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 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 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 –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

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

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 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

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 (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

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

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

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

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.