An Experimental Study of Autoignition in Turbulent Co-Flows 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 ξ MR (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 τ IGN (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 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

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 –Re turb : 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

INDEPENDENT VARIABLES: EXPERIMENTAL ACCURACY Set all rates to get a steady and repeatable flow –AIR- and FUEL-MFC (excellent, <0.6%) –N 2 Flow Meter (average, <5%) Measure all flow rates accurately –AIR- and FUEL-MFC (excellent, ~0.9% and ~1.9%) –N 2 Flow Meter (average, <6%) Set heaters to get steady temperature conditions –Active Heater Controllers (excellent, <1K) Measure T air and T fuel accurately –Air stream (excellent, <4K(random)+6K(systematic), or <1%) –N 2 -diluted fuel stream (good, <14K+2K, or <2-3%) –N 2 -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 2 nd Order Virial Equation of State (error<1%) for densities

INDEPENDENT VARIABLES: CHARACTERIZATION PITOT TUBE and HOT WIRE –Profiles at various axial locations for different Re turb –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

BULK BEHAVIOUR CTHC: Four regimes of operation identified for given Y fuel : 1.‘No Ignition’ 2.‘RANDOM SPOTS’ 3.‘Flashback’ 4.‘Lifted Flame’ CTHAJ: Similar, with exception of ‘SPOT-WAKE INTERACTIONS’ T U Random Spots Flashback No Ignition Lifted Flame Looking at effects of: –Fluid mechanics U air and U fuel –Chemistry T air and T fuel (*) Fuel dilution with N 2 (Y fuel ) Flow Direction Injector Quartz Tube

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

Confined Turbulent Flows of Hot Air Quartz Tube: 24.9mm Insulation: Blanket, ‘Jacketed’ Tube, Heat Exchanger Injectors: 2.24 &1.185mm Fuels: H 2, C 2 H 2, C 2 H 4, n-C 7 H 16 Mixing w/ Acetone PLIF and Link w/ L IGN MEASURE: L IGN, τ IGN and f IGN Quartz Tubes: 24.9&34.0mm Insulation: Blanket, ‘Jacketed’ Tube Injector & Bluff-bodies: 2.24&10.0/14.0mm Fuels: C 2 H 4 only MEASURE: L IGN ONLY and f IGN Confined Turbulent Hot Co-FlowsConfined Turbulent Hot Annular Jets REVIEW

OPTICAL MEASUREMENTS – I SPECTROSCOPY CTHC and CTHAJ similar 1.Nothing-to-Spots Transition: C 2 H 4 2.Random Spots: H 2 3.Comparison: C 2 H 2 and H

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

OPTICAL MEASUREMENTS – III PMT Fast imaging and PMT with all fuels including H 2 Reveal characteristic autoignition event profiles: explosion, propagation and quench Obtained f IGN from PMT timeseries; strong correlations with L IGN

OPTICAL MEASUREMENTS OVERVIEW Post-ignition flamelet propagation images consistent with DNS –Spherical shell shape –Propagation velocities ~ 15–20m/s for C 2 H 2 (not considered in depth) Life-span of spots ~ 0.1–0.2s for C 2 H 2 but can vary across fuels Autoignition kernel propagation velocities ~ U air Exposure times important because they determine the autoignition information that can be retrieved from the raw images

Flow direction Earliest Mean IMAGING 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 L RMS –Earliest L MIN Attempt to define corresponding times L MIN 〈 L IGN. 〉 Flow Direction

PRELUDE TO RESULTS 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”

CTHC RESULTS – I (H 2 ) Lengths: – Equal Velocity Case (U air = U fuel ) –Increased T air shifts autoignition UPSTREAM –Increased U shifts autoignition DOWNSTREAM L MIN ~ 60–70% of 〈 L IGN 〉 Times: –Define τ MIN “minimum autoignition time” simply as: L MIN /U (~ 1 ms) –Increased T air → 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 T air Increasing U U T Increasing U T U

CTHC RESULTS – II (H n C m ) 1.Effect of fuel dilution (C 2 H 2 &C 2 H 4 ): –L IGN decreases as Y fuel increases 2.Effect of U air (C 2 H 4 ): –τ IGN increases as U air increases 3.Effect of T air and small injector (C 2 H 2 ): –L IGN decreases as T air increases –Sensitivity of T air lost for small injector

On the effect of U air : – Autoignition delayed by increase in U air (and hence) u’, ( because u’ increases with U air 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’/L turb ξ’’ 2 also locally increase? PRELIMINARY DISCUSSION

TURBULENT MIXING: ΒACKGROUND Acetone PLIF for mixture fraction 266nm straight form Nd:Yag, 110mJ/pulse –Sheet thickness 0.15mm) Optimal linear de-noising (Wiener) of all images in the Wavelet domain before taking gradients for χ 2D Consider justification for extending to χ 3D We have,,, and (not shown) pdf(ξ), pdf(χ) –Also conditional,, pdf(χ|ξ)

TURBULENT MIXING: BELOW: –All are equal velocity cases ( U air = U fuel ) with varying Re turb RIGHT: –Jet case ( U fuel = 3 and 4 U air )

TURBULENT MIXING (U air =U fuel ):

TURBULENT MIXING (U air =U fuel ): MODELLING – Isotropy and C D LEFT: –Isotropy (Radial and Axial Components of χ 2D ) RIGHT: –Timescale ratio model for only valid away from the injector

CONCLUSIONS Length (both L MIN and 〈 L IGN. 〉 ): –Increase non-linearly with lower T air and/or higher U air –Increase with U fuel Residence Time until Autoignition: –Increases with lower T air and/or higher U air Enhanced turbulent mixing through u’ and through : 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.