CORSO DI LAUREA MAGISTRALE IN Ingegneria Aerospaziale PROPULSION AND COMBUSTION COMBUSTION SYSTEMS - EXAMPLE Cap. 9 AIAA AIRCRAFT ENGINE DESIGN www.amazon.com LA DISPENSA E DISPONIBILE SU www.ingindustriale.unisalento.it Prof. Ing. A. Ficarella antonio.ficarella@unile.it 1
EXAMPLE 2
The max dynamic pressure conditions (sea level, flight Mach number) establishes both the max gas temp. and max throughput condition. If design for relight will not be considered, single design point for both the man burner and the afterburner. From compressor and turbine design, mean radii at 3.1 and 4 have been established 3
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TWO LAYOUTS Fuel nozzles from Eq. 9.105 5
Because it has fewer fuel nozzles, design B will be selected However, because design A has smaller radial height H, mixing will be more effective 6
AR = A3.2/A3.1 = 3.49 < sweet spot 4 (fig. 9.21 9.22) eq. 9.66 and 9.67 give the total pressure loss coefficient and total pressure ratio, for any assumed diffusion efficiency Diffusion efficiency = 0.83 will satisfy the allocated pressure loss Pt3.1-Pt3.2 If a higher diffusion efficiency, the savings in total pressure can be allocated to the main burner to improve mixing Assuming ηdm = 0.9, eq. 9.103 gives the optimal area ratio for transition from flat-wall to dump diffuser however, since AR<4 simple flat-wall diffuser will be chosen L from eq. 9.61 reduced by splitter plates 7
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AIR PARTITIONING From tab. 9.E1 and eq. 9.90 the equivalence ratio at station 4 is calculated Liner material Tm=2110 R Cooling gas temperature Tc=Tt3.1=1660 R For any assumed gas temperature Tg using eq. 9.88-9.101 the air mass fraction μ for primary and secondary zones can be calculated, as well as the cooling mass fraction μc For example Tg=3200 R, primary+secondary air flow=96%, not enough air for cooling 9
Assuming εpz=0.7 (primary zone combustion efficiency) 10
The lack of cooling air stems from the fact that Tt4=3138.4 R is in the range of gas temperature that minimize CO and NOx (fig. 9.24). If transpiration/effusion cooling employed, the required air fraction is modest that Tg = 3500 R. For AAF engine, very wide range of flame stability primary zone equivalence ratio 0.8 well above lean blowout limit 0.4 0.5. By solving eq. 9.92 Tg = 3849 R. Mass flow fraction available for cooling 0.324 Cooling effectiveness 0.794 11
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DOME AND LINER eq. 9.109 optimal liner height for max secondary jet penetration 13
TOTAL PRESSURE LOSS eq. 9.79 to see if the allocated total pressure loss is sufficient for adequate stirring by the primary zone swirler 14
PRIMARY ZONE Reference velocity Ur from tab. 9.E1 Ur=163.90 ft/s Velocity for all jets from eq. 9.110 Assuming a hub radius to accommodate the atomizer, sharpedge hole, swirl blade angle eq. 9.111 15
Swirl number satisfactory Axial length of the primary zone eq. 9.50 16
KINETX The design of the primary zone validated by KINETX WSR model KINETX gives a residence time ts=3.376+10-5 s > blowout residence time tbo=4.403+10-6 s Predicted εpz=78.32% TPZ=3822 R compare favorably with assumed values 17
SECONDARY ZONE Calculate the various dynamic pressure Max penetration jet centerline eq. 0.107 18
Required single jet vena contracta area eq. 9.112 Total number of secondary holes eq. 9.113 Secondary jets angle fig. 9.15 eq. 9.115 19
Diameter of each secondary air hole eq. 9.43 or 9.114 Length of secondary zone eq. 9.116 20
DILUTION ZONE Same procedure except Ymax=HL/3 rather than HL/4 From eq. 9.117 Annulus airflow reduced, so that the dynamic pressure reduced to: Liner flow has been increased by secondary air: 21
Max penetration eq. 9.107 Vena contracta Entry angle Diameter of diluition hole eq. 9.43 or 9.114 number eq. 9.113 22
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AFTERBURNER DESIGN Length of both the mixer and afterburner minimized by making the outer radii as large as possible Outer diameter equal to fan inlet 24
CORSO DI LAUREA MAGISTRALE IN Ingegneria Aerospaziale PROPULSION AND COMBUSTION COMBUSTION SYSTEMS EXAMPLE Cap. 9 AIAA AIRCRAFT ENGINE DESIGN www.amazon.com LA DISPENSA E DISPONIBILE SU www.ingindustriale.unisalento.it Prof. Ing. A. Ficarella antonio.ficarella@unile.it 1
AFTERBURNER 2
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MIXER eq. 9.52 9.58 Lm = 12 ft!!! - low velocity ratio Diffuser operates within the mild transitory stall regime which in turn enhances the mixing Combined mixer + diffuser 4
DIFFUSER Annular flat wall + dump diffuser eq. 9.122-9.128 5
Total pressure loss eq. 9.66 Total pressure station 6.1 6
With the flow properties and geometry for 6.1 determined, design the hot section of the afterburner eq. 9.83 pressure loss 6.1 7 (W/H = 0.5) Pt7 design goal in tab. 9.E1 7
Static pressure eq. 9.62 9.64 Mach number station 6.1 8
eq. 9.137 reducing W/H from 0.5 to 0.4 the total pressure loss could be significantly reduced eq. 9.83 Total pressure 7 Above the target value 73.016 for wet operation 9
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FLAMEHOLDERS How large the vee-gutters must be How many rows eq. 9.134 tbo residence time at blow out in the mixing layer at the edge of the recirculating bubble U6.1 from the design of the afterburner diffuser TBO KINETX Find the composition of combustion product entering the turbine EQL (of KINETX) 11
At station 16 Combined streams 6A and 6.1 eq. 9.59 and 9.60 mixing layer entrains equal amounts of cold gases and recirculation product RR=0.5 TBO from KINETX eq. 9.134 for Hmin then Dmin for operating near blowout D = 10*Dmin 12
The max number of vee-gutter ring is determined by dividing the height of the total afterburner area H6.1 by the min vee-gutter channel height Hmin The effect of more rings is diminished as the n. of rings increases complexity of adding as many spray bars 13
ALTERNATE NO-MIX DESIGN Difficulty of mixing the core gas stream with the bypass fan air stream CO-FLOW Vee-gutter flameholders located at the entrance of the dump diffuser 14
Because both co-flowing streams are subsonic, the static pressure is always matched in both streams eq. 9.64 to find the areas of the two streams at station m Applying eq. 9.64 to either stream and assuming a diffusion efficiency ηd=0.9 mass continuity: 15
KINETX fixed afterburner exit temperature Tt=3600 R, the blowout residence times in the two stream tbocore and tbofan and the required fuel mass flow rate mfcore and mffan W/H=0.5, eq. 9.135 gives min channel widths for flameholding for the two streams The max n. of vee-gutter rings in each channel is determined with a safety margin of 10 eq. 9.137 for lateral dimension Dcore Shear layer residence time ts=hcore/umcore >> tbocore; ts=hfan/umfan>>tbofan 16
Some PROBLEMS Required fuel mass flows 5% greater than the fuel flow required for the mixed-stream design The height of the core stream is not quite large enough to accommodate two flameholders with good stability Radial spokes to give a residence time 10 times the blowout value to maintain the desired blockage B in the fan stream 17
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