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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, V ref Main burner dome height, H d Main burner length/dome height ratio, L mb /H d Passage velocity, V pass Number and spacing of fuel injectors Pattern factor correlation parameters, PF Profile factor correlation parameter, P f

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DEFINITION OF TERMS PATTERN FACTOR SYSTEM PRESSURE DROP LINER PRESSURE DROP (T EXIT ) MAX - (T EXIT ) AVE (T EXIT ) AVE - (T INLET ) AVE PF = (P INLET ) TOTAL - (P EXIT ) TOTAL (P INLET ) TOTAL DPS = (P INLET ) STATIC - (P EXIT ) STATIC (P INLET ) 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.) –2C 8 H 18 + 25(O 2 + 79/21 N 2 ) 16 CO 2 + 18H 2 O + 25(79/21)N 2 –f stoich = EQUIVALENCE RATIO,

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

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COMBUSTION PROCESS PROBLEM: want low (<1); can easily by 0.5 SOLUTION: locally rich mixture that’s burned then diluted and cooled to acceptable T t4 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 CLP Stable Unstable

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COMBUSTION PROCESS (Stability - CLP) Gives an estimate of combustor length A ref V ave = V ref L: distance required for combustion to be completed A ref : cross-sectional area normal to airflow t3 : approximate density of air entering combustor

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COMBUSTION PROCESS (Stability - CLP) Eq. 10-31: Design of “new” combustor based on “old” designs (Table 10-5) F100:L = 18.5 in D = 25 in P t3 = 366 psia T t4max = 3025 R Known Similar Reference New Design Note: this equation needs to be corrected in your book 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 = c pe T te - c pi T ti q D i e V i T ti V e T te

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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

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COMBUSTION PROCESS (Total Pressure Loss) M i or M 3 M e or M 4

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COMBUSTOR DIFFUSER (Total Pressure Loss) smooth-wall diffuser step (dump) diffuser 1 2 3 Smooth-WallDump A1A1 A2A2 A3A3 Set by Compressor Blade Height

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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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A ref = A pass + A comb riri roro H = r o - r i Main Burner Height, H Main Burner Areas, Heights, and Velocities rmrm

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COMBUSTOR DESIGN ITERATION Assume the following “typical” combustor geometry – Primary Combustor Volume, 3.5 ft 3 ( A comb *L comb ) – Combustor Reference Area, A ref = (r t 2 - r h 2 ) = 5 ft 2 – Dome Height, H = r t - r h = 7 in – Total Combustor Volume, Vol = 7.0 ft 3 Primary Volume Combustor Volume (includes Primary) H = r t -r h rhrh rtrt L mb = L diff + L comb

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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, V ref 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 (V ref )

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L mb = L diff + L comb Main Burner Lengths and Mass Flow Rates L mb L diff L comb L diff = L sm +L dump 3a 3b 3c Vol mb = 0.8L mb *A ref Vol comb = L comb *A comb

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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 Cooling Liner Zone 2 fuel spray ring Zone 3 fuel spray ring Zone 4 fuel spray ring Flame holder Splitter cone Fan flow Core flow Zone 1 fuel spray ring Zone 2 fuel spray ring Diffuser cone Linear louvered Linear perforated Station 6 Station 7 DiffuserCombustion Section

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Afterburners - Components Diffuser Spray Ring Flame Holder Recirculating Zone W H V2V2 d 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) 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 spray ring Ignition: spark or arc igniter pilot burner

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Flame Holder - Flame Stabilization Two main types – V-gutter Flame Holders – Pilot burners 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 Recirculating Zone W d L Mixing Zone V2V2 Flame Holder

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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 M Alt Screech Regime Rumble

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Variable Nozzle MFP - applied at Nozzle throat, M 8 = 1

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Single Flameholder Design d L W H D max = 35 in 1, i V2V2 V1V1 e Inlet Conditions (Typical) P t1 = 40 psia = 1.33 T t1 =1750 R m = 200 lb m /s Flameholder Geometry (Choice) half angle, = 30 deg d = 3.5 in local = 0.8 Exit Conditions (Typical) T te = 3800 R 2 = 1.3 f AB = 0.035

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Design Calculations 1. Find M 1 2. Check for flame stability for local = 0.8 Eq. 10-53 and Fig 10-89 Characteristic ignition time, t c

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Design Calculations (cont’d) 2. Flame stability (cont’d) eq 10-51: want something in terms of V 1c, H, and t c, where V 1c 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 1c above and compare to If V 1c > V 1, the flame will not blow out

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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: C D & T te /T ti - T te /T ti is given from calculations (Perf) - C D is estimated using equation 10-57 - Use equations 10-35 thru 10-38 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 10 6 Btu/(hr ft 3 atm)

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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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Combustion Chemistry Fuel JP 4 (CH 2.02 ) Propane (C 3 H 8 ) Methane (CH 4 ) Liquid Hydrogen Heating Value (Btu/lb m ) 18,400 1 19,944 2 21,518 2 51,593 2 (Equation not Valid) 1 EGTP, pg 827 2 Standard Handbook for Mechanical Engineers, pg 4-29, table 4.1.6 Estimate (Btu/lb m ) 18,579 19,436 21,203

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Combustion Chemistry - Non-Reacting Mixtures- Basic EquationsApplied Equations -Coefficients for C p 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. M 1 and V 1 2. V 1c (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: 1. T t4 /T t3 = 3.0 and C D = 0 (No Drag) 2. T t4 /T t3 = 1 and C D = 2.0 (No q) 3. T t4 /T t3 = 3.0 and C D = 2.0 (Both Drag and q) P t4 /P t3

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COMBUSTOR DIFFUSER (Total Pressure Loss) Station 1 to 2 (smooth-wall, sm) Station 2 to 3 (Dump) 1 2 Given: = 0.9, A 1 /A 3 = 0.20 M 1 = 0.5 Pick:A 1 /A 2 = ________ Find:P t2 /P t1 = __________ (Use Eq 9.17b) M 2 = _______ (Use MFP) L sm /H sm = ___________ (Use Fig 9.8) 2 3 Calc:A 2 /A 3 = ________ Find:P t3 /P t2 = __________ (Use Eq 9.18) M 3 = ___________ (Use MFP) L sm H sm Set by Compressor Blade Height Overall Pressure Ratio of Diffuser, P t3 /P t1 : _________ HDHD

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