1. 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.

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 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

3. 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

4. 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 –Re turb. : 90–160 Atmospheric Pressure Fuel continuously through S/Steel Injector (2.25mm): –Velocity: 20–120m/s –Temperature(*): 650–930K FUEL AIR N2N2 Electrical Heaters Grid Autoignition Autoignition Length = L IGN. Quartz Tube Injector T fuel U air U fuel T air

5. BULK BEHAVIOUR Four regimes of operation identified for given Y fuel : 1.‘No Ignition’ 2.‘Random Spots’ 3.‘Flashback’ 4.‘Lifted Flame’ 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

6. 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

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

8. 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”

9. RESULTS – 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. 〉 Jet in Co-Flow Case (U air < U fuel ): –Increased U jet shifts autoignition DOWNSTREAM –Otherwise similar inferences Increasing T air Increasing U fuel Increasing U U T U fuel

10. RESULTS – TIMES Equal Velocity: –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 –Effect of U jet partly explained Arrhenius Plots with high activation temperatures: 60,000–100,000K (as opposed to 30,000K) Increasing U T U

11. 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 (and correlation with autoignition) are available – In other words: u’ increases, but does χ ~ u’/L turb. ξ’’ 2 also locally increase? DISCUSSION

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

13. 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.