COMBUSTION SYSTEMS - EXAMPLE Cap. 9 AIAA AIRCRAFT ENGINE DESIGN

Transcription

1 CORSO DI LAUREA MAGISTRALE IN Ingegneria Aerospaziale PROPULSION AND COMBUSTION COMBUSTION SYSTEMS - EXAMPLE Cap. 9 AIAA AIRCRAFT ENGINE DESIGN LA DISPENSA E DISPONIBILE SU Prof. Ing. A. Ficarella [email protected] 1

2 EXAMPLE 2

3 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

4 4

5 TWO LAYOUTS Fuel nozzles from Eq

6 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

7 AR = A3.2/A3.1 = 3.49 < sweet spot 4 (fig ) eq 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 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 reduced by splitter plates 7

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9 AIR PARTITIONING From tab. 9.E1 and eq 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 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

10 Assuming εpz=0.7 (primary zone combustion efficiency) 10

11 The lack of cooling air stems from the fact that Tt4= 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 By solving eq Tg = 3849 R. Mass flow fraction available for cooling Cooling effectiveness

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13 DOME AND LINER eq optimal liner height for max secondary jet penetration 13

14 TOTAL PRESSURE LOSS eq to see if the allocated total pressure loss is sufficient for adequate stirring by the primary zone swirler 14

15 PRIMARY ZONE Reference velocity Ur from tab. 9.E1 Ur= ft/s Velocity for all jets from eq Assuming a hub radius to accommodate the atomizer, sharpedge hole, swirl blade angle eq

16 Swirl number satisfactory Axial length of the primary zone eq

17 KINETX The design of the primary zone validated by KINETX WSR model KINETX gives a residence time ts= s > blowout residence time tbo= s Predicted εpz=78.32% TPZ=3822 R compare favorably with assumed values 17

18 SECONDARY ZONE Calculate the various dynamic pressure Max penetration jet centerline eq

19 Required single jet vena contracta area eq Total number of secondary holes eq Secondary jets angle fig eq

20 Diameter of each secondary air hole eq or Length of secondary zone eq

21 DILUTION ZONE Same procedure except Ymax=HL/3 rather than HL/4 From eq Annulus airflow reduced, so that the dynamic pressure reduced to: Liner flow has been increased by secondary air: 21

22 Max penetration eq Vena contracta Entry angle Diameter of diluition hole eq or number eq

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24 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

25 CORSO DI LAUREA MAGISTRALE IN Ingegneria Aerospaziale PROPULSION AND COMBUSTION COMBUSTION SYSTEMS EXAMPLE Cap. 9 AIAA AIRCRAFT ENGINE DESIGN LA DISPENSA E DISPONIBILE SU Prof. Ing. A. Ficarella [email protected] 1

26 AFTERBURNER 2

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28 MIXER eq Lm = 12 ft!!! - low velocity ratio Diffuser operates within the mild transitory stall regime which in turn enhances the mixing Combined mixer + diffuser 4

29 DIFFUSER Annular flat wall + dump diffuser eq

30 Total pressure loss eq Total pressure station 6.1 6

31 With the flow properties and geometry for 6.1 determined, design the hot section of the afterburner eq pressure loss (W/H = 0.5) Pt7 design goal in tab. 9.E1 7

32 Static pressure eq Mach number station 6.1 8

33 eq reducing W/H from 0.5 to 0.4 the total pressure loss could be significantly reduced eq Total pressure 7 Above the target value for wet operation 9

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35 FLAMEHOLDERS How large the vee-gutters must be How many rows eq 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

36 At station 16 Combined streams 6A and 6.1 eq and 9.60 mixing layer entrains equal amounts of cold gases and recirculation product RR=0.5 TBO from KINETX eq for Hmin then Dmin for operating near blowout D = 10*Dmin 12

37 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

38 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

39 Because both co-flowing streams are subsonic, the static pressure is always matched in both streams eq to find the areas of the two streams at station m Applying eq to either stream and assuming a diffusion efficiency ηd=0.9 mass continuity: 15

40 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 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 for lateral dimension Dcore Shear layer residence time ts=hcore/umcore >> tbocore; ts=hfan/umfan>>tbofan 16

41 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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