INTRODUCTION

1. An Experimental Study of Autoignitionin Turbulent Co-Flows of Heated AirC.N. Markides & E. MastorakosHopkinson Laboratory, Department of Engineering,University of Cambridge, U.K.

2. 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

3. 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 understand 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) Investigate the temporal and topological features of the phenomenon Results directly available for modelling OBJECTIVES

4. APPARATUS • Air continuously through Perforated Grid (3mm, 44%) & Insulated Quartz Tube (24.9mm): • Velocity: up to 40m/s • Temperature: up to 1200K • Turbulence Intensity: 12–14% • Integral Length-scale: 3–4mm • Returb: 80 - 220 • Atmospheric Pressure • Fuel continuously through S/Steel Injector (2.24/1.185mm): • Velocity(*): 20–120m/s • Temperature(*): 650–1000K • Limited control of temperature • Bluff bodies (10.0 & 14.0mm): • Used with 24.8 & 34.0 mm tubes to give a single blockage ratio 0.17

5. INDEPENDENT VARIABLES:EXPERIMENTAL ACCURACY • Set all rates to get a steady and repeatable flow • AIR- and FUEL-MFC (excellent, <0.6%) • N2 Flow Meter (average, <5%) • Measure all flow rates accurately • AIR- and FUEL-MFC (excellent, ~0.9% and ~1.9%) • N2 Flow Meter (average, <6%) • Set heaters to get steady temperature conditions • Active Heater Controllers (excellent, <1K) • Measure Tair and Tfuel accurately • Air stream (excellent, <4K(random)+6K(systematic), or <1%) • N2-diluted fuel stream (good, <14K+2K, or <2-3%) • N2-diluted fuel stream & small injector (average, <14K+12K, or <3-4%) • Measure geometry accurately • Quartz tubes (excellent, <0.03mm or <0.1%) • Normal injectors (good, <0.03mm or <1%) • Small injectors (excellent, <0.005mm, or <0.4%) • Measure the ambient pressure • Use accurate 2nd Order Virial Equation of State (error<1%) for densities

6. INDEPENDENT VARIABLES:CHARACTERIZATION • PITOT TUBE and HOT WIRE • Profiles at various axial locations for different Returb • Mean velocity field uniformity • Magnitude of turbulence intensity • Integral lengthscale from Taylor hypothesis • Turbulence spectra estimation • Kolmogorov scales (dissipation) from variance of the velocity spatial gradients • THERMOCOUPLE • Profiles at various axial locations • Heat losses • Extent of thermal boundary layer (profile uniformity) • Estimate temperature fluctuations • HIGH TEMPERATURE HOT WIRE • Attempt to get simultaneous fluctuations of temperature and temperature/velocity fluctuation cross-correlations

7. U Lifted Flame No Ignition Flow Direction Quartz Tube Random Spots Flashback Injector T BULK BEHAVIOUR • CTHC: Four regimes of operation identified for given Yfuel: • ‘No Ignition’ • ‘RANDOM SPOTS’ • ‘Flashback’ • ‘Lifted Flame’ • CTHAJ: Similar, with exception of ‘SPOT-WAKE INTERACTIONS’ • Looking at effects of: • Fluid mechanics • Uair and Ufuel • Chemistry • Tair and Tfuel(*) • Fuel dilution with N2 (Yfuel)

8. UNSTEADY BEHAVIOUR • CTHAJ: ‘Spot-Wake Interactions’ • Velocity/Mixing PDFs crucial • CTHC: ‘Unsteady Regime’? • Velocity/Mixing PDFs crucial

9. REVIEW Confined Turbulent Flows of Hot Air Confined Turbulent Hot Co-Flows Confined Turbulent Hot Annular Jets Insulation: Blanket, ‘Jacketed’ Tube Insulation: Blanket, ‘Jacketed’ Tube, Heat Exchanger Quartz Tubes: 24.9&34.0mm Quartz Tube: 24.9mm Injector & Bluff-bodies: 2.24&10.0/14.0mm Fuels: C2H4 only Fuels: H2, C2H2, C2H4, n-C7H16 Injectors: 2.24 &1.185mm MEASURE: LIGN ONLY and fIGN MEASURE: LIGN, τIGN and fIGN Mixing w/ Acetone PLIF and Link w/ LIGN

10. 1 2 3 OPTICAL MEASUREMENTS – I SPECTROSCOPY • CTHC and CTHAJ similar • Nothing-to-Spots Transition: C2H4 • Random Spots: H2 • Comparison: C2H2 and H2

11. Flow Direction 2.5 mm ~ 4 mm ø Flow Direction Flow Direction Injector OPTICAL MEASUREMENTS – II IMAGING

12. OPTICAL MEASUREMENTS – IIIPMT • Fast imaging and PMT with all fuels including H2 • Reveal characteristic autoignition event profiles: explosion, propagation and quench • Obtained fIGN from PMT timeseries; strong correlations with LIGN

13. OPTICAL MEASUREMENTSOVERVIEW • Post-ignition flamelet propagation images consistent with DNS • Spherical shell shape • Propagation velocities ~ 15–20m/s for C2H2 (not considered in depth) • Life-span of spots ~ 0.1–0.2s for C2H2 but can vary across fuels • Autoignition kernel propagation velocities ~ Uair • Exposure times important because they determine the autoignition information that can be retrieved from the raw images

14. Flow Direction IMAGING DATA ANALYSIS PDFs from “OH Snapshots” Mean Flow direction Earliest Mean 〈LIGN.〉 Earliest LMIN • From PDF image get lengths: • Mean 〈LIGN.〉 and Standard Deviation LRMS • Earliest LMIN • Attempt to define corresponding times • Higher U (~ 26 m/s) • And/or Lower T (~ 1000 K) • Lower U (~ 20 m/s) • And/or Higher T (~ 1010 K)

15. In-homogenous autoignition of fuels in a turbulent co-flow of hot air with/without an additional bluff-body Various regimes possible, depending on conditions We concentrate on the ‘Random Spots’ Three types of experiments (mixing): Equal velocities in CTHC Jet in Co-Flow in CTHC Jet in CTHAJ (Mostly) 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” PRELUDE TO RESULTS

16. CTHC RESULTS – I (H2) • Lengths: • Equal Velocity Case (Uair = Ufuel) • Increased Tair shifts autoignition UPSTREAM • Increased U shifts autoignition DOWNSTREAM • LMIN~ 60–70% of 〈LIGN〉 • Times: • 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 Increasing U U T Increasing Tair U Increasing U T

17. CTHC RESULTS – II (HnCm) • Effect of fuel dilution (C2H2&C2H4 ): • LIGN decreases as Yfuel increases • Effect of Uair (C2H4): • τIGN increases as Uair increases • Effect of Tair and small injector (C2H2): • LIGN decreases as Tair increases • Sensitivity of Tair lost for small injector

18. PRELIMINARY DISCUSSION • 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 are considered • In other words: u’ increases, but does χ~ u’/Lturbξ’’2 also locally increase?

19. TURBULENT MIXING:ΒACKGROUND • Acetone PLIF for mixture fraction • 266nm straight form Nd:Yag, 110mJ/pulse • Sheet thickness <0.1mm (Kolmogorov Length scales are > 0.15mm) • Optimal linear de-noising (Wiener) of all images in the Wavelet domain before taking gradients for χ2D • Consider justification for extending to χ3D • We have <ξ>, <ξ‘2>, <χ>, <χ‘2> and (not shown) pdf(ξ), pdf(χ) • Also conditional <χ|ξ>, <χ|ξ‘2>, pdf(χ|ξ)

20. TURBULENT MIXING: <ξ> • BELOW: • All are equal velocity cases (Uair = Ufuel) with varying Returb • RIGHT: • Jet case (Ufuel = 3 and 4 Uair)

21. TURBULENT MIXING (Uair=Ufuel):<ξ>

22. TURBULENT MIXING (Uair=Ufuel):<ξ'2>

23. TURBULENT MIXING (Uair=Ufuel):<χ>

24. TURBULENT MIXING (Uair=Ufuel):MODELLING – Isotropy and CD • LEFT: • Isotropy (Radial and Axial Components of χ2D) • RIGHT: • Timescale ratio model for <χ2D> only valid away from the injector

25. Length (both LMIN and 〈LIGN.〉): Increase non-linearly with lower Tair and/or higher Uair Increase with Ufuel Residence Timeuntil Autoignition: Increases with lower Tair and/or higher Uair Enhanced turbulent mixing through u’ and through <χ>: DELAY AUTOIGNITION CONCLUSIONS

26. An Experimental Study of Hydrogen Autoignitionin a Turbulent Co-Flow of Heated AirC.N. Markides & E. MastorakosHopkinson Laboratory, Department of Engineering,University of Cambridge, U.K.