Jacqueline H. Chen Combustion Research Facility Sandia National Laboratories

Livermore, CA jhchen@sandia.gov

Princeton Combustion Institute Summer Shool June 23-28, 2019

Part 3: Lifted Flame Stabilization in Hot Coflow

Reactive Ethylene and Hydrogen Turbulent Lifted Jets in Heated Air Coflow

C.S.Yoo1,R.Sankaran2,T.Lu3,C.K.Law4,J.H.Chen51UNIST,S.Korea

2OakRidgeNa6onalLaboratory3UniversityofConnec6cut

4PrincetonUniversity5Combus6onResearchFacility,SandiaNa6onalLaboratories.

DNS of Lifted Ethylene-Air Flame in a Hot Coflow

•  3D slot burner configuration: –  Lx × Ly × Lz = 30 × 40 × 6 mm3 with –  1.28 billion grid points –  High fuel jet velocity (204m/s); coflow

velocity (20m/s)

–  Nozzle size for fuel jet, H = 2.0mm

–  Rejet = 10,000

–  Cold fuel jet (18% C2H4 + 82% N2) at 550K, ηst ≈ 0.27

–  Detailed C2H4/air chemistry, 22 species 18 global reactions, 201 steps

–  Hot coflow air at 1,550K

Ethylene-air lifted jet flame at Re=10000

C. S. Yoo, et al. Proc. Comb. Inst. 2011

Dynamics of lifted flame stabilization – Log(scalar dissipation rate) and temperature

Scalar Dissipation Rate, χ, Species Mass Fractions, and Mixture Fraction ξ

χ ξ OH HO2 CH2O

Favre Mean and Instantaneous Temperature and Species Mass Fractions (OH, CH2O, HO2)

T OH CH2O HO2

Temporal Evolution of OH at Stabilization Point

Temporal evolution of OH mass fraction isocontour at t/τj = 0.227 ~ 1.160

t/tj = 0.204 0.272 0.476 0.642

0.680 0.748 0.884 1.156

Displacement Speed Definition (Gibson, Phys. Fluids 1968)

sd u.n

YOH = 0.0005 used to track flame stabilization point

reaction Species Diffusion

•  Scalar gradient vanishes at point of thermal runaway – Sd* is unbounded

•  Sd*~ O(1) deflagration wave, reaction/diffusion balance •  Sd* >> O(1) spontaneous ignition front propagation

Temporal Tracking of Stabilization Point

x

-u.n

C. S. Yoo, et al. Proc. Comb. Inst. 2011

Conceptual Stabilization Mechanism

Temporal evolution of OH mass fraction isocontour at t/τj = 0.227 ~ 1.160

a) Ignition occurs in lean mixtures with low χ b) Stabilization point is advected downstream by high convective velocity c) Ignition occurs in another coherent jet structure

Convective velocity greater than displacement speed for ηst = 0.27

Su & Mungal

H2-Air Turbulent Jet Flames in Heated Coflow

•  Hydrogen/air case–  3D slot burner configuration: Lx ×

Ly × Lz = 24 × 32 × 6.4mm3 with 940M grid points

–  High fuel jet velocity (347m/s)–  Low coflow velocity (10m/s)–  Nozzle size for fuel jet, H = 1.92mm–  Rejet = 11,000; τj = 0.07ms–  Cold fuel jet (65% H2 + 35% N2) at

400K•  Stoichiometric mixture fraction, ξst ≈ 0.2

–  Hot coflow air at 1,100K

Inlet boundary conditions for temperature, species and velocity

Volume rendering of mixture fraction, scalar dissipation rate and mass fraction of OH an HO2 of hydrogen/air lifted jet flame in a heated coflow

C.S. Yoo, R. Sankaran, J.H. Chen, J. Fluid Mech., 2010

Hydrogen/Air Lifted Jet Flame C.S. Yoo, R. Sankaran, J.H. Chen, J. Fluid Mech., 2010.

Isocontours of temperature, heat release rate, YOH and YHO2. The red line represents the stoichiometric mixture fraction iso-lines

OH HO2

•  Flame base stabilizes in lean mixture•  HO2 radical in auto-ignition

–  Builds up upstream of OH and other intermediate radicals (H, H2O2)

–  Precursor of auto-ignition in hydrogen-air chemistry

–  Auto-ignition occurs at the flame base

•  Stabilization mechanism–  Ignition occurs in lean mixtures with low χ– The stabilization point propagates

upstream following a coherent jet structure– Local extinction occurs by high χ and the

point moves downstream–  Ignition occurs in another coherent jet

structure

Temporal evolution of the axial stabilization point with axial velocity, Sd (top) and mixture fraction, heat release rate,

and scalar dissipation rate (bottom)

Temporal Tracking of Stabilization Point (Lifted Hydrogen Jet Flame)

2 τj

C.S. Yoo, R. Sankaran, J.H. Chen, J. Fluid Mech., 2010.

Power Spectrum of Stabilization Point Fluctuation and the Axial Velocity Correlation Over 2δ1/2 Shows Coherent Jet Structure Role modulating

St ~ 2 τj

vv

C.S. Yoo, R. Sankaran, J.H. Chen, J. Fluid Mech., 2010.

Flame Stabilization Point Statistics (Hydrogen)

Re = 10,000 OH Mass Fraction

C.S. Yoo, R. Sankaran, J.H. Chen, J. Fluid Mech., 2010.

A Chemical Explosive Mode Analysis (CEMA)

•  Governing equations for a chemically reacting flow

•  The chemical Jacobian

•  Chemical mode

•  Positive eigenvalue, λexp, of Jω indicates chemical explosive mode

)()()( ysyωygy+==

dtd

y:thevectorofvariables(e.g.speciesconcentra6onandtemperature)ω:chemicalsourceterms:othersourceterms(e.g.diffusion)

gb ⋅=fyωJdd

=ω

b:aleOeigenvectorofJω

T. Lu, C.S. Yoo, J. H.Chen, C. K. Law, J. Fluid Mech. 2010

Composition of a Chemical Mode Based on the CSP Concepts

•  Explosion Index for Species

•  Participation Index for Reactions

|)|(||

expexp

expexp

baba

EIdiagsum

diag= a:therighteigenvector

Thecorrela6onofthespecieswiththechemicalexplosivemode

( )( ) |)(| exp

exp

RSbRSb

PI⊗⋅

⊗⋅=sum

S:thestoichiometriccoefficientmatrixR:thevectorofnetratesforthereac6ons⊗:element-wisemul6plica6on

Thecontribu6onofthereac6onstothechemicalexplosivemode

T. Lu, C.S. Yoo, J. H.Chen, C. K. Law, J. Fluid Mech. 2010

Chemical Explosive Mode Analysis, Da, Weighted Explosion Index

y/H

x/H

Weighted EI

-4 -2 0 2 40

5

10

15

y/H

x/H

sign(λexp) × log10(max(1, |λexp|), 1/s)

-4 -2 0 2 40

5

10

15

-4

-2

0

2

4

y/H

x/H

sign(λexp) × log10(max(1,|Da|))

-4 -2 0 2 40

5

10

15

-4

-2

0

2

4(a) (b) (c)

2.OH1.O

3.HO2

4.T

5.CO

6.CH3CHO

T. Lu, C.S. Yoo, J. H.Chen, C. K. Law, J. Fluid Mech. 2010 Z. Luo, C. S. Yoo, E. S. Richardson, J. H. Chen, C. K. Law, T. Lu, Comb.Flame 2010

Eigenvalue of CEM CEM Damköhler Number Explosion Index

Suggests framework for fundamental study of composition fluctuations. Illustrates how these fluctuations arise. Reduction of (co)variance matrix. Necessity of 2nd order closure. Testing of existing/proposed sub-models/modeling strategies

Analysis of 2nd Order CMC Applied to an Autoignitive H2 jet flame*

Second Order Conditional Moment Closure (CMC)

Volume rendering of HO2 mass fraction in an autoigniting H2 jet flame

* E.Richardson, C.S. Yoo, and J.H. Chen. Proc. Combust. Inst. 2009

CMC2 models conditional means and conditional variances and covariances. Examination of physics indicates appropriate simplifications Validation

Second-order CMC: Reaction Rate Closure

ηη jiji

YYYYWWW ʹ́ʹ́∂∂

∂+≈ QQY

2

21)()(

Cross stream conditionalaverage temperaturesource at x=6.5mm.

Richardson, Yoo, ChenProc. Combust. Inst., 31.

1) Need to include effects of conditional fluctuations in turbulent autoignition. 2) Second order contribution dominated by T, H, H2, OH conditional co-variances.

ηiY=iQSolve for conditionalaverage mass fractions:With reaction closure:

1st order 2nd order

Second-Order CMC: Where do the conditional fluctuations come from?

•  Dissipation rate fluctuations, S7, have a limited role in this lifted jet configuration.•  Chemistry and turbulent convection dominate – the mixture’s history is important.

§  Conditional variance generation by turbulent velocity fluctuation transport down conditional gradients is the prime mover S6.

§  Reaction term S4 amplifies HO2 fluctuations.

Budget for conditional variance of HO2,upstream of flame base: x=4.5mm.

DieselJetFlame:UnderstandingRoleofIgni9oninStabiliza9onofLi<edFlamesinHotCoflowatHighPressure

Chemiluminescence from diesel lift-off stabilization for #2 diesel, ambient 21% O2, 850K, 35 bar courtesy of Lyle Pickett, SNL

What is the role of ignition in lifted flame stabilization?

Lifted DME Jet Flame in Heated Coflow at 5 atm (Minamoto and Chen, 2016 Combustion and Flame)

•  11,700 jet Reynolds number

•  Turbulent Reynolds number of 1430

•  5 atm (NTC and low temperature heat release, LTHR)

•  DME reduced chemical model with 30 species (Bhagatwala et al. 2014) based on Zhao&Dryer

Minamoto et al. in prep (2015)

High- and Low-temperature Flame Structure

Negative Temperature Coefficient (NTC) & Two-stage Ignition in Dimethyl Ether (DME) at 5 atm

2nd ignition

1st ignition

Fuel stream: 0.1 DME+0.9 N2 (500 K) Oxidizer stream: 0.21 O2 + 0.79 N2 (1000 K)

Laminar DME Jet Flame Polybrachial Structure (Krisman et al. , Proc. Comb. Inst. 2015)

Heat release rate, * denotes stabilization point, square denotes Low temperature ignition, black line is stoichiometric condition

Laminar Lifted DME Jet Flame at 40 atm

Fuel Oxid Fuel stream: 0.3 DME + 0.7 N2 (400 K) Oxid stream: 0.21 O2 + 0.79 N2 (1300 K)

#1

#2

#3

#4 #5 “Pentabrachial flame structure” #1. Low-temperature reaction (LTHR) #2. High-temperature reaction (lean, NTC) #3. Lean premixed flame #4. Diffusion flame #5. Rich premixed flame

Objectives: What are the characteristics of a lifted jet flame in the presence of: •  Sheared Turbulence •  Mean velocity gradient •  Negative Temperature Coefficient Regime (NTC) •  Low temperature heat release (LTHR)

Krisman et al. 2015

Stabilization point

Laminar and Turbulent DME Flame Structure

Laminar pentabrachial flame, Log (heat release rate)

CH3OCH2O2 Log of heat release

Fuel

Air

Downstream Flame Branches in Turbulent Flame

x/H=7 x/H=16 Ultra lean

lean Rich Diffusion Lean

Premixed + Nonpremixed -

Ignition & Premixed Fronts Identified using CEMA

}  Fresh mixtures (pre-ignition): }  Products (post-ignition): }  Ignition points & premixed reaction fronts: }  Cool flames:

; ; peroxides (e.g. C12H25O2, C12OOH)

1-D premixed flames

P=60 atm K

Auto-ignition

n-Dodecan/air P=60 atm

Structure of the Lifted DME Jet Flame Visualized by CEMA

•  Flame Structure Segmentation by CEMA:

–  A non-premixed flame kernel

–  Lean premixed flamelets –  Rich premixed flame fronts

in the broken reaction zones regime (can be important for soot modeling)

–  A mixing layer with fresh mixtures (auto-igniting)

–  Pockets of cool flame

Positive eigenvalue, λexp, of Jacobian Jω indicates the chemical explosive mode y

ωJdd

=ω

T. Lu

Lu et al. 2009

CNF 2016

Structure of the Lifted DME Jet Flame Visualized by CEMA

•  Important flame features involved

–  A non-premixed flame kernel

–  Lean premixed flamelets –  Rich premixed flame fronts

in the broken reaction zones regime (can be important for soot modeling)

–  A mixing layer with fresh mixtures (auto-igniting)

–  Pockets of cool flame

Positive eigenvalue, λexp, of Jacobian Jω indicates the chemical explosive mode y

ωJdd

=ω

Edge Flame Propagation with Low Temperature Heat Release

Sd =Q

ρ0cp ∇T

+n ⋅∇ λn ⋅∇T( )ρ0cp ∇T

+λρ0cp

∇⋅n −ρρ0cp

∇T ⋅ cp,kDkWk

W∇Xk

1

N

∑%

&'

(

)*

Sd,R

Sd,N Sd,T Sd,cp à 0

Temperature (K)

Dis

plac

emen

t spe

ed (m

/s)

ξ = ξst Flame speed (m/s) Sref (m/s)

No radicals 0.56 1.06

t = t1 0.75 1.43

t = 0.01Δt2 + t1 0.79 1.50

t = 0.5Δt2 + t1 0.90 1.71

Sref =ρ0ρ1SL (ρ0, ρ1: unburnt and burnt densities on ξst)

FI

Premixed

Non-premixed

Time

Mas

s fra

ctio

n

Δt2 = t2 - t1

YCH3OCH2O2 YOH

t1 t2

Theoretical Propagation Speed (Ruetsch 1995)

Flame Displacement Speed Along ξst

Sref = 1.06 m/s where SL= 0.56 m/s

Summary of Lifted DME Jet Flame

•  Turbulent lifted DME flame structure is polybrachial consistent with laminar flame structure: •  Two upstream branches (NTC, LTHR) •  Triple flame (downstream) •  Lean stabilization point (NTC)

•  Radicals and heat produced at the upstream high temperature flame branch are unlikely to influence overall flame behavior.

•  First stage ignition enhances the laminar flame speed leading to a higher edge flame propagation speed.