1. Combustion Design Considerations
EGR 4347
Analysis and Design of Propulsion Systems

2. PROPERTIES OF COMBUSTION CHAMBERS
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

3. COMBUSTOR DESIGN GOALS ARE DEFINED BY THE ENGINE OPERATING REQUIREMENTS
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

4. CRITICAL DESIGN PARAMETERS
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

5. DEFINITION OF TERMS PATTERN FACTOR (TEXIT)MAX - (TEXIT)AVE PF =
(TEXIT)AVE - (TINLET)AVE
PF =
SYSTEM PRESSURE DROP
(PINLET)TOTAL - (PEXIT)TOTAL
(PINLET)TOTAL
DPS =
LINER PRESSURE DROP
(PINLET)STATIC - (PEXIT)STATIC
(PINLET)STATIC
DP =

6. 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.)
2C8H (O2 + 79/21 N2) CO2 + 18H2O + 25(79/21)N2
fstoich =
EQUIVALENCE RATIO,

7. ENGINE OPERATION AFFECTS INGNITION AND LEAN STABILITY
OPERATIONAL
ENVELOPE
DECELERATION
SCHEDULE
ALTITUDE
FUEL FLOW
IGNITION
ENVELOPE
STABLE
FLAMEOUT
MACH NO.
ENGINE SPEED

8. COMBUSTION PROCESS PROBLEM: want low f (<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

9. 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
Ignition Delay

10. 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
Unstable f
Stable
Unstable
CLP

11. COMBUSTION PROCESS (Stability - CLP)
Gives an estimate of combustor length
Aref
Vave = Vref
L: distance required for combustion to be completed
Aref: cross-sectional area normal to airflow
rt3: approximate density of air entering combustor

12. COMBUSTION PROCESS (Stability - CLP)
Eq :
Note: this equation needs to be corrected
in your book
Design of “new” combustor based on “old” designs (Table 10-5)
Known Similar Reference
New Design
F100: L = 18.5 in
D = 25 in
Pt3 = 366 psia
Tt4max = 3025 R
Thus: the length of main burners
varies with pressure and temperature

13. COMBUSTION PROCESS (Total Pressure Loss)
Heat interaction (Rayleigh Loss) + Friction/Drag (Fanno Loss)
q = cpeTte - cpiTti Vi
Tti D Ve
Tte e i q

14. COMBUSTION PROCESS (Total Pressure Loss)
Solution to these
3 equations:
exit, e
inlet, i 3
Equations thru on page 823

15. COMBUSTION PROCESS (Total Pressure Loss)
Me or M4
Mi or M3

16. COMBUSTOR DIFFUSER (Total Pressure Loss)3
Set by Compressor Blade Height 2 1 A1 A2 A3
smooth-wall
diffuser
step (dump)
diffuser
Smooth-Wall
Dump

17. 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

18. Main Burner Areas, Heights, and Velocitiesrm ro ri
H = ro - ri
Main Burner Height, H
Aref = Apass + Acomb

19. COMBUSTOR DESIGN ITERATION
Assume the following “typical” combustor geometry
Primary Combustor Volume, 3.5 ft3 ( Acomb*Lcomb)
Combustor Reference Area, Aref = p(rt2 - rh2) = 5 ft2
Dome Height, H = rt - rh = 7 in
Total Combustor Volume, Vol = 7.0 ft3 rt
H = rt-rh
Primary Volume
Combustor Volume
(includes Primary) rh
Lmb = Ldiff + Lcomb

20. COMBUSTOR DESIGN ITERATION
Can calculate from performance data the following:
Combustor Efficiency, hb
Check Stability by plotting CLP vs f
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.

21. COMBUSTOR EFFICIENCY (reaction rate parameter)

22. COMBUSTOR STABILITY (CLP)

23. SPACE HEAT RELASE (SR) and REFERENCE VELOCITY (Vref)

24. Main Burner Lengths and Mass Flow Rates
Lcomb
Ldiff = Lsm +Ldump
Ldiff 3c 3b 3a
Lmb
Volmb = 0.8Lmb*Aref
Lmb = Ldiff + Lcomb
Volcomb = Lcomb*Acomb

25. 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

26. Afterburners Components: Diffuser Spray Ring Flame Holder
Cooling Liner
Screech Liner
Variable Throat Nozzle

27. 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
spray ring
Zone 1 fuel
spray ring
Diffuser cone
Linear perforated
Linear louvered
Station 6
Station 7

28. Afterburners - Components
Spray Ring
Flame Holder V2
Diffuser H d
Recirculating Zone W L
Mixing Zone

29. 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

30. 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

31. Flame Holder - Flame StabilizationV2
Two main types
V-gutter Flame Holders
Pilot burners 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

32. Cooling and Screech Liner
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)
Hz,high heat loading & vibratory stresses
Rumble
Alt
Screech Regime M

33. Variable Nozzle
MFP - applied at Nozzle throat, M8 = 1

34. Single Flameholder Design
Dmax= 35 in V2 d V1 H W L
1, i e
Inlet Conditions (Typical)
Flameholder Geometry (Choice)
Pt1 = 40 psia g1 = 1.33
Tt1 =1750 R
m = 200 lbm/s
half angle, a = 30 deg
d = 3.5 in
flocal = 0.8
Exit Conditions (Typical)
Tte = 3800 R g2 = 1.3
fAB = 0.035

35. Design Calculations 1. Find M1
2. Check for flame stability for flocal = 0.8
Eq and Fig 10-89
Characteristic ignition time, tc

36. Design Calculations (cont’d)
2. Flame stability (cont’d)
eq 10-51:
want something in terms of V1c, H, and tc, where V1c
is the maximum entrance velocity for a stable flame
are functions of flameholder blockage
ratio, B = d/H - see Table 10-7
Solve for V1c above and compare to
If V1c > V1, the flame will not blow out

37. Design Calculations (cont’d)
3. Total Pressure Drop (pAB) - 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
- Use equations thru to determine
pressure ratio due to Rayleigh & Fanno losses

38. Design Calculations (cont’d)
4. Total Afterburner Length - Based on Fig 10-92
5. Space Heat Release Rate, SR
Vol = (total length x AB cross-sectional area)
Desired value near 8 x 106 Btu/(hr ft3 atm)

39. - Simple Approximation for Heating Value of the Fuel
Combustion Chemistry
- General Fuel-to-Air Stoichiometric Equation
- Simple Approximation for Heating Value of the Fuel
(Hill and Peterson, p. 221)

40. Heating Value (Btu/lbm)
Combustion Chemistry
Fuel
JP4 (CH2.02)
Propane (C3H8)
Methane (CH4)
Liquid Hydrogen
Heating Value (Btu/lbm)
18,4001
19,9442
21,5182
51,5932
(Equation not Valid)
Estimate (Btu/lbm)
18,579
19,436
21,203
1 EGTP, pg 827
2 Standard Handbook for Mechanical Engineers, pg 4-29, table 4.1.6

41. - Non-Reacting Mixtures-
Combustion Chemistry
- Non-Reacting Mixtures-
Basic Equations
Applied Equations
-Coefficients for Cp equation given in
Table 2-4 (pg 106) Mattingly
-Variation in properties given in
Figures 6-1 and 6-2

42. Combustion Chemistry
- Variation with Temp-

43. 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

44. 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)

45. COMBUSTOR DIFFUSER (Total Pressure Loss)
Station 1 to 2 (smooth-wall, sm)
Set by Compressor Blade Height 2
Given: h = 0.9, A1/A3 = 0.20
M1 = 0.5
Pick: A1/A2 = ________
Find: Pt2/Pt1 = __________ (Use Eq 9.17b)
M2 = _______ (Use MFP)
Lsm/Hsm = ___________ (Use Fig 9.8) 1
Hsm
Lsm 3
Station 2 to 3 (Dump)
Calc: A2/A3 = ________
Find: Pt3/Pt2 = __________ (Use Eq 9.18)
M3 = ___________ (Use MFP) HD 2
Overall Pressure Ratio of Diffuser, Pt3/Pt1: _________