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Mark A. Cappelli, Moon Soo Bak, G. Mungal Mechanical Engineering Department, Stanford University

Understanding the effectiveness of low-power nanosecond discharges in extending the lean flammability limit of premixed flames. Mark A. Cappelli, Moon Soo Bak, G. Mungal Mechanical Engineering Department, Stanford University. Outline. Experiments in air and pure N 2

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Mark A. Cappelli, Moon Soo Bak, G. Mungal Mechanical Engineering Department, Stanford University

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  1. Understanding the effectiveness of low-power nanosecond discharges in extending the lean flammability limit of premixed flames Mark A. Cappelli, Moon Soo Bak, G. Mungal Mechanical Engineering Department, Stanford University

  2. Outline • Experiments in air and pure N2 • question rate coefficients for dissociative quenching (0-D kinetics for revised rates) • Experiments in premixed methane/air flames straddling the LFL (  = 0.53) • Preliminary 2D simulations of plasma-assisted combustion below and above LFL

  3. Introduction • Nonequilibrium pulsed plasmas are candidates in enhancing energy conversion devices (ignition, stabilization) • Video clip from GE global research website • Mechanism attributed to; • Production of radicals (O, H, or OH) • Rapid gas heating by repetitive discharges • Extensive plasma reaction sets have been proposed and kinetic simulations have been carried out to describe various observables such as ignition delays1,2, O-radical production3, NO production4, etc. • Multi-scale simulations are rare, most not accounting for repetitive pulses and species diffusion/advection between the discharge and the surrounding flow on chemistry. Future Emphasis • I.N. Kosarev, et. al, Combustion and Flame, 2008 • I.N. Kosarev, et. al, Combustion and Flame, 2009 • M. Uddi, et. al, Proceedings of the Combustion Institute, 2008 • M. Uddi, et. al, Journal of Physics D: Applied Physics, 2009

  4. Basic ProblemPAC using Nanosecond Discharges “Hot” Electrons Electric Pulses “Excited” Molecules And Direct Dissociation Reactions Dissociative Quenching (believed to be a major path to O production and heat) Radicals + Heat “Enhanced” Combustion Plasma is a local source of radicals/practical configurations include jet diffusion and premixed flows

  5. Collisional quenching of N2* by N2 and O2 Role in radical production and explosive heating of the discharge volume through dissociative energy transfer to atomic oxygen

  6. Time-resolved emission measurements • Discharge conditions; • 1 mm diameter tungsten electrode separated by 1 mm • Gaussian voltage pulse with 10 ns FWHM • Vpeak, about 8 kV • 50 kHz repetition rate • Flow speed; • 4 m/s • N2 C-B/B-A emission spectra • Monochromator (2 ns gate) • Band-pass filters • Stark broadened Hb lineshape Serve as a means of measuring T(Trot) and ne to provide a bound on peak E/n for kinetic simulations

  7. OES results • Fast gas heating occurs during the pulse • Electron number density reaches to 4.51015 cm-3 • Small differences are found for quenching of N2 (C, B) between pure N2 discharges and air discharges

  8. 0-D kinetic simulations† • 10ns FWHM Gaussian E/n pulse is applied in air at 1 atm and 1300 K initial pressure and gas temperature. • E/npeak = 285 Td and 1013 cm-3 initial electron density provides a good agreement between the measured (rot) and simulated (trans) T and ne. • Propose revisions (factor of 2-5) to quench rates of N2 (C, B) by N2 and O2 commonly used (Kossyi et al 1992, Gordiets et al 1995, Capitelli et al 2000): kQ(N2 (C) by N2) = 2.5 (±0.3)x10-11 cm3s-1, kQ(N2 (C) by O2) = 10 (±0.3)x10-11 cm3s-1 kQ(N2 (B) by N2) = 1.6 (±0.3)x10-11 cm3s-1, kQ(N2 (B) by O2) = 4 (±0.3)x10-11 cm3s-1 † kinetics and energy equation discussed later within context of 2-D simulation

  9. Atomic O production mechanisms • 82% of atomic O are found to be produced via the dissociative quenching and only 5% from electron-impact. N2(A, B, a’, C) + O2→ N2(X) + 2O e + O2 → e + 2O • The atomic O produced is found to be 2.5 (±0.5)x1017 cm-3, corresponding to a mole fraction of 4.4 (±0.4)x10-2.

  10. Basic Problem “Hot” Electrons Electric Pulses “Excited” Molecules And Direct Dissociation Reactions Radicals + Heat “Enhanced” Combustion Stability enhancement and extended LFL

  11. Enhanced premixed laminar CH4/air combustion by pulsed discharges in the vicinity of the Lean Flammability Limit (LFL = 0.53)

  12. GC, thermocouple, OES measurement setup • Discharge conditions; • 1 mm diameter tungsten electrodes separated by 1 mm • Gaussian voltage pulse with 10 ns FWHM • Vpeak, about 8 kV • 10 - 50 kHz repetition rate • Flow speed; • About 42.5 cm/s • N2 C-B emission spectrum • 2 ns gate width • Gas Chromatography (GC) for major species (CO, H2, CH4) • TC (coated) measurements of T (not radiation corrected)

  13. GC and thermocouple results (30 kHz) CH4 consumption ignition T • For  < 0.52-0.53, major product species diluted downstream by surrounding flow (combustion is not sustained) • Plasma provides some benefit, but combustion efficiency is low quenching CO 1st stage (reforming) But subsequent dilution With surrounding flow H2

  14. GC (10 - 50 kHz) and OES results • The extension of LFL is improved with increased average power. 14W 2.5W • Fast gas heating occurs during the pulse. • For E/npeak = 278 (±1) Td and 1013 cm-3 initial electron density, measured temperatures agree well with those simulated (2-D).

  15. 2-D combustion simulations – (1/5) • Discharge size; • 1 mm height • 0.35 mm diameter • Discharges are assumed not to wander. • Constrain velocity to axial flow and assume constant pressure • Discharge conditions; • Gaussian pulse of E/n with 10 ns FWHM • E/npeak = 278 (±1) Td (Vpeak = 8 kV)and 1013 cm-3 initial electron number density • Repetition rate, 30 kHz • Flow (advection) speed; • 42.5 cm/s 0.175mm Uniform Grid 0.333mm

  16. 2-D combustion simulations – (2/5) • Species considered; • N2(X, A, B, a’, C), O2(X), O, N2+, O2+, CH4+, H2O+, CO2+, electron (e), and other species in a reduced methane/air reaction mechanisms (H2, H, OH, H2O, HO2, CH2, CH2(S), CH3, CH4, CO, CO2, HCO, CH2O, CH3O, C2H4, C2H5, C2H6) • Mechanism DRM-19 (Based on GRI-Mech 1.2) Frenklach et al. • Reaction set considered; • Electron-impact excitation and ionization of N2 • Electron-impact dissociation and ionization of O2 and CH4 • Electron-impact ionization of H2O and CO2 • Ion conversion • Recombination of electron and positive ions (diss recombination of O2+) • Quenching of N2* by N2 • Dissociative quenching of N2* by O2 and CH4 • Chemical transformations of neutral species • Reaction rate coefficients; • For reactions not involving electrons, the coefficients are adapted from previous plasma kinetic studies. • For reactions involving electrons, the coefficients are obtained as a function of E/n (using BOLSIG+).

  17. Reaction Mechanism(plasma excited components only) Coupled to GRI-MECH 1.2 reduced to 19 Species/84 Reactions in DRM-19 by Frenklach et al Plasma set adapted from: M. Capitelli, et al.,Plasma Kinetics in Atmospheric Gases, Springer, Berlin, 2000. A. Kossyi, et al., “Kinetic scheme of the non-equilibrium discharge in nitrogen-oxygen mixtures,” Plasma Sources Science and Technology, Vol. 1, No. 3, 1992. B. F. Gordiets, et al., “Kinetic Model of a Low-Pressure N2-O2 Flowing Glow Discharge,” IEEE Transactions on Plasma Science, Vol. 23, No. 4, August 1995.

  18. 2-D combustion simulations – (3/5) • Species conservations; • For each reaction, • Energy equation; • Ohmic dissipation is considered only at the discharge region. • Diffusion velocity of species j, VD,j; VD,j is the diffusion velocity of species j, and V *adv is the advection velocity, scaled as Tgas/Tgas,initial. me and E are the electron mobility and electric field, respectively, and l is the mixture-averaged thermal conductivity. and Djm is the mixture average diffusion coefficient of species j,and Qj is the thermal diffusion ratio of species j.

  19. 2-D combustion simulations – (4/5) • Mixture average diffusion coefficient for species j, Djm(Bird et al., 1960); • Mixture-averaged thermal conductivity, λ (Mathur et al., 1967); • Thermal diffusion ratio of species j , Qj(for MWj < 5) • Approximations; • Electron and positive ions locate only at a discharge region. • The species diffusion coefficients of electronically excited N2 and O2 are set to be equal to those for N2 and O2 in ground state, respectively. Dkj is the binary diffusion coefficient between species k and j. λj is the pure species conductivity for species j. qjk is the binary thermal diffusion ratio for species j into species k.

  20. 2-D combustion simulations– (5/5) • Numerical schemes used; • Central difference scheme for species diffusion and thermal conduction. • Upwind scheme for species advection and enthalpy diffusion and advection. • A system of ordinary differential equations are solved implicitly for each time step using backward difference formula (BDF). • The domain is divided into smaller subdomains, computed in parallel (32 CPUs). Each case takes approximately 24 hours of CPU. • Sundials CVODE with MPI support is used as solver, and each temporal solution is computed iteratively using Generalized Minimal Residual method (GMRES). • Initial conditions; • 1 atm and 296 K background pressure and gas temperature, initial guess from previous converged runs at similar conditions • Methane/air equivalence ratio, f (0.45 and 0.55) • 278 (±1) Td E/npeak with 1013 cm-3 initial electron number density • Boundary conditions; • Dirichlet boundary conditions for the domain bottom • Neumann boundary conditions (zero gradient) for the domain side and top

  21. Simulation results for CH4/air at f = 0.45 Spatial and temporal evolution of CH4 278 Td peak E/n 1013 cm-3 initial electron density Spatial and temporal evolution of CO2 Spatial and temporal evolution of H2O Spatial and temporal evolution of CO Spatial and temporal evolution of H2 Spatial and temporal evolution of O Spatial and temporal evolution of Gas temperature

  22. Simulation results for CH4/air at f = 0.55 Spatial and temporal evolution of CH4 279 Td peak E/n 1013 cm-3 initial electron density Spatial and temporal evolution of CO2 Spatial and temporal evolution of H2O Spatial and temporal evolution of CO Spatial and temporal evolution of H2 Spatial and temporal evolution of O Spatial and temporal evolution of Gas temperature

  23. Comparison to the measurements

  24. Kinetics in the discharge region (f = 0.55) • Most of methane is combusted because of the shorter t between pulses compared to the species diffusion t. • The dissociative quenching of N2*produces O radicals, leading to the production of other active species such as H, OH and H2.

  25. Summary: • Experiments/simulations carried out to refine important kinetics • Sampled chemistry/compared to preliminary 2D simulations • ignition/quenching (LFL) limit captured in the simulations Furthering this research: • Tighten discrepancy between experiments and simulations • - correct for gas reactions in sampling tube • Diagnostics/diagnostics/diagnostics • More strongly couple the plasma with surrounding flow • Refine the plasma simulations (sheaths, axial diffusion/drift) • - quasi-1D simulation under development • - need 2D simulation Thank You

  26. Premixed laminar CH4/air combustion by heating coil near LFL

  27. Experimental setup

  28. Simulation results for CH4/air at f = 0.55 Power input = 2W

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