Course Outline. Boundary Layer Flashback Core Flow Flashback and Combustion Induced Vortex Breakdown

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1 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 Bondary Layer Flashback Core Flow Flashback and Combstion Indced Vortex Breakdown Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 1

2 Flashback and Flameholding Flashback: Upstream propagation of a premixed flame into a region not designed for the flame to exist Occrs when the laminar and/or trblent flame speed exceeds the local flow velocity Reference flow speed and brning velocity? Flameholding: Flame stabilizes in an ndesired region of the combstor after a flashback/atoignition event Problem has hysteretic elements Wall temperatre effects Bondary layer and swirl flow stability effects Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 2

3 Flashback and Flameholding Mechanisms Flashback in the bondary layer Flame propagation into core flow We ll focs on swirl flows Combstion instabilities Strong acostic plsations lead to nearly reverse flow Note: p /p~ /c=m / i.e. /=(1/M)p /p Show video Heeger et al. Exp. In Flids 2010 Significance of above mechanisms is a strong fnction of: Fel composition Operating conditions Flid mechanics Kröner et al. CST 2007 Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 3

4 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 Bondary Layer flashback Core Flow Flashback and Combstion Indced Vortex Breakdown Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 4

5 Bondary Layer Flashback-Classical Treatment Neglects effects of Heat release (changes approach flow) Stretch (changes brning velocity) Flashback occrs if flame speed exceeds flow velocity at distance,, from the wall δ q ( y = δ ) = s ( y = δ ) x q d q Expanding velocity in a Taylor series, establish flashback condition: gδ x q ( y = δ ) δ = 1 y sd x q q g Assming, δq δf, define flashback Karlovitz nmber gδ F Ka = s d x δ q x y y = 0 flame Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 5

6 Bondary Layer Flashback Flashback Karlovitz nmber approach is well validated for open flames, sch as Bnsen brners Performed detailed kinetics calclations to determine flame speed and thickness for several data sets Shows how prior brning velocity, flame thickness tendencies can be sed to nderstand tendencies Pressre Preheat temperatre Stoichiometry g (1/s) CH 4 -air CH 4 -air (solid) and C 3 H 8 -air (dashed) Data for figres obtained from: Grmer Ind. & Eng. Chem Dgger Ind. & Eng. Chem 1955 Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 6

7 Bondary Layer Flashback Trblent Bondary Layers Mlti-zoned Near wall laminar sblayer, Basic scaling developed for laminar flows holds if: Most literatre data shows g, trblent 3 g, laminar Significant space-time variation dring flashback Images sggest flame interactions with bondary layer instabilities C. Eichler Exp. In Flids 2012 Show video Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 7

8 Copled Effects of Flame Crvatre and Gas Expansion Flame blging into reactants Approach flow decelerates Streamlines diverge Adverse pressre gradient Implications: Bondary layers adverse pressre gradients lead to separation Swirl flows adverse pressre gradients can lead to vortex breakdown Triple flames flame can propagate into region with velocity that is higher than flame speed Flame stability flame spontaneosly develops wrinkles Copyright T. Liewen 2013 Unathorized Reprodction Prohibited b x,0 x,0 A ξ 2π k Solid thick contors: positive pressre flctations Pressre Velocity kx 8

9 Heat Condction Inflences on Bondary Layers Reacting 1 Nonreacting Important implications for Scaling velocity gradients in shear layers Flame stretch rates max Shear layer instability freqencies acostic sensitivities Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 9

10 Heat Release and Stretch Effects Heat release modifies approach flow Stretch modifies brning velocity C. Eichler Trbo Expo 2011 Inner lip Unconfined Confined Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 10

11 Heat Release and Stretch Effects Particlarly important in explaining flameholding phenomenon Once a flashback event has occrred, difficlt to expel flame from combstor Leading point of advancing flashback event sbject to positive crvatre ( ) / s g confined nconfined φ Effect of gas expansion de to heat release on local flow velocity Copyright T. Liewen 2013 Unathorized Reprodction Prohibited C. Eichler Trbo Expo

12 Stretch Effects Leading point of advancing flashback event sbject to positive crvatre For Ma < 0, this can case: s ( y = δ ) >> s,0 d q d x x y y = 0 flame,0 s d can be a significant nderestimate of flame speed Positively crved flame δ q Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 12

13 Heat Release Effects Gas expansion across a crved flame alters the approach flow Reslting adverse pressre gradient ahead of flame decelerates flow In extreme cases, can case bondary layer separation Approach flow scks flame back into nozzle Flame Separated flow region 10 2 mm Figres: C. Eichler Trbo Expo 2011 Heeger et al. Exp. In Flids 2010 Copyright T. Liewen 2013 Unathorized Reprodction Prohibited Reactants Reactants 13

14 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 Bondary Layer flashback Core Flow Flashback and Combstion Indced Vortex Breakdown Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 14

15 Flow Stability and Vortex Breakdown The degree of swirl in the flow, S, has profond inflences on the flow strctre Most prominent featre of high swirl nmber flows is the occrrence of vortex breakdown, which is manifested as a stagnation point followed by reverse flow Stagnation points Billant et al., JFM, 1998 Sarpkaya, JFM, 1971 Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 15

16 Prominent Featres of Swirling Flows with Vortex Breakdown: Precessing Vortex Core The flow does not instantaneosly rotate abot the geometric centerline The location of zero azimthal velocity is referred to as the precessing vortex core (PVC) The freqency of rotation of the precessing vortex core scales with a Strohal nmber based on axial flow velocity and diameter Leads to a helical pattern in instantaneos axial flow velocity Important to differentiate the PVC from the other helical shear flow strctres which may also be present Copyright T. Liewen 2013 Unathorized Reprodction Prohibited Sorce: Syred, Prog. Energy and Comb. Sci.,

17 Prominent Featres of Swirling Flows: Shear Layer Instability Shear layers exist in both span- and streamwise directions Can be axisymmetric or helical Hang and Yang, Proc. Comb. Inst., 2005 Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 17

18 Flow Stability and Vortex Breakdown Vortex breakdown can be described as a fold catastrophe Bifrcation of the possible steady state soltions to the Navier-Stokes eqations In high Re flows, there is an intermediate swirl nmber range where flow is bi-stable and hysteretic i.e., either vortex breakdown or no vortex breakdown flow state possible Sorce: Lopez, Physics of Flids, 1994 Swirl Nmber Vortex Breakdown Either No Breakdown Vortex Core Size/Nozzle Radis Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 18

19 Flow Stability and Vortex Breakdown: Example calclation Vortex breakdown can be predicted for given velocity profile x,0 b,0 Q-vortex" velocity profile: rθ,0 b,0 x,0 2 b,0 b,0 2χ 5 r = 1+ exp( ( ) ) 1 χ 4 r 5 r rθ,0 Sv 4 rc = ( r/ r ) (1 exp( 5 / 4)) c 2 (1 exp( ( ) )) c rr c Axial and azimthal velocity profiles sed for vortex breakdown calclation, sing S v =0.71 for θ,0 plot. χ = a,0 b,0 + a,0 b,0 Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 19

20 Flow Stability and Vortex Breakdown: Example Calclation Wake Jet Breakdown Breakdown S v 0.6 S v No breakdown 0.5 No breakdown r c /a χ Following Z. Rsak Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 20

21 Core Flow Flame Propagation A B Stable Flame U f CIVB r z Swirl Generator Mixing Tbe Combstion Chamber Image reprodced from T. Sattelmayer Vortex breakdown flame interaction Can occr even if flame speed everywhere less than flow speed Gas expansion across a crved flame: 1. Adverse pressre gradient & radial divergence imposed on reactants 2. Low/negative velocity region generated pstream of flame 3. Flame advances frther into reactants 4. Location of vortex breakdown region advances pstream De to bi-stable natre of vortex breakdown bondaries CIVB itself not necessarily bi-stable Copyright T. Liewen 2013 Unathorized Reprodction Prohibited 21