Lecture I: Reviewcc.sjtu.edu.cn/Upload/20160505155848658.pdf · 2016. 5. 5. · 1.1 ME 200...
Transcript of Lecture I: Reviewcc.sjtu.edu.cn/Upload/20160505155848658.pdf · 2016. 5. 5. · 1.1 ME 200...
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1.1
ME 200 –Thermodynamics I
Lecture 44: Review Thermodynamics I
Yong Li
Shanghai Jiao Tong University
Institute of Refrigeration and Cryogenics
800 Dong Chuan Road Shanghai, 200240, P. R. China
Email : [email protected]
Phone: 86-21-34206056; Fax: 86-21-34206056
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1.2
What is Thermodynamics?
Science to study how one energy changes from one to
another
Thermodynamics = Therme(heat) + dynamis(force)
Energy exists in several forms, e.g., potential, kinetic,
chemical, thermal, electrical, nuclear among many others
During interactions in nature, energy simply changes from
one form to another; but the total energy remains constant
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1.3
Basic Principles
First law of thermodynamics
» A statement of conservation of energy principle
» Energy is a thermodynamic property; quantifies energy
Second law of thermodynamics
» Energy has quality as well as quantity. Actual processes occur in
direction of decreasing quality of energy
» Establishes direction and possibility for process
» Provides means for measuring the quality of energy
» Determines theoretical limits regarding the performance
of engineering devices
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1.4
Terms and Concepts
»» System – Thermodynamic system, Closed system, Open (flow) system
– Surroundings,
– System boundary, Adiabatic (insulated) , Rigid, Isolated
»» Property Intensive, Extensive Specific properties
»» State
»» Phases
»» Equilibrium, Thermodynamic equilibrium Mechanical Eq.----- Thermal Eq. -----Phase Eq.-----Chemical Eq.
»» Process Isothermal, Isobaric, Isochoric ,
Quasi-Equilibrium Process
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1.5
First law of Thermodynamics Open System
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1.6
Important Equipments
Turbines
compressors
pumps
Nozzles,
diffusers, Throttling
valve
Heat
exchanger
Turbines
compressors
pumps
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1.7
Second Law of Thermodynamics
Clausius (C) statement
It is impossible for any system to operate in
such a way that the sole result would be an
energy transfer by heat from acooler to a
hotter body.
Kelvin–Planck (K-P) statement
It is impossible for any system to operate in a
thermodynamic cycle and deliver a net
amount of energy by work to its surroundings
while receiving energy by heat transfer from
a single thermal reservoir..
» Analytical form of the K-P statement
Irreversibility
Heat transfer through a finite
temperature difference
Unrestrained expansion of a gas or
liquid to a lower pressure
Spontaneous chemical reaction
…….
Reversible cycle
» there are no irreversibilities within the
system as it undergoes the cycle
» heat transfers between the system and
reservoirs occur reversibly.
Two Carnot corollaries
irrev rev rev1 = rev2
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1.8
Thermal Efficiency
A reversible power cycle operating between two
thermal reservoirs.
Four internally reversible processes: two adiabatic
processes alternated with two isothermal processes.
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1.9
Entropy
The integral of dQ/T gives S only if the integration is carried out along an
internally reversible path between the two states.
Entropy is a property, it has fixed values at fixed states. S
between two specified states is the same no matter
what path, reversible or irreversible.
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1.10
Entropy Balance
Closed system entropy balance
Other forms of the entropy balance
Increase of entropy principle
» the entropy of an isolated system during a process always increases or, in the limiting case of a
reversible process, remains constant. In other words, it never decreases.
» Control volume entropy rate balance
Steady state
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1.11
Triple point ::: the triple line of the three-
dimensional p–v–T surface projects onto a
point on the phase diagram.
water, triple point defined
at 0.01oC 0.6113 kPa
p-v-T Surface
Subcooled liquid=compressed liquid
Saturated Liquid
Liquid‐Vapor Mixture
Saturated Vapor
Superheated Vapor
water, pcr ~ 221 bar; Tcr ~ 374.1C
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1.12
Incompressible Substance model
Incompressible Substance model::: An
idealization to simplify evaluations of liquids or
solids, the v () is assumed to be constant and the u
assumed to vary only with T.
v =const
Concepts
≈0
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1.13
u, h, c of Ideal Gases
specific internal energy depends only on T
specific enthalpy depends only on T
Important relation
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1.14
Entropy
cv and cp are constants
Ideal Gas
liquids and solids modeled as
incompressible.
Variable cv and cp
Compressed liquid liquid–vapor mixture
Saturated liquid to saturated vapor at constant T and p
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1.15
Isentropic Processes of air (IG)
Isentropic process for air modeled as ideal gas
relative pressure. )(]/)(exp[ TpRTs ro
)(/)( TpRTTv rr relative volume.
reduced pressure.
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1.16
Isentropic Processes of air (IG) with constant c
constant1 kvT
constantkvp
constant/)1( kkpT
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1.17
Polytropic Processes on p–v and T–s Diagrams
cpvn
cpn 0
cvpn /1
11 cTcpvRTn
cscpvkn k
cvn
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1.18
Isentropic Efficiencies
Isentropic Efficiencies ::: Comparison between the actual
performance of a device and the performance that would be achieved under
idealized circumstances for the same inlet state and the same exit pressure.
Turbine
isentropic turbine efficiency
h2 > h2s ηt
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1.19
Expressions for the Work
Control
Volumes
One-inlet, one-exit
steady-state flow
Internally reversible
e
ee
ee
i
ii
iicvcvcv gz
Vhmgz
VhmWQ
dt
dE
22
22
2
1
revint
Tdsm
Qcv
)(2
)( 21
2
2
2
121 zzg
VVhh
m
Q
m
W cvcv
)(2
)( 21
2
2
2
121
2
1int
zzgVV
hhTdsm
W
rev
cv
vdpdhTds
2
112
2
1vdphhdsT
)(2
21
2
2
2
12
1int
zzgVV
vdpm
W
rev
cv
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1.20
Analyzing Rankine Cycle---I
Turbine
Condenser
Pump
Boiler
Thermal efficiency of the power cycle
Back work ratio
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1.21
Superheat and Reheat
Superheat :
» Reason: Increase average temperature for
heat
addition at a given boiler pressure
increase in performance
Reheat: High quality (or superheated vapor)
existing the turbine without large
superheat
For a given TH can increase Tb without
reducing quality
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1.22
Refrigeration Cycle
T
s
2
1
3
4
Tcond
Tevap
TH
TL
subcooling
Tsc
superheat: Tsh
COP=
Evaporator:
The heat transfer rate is referred to
as the refrigeration capacity. ( kW).
» Another unit for the refrigeration
capacity is the ton of refrigeration, =
211 kJ/min.
Compressor
Condenser
Throttling process
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1.23
Air Standard Cycles
Air standard cycles are idealized cycles based on the
following approximations:
A fixed amount of air modeled as an ideal gas (working fluid).
The combustion process is replaced by a heat transfer from an
external source. There are no exhaust and intake processes as in
an actual engine.
The cycle is completed by a constant-volume heat transfer
process taking place while the piston is at the bottom dead center
position.
All processes are internally reversible.
Cold air-standard analysis The specific heats are assumed constant at Ta.
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1.24
Otto Cycle and Diesel Cycle
Air Standard Cycle for CI Engines:
3 BDC1c
2 2 TDC
V VVDefine : r "cutoff ratio" compression ratio r
V V V
3xp
2 2
ppr pressure ratio
p p
k
cth k 1
c
1
th,Diesel th,Otto
r 11Then, 1
r k(r 1)
Thus, for a given r : !
th k 1
11
r
net
max min
W net work for one cycleMEP
V V displacement volume
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1.25
Brayton Cycle
4 5x 2
4 2 4 2
h hh hactual heat transfer
maximum heat transfer h h h h
1
p 2 1 22th,R
4p 3 4 3
3
k 1 k 1
k k4 4 1 1
3 3 2 2
For constant specific heats:
T1
c (T T ) TT1 1
Tc (T T ) T1
T
Also, assuming ideal gas and isentropic expansion and compression:
T p p T
T p p T
Notes:
-For cycles with regeneration:
qin relatively constant
qin = (h3-hx)+(h3-hx) ~ h3-hxo
wnet increases (by 4-5-6-6o)
Reheater increases th,R
- For cycles without regen.:
qin increases by h5-h4 and
wnet increases (by 4-5-6-6o)
Reheater reduces th,R