
MnMn
n
mm
n
mM
yyxxyx
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-18- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
-공기조화에서 습공기는 이상기체인 건조공기와 이상기체인 수증기의 혼합물로 생각함. - 공기의 분자량 및 기체상수 - 공기의 비엔탈피(specific enthalpy):
97.2884.282879.03221.0
n
MnMnM
yyxx
air
Celsius)in (t
[kJ/kg] 004.1 ttch paa
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-19- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.12 Dry Air
-실제가스의 p-v-T 관계를 간단한 compressibility factor(Z)를 이용하여 묘사. -generalized compressibility chart: reduced property를 사용 하여 압축성인자 Z를 구함. - 저압, 고온: Z 1(이상기체)
ideal
real
v
v
RT
pvZ
c
r
c
r
c
rrrv
vv
T
TT
p
pppTzz ,, ),,(
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-20- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.13 Properties of water and steam
- Clapeyron equation: 증기압 곡선을 미분(기울기)형태로 표시함. -포화증기압곡선 (vapor-pressure curve)
fg
fg
fg
fg
sat
sat
Tv
h
v
s
dT
dP
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-21- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
- 건도(quality); x - 기-액 혼합물의 물성치 specific volume
m
mx
g
fgffgfgf
ggfgggff
xvvvvxvxvvx
m
vmvmm
m
vmvm
m
Vv
)()1(
( )
fgf
fgf
fgf
xsss
xuuu
xhhh
:entropy specific
:energy internal specific
:enthalpy specific
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-22- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.14 Low-pressure water vapor
- 물의 분자량 및 기체상수
- 수증기의 비엔탈피(specific enthalpy):
015.18wM
KkgkJM
RR
w
w /4615.0015.18
314.8
Celsius)in (t
[kJ/kg] 86.12501)0( , ttcChh wvp
o
gwv
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-23- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.15 Thermodynamic properties of refrigerants
- 1법칙 에서 가역과정을 가정하면 - Tds-equation
wqdu
pdvwTdsq ,
Tds du pdv
Tds dh vdp
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-24- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.16 The steady-flow energy equation
- 제1법칙: 에너지 보존 (energy conservation) - 정상상태, 입출구 1개씩인 경우 (steady state, single inlet and single exit)
dE
dtQ m h
VgZ m h
VgZ W
c v
c v i i
i
i e e
e
e c v
. .
. . . .
2 2
2 2
e
eei
ii gZ
VhgZ
Vhwq
220
22
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-25- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2
2
221
2
111212
220 gZ
VhgZ
Vhwq
3
2
332
2
222323
220 gZ
VhgZ
Vhwq
4
2
443
2
333434
220 gZ
VhgZ
Vhwq
4
2
444
2
444141
220 gZ
VhgZ
Vhwq
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-26- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.17 The steady-flow entropy equation
-Kelvin-Planck statement: 사이클로 작동하며 하나의 열저장조 (thermal reservoir)와 열교환을 하여 물체를 들어올리는 것(일) 이외의 효과는 발생하지 않는 장치를 구성하는 것은 불가능하다. 열은 100% 일로 변환 할 수 없다.
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-27- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
-Clausius statement: 사이클로 작동하며 낮은 온도의 물체로부터 높은 온도의 물체로 흐르게 하는 것 이외의 효과는 발생하지 않는 장치를 구성하는 것은 불가능하다. 열은 높은 온도에서 낮은 온도로 흐른다.
-Kelvin-Planck statement와 Clausius statement는 표현 방법이다르나 실제로 동일한 내용을 기술하고 있음.
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-28- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
-열역학 제2법칙의 정량적인 formulation을 위해 엔트로피( entropy, S)의 개념 도입. - 닫힌계에서 entropy 변화: 열전달과 비가역성에 의해 변화. - Entropy의 정의: - Clausius의 부등식:
0 , gengen SST
QdS
rev
T
QdS
0T
Q
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-29- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
- 비압축성에 가까운 액체 및 고체의 entropy: 비열이 일정한 경우 - 이상기체(ideal gas) 의 entropy: 비열이 일정한 경우
T
CdTds
2
1 12
T
CdTss
ln1
212
T
TCss
p
dpR
T
dTCds
po ln
1
22
1 12
p
pR
T
dTCss
po
lnln1
2
1
212
p
pR
T
TCss po
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-30- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
T-s 선도 (T-s Diagram)
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- p-h 선도
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-32- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.18 Heat conduction
열전달의 종류
-전도(conduction) -대류(convection) -복사(radiation)
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-33- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
전도 열전달
- 평판의 경우 전도 열전달식 - 환형 실린더의 경우 전도 열전달식
x
TTkA
dx
dTkAQx
21
i
o
oi
r
r
TTkLQ
ln
)(2
Refrigeration and Air Conditioning
-34- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.19 Force convection for internal flows
- Reynols number에 따라 유동의 형태 구분, 층류(laminar flow): 난류(turbulent flow): - 대류열전달식: - 열전달계수(heat transfer coefficient, h)는 Nu-Re,Pr 관계식으로부터 구함. 많은 난류 유동에 대해
Vdd Re
200,2Re0 d
000,10Re d
)( 21 TThAQ
4.03.08.0
023.0
PrRe
k
cVd
k
hd
CNu
p
nm
d
Refrigeration and Air Conditioning
-35- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.20 Forced convection for external flows
- 평판(Ozisik, ) - 실린더 주변 강제 대류(Whitacker, )
5102Re L
25.0
8.043.0 )9200(RePr36.0
w
Lm
Lk
LhNu
510Re40 d
25.0
4.03/25.0 Pr)Re06.0Re4.0(
w
ddm
dk
dhNu
Refrigeration and Air Conditioning
-36- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.21 Natural convection for external flows
-수직 평판의 경우 열전달은 Reynolds number보다 Rayleigh number ( )에 지배적으로 영향을 받음. * : volume expansivity, :thermal diffusivity 층류(laminar flow): 난류(turbulent flow): - 수직 평판 자연대류 열전달식(Ozisik, )
THgH
3
Ra
pT
v
vβ
1
pρc
kα
810Ra0 H
910Ra H
5102Re L
15/1
153/1
1512/5
5/3
1/4
H 1.0Pr
559.01Ra67.0
H
mH Ra
k
HhNu
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-37- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.22 Natural convection for cavity flows
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2.23 Pool boiling
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2.24 Forced-convection boiling
-Gungor and Winterton(1987), Kandilikar(1999), Shah(1982) 등 다양한 식이 제안되어 있으나 모든 경우에 대해 검증된 식은 없음.
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-40- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.25 Condensation on external surfaces
- 직경 d인 수평 튜브가 수직방향으로 N 개가 겹쳐 있는 경우:
25.023
1
25.0
1 ,79.0
l
llfg gkF
TNd
hFh
Values for F Refrigerant
Temperature(oC)
F1
R-22
24 80.3
38 75.5
68 69.2
Ammonia
24 214.5
38 214
68 214
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-41- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.26 Condensation in internal flows
-Shah(1979), Cavallini and Zecchin(1974), Fujii(1995) 등 다양한 식이 제안되어 있으나 모든 경우에 대해 검증된 식은 없음.
Refrigeration and Air Conditioning
Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.27 Thermal radiation
-흑체(black body): 모든 파장과 모든 방향의 incident radiation을 흡수. - Stefan-Boltzmann law: - Planck’s distribution: - 흑체간의 복사 열전달
1
4TEb
1/
5
1
2
TCbe
CE
2
2
2
1
211
1
4
2
4
1121
111
)(
A
A
F
TTAQ
Refrigeration and Air Conditioning
Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
- 복사 열전달계수
)( 21 TTAhQ rrad
2
2
2
1
211
1
21
4
2
4
1
111
)/()(
A
A
F
TTTThr
2
2
2
1
211
1
3
111
)(4
A
A
F
Th
avg
r
- 대부분의 HVAC application의 경우
Refrigeration and Air Conditioning
2.28 Combined modes of heat transfer
( ) ( )
( )
( )
( )
c r c w a r w s
w a
w sc r
w a
q q q h t t h t t
h t t
t th h h
t t
Refrigeration and Air Conditioning
Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
2.29 Overall transfer of heat
- 관내외의 대류 및 전도에 의한 저항을 모두 고려한 열전달계수 - : 오염계수(fouling factor)
- : log mean temperature difference - 평행류(parallel flow) 또는 대향류(counter-current flow)의 경우
moo TAUQ o
o
i
oo
i
i
o
ii
o
o hf
k
dd
d
fd
d
hd
d
U
1
2
ln1
oi ff and
mT
B
A
BAm
T
T
TTT
ln
Refrigeration and Air Conditioning
-46- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
- 1법칙:
- 2법칙:
적분하면 - 위의 두 식에서 q를 소거하면
wgZV
hgZV
hq ee
eii
i )2
()2
(22
gensTqvdpdhTds
e
igen
e
iie sTqvdphh
e
igenLLei
ee
i
i sTwwZZgVV
vdpw
2
2
,)()22
(
2.30 Fluid flow in ducts and pipes
Refrigeration and Air Conditioning
-47- Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
- 가역과정, 비압축성, w=0을 가정하면: Bernoulli equation
e
e
ei
i
i
ei
ei
ei
gZV
pgZV
p
ZZgVV
ppo
22
)(2
)()(
22
22
Refrigeration and Air Conditioning
Thermal Engineering Lab., Dept. of Mechanical Engineering, Sogang University
- 비압축성 유체의 경우
LwZZgVVpp
w
)(2
)(21
2
2
2
121
LwZZgVV
ppw
)(2
)()( 21
2
2
2
121
g
wZZ
g
VV
g
pp
g
w L
)(2
)(21
2
2
2
121
: Energy
: Pressure
: Head
- 마찰손실
duct or pipe: minor losses:
2
2V
d
Lfpw LossL
2
2VKpw LossL