Combustion Design ConsiderationsEGR 4347Analysis and Design of Propulsion Systems

PROPERTIES OF COMBUSTION CHAMBERSComplete combustionLow total pressure lossStability of combustion processProper temperature distribution at the exit with no hot spotsShort length and small cross sectionFreedom from flameoutRelightabilityOperation over a wide range of mass flow rates, pressure and temperatures

COMBUSTOR DESIGN GOALS ARE DEFINED BY THE ENGINE OPERATING REQUIREMENTSLEAN BLOW OUT FUEL-AIR RATIOIGNITION FUEL-AIR RATIOPATTERN FACTORRADIAL PROFILE FACTORPRESSURE DROP (SYSTEM AND LINER)COMBUSTION EFFICIENCYMAXIMUM WALL TEMPERATURESMOKE AND GASEOUS EMISSIONS

CRITICAL DESIGN PARAMETERSEquivalence ratio, Combustor loading parameter, CLPSpace heat release rate, SRReference velocity, VrefMain burner dome height, HdMain burner length/dome height ratio, Lmb/HdPassage velocity, VpassNumber and spacing of fuel injectorsPattern factor correlation parameters, PFProfile factor correlation parameter, Pf

DEFINITION OF TERMSPATTERN FACTORSYSTEM PRESSURE DROPLINER PRESSURE DROP

COMBUSTION PROCESSREACTION RATE - f(Temp, Press)T & P high fast reaction ratelimited by rate at which fuel is vaporizedFUEL/AIR RATIO (OCTANE e.g.)2C8H18 + 25(O2 + 79/21 N2) 16 CO2 + 18H2O + 25(79/21)N2fstoich = EQUIVALENCE RATIO,

ENGINE OPERATION AFFECTS INGNITION AND LEAN STABILITYIGNITIONENVELOPEALTITUDEMACH NO.DECELERATIONSCHEDULEOPERATIONALENVELOPESTABLEFLAMEOUTENGINE SPEEDFUEL FLOW

COMBUSTION PROCESSPROBLEM: want low f (

COMBUSTION PROCESS(Ignition)Requires fuel/air mixture be within flammability limitsSufficient residence timeIgnition source in vicinity of combustible mixtureIf mixture is below Spontaneous Ignition Temperature (SIT), an ignition source is required to bring temp up to SIT (Spark Plug)Ignition energy - fig 10-68Ignition Delay

COMBUSTION PROCESS (Stability)Ability of the combustion process to sustain itselfPROBLEMS: Too lean or too richTemp & reaction rates drop below that required to heat and vaporize the fuel/air mixtureCLP (Combustion Loading Parameter)Indication of stability based on mass flow, pressure (n = 1.8 for typical fuels), and combustor volume CLP fStableUnstableUnstable

COMBUSTION PROCESS (Stability - CLP)Gives an estimate of combustor lengthArefVave = VrefL: distance required for combustion to be completedAref: cross-sectional area normal to airflowrt3: approximate density of air entering combustor

COMBUSTION PROCESS (Stability - CLP)Eq. 10-31:Design of new combustor based on old designs (Table 10-5)F100:L = 18.5 inD = 25 inPt3 = 366 psiaTt4max = 3025 RKnown Similar ReferenceNew DesignNote: this equation needs to be corrected in your bookThus: the length of main burners varies with pressure and temperature

COMBUSTION PROCESS (Total Pressure Loss) Heat interaction (Rayleigh Loss) + Friction/Drag (Fanno Loss)q = cpeTte - cpiTtiqDieViTtiVeTte

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

COMBUSTION PROCESS (Total Pressure Loss)Mi or M3Me or M4

COMBUSTOR DIFFUSER (Total Pressure Loss)smooth-walldiffuserstep (dump)diffuser123Smooth-WallDumpA1A2A3 Set by Compressor Blade Height

COMBUSTOR DESIGN ITERATIONEstimate the combustor geometryCheck 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

Aref = Apass + AcombriroMain Burner Areas, Heights, and Velocitiesrm

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 ft3Primary VolumeCombustor Volume(includes Primary)H = rt-rhrhrtLmb = Ldiff + Lcomb

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.

COMBUSTOR EFFICIENCY (reaction rate parameter)

COMBUSTOR STABILITY (CLP)

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

Lmb = Ldiff + LcombMain Burner Lengths and Mass Flow RatesLmbLdiffLcombLdiff = Lsm +Ldump3a3b3cVolmb = 0.8Lmb*ArefVolcomb = 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

AfterburnersComponents: Diffuser Spray Ring Flame Holder Cooling Liner Screech Liner Variable Throat Nozzle

Afterburners - ComponentsCooling LinerZone 2 fuel spray ringZone 3 fuel spray ringZone 4 fuel spray ringFlame holderSplitter coneFan flowCore flowZone 1 fuel spray ringZone 2 fuel spray ringDiffuser coneLinear louveredLinear perforatedStation 6Station 7DiffuserCombustion Section

Afterburners - ComponentsDiffuserSpray RingFlame HolderRecirculating ZoneWHV2dLMixing Zone

Diffuser Balance between low total pressure lossduring combustion (loss Mach no) andAB cross-sectional area (no larger thanlargest diameter upstream)

Short diffuser to reduce AB length with lowtotal pressure loss

Analysis - same as combustor diffuser

Spray Ring - Injection, Atomization, Vaporization, & Ignition Injection: core stream first (high temp)Fuel is injectedperpendicular to air stream & ripped into micron-sized droplets (atomized).Fuel is vaporized then ignited prior tobeing trapped in downstream flameholdersprayring Ignition: spark or arc igniter pilot burner

Flame Holder - Flame Stabilization Two main types V-gutter Flame Holders Pilot burners Bluff body that generates a low-speed mixingregion just downstream of fuel injection high local equivalence ratio (~ 1) 2 zones: 1) Mixing - turbulent flow with very high shearsharp temp gradients and vigorous chemical reactions; 2) Recirculating - strong recirculation, low reaction ratesand temps very near stoiciometricRecirculating ZoneWdLMixing ZoneV2Flame Holder

Cooling and Screech Liner Cooling Isolates the very high temperatures from outer casing. In F119all the fan air is used to cool the AB and Nozzle duringAB operation. Screech Attenuates high frequency oscillations associated with combustion instability (high heat release rates) 200-20000 Hz,high heat loading & vibratory stressesMAltScreech RegimeRumble

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

Single Flameholder DesigndLWHDmax= 35 in1, iV2V1eInlet Conditions (Typical)Pt1 = 40 psia g1 = 1.33Tt1 =1750 Rm = 200 lbm/sFlameholder Geometry (Choice)half angle, a = 30 degd = 3.5 inflocal = 0.8Exit Conditions (Typical)Tte = 3800 R g2 = 1.3fAB = 0.035

Design Calculations1. Find M12. Check for flame stability for flocal = 0.8Eq. 10-53 and Fig 10-89Characteristic ignition time, tc

Design Calculations (contd)2. Flame stability (contd)eq 10-51: want something in terms of V1c, H, and tc, where V1cis the maximum entrance velocity for a stable flameare functions of flameholder blockage ratio, B = d/H - see Table 10-7If V1c > V1, the flame will not blow out

Design Calculations (contd)3. Total Pressure Drop (pAB) - Target Values: Fig 10-90Diffuser: combination of smooth wall & dump - same approach as main combustor diffuser using equations 10-42a&b and 10-43Rayleigh + Fanno: CD & Tte/Tti- Tte/Tti is given from calculations (Perf)- CD is estimated using equation 10-57- Use equations 10-35 thru 10-38 to determine pressure ratio due to Rayleigh & Fanno losses

Design Calculations (contd)4. Total Afterburner Length - Based on Fig 10-925. Space Heat Release Rate, SRVol = (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- Simple Approximation for Heating Value of the Fuel (Hill and Peterson, p. 221)

Combustion ChemistryFuelJP4 (CH2.02)Propane (C3H8)Methane (CH4)

Liquid HydrogenHeating Value (Btu/lbm)18,400119,944221,5182

51,5932(Equation not Valid)1 EGTP, pg 8272 Standard Handbook for Mechanical Engineers, pg 4-29, table 4.1.6Estimate (Btu/lbm)18,57919,43621,203

Combustion Chemistry- Non-Reacting Mixtures-Basic EquationsApplied Equations-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-

Design ExampleFor 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: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)Pt4/Pt3

COMBUSTOR DIFFUSER (Total Pressure Loss)Station 1 to 2 (smooth-wall, sm)Station 2 to 3 (Dump)12Given:h = 0.9, A1/A3 = 0.20M1 = 0.5Pick:A1/A2 = ________Find:Pt2/Pt1 = __________ (Use Eq 9.17b)M2 = _______ (Use MFP)Lsm/Hsm = ___________ (Use Fig 9.8)23Calc:A2/A3 = ________Find:Pt3/Pt2 = __________ (Use Eq 9.18)M3 = ___________ (Use MFP)LsmHsmSet by Compressor Blade HeightOverall Pressure Ratio of Diffuser, Pt3/Pt1: _________HD