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**Combustion Design Considerations**

EGR 4347 Analysis and Design of Propulsion Systems

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**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

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**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

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**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

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**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 =

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**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,

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**ENGINE OPERATION AFFECTS INGNITION AND LEAN STABILITY**

OPERATIONAL ENVELOPE DECELERATION SCHEDULE ALTITUDE FUEL FLOW IGNITION ENVELOPE STABLE FLAMEOUT MACH NO. ENGINE SPEED

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**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

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**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

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**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

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**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

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**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

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**COMBUSTION PROCESS (Total Pressure Loss)**

Heat interaction (Rayleigh Loss) + Friction/Drag (Fanno Loss) q = cpeTte - cpiTti Vi Tti D Ve Tte e i q

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**COMBUSTION PROCESS (Total Pressure Loss)**

Solution to these 3 equations: exit, e inlet, i 3 Equations thru on page 823

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**COMBUSTION PROCESS (Total Pressure Loss)**

Me or M4 Mi or M3

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**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

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**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

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**Main Burner Areas, Heights, and Velocities**

rm ro ri H = ro - ri Main Burner Height, H Aref = Apass + Acomb

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**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

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**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.

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**COMBUSTOR EFFICIENCY (reaction rate parameter)**

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**COMBUSTOR STABILITY (CLP)**

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**SPACE HEAT RELASE (SR) and REFERENCE VELOCITY (Vref)**

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**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

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**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

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**Afterburners Components: Diffuser Spray Ring Flame Holder**

Cooling Liner Screech Liner Variable Throat Nozzle

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**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

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**Afterburners - Components**

Spray Ring Flame Holder V2 Diffuser H d Recirculating Zone W L Mixing Zone

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**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

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**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

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**Flame Holder - Flame Stabilization**

V2 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

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**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

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

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**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

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**Design Calculations 1. Find M1**

2. Check for flame stability for flocal = 0.8 Eq and Fig 10-89 Characteristic ignition time, tc

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**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

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**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

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**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)

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**- 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)

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**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

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**- 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

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Combustion Chemistry - Variation with Temp-

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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

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**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)

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**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: _________

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