KAIST 기계공학과 · 2017-01-07 · •Conservation laws of fluid mechanics - Conservation of mass ... #3 F.M. White, Fluid Mechanics, 7th ed., McGaw-Hill, 2009, p. 26 #4 çengel,

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


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

• 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









#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/










물과 공기로 움직이는 기계


정 상 권

[ 학 습 목 차 ]


• Work production

• Heat engine

• Refrigerator

• System modeling


Ein - Eout =(Qin -Qout ) + (Win -Wout ) = Esystem


The first law of thermodynamics

#1


#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 ]


• 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






#7 Car, https://upload.wikimedia.org/

#8 Car, http://3.bp.blogspot.com/





자료 출처 #13 çengel, Y.A. and Boles, M.A. Thermodynamics – An Engineering Approach, 5th Edition, McGraw-Hill, 2006, p.288












사라지지 않는 에너지 / 증가하는 엔트로피


정 상 권

[ 학 습 목 차 ]


• The second law of thermodynamics

• Entropy

• Application of the second law of thermodynamics

• Refrigerator


Ein - Eout =(Qin -Qout ) + (Win -Wout ) + (Emass, in- Emass, out ) = Esystem


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


• Bernoulli equation (by energy conservation)

#1

2 2

1 1 1 2 2 2

1 1

2 2p V gz p V gz

>


• 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


• 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



#7



#8



#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


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.



#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.


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.


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.


• Entropy can be generated.

3. Entropy

#12

3. Entropy

#13

3. Entropy


• 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


#16

By the first law of thermodynamics,

Q = 0, W = 0, Hin = Hout

and Uin > Uout in non-ideal gas

Tin > Tout


Enthalpy H is a property and is defined

as the sum of the internal energy U

and the PV product.

#17


5. Refrigerator

#18 #19

5. Refrigerator

Application of

thermodynamics !

#20

[ 학 습 목 차 ]


• 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







자료 출처










#20 Refrigerator, https://encrypted-tbn2.gstatic.com/i

Documents

KAIST 기계공학과 · 2017-01-07 · •Conservation laws of fluid mechanics - Conservation of mass ... #3 F.M. White, Fluid Mechanics, 7th ed., McGaw-Hill, 2009, p. 26 #4 çengel,