An Experimental Study of Hydrogen Autoignition

in a Turbulent Co-Flow of Heated Air

C.N. Markides & E. Mastorakos

Hopkinson Laboratory, Department of Engineering,University of Cambridge, U.K.

INTRODUCTION• Theory:

Motivated by the DNS work of Mastorakos et al, 1997(and similar)– Re-examination of laminar, inhomogeneous Linan, Linan/Crespo, mid-70’s– Maximizing local reaction rate through ξMR (most reactive mixture fraction)

– AND –– Minimizing local heat losses through χ (effect of scalar dissipation rate)– “Turbulence” may accelerate autoignition– Autoignition was always observed at a finite τign. (ignition delay time)

• Experiment:Turbulent, inhomogeneous counterflows of Law et al, from late-90’s(and similar)– Turbulent, hot air opposite cold fuel, including hydrogen (elliptic problem)– Enhanced turbulence and increased strain rate increase “autoignition

temperature” necessary for autoignition – and even more interestingly –– Higher strain rates completely preclude autoignition

OBJECTIVES• Aforementioned results are not entirely consistent and there is

an inability to properly explain why

• This is a reflection of a more general situation:– Insufficient current knowledge concerning turbulent,

inhomogeneous autoignition– Limited number of relevant, well characterized experiments for

validation– THUS –

• In order to understanding the fundamental underlying physics of the coupling between turbulent mixing and the chemistry of autoignition, we experimentally:– Observe autoignition in a turbulent, co-flow configuration

(parabolic problem, easier to model)– Results directly available for modelling– Investigate the temporal and topological features of the

phenomenon

APPARATUS• Air continuously through

Perforated Grid (3mm, 44%) & Quartz Tube (0.5m x 25mm):– Velocity: up to 35m/s– Temperature: up to 1015K– Turbulence Intensity: 12–13%– Integral Length-scale: 4–5mm– Returb.: 90–160

• Atmospheric Pressure

• Fuel continuously throughS/Steel Injector (2.25mm):– Velocity: 20–120m/s– Temperature(*): 650–930K FUEL

AIR

N2

Electrical Heaters

Grid

Autoignition

Autoignition Length =

LIGN.

QuartzTube

Injector

Tfuel

Uair

Ufuel

Tair

BULK BEHAVIOUR• Four regimes of operation

identified for given Yfuel:1. ‘No Ignition’2. ‘Random Spots’3. ‘Flashback’4. ‘Lifted Flame’

T

U

RandomSpots

Flashback

NoIgnition

LiftedFlame

• Looking at effects of:– Fluid mechanics

• Uair and Ufuel

– Chemistry• Tair and Tfuel(*)• Fuel dilution with N2 (Yfuel)

Flow Direction

Injector

Quartz Tube

AUTOIGNITION MEASUREMENT

Injector

2.5 mm

~ 4 mm ø

• Fast imaging at 13.5 kHz (acetylene)

• Life-span ~ 100–200μs

• Images consistent with DNS

• Capture many (up to 2000) OH chemiluminescence “snapshots” of “autoignition spots” (exposure times 50–150μs)

Flow Direction

Flow Direction

Flow directionEarliest

Mean

DATA ANALYSIS

Earliest

Mean

• Lower U (~ 20 m/s)• And/or Higher T (~ 1010

K)

• Higher U (~ 26 m/s)• And/or Lower T (~ 1000

K)

PDFs from“OH Snapshots”

• From PDF image get lengths:– Mean L⟨ IGN. and ⟩ Standard Deviation LRMS

– Earliest LMIN

• Attempt to define corresponding times

LMIN

⟨LIGN.⟩

Flow Direction

REVIEW• In-homogenous autoignition of hydrogen

in a turbulent co-flow of hot air

• Four regimes possible, depending on conditions– We concentrate on the ‘Random Spots’

• Two types of experiments (mixing):– Equal velocities– Jet in Co-Flow

• Optical OH chemiluminescence measurements (images)– To get PDF of autoignition– Define suitable “autoignition lengths”– And calculate corresponding “residence times until

autoignition” or “autoignition delay times”

RESULTS – LENGTHS• Equal Velocity Case (Uair =

Ufuel):– Increased Tair shifts

autoignition UPSTREAM– Increased U shifts

autoignition DOWNSTREAM• LMIN ~ 60–70% of L⟨ IGN.⟩

• Jet in Co-Flow Case (Uair < Ufuel):

– Increased Ujet shifts

autoignition DOWNSTREAM– Otherwise similar inferences

IncreasingTair

IncreasingUfuel

IncreasingU

U

T

Ufuel

RESULTS – TIMES• Equal Velocity:

– Define τMIN “minimum autoignition time” simply as: LMIN/U (~ 1 ms)– Increased Tair → EARLIER autoignition– Increased U → DELAYED autoignition

• Similarly for Jet in Co-Flow:– Not easy to define an unambiguous “autoignition time”– Consider the centreline velocity decay in the jet and integrate– Effect of Ujet partly explained

• Arrhenius Plots with high activation temperatures: 60,000–100,000K(as opposed to 30,000K)

Increasing U

T

U

• On the effect of Uair:– Autoignition delayed by increase in Uair (and hence) u’,

(because u’ increases with Uair so that u’/U ~ const. behind the

grid)

– BUT –

– Direct comparison with DNS pre-mature until ξ and χ measurements (and correlation with autoignition) are available

– In other words:u’ increases, but does χ ~ u’/Lturb.ξ’’2 also locally increase?

DISCUSSION

CONCLUSIONS

• Length (both LMIN and L⟨ IGN.⟩):– Increases non-linearly with lower Tair and/or higher Uair

– Increases with Ufuel

• Residence Time until Autoignition:– Increases with lower Tair and/or higher Uair

• Enhanced turbulent mixing (through u’) seems to:DELAY AUTOIGNITION

An Experimental Study of Hydrogen Autoignition

in a Turbulent Co-Flow of Heated Air

C.N. Markides & E. Mastorakos

Hopkinson Laboratory, Department of Engineering,University of Cambridge, U.K.