Combustion Design
Considerations
EGR 4347
Analysis and Design of Propulsion
Systems
PROPERTIES OF COMBUSTION CHAMBERS
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Complete combustion
Low total pressure loss
Stability of combustion process
Proper temperature distribution at the exit with no
“hot spots”
Short length and small cross section
Freedom from flameout
Relightability
Operation over a wide range of mass flow rates,
pressure and temperatures
COMBUSTOR DESIGN GOALS ARE DEFINED
BY THE ENGINE OPERATING
REQUIREMENTS
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LEAN BLOW OUT FUEL-AIR RATIO
IGNITION FUEL-AIR RATIO
PATTERN FACTOR
RADIAL PROFILE FACTOR
PRESSURE DROP (SYSTEM AND LINER)
COMBUSTION EFFICIENCY
MAXIMUM WALL TEMPERATURE
SMOKE AND GASEOUS EMISSIONS
CRITICAL DESIGN PARAMETERS
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Equivalence ratio, 
Combustor loading parameter, CLP
Space heat release rate, SR
Reference velocity, Vref
Main burner dome height, Hd
Main burner length/dome height ratio, Lmb/Hd
Passage velocity, Vpass
Number and spacing of fuel injectors
Pattern factor correlation parameters, PF
Profile factor correlation parameter, Pf
DEFINITION OF TERMS
• PATTERN FACTOR
(TEXIT)MAX - (TEXIT)AVE
PF =
(TEXIT)AVE - (TINLET)AVE
• SYSTEM PRESSURE DROP
(PINLET)TOTAL - (PEXIT)TOTAL
DPS =
(PINLET)TOTAL
• LINER PRESSURE DROP
(PINLET)STATIC - (PEXIT)STATIC
DP =
(PINLET)STATIC
COMBUSTION PROCESS
• REACTION RATE - f(Temp, Press)
– T & P high
fast reaction rate
– limited by rate at which fuel is vaporized
• FUEL/AIR RATIO (OCTANE e.g.)
– 2C8H18 + 25(O2 + 79/21 N2)
– fstoich =
16 CO2 + 18H2O + 25(79/21)N2
2(96  18)
 0.0664
25(32  79 21  28)
• EQUIVALENCE RATIO,
 ff
stoich
ENGINE OPERATION AFFECTS INGNITION
AND LEAN STABILITY
IGNITION
ENVELOPE
FUEL FLOW
ALTITUDE
OPERATIONAL
ENVELOPE
DECELERATION
SCHEDULE
STABLE
FLAMEOUT
MACH NO.
ENGINE SPEED
COMBUSTION PROCESS
10
Equivalence Ratio, 
6
4
2
Rich flammability limit
Flammable
mist
Flammable
vapor
1.0
0.6
0.4
Spontaneous
ignition
Lean flamm
abilit y lim
it
Flash point
0.2
0.1
50
100
150
200
250
T (C)
• PROBLEM: want low  (<1); can easily by 0.5
SOLUTION: locally rich mixture that’s burned then diluted and
cooled to acceptable Tt4
• PROBLEM: want stationary flame within a moving flow
SOLUTION: Recirculating region at front of combustor, or “flame
holders” in AB
COMBUSTION PROCESS
(Ignition)
• Requires fuel/air mixture be within flammability
limits
• Sufficient residence time
• Ignition source in vicinity of combustible mixture
– If mixture is below Spontaneous Ignition
Temperature (SIT), an ignition source is
required to bring temp up to SIT (Spark Plug)
– Ignition energy - fig 10-68
 const  1
– Ignition Delay
tign  exp 
, 
 T  P
COMBUSTION PROCESS
(Stability)
• Ability of the combustion process to sustain itself
• PROBLEMS: Too lean or too rich
– Temp & reaction rates drop below that required to heat
and vaporize the fuel/air mixture
• CLP (Combustion Loading Parameter)
– Indication of stability based on mass flow, pressure (n =
1.8 for typical fuels), and combustor volume
CLP 
m
P n Vol 
Unstable

Stable
Unstable
CLP
COMBUSTION PROCESS
(Stability - CLP)
• Gives an estimate of combustor length
L  Vref t res

t 3A ref t res

m
L
Aref
2"
Vave = Vref
m

L: distance required for combustion to be completed
Aref: cross-sectional area normal to airflow
1

t3: approximate density of air entering combustor t 3  Pt 3
t res  Pt3 n
c
COMBUSTION PROCESS
(Stability - CLP)

Eq. 10-31: L 
c
1 
Pt 3
c
Tt 4
A4
t res
A ref
Note: this equation needs to be corrected
in your book
Design of “new” combustor based on “old” designs (Table 10-5)
Known Similar Reference
F100: L = 18.5 in
D = 25 in
Pt3 = 366 psia
Tt4max = 3025 R
 c 1
L  const
c
Pt 3
Tt 4
Pt3n
New Design
Pt3r
L  Const
Tt 4
   1
where r  n  c
 c 

Thus: the length of main burners
varies with pressure and temperature
COMBUSTION PROCESS
(Total Pressure Loss)
• Heat interaction (Rayleigh Loss) + Friction/Drag (Fanno Loss)
1
Drag  i Vi2 C D A
q = cpeTte - cpiTti
2
Vi
Tti
D
Ve
Tte
e
i
q
Cont: Vi i  Vee
i Vi2
eVe2 1
Mom: Pi 
 Pe 
 i Vi2CD
gc
gc
2
Energy: q = c peTte - c pi Tti
COMBUSTION PROCESS
(Total Pressure Loss)
• Solution to these
3 equations:
exit, e  4
inlet, i  3
• Equations 10-35 thru
10-38 on page 823
Tt4 q  c p3Tt3
1.

Tt3
c p4 Tt3
  1 2
 3 M 32 1  3
M3 
2

 Tt4
2.  =
2
 4 1 +  3 M 32  1  CD  Tt3

3. M 42 
P
4. 4 
P3

2
1 - 2 4   1 - 2  4  1

1 +  3 M 32 1 
CD
2

1   4 M 42
4
 4  1 2   4 1
M4 
1 
Pt4 P4 
2

5.

3  1
Pt3 P3
1   3  1 M 2   3 1

3
2


COMBUSTION PROCESS
(Total Pressure Loss)
Pte / Pti
or
Pt4 / Pt3
1.00
1.0
0.95
0.80
0.90
0.6
Me or M4
0.85
0.4
0.80
0.2
0.75
0.0
0.3
0
0.05
0.1
0.15
0.2
Mi or M3
0.25
COMBUSTOR DIFFUSER
(Total Pressure Loss)
3
Set by Compressor Blade Height
2
1
A2
A1
A3
smooth-wall
diffuser
step (dump)
diffuser
Smooth-Wall


2
 2


A1
M1 1  1 
A 2 
Pt 2
2

 1

Pt1
1    1 M 2   1
1

2
Dump
2
6 







Pt 3

A
A

2
2
2
 exp  M 2 1 
  1 
 
Pt 2
2
A
A




3
3 




COMBUSTOR DESIGN ITERATION
• Estimate the combustor geometry
– Check Combustion Stability (at all flight conditions)
– Determine Combustion Efficiency (at all flight
conditions)
– Calculate Space Rate Heat Release (at all flight
conditions)
– Determine Combustor Reference Velocity (at all flight
conditions)
• NEXT: Modify design based on the above
calculations and typical/target values
Main Burner Areas, Heights, and Velocities
rm
Vref
Aref
Acomb
Vcomb~ 30 ft/s
ro
Apass
Vpass~ 150 ft/s
ri
Main Burner Height, H
Aref = Apass + Acomb
m 3a
Vref 
 t 3 Aref
H = ro - ri
Aref
H
2  rm
COMBUSTOR DESIGN ITERATION
• Assume the following “typical” combustor geometry
– Primary Combustor Volume, 3.5 ft3 ( Acomb*Lcomb)
– Combustor Reference Area, Aref = (rt2 - rh2) = 5 ft2
– Dome Height, H = rt - rh = 7 in
– Total Combustor Volume, Vol = 7.0 ft3
rt
Vref
H = rt-rh
Acomb
Aref
Primary Volume
Combustor Volume
(includes Primary)
rh
Lmb = Ldiff + Lcomb
COMBUSTOR DESIGN ITERATION
• Can calculate from performance data the following:
– Combustor Efficiency, b
– Check Stability by plotting CLP vs 
– Calculate Space Rate or Space Heat Release Rate -- measure of
intensity of energy release
– Calculate the Reference Velocity, Vref
• Review literature to determine acceptable values for the
above parameters then adjust the design choices
such as Volumes, Areas, and Height.
COMBUSTOR EFFICIENCY
(reaction rate parameter)

1.75
t3
P
Tt 3
b
Aref e H
.
x 10 5

m3
"" when   1.03

where b  382 2  ln  / 1.03
"" when   1.03
COMBUSTOR STABILITY (CLP)
CLP 
m

P Vol 
n
SPACE HEAT RELASE (SR) and
REFERENCE VELOCITY (Vref)
 f AB hPR 3600
m
SR 
Pt (Vol ) AB
.
Vref
m3

 t 3 Aref
Main Burner Lengths and Mass Flow Rates
Lcomb
Ldiff = Lsm +Ldump
Ldiff
local = /50%
m3a*50%
Primary Comb
Zone
3b
3c
m3a
Passage
m3a*50%
3a
Lmb
Lmb = Ldiff + Lcomb
Pt3r
Lmb  const
Tt 4
Volmb = 0.8Lmb*Aref
Volcomb = Lcomb*Acomb
Afterburner Design Requirements
*Large temperature rise
*Low dry loss (non-AB thrust)
*Wide temperature modulation (throttle)
*High combustion efficiency
*Short length; light weight
*Altitude light-off capability
*No acoustic combustion instabilities
*Long life, low cost, easy repair
Afterburners
Components:
• Diffuser
• Spray Ring
• Flame Holder
• Cooling Liner
• Screech Liner
• Variable Throat Nozzle
Afterburners - Components
Diffuser
Combustion Section
Zone 4 fuel spray ring
Zone 3 fuel spray ring
Zone 2 fuel spray ring
Fan flow
Splitter cone
Flame holder
Cooling Liner
Core flow
Zone 2 fuel Zone 1 fuel
spray ring spray ring
Diffuser cone
Linear perforated
Linear louvered
Station 6
Station 7
Afterburners - Components
Spray Ring
Diffuser
Flame Holder
V2
d
Recirculating Zone
H
W
L
Mixing Zone
Diffuser
• Balance between low total pressure loss
during combustion (loss Mach no) and
AB cross-sectional area (no larger than
largest diameter upstream)
• Short diffuser to reduce AB length with low
total pressure loss
• Analysis - same as combustor diffuser
Spray Ring - Injection, Atomization,
Vaporization, & Ignition
• Injection: core stream first (high temp)
spray
ring
Fuel is injected
perpendicular to air stream &
ripped into micron-sized droplets (atomized).
Fuel is vaporized then ignited prior to
being trapped in downstream flameholder
• Ignition: spark or arc igniter
pilot burner
Flame Holder - Flame Stabilization
• Two main types
– V-gutter Flame Holders
– Pilot burners
V2
d
Recirculating Zone
W
L
Flame Holder
Mixing Zone
• Bluff body that generates a low-speed mixing
region just downstream of fuel injection
– high local equivalence ratio (~ 1)
– 2 zones: 1) Mixing - turbulent flow with very high shear
sharp temp gradients and vigorous chemical reactions;
2) Recirculating - strong recirculation, low reaction rates
and temps very near stoiciometric
Cooling and Screech Liner
• Cooling
– Isolates the very high temperatures from outer casing. In F119
all the fan air is used to cool the AB and Nozzle during
AB operation.
• Screech
– Attenuates high frequency oscillations associated with
combustion instability (high heat release rates)
– 200-20000 Hz,high heat loading & vibratory stresses
Rumble
Alt
Screech Regime
M
Variable Nozzle
• MFP - applied at Nozzle throat, M8 = 1
m
 8 Tt 8
A8 
Pt 8MFP(M8 )
Single Flameholder Design
Dmax= 35 in
V1
d
V2
L
1, i
Inlet Conditions (Typical)
Pt1 = 40 psia
Tt1 =1750 R
m = 200 lbm/s
1 = 1.33
Exit Conditions (Typical)
Tte = 3800 R
2 = 1.3
fAB = 0.035
W
H
e
Flameholder Geometry (Choice)
half angle, a = 30 deg
d = 3.5 in
local = 0.8
Design Calculations
1. Find M1
m
 1 Tt1
MFP ( M1 ) 
Pt1A1
2. Check for flame stability for local = 0.8
Tt1
T1 
  1 1 2 
M1 
1 
2


P1 
Pt1
  1 1 2 
M1 
1 
2


Eq. 10-53 and Fig 10-89
Characteristic ignition time, tc
tc 
k ( )
PT
2.5

t c ref 
2.5
t c ref Pref Tref
PT 2.5
1
( 1 1)
k ()
2.5
Pref Tref
Design Calculations (cont’d)
2. Flame stability (cont’d)
eq 10-51:
tc 
L
V2c
want something in terms of V1c, H, and tc, where V1c
is the maximum entrance velocity for a stable flame
V1c L W  V1  L  W 
 V1ctc 

     eq 10  54


 H  Blowout H V2c W  V2  W  H 
V1 W
,
V2 H
are functions of flameholder blockage
ratio, B = d/H - see Table 10-7
L
4
W
Solve for V1c above and compare to V1  M1  1RT1gc
If V1c > V1, the flame will not blow out
Design Calculations (cont’d)
3. Total Pressure Drop (AB) - Target Values: Fig 10-90
Diffuser: combination of smooth wall & dump
- same approach as main combustor diffuser
using equations 10-42a&b and 10-43
Rayleigh + Fanno: CD & Tte/Tti
- Tte/Tti is given from calculations (Perf)
- CD is estimated using equation 10-57
V 
C D  B 2 
 V1 
2
- Use equations 10-35 thru 10-38 to determine
pressure ratio due to Rayleigh & Fanno losses
Design Calculations (cont’d)
4. Total Afterburner Length - Based on Fig 10-92
5. Space Heat Release Rate, SR
 f AB hPR 3600
m
SR 
Pt (Vol ) AB
Vol = (total length x AB cross-sectional area)
Desired value near 8 x 106 Btu/(hr ft3 atm)
Combustion Chemistry
- General Fuel-to-Air Stoichiometric Equation
y z
y z
y
y z



C x H y Oz   x   O2  3.76 x    N 2  xCO2  H 2O  3.76 x    N 2
4 2
4 2
2
4 2



f stoich 
m fuel
mair

12 x  1y  16 z
y z

 x   32  3.76  28.16
4 2

- Simple Approximation for Heating Value of the Fuel
(Hill and Peterson, p. 221)
H
hPR  15,900  15,800    in BTU
lb m
C
H 1.008  m
where

for Hydrocarbo n C n H m
C 12.01 n
Combustion Chemistry
Fuel
Heating Value (Btu/lbm) Estimate (Btu/lbm)
JP4 (CH2.02)
Propane (C3H8)
Methane (CH4)
Liquid Hydrogen
1
18,4001
19,9442
21,5182
18,579
19,436
21,203
51,5932
(Equation not Valid)
EGTP, pg 827
2 Standard Handbook for Mechanical Engineers, pg 4-29, table 4.1.6
Combustion Chemistry
- Non-Reacting MixturesBasic Equations
Applied Equations
k
mass: m m   mi
i 1
R
k
Mole Number : N m  N i
C p  A0  A1T  A2T 2  A3T 3  A4T 4
i 1
k
Mass Fraction: mf i   mf i ,
i 1
Mole Fraction:  i 
Ni
,
Nm
1.9857117 (Btu/lb mole R)
28.97 - f  0.946186lb m /lb mole 
k
 mf i  1
i 1
k
 i  1
i 1
 A5T 5  A6T 6  A7T 7
C pm 
C pair  f  C pprod
1 f

R 
  1  m 
 C pm 
1
k
Mass: mi  N i M i ; m m    i M i
i 1
Gas Constant: R m 
Ru
Mm
-Coefficients for Cp equation given in
Table 2-4 (pg 106) Mattingly
-Variation in properties given in
Figures 6-1 and 6-2
Combustion Chemistry
- Variation with Temp-
 versus Temp for JP-4
Cp versus Temp for JP-4
1.42
0.36
f=0
f = 0.02
1.4
0.34
f = 0.04
f = 0.06
1.38
f = 0.0676
0.32
0.3
1.34

Cp (Btu/lb mR)
1.36
1.32
0.28
1.3
f=0
0.26
f = 0.02
1.28
f = 0.04
0.24
f = 0.06
1.26
f = 0.0676
0.22
1.24
0
500
1000
1500
2000
2500
Temp (R)
3000
3500
4000
4500
0
500
1000
1500
2000
2500
Temp (R)
3000
3500
4000
4500
Design Example
For the information given on the 1st slide, find the following:
1. M1 and V1
2. V1c (check stability)
3. Pressure ratio due to Rayleigh and Fanno losses
4. AB length
5. SR
COMBUSTION PROCESS
(Total Pressure Loss)
Example: What is the pressure ratio across the
burner for the following conditions:
Pt4/Pt3
1. Tt4/Tt3 = 3.0 and CD = 0 (No Drag)
2. Tt4/Tt3 = 1 and CD = 2.0 (No q)
3. Tt4/Tt3 = 3.0 and CD = 2.0 (Both Drag and q)
COMBUSTOR DIFFUSER
(Total Pressure Loss)
Set by Compressor Blade Height
Station 1 to 2 (smooth-wall, sm)
Given:
 = 0.9, A1/A3 = 0.20
M1 = 0.5
Pick:
A1/A2 = ________
Find:
Pt2/Pt1 = __________ (Use Eq 9.17b)
2
1
Hsm
Lsm
M2 = _______ (Use MFP)
Lsm/Hsm = ___________ (Use Fig 9.8)
3
Station 2 to 3 (Dump)
Calc:
A2/A3 = ________
Find:
Pt3/Pt2 = __________ (Use Eq 9.18)
2
HD
M3 = ___________ (Use MFP)
Overall Pressure Ratio of Diffuser, Pt3/Pt1: _________
L D  HD