Flame Stretch, Edge Flames, and Flame Stabilization

Transcription

1 Flame Stretch, Edge Flames, and Flame Stabilization Natarajan et al. Combstion and Flame 2007 Flame Anchoring -1

2 Corse Otline A) Introdction and Otlook B) Flame Aerodynamics and Flashback C) Flame Stretch, Edge Flames, and Flame Stabilization Concepts D) Distrbance Propagation and Generation in Reacting Flows E) Flame Response to Harmonic Excitation Introdctory Concepts Flame Stretch Edge Flames Flame Stabilization in Shear Layers Flame Stabilization by Stagnation Points Flame Anchoring -2

3 Corse Otline A) Introdction and Otlook B) Flame Aerodynamics and Flashback C) Flame Stretch, Edge Flames, and Flame Stabilization Concepts D) Distrbance Propagation and Generation in Reacting Flows E) Flame Response to Harmonic Excitation Introdctory Concepts Flame Stretch Edge Flames Flame Stabilization in Shear Layers Flame Stabilization by Stagnation Points Flame Anchoring -3

4 Flame Stabilization and Blowoff Flame stabilization reqires: A point where the local flame speed and flow velocity match: s d = Typically fond in regions with: Low flow velocity» Aerodynamically decelerated regions (VBB) High shear» Locations of flow separation (ISL & OSL) Figres: Natarajan et al. Combstion and Flame 2007 Petersson et al. Applied Optics 2007 Flame Anchoring -4

5 Premixed Flame Stabilization: Basic Effects Flame stabilization: Balance combstion wave propagation with flow velocity Brning velocity, edge speed, atoignition front? Sggests that stable flames are rare However, flames have self-stabilizing mechanisms Shear layer stabilized: Upstream flame propagation increases wall qenching Aerodynamic stabilization velocity profiles ( cm s ) Flow δ q c s d b a x Lewis & Von Elbe 1987 Flame Anchoring -5

6 Flame Anchoring Locations Complex flows can provide mltiple anchor locations II: VBB/ISR I: VBB IV: OSR III: ISR Flame Anchoring -6

7 Flame Stabilization and Blowoff Stabilization locations determine location/spatial distribtion of flame Flame Shape Flame Length Combstor operability, drability, and emissions directly tied to these fndamental characteristics affecting: Heat loadings to combstor hardware Combstion instability bondaries Blowoff limits Flame Anchoring -7

8 Review of the Idealized Premixed Flame Simplest possible premixed flame configration 1-dimensional, planar Adiabatic T0 b = T ad Adiabatic Flame Temperatre ρ = ρ S0 L = ρ b b Mass Brning Flx What are the controlling parameters? Thermal and mass diffsivities Reaction rates Temperatre of reactants Pressre Exothermicity of fel/oxidizer S0 L Fndamental property of fel/oxidizer mixtre Flame Anchoring -8

9 What Happens if a Flame isn t Flat? Flame Anchoring -9

10 What Happens if Flow Field isn t 1-D? Common theme to 2 problems: misaligned convective and diffsive flxes Flame Anchoring -10

11 Corse Otline A) Introdction and Otlook B) Flame Aerodynamics and Flashback C) Flame Stretch, Edge Flames, and Flame Stabilization Concepts D) Distrbance Propagation and Generation in Reacting Flows E) Flame Response to Harmonic Excitation Introdctory Concepts Flame Stretch Edge Flames Flame Stabilization in Shear Layers Flame Stabilization by Stagnation Points Flame Anchoring -11

12 Overview of Flame Stretch Consider a stationary flame; focs on a C.V. intersection of a streamtbe, and the flame. Steady state energy balance (no viscos effects, no body forces) ρ h T = q Enthalpy Convection EnthalpyDiffsion Constittive relation of enthalpy flx (Fickian diffsion, no radiative heat transfer, no Soret or DFor effects) N q = kt T ρ hid i Yi i= 1 Heat Flx Mass Flx Flame Anchoring -12

13 Overview of Flame Stretch Flame stretch effects: is there a net enthalpy loss/gain or change in composition inside the C.V. becase of diffsion flxes throgh its lateral srface? Two mechanisms: Lewis Nmber effects Le =α /D Diffsion of mass and of heat are nbalanced. Differential diffsion effects D Fel /D ox Lighter species diffse faster than heavier species: eqivalence ratio or dilent/reactant ratio inside the C.V. can change. Negative stretch Positive stretch Flame Anchoring -13

14 Lewis nmber effects If Le = 1 then α =D : no net enthalpy loss throgh the lateral srface Energy Eq.: ρ ht = ht ( ρd ) If Le > 1 then α >D : Heat flx > Mass flx o Positive stretch: net enthalpy flx ot of the C.V. o Negative stretch: net enthalpy flx into the C.V. Negative stretch If Le < 1 then α <D : Heat flx < Mass flx o Positive stretch: net enthalpy flx into the C.V. o Negative stretch: net enthalpy flx ot of the C.V. Positive stretch Flame Anchoring -14

15 Differential diffsion effects Consider the temperatre at the tip of a Bnsen Flame T tip : CH 4 /Air: CH 4 is lighter than O 2, ths diffses faster (D Fel >D Ox ) and its concentration inside the C.V. decreases. o If overall eqivalence ratio is lean, then locally in the C.V. φ is made leaner: T tip is lower then T ad calclated at the overall φ. o If overall eqivalence ratio is rich, then locally in the C.V. φ plled toward stoichiometric: T tip is then higher then T ad calclated at the overall φ. C 3 H 8 /Air: C 3 H 8 is heavier than O 2, ths diffses more slowly (D Fel <D Ox ) and its concentration inside the C.V. increases. o The dependence of T tip on the overall φ is opposite to that of CH 4 /Air. Flame Anchoring -15

16 Bnsen Tip Flame Temperatre Data From C.K. Law, Combstion Physics Flame Anchoring -16

17 Example: Tips of Bnsen Flames Propane(C 3 H 8 ) Methane(CH 4 ) d=10mm φ = Ref: Mizomoto, Asaka, Ikai and Law, Proc. Combst. Inst. 20, 1933 (1984) Slide cortesy of J. Seitzman Flame Anchoring -17

18 Stretched Flames: Non-nity Lewis Nmbers Φ=0.27 Φ=0.27 Φ=0.37 T =298K P=1atm Le i >1 Le i 1 Le i <<1 Bell et al. Proceedings of the Combstion Institte (2007) Flame Anchoring -18

19 Mathematical School of Aerospace Engineering expressions of stretch κ κ 1 da A dt Williams (1975) Flame stretch rate is defined as the normalized differential change with respect to time of an infinitesimal flame srface area element Flame stretch rate qantifies the degree of stretch imposed on a differential flame srface element Lagrangian qantity Units of flame stretch are 1/s i.e. an inverse time scale Flame Anchoring -19

20 Mathematical expressions of stretch κ Expression for κ in terms of flow velocity,, and flame sheet velocity, v F : t1 t2 κ = + + ( vf n)( n) = t t + ( vf n)( n) t t 1 2 Hydrodynamic stretch κ a : variation of tangential flow velocity in the tangential direction (t 1, t 2 ) or, eqivalently (by continity), variation of normal flow velocity in the direction normal (n) to the flame. κ a κ b κ = Hydrodynamic stretch 1 da A dt Unsteady Crvatre stretch κ b : non-stationary flames o Positive κ: divergent tangential velocities or expanding flame flame area increases o Negative κ: convergent tangential velocity or contracting flame flame area decreases Unsteady crvatre stretch Flame Anchoring -20

21 Mathematical expressions of stretch κ Stationary flames vf = 0 κ = t t = n n since = n n ( ) ( ) Hydrodynamic stretch can be interpreted as a variation of the angle between flow velocity and flame normal or, eqivalently, as a variation in tangential flow magnitde along the flame srface. Alternative flame stretch expression t κ = nn : + s ( n) = n S n+ s ( n) 1 κ S κ crv T S = ( + ) Flow Strain, s n = ( vf ) 2 o κ s : stretch de to flow non niformities o κ crv : stretch of a crved flame in a niform approach flow κ can be non zero also when the flow strain is zero Flame Anchoring -21

22 Unsteady Effects Motion of Crved Flames Crvatre is present bt flow velocities align with flame srface normal Stationary spherical flame wold be stretchless t1 t2 κ = + + ( vf n)( n) = t t + ( vf n)( n) t t 1 2 κ a κ b Combstion Physics by C.K. Law (Cambridge University Press, 2006) Flame Anchoring -22

23 Weak stretch effects Asymptotic analysis shows that in the linear limit of weak stretch the effect of varios type of stretch (κ, κ a, κ b, κ S, κ crv ) on flame characteristics (s, δ F, ) is the same. For the flame speed measred in a reference frame attached to the nbrned gases, s, we can write: M Definition of Markstein length is not niqe bt depends on the isosrface sed to define it; e.g. for the flame speed measred in a reference frame attached to the brned gasses s b we have: Dimensionless qantities Markstein nmber Ma Karlovitz nmber Ka s,0 s = s ( κ) s κ= 0 + κ= 0κ = s δmκ κ δ Markstein length s s δ κ δ δ b b,0 b b M M M 0 M δfκ s 0,0,0 F s s δ Ma = Ka = = 1 Ma Ka δ Flame Anchoring -23

24 Weak stretch effects The Markstein nmber contains all the stretch effects described previosly Ma= Ma(Le, D Fel /D Ox, φ, ) For lean mixtres of fels lighter than air (e.g. H 2, CH 4 ) and rich mixtres of fels heavier than air (e.g. C 3 H 8 ), Ma < 0 Conversely, for lean mixtres of heavier than air fels and rich mixtres of lighter than air fels, Ma > 0 Ma vales are sensitive to the position at which flow and flame speeds are measred For a specific mixtre, measred Ma nmbers in the literatre can vary widely among different investigators. Flame Anchoring -24

25 Application Stoichiometry Effects C 3 H 8 /Air p = 1 atm, T = 300 K Tseng et al., Comb. Flame, 95(2), 1993 Flame Anchoring -25

26 Application Fel Effects n-alkanes/air p =1atm, T =300K n-c 8 H 18 /air Ref: Tseng et al., Comb. Flame, 95(2), 1993 Ref: Halter et al. Comb and Flame, 157 (2010) Flame Anchoring -26

27 Application Pressre Effects atm 10 atm 15 atm 5 atm κ (1/s) atm 1 atm 10 atm 15 atm Ka=δ κ s 0,0 F H 2 /CO 30/70 (by vol.) in air, T = 300 K Conterflow twin flame φ adjsted at different p to maintain s,0 = 34 cm/s = const Flame Anchoring -27

28 Strong Stretch Effects PREMIXED FLAME CONCEPTS FLAME STRETCH AND FLAME EXTINCTION Flame Anchoring -28

29 Displacement Speed s d and Consmption Speed s c Displacement speed s d : speed at which the flame is moving along its normal relative to the flow The vale of s d depends on the reference srface: Low κ : the approach flow varies weakly pstream of the flame and the iso-srface choice is not to problematic High κ : velocity gradients occrs on a scale comparable to the flame thickness; s d definition becomes ambigos Displacement speed can also become negative when diffsive flxes are strong enogh to contrast the blk convection in the opposite direction Flame Anchoring -29

30 Displacement Speed s d and Consmption Speed s c Consmption speed s c : spatial integral of chemical rates Can be obtained integrating along a streamline the heat prodction rate and normalizing by the total change in sensible enthalpy across the flame Alternatively, the integrated qantity can be a reactant species consmption rate normalized by the total change in reactant species mass density Example: 1D steady flame sensible enthalpy balance (no viscos and body forces) N d( ρh x sens ) x d 0 b b dx = qdx ( ρ hf, iyi x, D, i ) dx ρ sc ρ sc qdx hsens, hsens, dx + dx dx = = i= 1 b b ρ hsens, ρ hsens, = ρ s b b c h sens h sens ρ = ρ (,, ) dq Similarly from species eq. sc = w idx ρ Yi Yi = 0 (,, ) ( ) Flame Anchoring -30

31 Displacement School of Aerospace Engineering Speed s d and Consmption Speed s c s c0 =s d 0 bt their vales differ at non-zero κ vales H 2 /CO 30/70 (by vol.) in air φ = 0.75, T = 300 K, p = 5 atm s (cm/s) s d s c ( ) H 2 s c (enthalpy ) κ (1/s) Flame Anchoring -31

32 Extinction stretch rate κ ext κ ext : maximm stretch that a flame can sstain before extingishing H 2 /CO 30/70 (by vol.) in air φ = 0.75, T = 300K atm 1 atm 10 atm 15 atm Ka=δ κ s 0,0 F Flame Anchoring -32

33 Example: Pressre Effects Most of the available data are for steady symmetric opposed flow flames : o κ ext depends on flame chemical time τ chem =δ F /s,0 (eg. p effects at fixed s,0 ) H 2 /CO 30/70 (by vol.) in air T = 300 K φ adjsted at different p to maintain s,0 = 34 cm/s = const Flame Anchoring -33

34 Example: Fel and Stoichiometry Effects Ref: Jackson et al., Comb. Fl., 1994, 25(1) Flame Anchoring -34

35 Example: Preheat Effects At high diltion/preheating levels, the flame does not "extingish increases in reactant temperatre are eqivalent to a redction in dimensionless activation energy Example: calclation of CH 4 /air flame stagnating against hot prodcts, whose temperatre is indicated on the plot (cm/s) [cm/s] s c K 1400 K 1450 K (cm/s) [cm/s] s d K 1400 K 1450 K Strain Strain Rate rate [1/s] (1/s) Strain rate Rate (1/s) [1/s] Flame Anchoring -35

36 Caveats on Stretch Sensitivity of Highly Stretched flames Stretch sensitivities and κ ext vales are not intrinsic to mixtre bt also depend on manner in which stretch is applied Example: it depends also on flame geometry and configration: o Velocity profile across the flame thickness (eg. κ ext for opposed flow flames depends on jets distance) o Type of stretch (κ, κ a, κ b, κ S, κ crv ) o Length and time scale of flame/stretch interaction H 2 /air, φ = 0.37, T = 298 K, p = 1 atm Flame Anchoring -36

37 Corse Otline A) Introdction and Otlook B) Flame Aerodynamics and Flashback C) Flame Stretch, Edge Flames, and Flame Stabilization Concepts D) Distrbance Propagation and Generation in Reacting Flows E) Flame Response to Harmonic Excitation Introdctory Concepts Flame Stretch Edge Flames Flame Stabilization in Shear Layers Flame Stabilization by Stagnation Points Flame Anchoring -37

38 Overview Real flames have edges strctre is different than continos flames previosly considered: Non-premixed flames do not propagate, bt their edges do; Premixed flame edge velocity is different from the laminar brning rate (e.g., can be negative) Applications Stabilization of non-premixed (a) and premixed flames (b) Propagation of an ignition front (c) Flame propagation after local extinction (d) Edge flame can be advancing, retreating or stationary Attention has to be paid to the observer reference frame. Flame Anchoring -38

39 Edge flame examples Piloted Bnsen flame A retreating flame edge that is stationary in lab coordinate High φ Low φ v flow v F Rajaram et al., Comb. Sci.Tech,(175) 2003 Flame Anchoring -39

40 Edge flame examples Premixed blff body stabilized flame near blow-off Chadhry et al., Comb. Flame (158), 2011 v flow v F Flame Anchoring -40

41 Cabra brner Edge flame examples Dnn et. al, Comb. Flame,(151) 2007 Vitiated coflow 1500K, 0.8m/s 200 Central jet velocity, (m/s) Downstream extinction observed Continos flame Natral gas/air Eqivalence Ratio,φ 0.8 Flame Anchoring -41

42 Edge Flame Concepts Illstrative Model Problem Flame Anchoring -42

43 Bckmaster s Edge flame model problem Generalize the one dimensional nonpremixed chambered flame (z + ). z x Oxidizer T a Y Ox, a, Flame edge Nonpremixed flame T b Y Fel b Fel 2 2 T T T T ρcp kt = k 2 T ρc 2 p x Q w Fel t z x x ρy Transverseflxes = B MW Fel Ea RT YOxe Fel z = 0 v F L Flame Anchoring -43

44 Bckmaster s Edge flame model problem Generalize the one dimensional nonpremixed chambered flame (z + ). z x Oxidizer T a Y Ox, a, Flame edge Nonpremixed flame T b Y Fel b Fel 2 2 T T T T ρcp kt = k 2 T ρc 2 p x Q w Fel t z x x ρy approximate as k T ( T T ) L b 2 = B MW Fel Ea RT YOxe Fel -Approximate transverse flx terms by convective loss-like term -L characterizes the scale of gradients normal to the flame, sch as de to strain. z = 0 v F L Flame Anchoring -44

45 Edge flame model problem Dimensionless eqation (Le=1) E z T Y Q τ = = = = = = R T L T ct τ a Fel, b E, z, T, Q, Da b b p b chem v F 2 dt d T = F( T, Da) 2 dz dz ρcl 2 flow p T 1B k z x Oxidizer T a Y Ox, a, Flame edge z = 0 v F L Nonpremixed flame T b Y Fel b Fel 2 (1 T + Q ) E where F( T, Da) = 1 T + YFel, bda exp and v F = Q T v F ρcl k T p Flame Anchoring -45

46 Soltion Limit for Edge-less Flame Steady state soltion with no z- direction variation: F( T, Da ) = 0 the problem becomes the same as the steady well stirred reactor; recover the same S-crve behavior. 2 (1 T + Q ) E F( T, Da) = 1 T + YFel, bda exp = 0 Q T E Da = 534 Da = 12, 070 I = 14, Q = 3 II We will focs on Da I <Da<Da II range, where propagating flame edges can occr Three possible behaviors: propagates (v F >0) retreats (v F <0) stays stationary (v F =0) Flame Anchoring -46

47 Edge flame model problem Edge flame velocity Thigh 2 dt v F = F( T, Da) dt dz dz T low E = 14, Q = 3 Flame Anchoring -47

48 Edge flame model problem Edge flame velocity T high 2 dt v F = F( T, Da) dt dz dz T low v F sign depends on the nmerator: v F >0 for Da>Da V and v F <0 for Da<Da V E = 14, Q = 3 v = F 0 Da V Physical meaning: Bi-stable steady state region frther divided into 2 regions if an edge exists for Da>Da V the flame edge acts as an ignition sorce; if the flame locally develops an hole this will close; for Da<Da V the nbrned gasses qench the flame; if the flame develops an hole this will spread ntil the whole flame is extingished. Flame Anchoring -48

49 Edge Flame Concepts Edge Strctre and Velocity Flame Anchoring -49

50 Edge flames strctre Non-premixed flames: Advancing (ignition front, v F >0) have often a triple flame strctre; Retreating (extinction front, v F <0) generally consist of a single edge. v F >0 v F ~ 0 v F < 0 Compted Reaction rate contors Dao et al., Proc. Comb. Inst.,(29) 2002 Premixed flames: generally have a single edge bt can have significant hook-like strctres Image conterflow brner Li et al., Comb. Sci. Tech.,(144) 1999 Compted Reaction rate contors Verdarajan et al., Comb. Flame,(114) 1998 Also stretch and heat losses inflence the flame edge strctre Low Stretch High Stretch False color images Cha et al., Comb. Flame,(146) 2006 Flame Anchoring -50

51 Flame edge velocity Gas Expansion Effects Velocity of edge flames v F depends on density ratio σ ρ (=ρ b /ρ ) across the flame, Damkohler nmber Da, heat losses, Le, D Fel /D Ox and Z st. Dependence on σ ρ : for a convex flame the deceleration of the approach flow in front of the flame monotonically increases with σ ρ (to be discssed later) Retsch et al. s nonpremixed flame scaling for Da v F Da s σ ρ Flame Anchoring -51

52 Flame edge velocity Heat Loss Effects Heat losses may be important process in edge flames stabilized near metal srfaces Heat losses can case v F <0 at high Da Example: Non-Premixed conterflow flame Cha, Ronney, Comb. Fl., 2006, 146 (1-2) Flame Anchoring -52

53 Conditions at the flame edge Consider a premixed flame sbject to a spatially varying stretch rate κ at t=0. Hole will form at points where κ >κ ext edges will retreat to points where κ=κ(v F =0)=κ edge κ κ ext κ edge time Spatial coordinate Flame Anchoring -53

54 Conditions at the flame edge From edge flame model problem κ κ e R T (1 T / T ) E 2 edge ad ext 0 ad a For T 0 /T ad =0.2 and E a /R T ad = <κ edge /κ ext <1 κ κ ext κ edge time Spatial coordinate Conclsion: once flame hole opens, it grows larger than wold be expected based on continos flame concepts Flame Anchoring -54

55 Conditions at the flame edge Experiments show additional physics: Tangential flows of hot gases (e.g., in a lab stationary, retreating edge flame) can increase v F and case κ edge >κ ext CH 4 Air, φ = eqilibrim prodcts ( T = 450K p = 1 atm) When a hole forms in a premixed flames, reactants and prodcts can mix: Mass brning rate increases becase of presence of radicals and increased initial temperatre. Peak heat release rate changes little across different diltion levels; The flame looses its S-crve character. Flame Anchoring -55

56 Corse Otline A) Introdction and Otlook B) Flame Aerodynamics and Flashback C) Flame Stretch, Edge Flames, and Flame Stabilization Concepts D) Distrbance Propagation and Generation in Reacting Flows E) Flame Response to Harmonic Excitation Introdctory Concepts Flame Stretch Edge Flames Flame Stabilization in Shear Layers Flame Stabilization by Stagnation Points Flame Anchoring -56

57 Stretch Effects on Shear Layer Flames Flame will extingish when flame stretch rate exceeds κ ext As expected, higher flow velocities reslt in flame extinction occrring at higher vales of κ ext Flame Anchoring -57

58 Shear Layer Stabilized Flames In high speed flows, althogh locally low velocities exist within the shear layer, flame extinction typically leads to liftoff/blowoff Limited by the amont of flame stretch which they can withstand before extinction e.g, 50 m/s jet with 1 mm shear layer thickness shear~d z /dx ~ s -1 Mch greater than typical extinction strain rates, κ ext How is flame stretch related to flow strain in a shear layer? Flame Anchoring -58

59 Sorces of Flame Stretch 1. Flame crvatre 2. Unsteadiness in flame and flow 3. Hydrodynamic strain: κ ( δ nn ) = s, strain ij i j x i j Q. Zhang et al. J. Eng. for Gas Trbines & Power 2010 For reference, flid strain rate given by tensor: S ij 1 i = + 2 xj x i j Flame Anchoring -59

60 Flame Stretch De to Flid Strain General expression can be redced by assming 2-D flow and incompressibility (pstream of flame): 2 2 z x κ s = ( nx nz ) nn x z + z z x z Normal Strain Contribtion Shear Strain Contribtion Flame Anchoring -60

61 Flame Stretch De to Flid Strain Shear Strain Contribtion κ x = nn + z s, shear x z x z Flame strain occrs de to variations in tangential velocity Leads to positive stretch Flame Anchoring -61

62 Flame Stretch De to Flid Strain Normal Strain Contribtion κ ( 2 2 n n ) = s, normal x z z Jet flows typically decelerate prodcing normal strain Leads to negative stretch z Flame Anchoring -62

63 Flame Stretch De to Flid Strain For high speed flows: κ z θ 1 nx Also assme: x z << z x 2 3 = 1 + O( θ ) nz = θ + O( θ ) x = nn + z s, shear x z κ s d s, shear & θ x x z z n z Expressions for shear and normal stretch can be simplified as follows: κ n x n θ 2 2 ( n n ) = s, normal x z κ s, normal z z z z Flame Anchoring -63

64 Stretch from Shear Strain κ s, shear θ x z If θ s d z then κ s, shear s d z x z κ ~ s, shear sd δ sh Stretch rate de to shear ~indep. of? increasing shear bt θ bt can inflence shear layer: δ sh -1/2 Shearing flow velocity profile Flame Anchoring -64

65 Stretch from Normal Strain κ s, normal ~ z z ~ L z char Obtain traditional flow time scaling approach Decelerating flow velocity profile Flame Anchoring -65

66 Total Stretch from Strain Total stretch de to strain κ s z z + θ x z Shearing flow velocity profile Opposite signs if decelerating flow which term dominates - θ small? x z even if << x z vs. θ?? z x z x Decelerating flow velocity profile Flame Anchoring -66

67 Flame Stretch Distribtion in Swirl Flame Dimensionless vale of 1= 4,125 1/s Shows dominance of deceleration term in first 10 mm Shear term dominates farther downstream Normalized Strain Rate (1/s) Axial Location (mm) 22 n-n * / r r x Zhang, Q., Shanboge, S., Shreekrishna,O Connor, J., Liewen, T., Strain Characteristics Near the Flame Attachment Point in a Swirling Flow, Combstion Science and Technology, Vol. 83, 2011, x 2 z 2 n-n z * v/ z z z n-n x r *n nz z * / z x z n-n r *n z * v/ r x nz z x κ κ s s x Flame Anchoring -67

68 Piloting or Flow Recirclation Effects Flame stabilization can be enhanced throgh: Pilot flames Recirclation zones Transport hot prodcts to the attachment point of a flame Flame Anchoring -68

69 Diltion/Liftoff Effects At high diltion/preheating levels, the flame does not "extingish" Increases in reactant temperatre are eqivalent to a redction in dimensionless activation energy Example: calclation of CH 4 /air flame stagnating against hot prodcts with indicated temperatre (cm/s) [cm/s] s c K 1400 K 1450 K (cm/s) [cm/s] s d K 1400 K 1450 K Strain Strain Rate rate [1/s] (1/s) Strain rate Rate (1/s) [1/s] Flame Anchoring -69

70 Blowoff of Blff Body Stabilized Flames Stages of blowoff Stage 1: Flame is continos bt marked by local extinction events Stage 2: Changes in wake dynamics as large scale strctres become visible Stage 3: Blowoff of flame Da Approach: Scaling captres onset of flame extinction events Ability to captre blowoff dependent on link between extinction events and blowoff physics Nair J. Prop. Power 2007 Images cortesy of D. R. Noble Flame Anchoring -70

71 Corse Otline A) Introdction and Otlook B) Flame Aerodynamics and Flashback C) Flame Stretch, Edge Flames, and Flame Stabilization Concepts D) Distrbance Propagation and Generation in Reacting Flows E) Flame Response to Harmonic Excitation Introdctory Concepts Flame Stretch Edge Flames Flame Stabilization in Shear Layers Flame Stabilization by Stagnation Points Flame Anchoring -71

72 Flame Anchoring Locations and Flame Shapes in Swirling Flows (c) IRZ & Oter Nozzle (d) Centerbody & Oter Nozzle Flame Anchoring -72

73 Flame Anchoring Locations and Flame Shapes in Annlar Nozzle Geometries Flame stabilizes in front of stagnation point of vortex breakdown bbble Stagnation point apparently precesses, probably also moves p and down i.e., flame anchoring position highly nsteady, in contrast to stabilization at edges/corners Under what circmstances can sch flames exist? Not always observed; flames may blowoff directly withot reverting to a free floating configration Flow mst have interior stagnation point From Kmar and Liewen Flame Anchoring -73

74 Flame Anchoring -74

75 Vortex Breakdown in Annlar Geometries Natre of centerbody wake/ VBB changes with geometry, swirl #, and Reynolds # Recirclation zone with vortex tbe Recirclation zone with bbble-like breakdown above Merged recirclation zones Sheen et al., Phys. Flids, 1996 Swirl nmber/ Centerbody Diameter Flame Anchoring -75

76 Vortex Breakdown in Annlar Geometries Natre of centerbody wake/ VBB changes with geometry, swirl #, heat release parameter, and Reynolds # Flame Anchoring -76

77 Vortex Breakdown in Annlar Geometries Natre of centerbody wake/ VBB changes with geometry, swirl #, and Reynolds # Flame Anchoring -77

78 Flame Stabilization Inflenced by Downstream Bondary Conditions Flid rotation introdces inertial wave propagation mechanism sb- and spercritical flow distinction Exit bondary condition has significant inflence on vortex breakdown bbble topology Emara et. al 2009 Flame Anchoring -78