Lecture note for general thermodynamics, 2003
OVERVIEW
POWER CYCLE
•The Rankine Cycle
•thermal efficiency
•effects of pressure and temperature
•Reheat cycle
•Regenerative cycle
•Losses and Cogeneration
•Air-Standard Power Cycles (open cycle)
•The Brayton cycle
•Simple gas-turbine cycle with regenerator
•Gas turbine power cycle configurations
•Jet propulsion
•Reciprocating Engine Power Cycles
•Otto cycle
•Diesel cycle
•Stirling cycle
REFRIGERATION SYSTEMS
•Vapor-compression refrigeration cycle
•Actual vapor-compression refrigeration cycle
•Ammonia absorption refrigeration cycle
•Air-standard refrigeration cycle
OTHER SYSTEMS : combined-cycle power and refrigeration systems
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
11.8 AIR-STANDARD POWER CYCLES
So far, we studied idealized four-process cycles
A. External-combustion engine : liquid – phase change – gas : closed cycle
B. Internal combustion engine – ex) automotive engine (diesel, gasoline engines),
gas-turbine engines ; working fluid is always gas. : open cycle – inlet to exhaust
(focuses our attention on air pollution problem.
IC engine operates on the so-called open cycle – but we may consider closed cycle
that closely approximate the open cycles. : air-standard cycle based on the
following assumptions
- A fixed mass of air is the working fluid throughout the entire cycle, and the air is
always an ideal gas. Thus, there is no inlet process or exhaust process.
- The combustion process is replaced by a process transferring heat from an
external source.
- The cycle is completed by heat transfer to the surroundings ( in contrast to the
exhaust and intake process of an actual engine)
- All processes are internally reversible
- An additional assumption is often made that air has a constant specific heat,
recognizing that this is not the most accurate model.
Main goal of this approach is to examine qualitatively the influence of a number of
variables on performance. : mep (mean effective pressure), efficiency
A. Brayton cycle
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
Standard Brayton cycle
• Two const-P processes (combustor, and approximated condensed process)
+ two isentropic processes (compressor, turbine)
• Rankine cycle using a single phase, gaseous working fluid – Brayton cycle
• Ideal cycle for the simple gas turbine
C (T − T )
T ( T / T − 1)
Q
η th = 1 − L = 1 − P 4 1 = 1 − 1 4 1
QH
C P ( T3 − T2 )
T2 ( T3 / T2 − 1)
= 1−
1
T1
= 1−
k −1 / k
T2
( P2 / P1 )
here , we note
T4
T
−1 = 3 −1
T1
T2
∵
P3 P2
=
P4 P1
P2 ⎛ T2 ⎞
=⎜ ⎟
P1 ⎝ T1 ⎠
T2 T3
=
T1 T4
k / ( k − 1)
ηcomp =
h2 s − h1
h2 − h1
η turb =
h3 − h4
h3 − h4 s
P ⎛T ⎞
= 3 =⎜ 3⎟
P4 ⎝ T4 ⎠
k / ( k − 1)
• Large amount of compressor work
• Exam 11.6
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
11.10 GAS-turbine cycle with a regenerator
η th =
wnet wt − wc
=
qH
qH
qH = C P (T3 − Tx )
wt = C P (T3 − T4 )
For ideal regenerator
η th = 1 −
wt = qH , T4 = Tx
C (T − T )
T ( T / T − 1)
wc
= 1− P 2 1 = 1− 1 2 1
qH
C P (T3 − T4 )
T3 (1 − T4 / T3 )
⎡( P2 / P1 )( k −1) / k − 1⎤
T
⎦
= 1− 1 ⎣
T3 ⎡1 − ( P1 / P2 )( k −1) / k ⎤
⎣
⎦
T ⎛P ⎞
η th = 1 − 1 ⎜ 2 ⎟
T3 ⎝ P1 ⎠
( k −1) / k
11.11 Gas-turbine power cycle
configurations
• Ericsson cycle
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
JET PROPULSION
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
11. 13 Reciprocating Engine Power Cycles
• Otto cycle
• Diesel cycle
Bore B : cylinder diameter
• Stirling cycle
Crank angle
TDC : Top dead center
Some definitions and
BDC : bottom dead center
terms
Clearance volume
Displacement volume
Compression ratio
Air-fuel ratio
Mean effective pressure (mep)
S = 2 Rcrank
Vdispl = N cyl (Vmax − Vmin ) = N cyl Acyl S
rv = CR = Vmax / Vmin
wnet =
∫ Pdv = P
mef
f (vmax − vmin )
Wnet = mwnet = Pmeff (Vmax − Vmin )
W = N cyl mwnet
RPM
RPM
− Pmeff Vdispl
60
60
11. 14 The Otto cycle
T2 ⎛ V1 ⎞
=⎜ ⎟
T1 ⎝ V2 ⎠
T T
∴ 3 = 4
T2 T1
η th =
k −1
⎛V ⎞
=⎜ 1⎟
⎝ V3 ⎠
QH − Q L
Q
mC v (T4 − T1 )
(T − T )
= 1− L = 1−
= 1− 4 1
QH
QH
mC v (T3 − T2 )
(T3 − T2 )
School of Mechanical Engineering, ChungAng University
k −1
=
T3
T4
Lecture note for general thermodynamics, 2003
Thermal efficiency of the Otto
cycle as a function of compression
ratio
η th = 1 −
T1
1
1− k
= 1 − ( rv ) = 1 −
k −1
T2
( rv )
where , rv =
V1 V4
=
V2 V3
NOTE 1;
1. higher compression ratio, higher
thermal efficiency
2. detonation occurs at very high
compression ratio, - negative
respect in actual engines : strong
pressure wave (spark knock)
NOTE 2 ; Deviation of actual engine from air-standard cycle
1. specific heat – increases with temperature
2. combustion process is present – incomplete : producing pollutant
such as Nox, Soot, and particulate matter (PM)
3. inlet and outlet processes + a certain amount of work is required
because of pressure drops
4. considerable heat transfer
5. irreversibilities (pressure and temperature gradients)
11. 15 The Diesel Cycle (Compression Ignition – CI engine)
η th = 1 −
= 1−
QL
C (T − T )
= 1− P 4 1
QH
C P (T3 − T2 )
T1 (T4 / T1 − 1)
kT2 (T3 / T2 − 1)
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
NOTE
1. there is no knocking problem because only air is compressed
during the compression stroke
2. constant pressure – heat transferring (combustion process)
Cf) Otto cycle – constant volume process
Some losses
- pumping loss
- some losses during inlet and exhaust processes
- heat transfer
- not constant pressure process during combustion process
11. 16 Stirling cycle
NOTE
1. Strictly, the Stirling cycle engine is not an internal-combustion engine
but external-combustion engine with regeneration
2. Two gas chambers are connected to pistons
3. Constant volume process – heat transferred by external combustors
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
OVERVIEW
POWER CYCLE
•The Rankine Cycle
•thermal efficiency
•effects of pressure and temperature
•Reheat cycle
•Regenerative cycle
•Losses and Cogeneration
•Air-Standard Power Cycles (open cycle)
•The Brayton cycle
•Simple gas-turbine cycle with regenerator
•Gas turbine power cycle configurations
•Jet propulsion
•Reciprocating Engine Power Cycles
•Otto cycle
•Diesel cycle
•Stirling cycle
REFRIGERATION SYSTEMS
•Vapor-compression refrigeration cycle
•Actual vapor-compression refrigeration cycle
•Ammonia absorption refrigeration cycle
•Air-standard refrigeration cycle
OTHER SYSTEMS : combined-cycle power and refrigeration systems
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
11.18 Vapor-Compression Refrigeration Cycle
4 processes
1-2 : isentropic compression (pump)
2-3 : constant pressure heat rejection (condenser)
3-4 : adiabatic throttling process (irreversible)
4-1 : constant pressure evaporation (heat absorption)
Cycle performance : Coefficient of Performance (COP)
β=
qL
q
, β′ = H
wc
wc
Working Fluids (Refrigerants)
Ammonia & Sulfur-Dioxide (early days) – but not used ; highly toxic and
dangerous
Chlorofluorocarbons (CFCs) – CCl2F2 (Freon-12, Genatron-12) ; R-11 and R-12
: but destroying the protective ozone layer of the stratosphere
The most desirable fluids – HFCs (CFCs containing hydrogen) R-22
Two important considerations when selecting refrigerant working fluids
A. Temperature at which refrigeration is needed
B. Type of equipment to be used
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
Deviation of the Actual Vapor-Compressor Refrigeration Cycle
from the Ideal Cycle
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
Ammonia Absorption Refrigeration Cycle – 흡수식 냉동기
The Air-Standard Refrigeration Cycle
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
The Air-Standard Refrigeration Cycle (for aircraft cooling)
The Air Refrigeration Cycle utilizing a heat exchanger
Combined-Cycle Power and Refrigeration System
School of Mechanical Engineering, ChungAng University
Lecture note for general thermodynamics, 2003
Combined Brayton/Rankine Cycle Power System
School of Mechanical Engineering, ChungAng University