Combustion Design Considerations EGR 4347 Analysis and Design

Combustion Design Considerations EGR 4347 Analysis and Design of Propulsion Systems

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

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

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

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. ) – 2 C 8 H 18 + 25(O 2 + 79/21 N 2) 16 CO 2 + 18 H 2 O + 25(79/21)N 2 – fstoich = • EQUIVALENCE RATIO,

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

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 Tt 4 • 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 – Ignition Delay

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

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 rt 3: approximate density of air entering combustor

COMBUSTION PROCESS (Stability - CLP) Eq. 10 -31: 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 F 100: L = 18. 5 in D = 25 in Pt 3 = 366 psia Tt 4 max = 3025 R Thus: the length of main burners varies with pressure and temperature

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

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) Me or M 4 Mi or M 3

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

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

COMBUSTOR DESIGN ITERATION • Assume the following “typical” combustor geometry – Primary Combustor Volume, 3. 5 ft 3 ( Acomb*Lcomb) – Combustor Reference Area, Aref = p(rt 2 - rh 2) = 5 ft 2 – Dome Height, H = rt - rh = 7 in – Total Combustor Volume, Vol = 7. 0 ft 3 rt H = rt-rh Primary Volume Combustor Volume (includes Primary) rh Lmb = 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)

Main Burner Lengths and Mass Flow Rates Lcomb Ldiff = Lsm +Ldump Ldiff 3 b 3 c 3 a Lmb = Ldiff + Lcomb Volmb = 0. 8 Lmb*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 Diffuser cone Linear perforated Linear louvered Station 6 Station 7

Afterburners - Components Spray Ring Diffuser Flame Holder d V 2 Recirculating Zone L H W 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 V 2 d Recirculating Zone L Flame Holder W 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 F 119 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, M 8 = 1

Single Flameholder Design Dmax= 35 in V 1 d V 2 L 1, i Inlet Conditions (Typical) Pt 1 = 40 psia g 1 = 1. 33 Tt 1 =1750 R m = 200 lbm/s Exit Conditions (Typical) Tte = 3800 R g 2 = 1. 3 f. AB = 0. 035 W H e Flameholder Geometry (Choice) half angle, a = 30 deg d = 3. 5 in flocal = 0. 8

Design Calculations 1. Find M 1 2. Check for flame stability for flocal = 0. 8 Eq. 10 -53 and Fig 10 -89 Characteristic ignition time, tc

Design Calculations (cont’d) 2. Flame stability (cont’d) eq 10 -51: want something in terms of V 1 c, H, and tc, where V 1 c is the maximum entrance velocity for a stable flame are functions of flameholder blockage ratio, B = d/H - see Table 10 -7 Solve for V 1 c above and compare to If V 1 c > V 1, the flame will not blow out

Design Calculations (cont’d) 3. Total Pressure Drop (p. AB) - Target Values: Fig 10 -90 Diffuser: combination of smooth wall & dump - same approach as main combustor diffuser using equations 10 -42 a&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 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 Vol = (total length x AB cross-sectional area) Desired value near 8 x 106 Btu/(hr ft 3 atm)

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

Combustion Chemistry Fuel JP 4 (CH 2. 02) Propane (C 3 H 8) Methane (CH 4) Heating Value (Btu/lbm) Estimate (Btu/lbm) Liquid Hydrogen 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 1

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

Combustion Chemistry - Variation with Temp-

Design Example For the information given on the 1 st slide, find the following: 1. M 1 and V 1 2. V 1 c (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: Pt 4/Pt 3 1. Tt 4/Tt 3 = 3. 0 and CD = 0 (No Drag) 2. Tt 4/Tt 3 = 1 and CD = 2. 0 (No q) 3. Tt 4/Tt 3 = 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: h = 0. 9, A 1/A 3 = 0. 20 M 1 = 0. 5 Pick: Hsm A 1/A 2 = ____ Find: 2 1 Pt 2/Pt 1 = _____ (Use Eq 9. 17 b) Lsm M 2 = _______ (Use MFP) Lsm/Hsm = ______ (Use Fig 9. 8) 3 Station 2 to 3 (Dump) Calc: A 2/A 3 = ____ Find: HD Pt 3/Pt 2 = _____ (Use Eq 9. 18) 2 M 3 = ______ (Use MFP) Overall Pressure Ratio of Diffuser, Pt 3/Pt 1: _____