kaist 기계공학과 · 2017-01-07 · •conservation laws of fluid mechanics - conservation of...
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열과 유체, 에너지와 친해지기
KAIST 기계공학과
정 상 권
• 열역학 - 세상을 움직이는 스마트한 법칙
• 물과 공기로 움직이는 기계
• 사라지지 않는 에너지 / 증가하는 엔트로피
이번 시간에는!
열역학 - 세상을 움직이는 스마트한 법칙
KAIST 기계공학과
정 상 권
[ 학 습 목 차 ]
• Thermofluids
• Energy conservation principle
• Energy
• Work (boundary work)
1. Thermofluids
#1
Phase : identified as having a distinct molecular arrangement that is homogeneous
throughout and separated from the others by easily identifiable boundary surfaces
1. Thermofluids
#2
• What is a fluid?
- Fluid is a material whose shape is determined by the shape of a container.
- A substance which moves and deforms “continuously” as a result of an applied shear stress of any magnitude.
- A solid can resist shear stress by static deflection; a fluid cannot resist shear stress.
1. Thermofluids
• Thermodynamic properties
- Thermodynamic properties describe the state of a system.
- Three primary thermodynamic properties are 1) pressure, 2) temperature 3) density.
1) Pressure (p): compressive stress at a point in a static fluid (Pa, psi).
2) Temperature (T): related to the internal energy level of a fluid (˚C, ˚F).
3) Density (ρ): mass per unit volume.
Air : ρ (at 1 atm, 4˚C) = 1.205 kg/m3
Water: ρ (at 1 atm, 4˚C) = 1000 kg/m3
1. Thermofluids
du
dy μ : the coefficient of viscosity
A Newtonian fluid has a constant coefficient of viscosity.
1. Thermofluids
• Newtonian fluid : Shear stress is linearly proportional to the rate of shearing strain (e.g. water, air, and oil).
#3
μ: coefficient of viscosity (dynamic viscosity)
Water at 1 atm, 20°C = 1.0× 10-3 kg/m·s
Air at 1 atm, 20°C = 1.8× 10-5 kg/m·s
ν: kinematic viscosity = μ / ρ
Water at 1 atm, 20°C = 1.0× 10-6 m2/s
Air at 1 atm, 20°C = 1.5× 10-5 m2/s
1. Thermofluids
• Coefficient of viscosity
Temperature (ºC)
Abso
lute
Vis
cosi
ty, μ
N.s
/m2
0 20 40 60 80 100
10-5
10-4
10-3
10-2
Water
Air
1. Thermofluids
#4 #5 #6 #7 #8
1. Thermofluids
#9
1. Thermofluids
#10
1. Thermofluids
#11
1. Thermofluids
#12
Temperature
Celsius
(ºC)
Absolute
(K)
Tropics
Human body
Room temperature
Ice point
Salt + water (cryogen)
Antarctic winter
Solid carbon dioxide
Liquid oxygen
Liquid nitrogen
Liquid helium
Absolute zero
45
37
20
0
- 18
- 50
- 78
- 183
- 196
- 269
- 273
318
310
293
273
255
223
195
90
77
4
0
영국 일간 가디언은 최고 최저 온도 기록을 소개했다. 1922년 리비아 알-아지지야는 57.7ºC, 1983년 7월 남극 보스토크는 -89ºC를 기록했다.
1. Thermofluids
• Some typical low temperature
• Basket ball
Circumference: 75∼78 cm
Weight: 600 ~ 650 g
Optimal pressure: 0.6 ~ 0.7 kg/cm2.gauge
• Volley Ball
0.42 ~ 0.48 kg/cm2.gauge
• Soccer Ball
Circumference 68~70cm
Weight 410~430g
Pressure 0.6~1.1 bar.gauge
1. Thermofluids
#13
#14
#15
• PRESSURE
1. Thermofluids
#16 #17
1. Thermofluids
• Control volume
System vs. Control volume:
- System: a fixed mass with a boundary
- Control volume: a "window" for observation in the flow: region of interest
System
System boundary
Control
Volume
Control Surface
1. Thermofluids
• Conservation laws of fluid mechanics
- Conservation of mass
- Conservation of linear momentum
- Conservation of angular momentum
- Conservation of energy
2. Energy conservation principle
Ein - Eout =(Qin -Qout ) + (Win -Wout ) = Esystem
Internal energy, U
• Energy can be neither created nor destroyed; it can only change forms.
#18
3. Energy
#19
Internal energy, U
3. Energy
#20
• Internal energy : U
• Specific heat
3. Energy
#21
• Specific heat
3. Energy
#22
Liquid and solid are incompressible (compared to gas)
CCC vp
Lead 0.128 Mercury 0.139 Argon 0.520
Tin 0.217 R-12 0.917 CO2 0.846
Copper 0.386 Methanol 2.550 Air 1.005
Iron 0.450 Water 4.184 Steam 1.8723
Wood 1.760 Ammonia 4.800 H2 14.31
3. Energy
Unit : kJ/kg.K ( )
Boundary work
(PdV work or moving
boundary work) is the
work associated with
the expansion or
compression of a gas
in a piston-cylinder
device.
4. Work (boundary work)
#23 #24
[ 학 습 목 차 ]
• Thermofluids
• Energy conservation principle
• Energy
• Work (boundary work)
자료 출처 #1 Earth, https://static.pexels.com/
#2 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.113
#3 F.M. White, Fluid Mechanics, 7th ed., McGaw-Hill, 2009, p. 26
#4 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.114
#5 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.114
#6 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.114
#7 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.115
#8 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.115
#9 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.115
#10 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.116
#11 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.119
#12 Solar system, https://camo.githubusercontent.com/
자료 출처 #13 Basketball, https://upload.wikimedia.org/
#14 Volley ball, https://upload.wikimedia.org/
#15 Soccer ball, http://wwwchem.uwimona.edu.jm/
#16 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.23
#17 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.25
#18 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.174
#19 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.119
#20 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.126
#21 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.178
#22 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.178
#23 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.166
#24 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.167
물과 공기로 움직이는 기계
KAIST 기계공학과
정 상 권
[ 학 습 목 차 ]
• Energy conservation principle
• Work production
• Heat engine
• Refrigerator
• System modeling
1. Energy conservation principle
Ein - Eout =(Qin -Qout ) + (Win -Wout ) = Esystem
• Energy can be neither created nor destroyed; it can only change forms.
The first law of thermodynamics
#1
1. Energy conservation principle
#2
2. Work production
#3 #4
2. Work production
#5 #6
2
1
1 2
에너지 변환 과정으로 움직이는 기계
2. Work production
#8
#7
Heat engines are devices that convert heat to work.
Heat engines differ considerably from one another,
but all can be characterized by the following:
1. They receive heat from a high-temperature source
(solar energy, oil furnace, nuclear reactor, etc.).
2. They convert part of this heat to work
(usually in the form of a rotating shaft).
3. They reject the remaining waste heat
to a low-temperature sink (the atmosphere, rivers, etc.).
4. They operate on a cycle.
3. Heat engine
#9
Working fluid is the fluid to and from which heat and work is transferred
while undergoing a cycle in heat engines and other cyclic devices.
Thermal efficiency is a measure of the performance of a heat engine and is the
fraction of the heat input to the heat engine that is converted to net work output.
Thermal efficiency th is the ratio of the net work produced by a heat engine
to the total heat input, th = Wnet/Qin.
3. Heat engine
3. Heat engine
#10
3. Heat engine
#11 #12
Refrigerators are cyclic devices
which allow the transfer of heat
from a low-temperature medium
to a high-temperature medium.
4. Refrigerator
#13
Refrigerant is the working fluid used in the
refrigeration cycle.
Coefficient of performance COP is the measure
of performance of refrigerators and heat pumps.
COP =Desired output
Required input=
QL
Wnet,in
4. Refrigerator
#14
5. System modeling
#15
5. System modeling
#16
5. System modeling
#17
#18
5. System modeling
Otto cycle is the ideal cycle for spark-
ignition reciprocating engines.
It consists of four internally reversible
processes:
1-2 Isentropic compression,
2-3 Constant volume heat addition,
3-4 Isentropic expansion,
4-1 Constant volume heat rejection.
#19
5. System modeling
#20
5. System modeling
Brayton cycle is used for gas turbines,
which operate on an open cycle, where both
the compression and expansion processes take
place in rotating machinery.
Aircraft propulsion & electric power
generation.
1-2 Isentropic compression (in a compressor),
2-3 Constant pressure heat addition,
3-4 Isentropic expansion (in a turbine),
4-1 Constant pressure heat rejection. #21
5. System modeling
#22
#23
5. System modeling
Application of
thermodynamics!
#24
5. System modeling
[ 학 습 목 차 ]
• Energy conservation principle
• Work production
• Heat engine
• Refrigerator
• System modeling
자료 출처 #1 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.174
#2 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.10
#3 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.166
#4 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.166
#5 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.167
#6 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.167
#7 Car, https://upload.wikimedia.org/
#8 Car, http://3.bp.blogspot.com/
#9 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.282
#10 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.283
#11 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.284
#12 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.284
자료 출처 #13 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.288
#14 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.288
#15 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.488
#16 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.488
#17 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.492
#18 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.500
#19 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.497
#20 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.508
#21 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.508
#22 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.508
#23 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.509
#24 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.4
사라지지 않는 에너지 / 증가하는 엔트로피
KAIST 기계공학과
정 상 권
[ 학 습 목 차 ]
• Energy conservation principle
• The second law of thermodynamics
• Entropy
• Application of the second law of thermodynamics
• Refrigerator
1. Energy conservation principle
Ein - Eout =(Qin -Qout ) + (Win -Wout ) + (Emass, in- Emass, out ) = Esystem
• Energy can be neither created nor destroyed; it can only change forms.
The first law of thermodynamics
By mass conservation, smaller area → higher speed (V1 < V2)
By Bernoulli equation, higher speed → lower pressure (p1 < p2)
Venturi tube
1. Energy conservation principle
• Bernoulli equation (by energy conservation)
#1
2 2
1 1 1 2 2 2
1 1
2 2p V gz p V gz
>
1. Energy conservation principle
• Boundary layer : the layer of reduced velocity in fluids, that is immediately adjacent to the surface of a solid past which the fluid is flowing. No slip condition !
#2 #3
1. Energy conservation principle
• Boundary layer : the layer of reduced velocity in fluids, that is immediately adjacent to the surface of a solid past which the fluid is flowing.
#4
2. The second law of thermodynamics
• Direction of the process
#6
#5
2. The second law of thermodynamics
• Direction of the process
#7
2. The second law of thermodynamics
• Direction of the process
#8
2. The second law of thermodynamics
• Direction of the process
#9
• Clausius statement of the second law :
It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.
#14
2. The second law of thermodynamics
Entropy (from a classical thermodynamics point of view) is a property
designated S and is defined as dS =(Q/T)int rev.
Entropy (from a statistical thermodynamics point of view) can be viewed
as a measure of molecular disorder, or molecular randomness. The entropy of
a system is related to the total number of possible microscopic states of that
system, called thermodynamic probability p, by the Boltzmann relation,
expressed as S = k ln p where k is the Boltzmann constant.
Boltzmann’s constant, k has the value of 1.3806 1023 J/K.
2. The second law of thermodynamics
2. The second law of thermodynamics
#10
Second law of thermodynamics the entropy of an isolated system
during a process always increases or, in the limiting case of a reversible
process, remains constant
Entropy generation Sgen is entropy generated or created during an
irreversible process, is due entirely to the presence of irreversibilities.
Entropy generation is always a positive quantity or zero.
Its value depends on the process, and thus it is not a property.
2. The second law of thermodynamics
Reversible process is defined as a process
that can be reversed without leaving any trace on the surroundings.
Irreversible processes are processes which,
once having taken place in a system, cannot spontaneously reverse themselves
and restore the system to its initial state.
Irreversibilities are the factors that cause a process to be irreversible.
They include friction, unrestrained expansion, mixing of two gases,
heat transfer across a finite temperature difference, electric resistance,
inelastic deformation of solids, and chemical reactions.
2. The second law of thermodynamics
3. Entropy
#11
3. Entropy
Tds relations relate the Tds product to other thermodynamic properties.
The first Gibbs relation is Tds = du + Pdv.
The second Gibbs relation is Tds = dh – vdP.
• Energy can be neither created nor destroyed; it can only change forms.
• Entropy can be generated.
3. Entropy
#12
3. Entropy
#13
3. Entropy
• Energy can be neither created nor destroyed; it can only change forms.
• Entropy can be generated.
#14
4. Application of the second law of thermodynamics
• Clausius statement of the second law :
It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.
#15
Throttling process !
Abrupt pressure drop of flow
4. Application of the second law of thermodynamics
#16
By the first law of thermodynamics,
Q = 0, W = 0, Hin = Hout
and Uin > Uout in non-ideal gas
Tin > Tout
4. Application of the second law of thermodynamics
Enthalpy H is a property and is defined
as the sum of the internal energy U
and the PV product.
#17
4. Application of the second law of thermodynamics
5. Refrigerator
#18 #19
5. Refrigerator
Application of
thermodynamics !
#20
[ 학 습 목 차 ]
• Energy conservation principle
• The second law of thermodynamics
• Entropy
• Application of the second law of thermodynamics
• Refrigerator
자료 출처
#1 Venturi tube, https://encrypted-tbn1.gstatic.com/
#2 Homsy et al., Multimedia Fluid Mechanics, 2nd ed., Cambridge University Press
#3 Homsy et al., Multimedia Fluid Mechanics, 2nd ed., Cambridge University Press
#4 F.M. White, Fluid Mechanics, 7th ed., McGaw-Hill, 2009, p. 266
#5 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.281
#6 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.280
#7 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.280
#8 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.280
#9 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.281
#10 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.335
자료 출처
#11 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.338
#12 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.340
#13 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.340
#14 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.377
#15 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.288
#16 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.239
#17 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.176
#18 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.288
#19 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.288
#20 Refrigerator, https://encrypted-tbn2.gstatic.com/i
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