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C.N. Markides, G. De Paola, E. Mastorakos ( em257@engm.ac.uk ) engm.ac.uk/~em257/

Measurements and simulations of mixing and autoignition on an n-heptane plume in a turbulent flow of heated air. C.N. Markides, G. De Paola, E. Mastorakos ( em257@eng.cam.ac.uk ) http://www.eng.cam.ac.uk/~em257/. Introduction. Structure of presentation: Experimental Apparatus

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C.N. Markides, G. De Paola, E. Mastorakos ( em257@engm.ac.uk ) engm.ac.uk/~em257/

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  1. Measurements and simulations of mixing and autoignition on an n-heptane plume in a turbulent flow of heated air C.N. Markides, G. De Paola, E. Mastorakos (em257@eng.cam.ac.uk) http://www.eng.cam.ac.uk/~em257/

  2. Introduction Structure of presentation: • Experimental • Apparatus • Bulk observations • Simulations • The CFD • The CMC model • Results • Ignition lengths • Explanation of trends • Implications • Conclusions & suggestions for the future

  3. Why is autoignition important? LPP gas turbines: Premixing for low NOx, but danger of autoignition! Diesel & HCCI engines: Fast mixing for low emissions, but need to predict autoignition!

  4. grid Air in, hot Fuel in, cold Experiments Atmospheric pressure Air T up to 1100K Bulk velocities up to 30m/s Fuels: H2/N2, C2H2/N2 C7H16/N2 Techniques: Hot wire for initial conditions PLIF of acetone for x 2D image of OH* with ICCD 1.Apparatus Turbulence intensity boosted by grids. “Diffusion from point source”. (Markides & Mastorakos, 2005, Proc. Comb. Inst. 30)

  5. Fuel Hot air Experiments Ignition spot appears and then disappears. Location of ignition spot is random. 2.Visualization C2H2 ignition, natural light (1/125s exp.) OH chemiluminescence (0.2 ms exp.): Individual spots, not connected flame Ignition spot development at 20kHz: nothing, spot, spherical flame, nothing (consistent with DNS!)

  6. U Lifted Flame No Ignition Random Spots Flashback T Experiments Qualitative regimes of operation (for all Ujet/Uair tested between 1 and 5): 2.Visualization Autoignition not happening due to high strain? Continuous flame sheet ? Stabilisation in mixture “almost ready to ignite”? This regime more likely at high Ujet/Uair. Similar to “Cabra” burner Individual short-lived autoignition kernels Quick propagation back to nozzle

  7. Experiments 2.Visualization Localised autoignition Statistically-steady If ignition happens close, then it happens often They always come in bursts

  8. Experiments 3.Mixing Mean and variance of mixture fraction as expected Two-component scalar dissipation measured at Kolmogorov resolution <c> satisfies global conservation (Bilger, 2004) Data used for validating CFD & CMC model (Markides & Mastorakos, to appear in Chem Eng Sci) x <x> <c>

  9. Calculations 1. CFD – for mixing, neglecting reactions STAR-CD k-e model Very good resolution close to nozzle needed Use experimental initial conditions Use experimental Cd in model for <c>

  10. Calculations 2. CMC model Conditional Moment Closure equations: Diffusion in h-space & chemistry, closed at 1st order Conditional convection Conditional turbulent flux

  11. Calculations 2. Formulation of the CMC model for plume Averaged across plume:

  12. Calculations 3. Code, chemistry, validation 31-scalar reduced heptane chemistry (Bikas, PhD Thesis, Aachen) Ignition times of homogeneous mixtures OK Ignition times of spray with CMC OK (Wright, De Paola, Boulouchos, Mastorakos, Comb. Flame, to appear)

  13. Results 1. Mixing: Good agreement <x> x variance <c|h> <c>

  14. Results 2. Autoignition lengths: reasonably good agreement • Physics: • As U increases, ignition length L increases, but also L/U increases. Hence, not simply chemistry-controlled! • Trend captured by model

  15. Results 2. Conditional statistics Low T: long L High T: short L xMR Ignition at the most-reactive mixture fraction, not at stoichiometry. As L increases, P(h)d(h-xwell-mixed). Ignition time becomes long as P(xMR)  0.

  16. Results 3. Discussion Flamelet or CMC: ignition time increases as N increases In our flow: N increases with U Hence: Ignition time in our flow increases as U increases Also: N<Ncritical hence 2nd-order CMC not needed

  17. Conclusions A novel autoignition rig is operational and has produced results for various fuels Intense turbulence can delay autoignition due to increasing scalar dissipation rate CMC model can capture all experimental trends Crucial aspect: modelling of scalar dissipation Future: Transport equation for <c> 2D-CMC to capture spatial diffusion / flashback conditions LES, PDF calculations

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