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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.