1.1 Thermodynamics I Lecture 42: Brayton cycle 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
1.1
Thermodynamics I
Lecture 42: Brayton cycle
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
1.2
Brayton Cycle
Open Brayton cycle Closed Brayton cycle
Gas-cooled
nuclear reactor
1.3
Ideal Brayton Cycle
Description
Process Closed cycle Open cycle
12 Isentropic compression Isentropic compression
23 Constant pressure heat input Heat librated from fuel
during combustion process
34 Isentropic expansion Isentropic expansion
41 Constant pressure heat Enthalpy difference between
rejection exhaust products and intake air
P-v diagram T-s diagram
1.4
Continue Brayton Cycle
V
p
1
2
4
3
qin
qout
s
T
1
2 4
3
s1=s2 s3=s4
qin
qout
V2 V3 V4 V1
Ideal Brayton Cycle
1.5
Continue Brayton Cycle
Comparison with vapor power cycle:
» lighter and more compact (air vs. water
density)
high power output-to-weight ratio
» lower pressure ratios, higher volume based on
gas vs. liquid compression
» lower efficiencies based on non-isothermal heat
addition and heat rejection
Typical applications:
» Aircraft propulsion
» Power plant for marine,
space, etc.
» Stationary peaking
power plant
» Ground transportation ??
1.6
Continue Brayton Cycle
Comparison with Otto and Diesel cycles:
4 1: constant pressure vs. constant volume
process (both Otto and Diesel cycles)
2 3: constant pressure vs. constant volume process (Otto cycle only)
» In general, pressure rise much smaller for turbo-machinery than
for reciprocating devices
Otto cycle Brayton cycle
Diesel cycle
1.7
Continue Brayton Cycle
inQ
Heat Exchanger
Turbine cycleW
1
2 3
4
Comp
Heat Exchanger
outQ
C 2 1
3 4T
W h hbwr
h hW
T3 4
Wh h
m C
2 1
Wh h
m and
in3 2
Qh h
m out
4 1
Qh h
m and
1st law, energy balances, for each component:
For gas turbine, bwr typically 40~80%
For vapor cycle, bwr 1~2%
Back Work Ratio:
1.8
Continue Brayton Cycle
Thermal efficiency:
in out outnet T Cth
in in in in
q q qw w w1
q q q q
closed cycle, SSSF, anFor d KE PE 0
out 4 1th
in 3 2
q h h1 1
q h h
constant specific heFor ats:
p 4 1
th
p 3 2
c (T T )1
c (T T )
4
11
32
2
T1
TT1
TT 1T
1.9
Continue Brayton Cycle
Continue thermal efficiency: pk
v
cFor an isentropic process : pv constant, with k
c
k k
1 1 2 2For process 1 2 : p v p v k
1 1k
k k11 2 1 2 2 1 2
kk
2 1 2 1 1 2 12
2
RT
pv p T p p T p
v p T p p T pRT
p
1 k k 1 k 1
k k k k k2 1 2 2 2 2
1 2 1 1 1 1
T p p p p p
T p p p p p
k 1 k 1
k k4 4 1
3 3 2
T p pAlso, using same analysis:
T p p
1.10
Continue Brayton Cycle thermal efficiency
k 1 k 1
k k2 1 2 4
1 2 1 3
p p T TThen, 1
p p T T
4 3 1th
1 2 2
T T T1
T T T
2p
1
pressurep
Define :o rratip
th k 1
kp
11
r
4
11th
32
2
T1
TT1
TT 1T
1.11
Continue Brayton Cycle
Thermal efficiency versus rp (air k=1.4):
th k 1
kp
11
r
1.12
Continue Brayton Cycle
For isentropic process of an ideal gas with varying specific heats:
2 r2
1 r1
p p (T)
p p (T) 4 r4
3 r3
p p (T)
p p (T)
4 1 2 3Also, since p p and p p :
1 4 r4 r1
2 3 r3 r2
p p p (T) p (T)
p p p (T) p (T)
r
o
where p is the relative pressus (T)
ere defined as xpR
1.13
Continue Brayton Cycle
Trade off between efficiency and power:
V
p
1 4
s
T
1
2
4
3
2 3
4’
4’
2’
3’
2’ 3’
Upper temperature limit
Which cycle has greater efficiency?
Which cycle has greater work output?
1.14
Continue Brayton Cycle
Note: Cycle B will require smaller device to produce
the same work output as cycle A. Need to
consider this for transport application!
Task: Maximize work output, i.e., find optimal T2 that
maximizes work output for given T1 and T3. T
s 1
2
4
3
2’
3’
T1
T3
4’
Limiting Cases:
1.) T2 = T3 wnet = 0
th = max
2.) T2 = T1 wnet = 0
th = 0
1.15
Continue Brayton Cycle
maximize work output:
net T C in outw w w q q
For constant specific heats:
net p 3 2 p 4 1w c (T T ) c (T T )
netFor maximum w :net 3
p
2 2
dw dT0 c
dT dT 4 1
2 2
dT dT1
dT dT
4 3
1 2
T TNow recall:
T T 1 3
4
2
T TT
T 4 1 3
2
2 2
dT T T
dT T
net 4 1 3
2
2 2 2
dw dT T T0 1 1
dT dT T
2 1 3T T T
For non-constant specific heats: use air tables!
1.16
Continue Brayton Cycle
Irreversibilities of Brayton cycles:
Deviation of actual gas-turbine
cycles from idealized ones:
s 2s 1C
a 2a 1
w h h
w h h
Note:
T2a > T2s if p2a = p2s
T4 > T4s if p4a = p4s
irreversibilities change
mechanical energy to thermal
energy through friction.
a 3 4aT
s 3 4s
w h h
w h h
1.17
Continue Brayton Cycle
Continue irreversibilities of Brayton cycles:
Example:
T1 = 22oC, p1 = 0.95 bars, rp = p2/p1 = 6, T3 = 1100 K
wC/wT
Ideal 0.38 0.44
C = 0.82 and T = 0.85 0.23 0.62
Notes:
» The overall efficiency is reduced by 40%!
» Gas turbine performance is very sensitive to turbine and
compressor efficiencies!
1.18
Example: Ideal Brayton Cycle
Known: rp = 10, state 1: p1 =100 kPa, volumetric
flow rate 5 m3/s, T1 =300 K, T3 =1400 K
Find: bwr, ηth and wnet
Assumptions: 1) Air-Standard assumptions. 2)
ΔKE=ΔPE =0. 3)Variable cp 4) ΔP =0 in HX, 5)
isentropic efficiency=1
Analysis:
The results with constant k and
ηt = ηc = 80% (Text Example 9.6)
η= 24.9%
bwr=61.8%
Wcycle=1254 kW