SELF-IGNITION IN SUPERSONIC CONFINED FLOWS: HYDROGEN VERSUS HYDROCARBONS/HYDROGEN MIXTURES - CHEMISTRY/MIXING INTERPLAY

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1 SELF-IGNITION IN SUPERSONIC CONFINED FLOWS: HYDROGEN VERSUS HYDROCARBONS/HYDROGEN MIXTURES - CHEMISTRY/MIXING INTERPLAY Vladmr SABELNIKOV ONERA, DEFA/PRA, Palaseau, France wth collaboraton P. Magre, E. George GDR Turbulence (GDR CNRS 2865) Réunon de Poters octobre

2 Content I. ONERA s LAERTE test faclty for fundamental study of supersonc combuston. Model supersonc combustor (scramjet). Expermental data Operatng condtons Pressure dstrbuton: a) weak and b) abrupt self-gnton modes Instantaneous OH-PLIF mages Two types of self-gnton : a) weak self-gnton mode, b) abrupt self-gnton mode II. LES smulaton of weak self-gnton mode. Comparson wth experment III. Unsteady RANS smulaton of abrupt self-gnton mode. Comparson wth experment Conclusons 2

3 LAERTE test faclty (1/3) d=6 mm, nternal dameter Coaxal fuel njecton: adapted statc pressures njecton Constant secton durng the frst 370 mm of the duct, after dverges wth a half-angle of 1.15 Operatng condtons Ar H 2 26% C 2 H 4 / 74% H 2 (molar fractons) 50% C 2 H 4 / 50% H 2 (molar fractons) Mach number Statc Pressure (MPa) Total Temperature (K) Statc Temperature (K) Mass flow rate (g/s) (C 2 H 4 ) + 2.6(H 2 ) 16.2(C 2 H 4 )+ 1.2(H 2 ) velocty (m/s)

4 LAERTE test faclty (2/3) Optcal access through the test secton H 2 X/d=6 X/d=15 X/d=26 X/d=34 X/d=43 X = 0 Wndows n Slce-UV Pressure transducers 4

5 Expermental optcal faclty (OH-Ac PLIF) Fltre OH Laser Fltre ICCD 1 laser : 2 caméras 5

6 Results : jont OH and Acetone PLIF LIF OH et Ac. 1 λ N OH et Nac. Acetone + H 2 Acetone Acetone + H 2 Acetone + H 2 Sum λ=282 nm OH 6

7 Structure of supersonc mxng layer OH-PLIF 2 dfferent regons n the flow 6<x/d<14 26<x/d<34 close from the njector far from the njector pockets wth sze less 1 mm n ntermttent combuston. Pockets composed of fuel and ar comng from the external boundary layer of the njector. Temperature of ths ar: 1600K (close from stagnaton temperature de la temperature of 1850 K) «large» pockets wth a characterstc sze of 5 mm n combuston, composed of hot ar, fresh fuel and burnt gases. 7

8 Instantaneous OH-PLIF mages (pure H 2 jet) close to the njector 6<(x-x nj )/d<15 OH far from the njector 26<(x-x nj )/d<34 (The flow s from left to rght) Man characterstcs of the self-gnton process: self-gnton has a spotty character and appears n the form of random pockets self-gnton nvolves the ntrcate nterplay between the mxng and the chemstry each pocket has ts unque hstory, as a consequence mxng and self-gnton chemstry cannot be decoupled flamelet models are not adequate for LES modelng self-gnton of supersonc fuel jet n hot supersonc ar stream 8

9 LAERTE test faclty (3/3) Wall pressure measurements Pressure transducers 80 channels dstrbuted on the top and on the bottom of the combuston chamber Locatons: step of 10 mm on the frst 150 mm of the duct step of 15 mm between 150 mm < x < 370 mm step of 30 mm between 370 mm < x < 870 mm Pressure rse characterzes the amount of heat release evaluaton of auto-gnton by comparng reactng and nonreactng cases 9

10 Two types of autognton weak ("smooth") mode: progressve and moderate heat release wall pressure profle wth a low rse of pressure due to combuston "abrupt " mode: sudden and brutal heat release wall pressure profle wth a large rse of pressure due to combuston 10

11 Expermental data (1/2) Weak (smooth) mode of self-gnton. Pressure dstrbutons along the combustor Pure hydrogen fuel jet: mass rate 6.2 g/s, x 0 =33mm Self-gnton length 15 cm 26% C 2 H 4 / 74% H 2 mxture fuel jet: mass rate 12.8 (C 2 H 4 ) g/s+ 2.6 (H 2 ) g/s Self-gnton length 25 cm The flow s supersonc throughout the combustor 11

12 Expermental data (1/2) Pure hydrogen jet (1998), mass rate: 6.2 g/s Self-gnton lengths are: 15 cm for H 2, 73% methane/27% hydrogen jet (mass fractons) mxture (2004), mass rate: 7.4 g/s CH g/s H 2 33 cm for CH 4 - H 2 mxture 12

13 Expermental data (2/2) Abrupt mode of self-gnton. Pressure dstrbuton along the combustor 50% C 2 H 4 / 50% H 2 mxture fuel jet mass rate: 16.2 (C 2 H 4 ) g/s (H 2 ) g/s Self-gnton length s about 35 cm Abrupt mode arses f content C 2 H 4 exceeds 29% (molar fracton) 1D calculaton. Mach number dstrbuton for 50% C 2 H 4 / 50% H 2 mxture fuel jet thermal chockng takes place subsonc regon appears between 0.4 m < x-x o < 0.63 m 13

14 Synthess of expermental results 2 scenaros based on observatons Smooth mode at the stagnaton temperature 1850K and entrance Mach numberm=2, the selfgnton of pure hydrogen starts smoothly wth the ncrease of the ethylene concentraton n the ethylene/hydrogen mxture and for CH4/H2 mxture the delay length of self-gnton ncreases Abrupt mode at some crtcal value of ethylene concentraton self-gnton starts suddenly and strongly. Importance of mxng tme wth ncreasng of an ar/fuel premxng (.e. after fuel njecton), self-gnton can be brutal such as for premxed mxtures nteracton between chemstry an mxng (turbulence) controls the self-gnton regme 14

15 LES smulatons of weak self-gnton mode LES subgrd flow model Smagornsk model LES combuston model PaSR-Vuls model Chemcal mechansms: H 2 : Jachmowsk - 7 speces / 9 reactons CH 4 /H 2 : Davdenko 21 speces / 79 reactons C 2 H 4 /H 2 : Sngh - 9 speces /10 reactons 15

16 16 LES Combuston Model: PaSR Vuls model Mult-scale model based on the assumpton that reactons take place on the smallest fne structures (*) embedded n the surroundngs ( 0 ) Subgrd balance equatons ),, ( ),, ( ) ~~ ( ),, ( ) ~ ( * * * *, 1 * * 1 * * * T Y w w T Y w h Yh Y h T Y w Y Y f N m N m ρ ρ τ ρ ρ τ ρ θ & & & & = = = Σ = Σ = *), ( ) ) / (( ) ~ ~ ( ) ~ ( Y * T M P w Y Sc D Y Y j j k k t & + + = + µ ρ ρ v j j m S S ~ ~ 2 1 = τ + = k l l k k k x u x u x u S δ ~ 3 2 ~ ~ 2 1 ~

17 LES smulatons: pure H2 jet pure hydrogen reactng case (experment 1998): wall-pressure profles good estmaton of gnton delay and pressure level for pure hydrogen (same behavour for 2004 case) no nfluence of PaSR-VM: hydrogen s hghly reactve - no need to been well mxed to ar for burnng t sm =10 ms 17

18 LES smulatons: pure H2 jet nstantaneous snapshots Statc pressure OH radcal (red) and vortcty feld H 2 feld H 2 O feld Statc temperature 18

19 LES smulatons: pure H2 jet combuston η c and mxng η m effcences 19

20 LES smulatons: CH 4 /H 2 mxture jet Self-gnton lengths: 33 cm n the experment 33 cm wth EVM 25 cm wthout EVM LES wthout EVM overestmates the pressure rse due to heat release n the regon 25 cm <(x-x nj )/d< 50 cm 20

21 LES smulatons: CH 4 /H 2 mxture jet 21

22 LES smulatons: C 2 H 4 /H 2 mxture jet wall-pressure profles t sm =10 ms Pressure levels are n both cases underestmated EVM gves better self-gnton delay Smulaton tme mght be too short 1D analyss shows to possblty of thermal shockng (heat release greater than the crtcal value) LES s too expensve, RANS wll be used to study the thermal shockng regme 22

23 URANS smulatons of abrupt self-gnton mode: C 2 H 4 /H 2 mxture 50% C2H4 / 50% H2 mxture fuel jet mass rate: 16.2 (C2H4) g/s (H2) g/s 23

24 One-dmensonal analyss of the flow Thermal shockng nvestgaton (1/2) Advantage to be easy, cheap & gves good nformaton on crucal parameters of the flow. Takes nto account the skn frcton & heat losses of the expermental combuston chamber (based on experments) Q cr = 0.50 Crtcal value for heat release n the test tube pure hydrogen = 0.47 methane/hydrogen mxture = 0.43 ethylene/hydrogen =

25 One-dmensonal analyss of the flow Thermal shockng nvestgaton (2/2) Mach number dstrbuton for the dfferent fuels studed Mach number dstrbuton Heat release dstrbuton The ethylene/hydrogen mxture s able to be thermal shocked Thermal chokng could explan the dfferent modes of combuston RANS approach s chosen for ths study of thermal shockng regme 25

26 URANS smulaton results (1/4) Observatons: Self-gnton s premature that explaned by neglectng turbulence-chemstry nteracton Transent perod from ntal state lasts a long tme 35 ms (35 resdence tme) Maxmum pressure rse s n good agreement wth experments Pressure s essentally non-steady Pressure FFT Pressure waves moves upstream and downstream wth ampltude about 3 cm, and 4.2 bar ampltude Power pressure spectrum has the peak of 580 Hz (1.72 ms oscllaton perod) 26

27 URANS smulaton results (2/4) Instantaneous pressure ant statc temperature snapshots Unsteady behavour of self-gnton (heat release n the duct). Mach structures are observed Observatons: Unsteady behavour of self-gnton (heat release n the duct) Bfurcated shocks (Mach structure) Shock tran regon s formed n whch pressure buld-up develops The formaton of OH starts at (x-x o ) 21 cm 27

28 URANS smulaton results (3/4) Axal evoluton of velocty profles n transonc zone Transversal Mach number profles n the transonc zone, arrow ponts n the drecton of x rse Cross-area mean Mach number axal dstrbuton Observatons: Mean Mach number M F s less than one n the doman 37.5 cm < (x-x o ) < 49 cm 28

29 URANS smulaton results (4/4) Combuston η c and mxng η m effcences Maxmal total temperature along the combsutor Combuston and mxng effcences along the combustor Observatons: The combuston s mxng controlled Practcally overall heat release occurs on 5 cm length 29

30 Mxng/Combuston Effcences mxng chemstry methane/hydrogenm xture pure hydrogen ethylene/hydrogen mxture (LES) ethylene/hydrogen mxture (RANS) The more hydrocarbon we have, the more mxng s mportant for premxng followed by burnng 20 cm for H 2, 10 cm for CH 4 /H 2 and 6 cm for C 2 H 4 /H 2 30

31 Conclusons Self-gnton of H2, C2H4/H2 and CH4/H2 jets n a supersonc vtated confned flow s studed. Two modes are found - weak mode thermal chockng s absent - abrupt mode wth followng thermal chockng Weak mode was smulated wth LES Abrupt mode was smulated wth URANS - abrupt mode s essentally non-steady - the flow oscllatons are drven by nteracton between thermal chokng and upstream fuel-ar mxng Addtonal study s needed to establsh whch mechansm between two : - acoustc, through local subsonc zone - pure gas dynamc, by mpact of heat release fluctuatons on the thermal chockng poston s n the orgn of flow oscllatons 31