Slide 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 Slide 2 Outline Experiments in air and pure N 2 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 Slide 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 delays 1,2, O-radical production 3, NO production 4, 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. 1.I.N. Kosarev, et. al, Combustion and Flame, 2008 2.I.N. Kosarev, et. al, Combustion and Flame, 2009 3.M. Uddi, et. al, Proceedings of the Combustion Institute, 2008 4.M. Uddi, et. al, Journal of Physics D: Applied Physics, 2009 Future Emphasis Slide 4 Basic Problem PAC using Nanosecond Discharges Dissociative Quenching (believed to be a major path to O production and heat) Electric Pulses Hot Electrons Excited Molecules And Direct Dissociation Reactions Radicals + Heat Enhanced Combustion Plasma is a local source of radicals/practical configurations include jet diffusion and premixed flows Slide 5 Collisional quenching of N 2 * by N 2 and O 2 Role in radical production and explosive heating of the discharge volume through dissociative energy transfer to atomic oxygen Slide 6 Time-resolved emission measurements Discharge conditions; 1 mm diameter tungsten electrode separated by 1 mm Gaussian voltage pulse with 10 ns FWHM V peak, about 8 kV 50 kHz repetition rate Flow speed; 4 m/s N 2 C B/B-A emission spectra Monochromator (2 ns gate) Band-pass filters Stark broadened H lineshape Serve as a means of measuring T(T rot ) and n e to provide a bound on peak E/n for kinetic simulations Slide 7 OES results Fast gas heating occurs during the pulse Small differences are found for quenching of N 2 (C, B) between pure N 2 discharges and air discharges Electron number density reaches to 4.5 10 15 cm -3 Slide 8 10ns FWHM Gaussian E/n pulse is applied in air at 1 atm and 1300 K initial pressure and gas temperature. E/n peak = 285 Td and 10 13 cm -3 initial electron density provides a good agreement between the measured (rot) and simulated (trans) T and n e. Propose revisions (factor of 2-5) to quench rates of N 2 (C, B) by N 2 and O 2 commonly used (Kossyi et al 1992, Gordiets et al 1995, Capitelli et al 2000): k Q (N 2 (C) by N 2 ) = 2.5 (0.3)x10 -11 cm 3 s -1, k Q (N 2 (C) by O 2 ) = 10 (0.3)x10 -11 cm 3 s -1 k Q (N 2 (B) by N 2 ) = 1.6 (0.3)x10 -11 cm 3 s -1, k Q (N 2 (B) by O 2 ) = 4 (0.3)x10 -11 cm 3 s -1 0-D kinetic simulations kinetics and energy equation discussed later within context of 2-D simulation Slide 9 82% of atomic O are found to be produced via the dissociative quenching and only 5% from electron-impact. N 2 (A, B, a, C) + O 2 N 2 (X) + 2O e + O 2 e + 2O The atomic O produced is found to be 2.5 (0.5)x10 17 cm -3, corresponding to a mole fraction of 4.4 (0.4)x10 -2. Atomic O production mechanisms Slide 10 Basic Problem Stability enhancement and extended LFL Electric Pulses Hot Electrons Excited Molecules And Direct Dissociation Reactions Radicals + Heat Enhanced Combustion Slide 11 Enhanced premixed laminar CH 4 /air combustion by pulsed discharges in the vicinity of the Lean Flammability Limit (LFL = 0.53) Slide 12 GC, thermocouple, OES measurement setup Discharge conditions; 1 mm diameter tungsten electrodes separated by 1 mm Gaussian voltage pulse with 10 ns FWHM V peak, about 8 kV 10 50 kHz repetition rate Flow speed; About 42.5 cm/s N 2 C B emission spectrum 2 ns gate width Gas Chromatography (GC) for major species (CO, H 2, CH 4 ) TC (coated) measurements of T (not radiation corrected) Slide 13 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 GC and thermocouple results (30 kHz) CH 4 consumption CO H2H2 T ignition quenching 1 st stage (reforming) But subsequent dilution With surrounding flow Slide 14 The extension of LFL is improved with increased average power. GC (10 50 kHz) and OES results For E/n peak = 278 (1) Td and 10 13 cm -3 initial electron density, measured temperatures agree well with those simulated (2-D). Fast gas heating occurs during the pulse. 2.5W 14W Slide 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/n peak = 278 (1) Td (V peak = 8 kV) and 10 13 cm -3 initial electron number density Repetition rate, 30 kHz Flow (advection) speed; 42.5 cm/s 0.175mm 0.333mm Uniform Grid Slide 16 2-D combustion simulations (2/5) Species considered; N 2 (X, A, B, a, C), O 2 (X), O, N 2 +, O 2 +, CH 4 +, H 2 O +, CO 2 +, electron (e), and other species in a reduced methane/air reaction mechanisms (H 2, H, OH, H 2 O, HO 2, CH 2, CH 2 (S), CH 3, CH 4, CO, CO 2, HCO, CH 2 O, CH 3 O, C 2 H 4, C 2 H 5, C 2 H 6 ) Mechanism DRM-19 (Based on GRI-Mech 1.2) Frenklach et al. Reaction set considered; Electron-impact excitation and ionization of N 2 Electron-impact dissociation and ionization of O 2 and CH 4 Electron-impact ionization of H 2 O and CO 2 Ion conversion Recombination of electron and positive ions (diss recombination of O 2 +) Quenching of N 2 * by N 2 Dissociative quenching of N 2 * by O 2 and CH 4 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+). Slide 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 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. Plasma set adapted from: Slide 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, V D,j ; e and E are the electron mobility and electric field, respectively, and is the mixture-averaged thermal conductivity. V D,j is the diffusion velocity of species j, and V * adv is the advection velocity, scaled as T gas /T gas,initial. D jm is the mixture average diffusion coefficient of species j, and j is the thermal diffusion ratio of species j. and Slide 19 2-D combustion simulations (4/5) Mixture average diffusion coefficient for species j, D jm (Bird et al., 1960); Mixture-averaged thermal conductivity, (Mathur et al., 1967); Thermal diffusion ratio of species j, j (for MW j < 5) Approximations; Electron and positive ions locate only at a discharge region. The species diffusion coefficients of electronically excited N 2 and O 2 are set to be equal to those for N 2 and O 2 in ground state, respectively. D kj is the binary diffusion coefficient between species k and j. j is the pure species conductivity for species j. jk is the binary thermal diffusion ratio for species j into species k. Slide 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, (0.45 and 0.55) 278 (1) Td E/n peak with 10 13 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 Slide 21 Simulation results for CH 4 /air at = 0.45 278 Td peak E/n 10 13 cm -3 initial electron density Spatial and temporal evolution of CH 4 Spatial and temporal evolution of CO 2 Spatial and temporal evolution of H 2 O Spatial and temporal evolution of CO Spatial and temporal evolution of H 2 Spatial and temporal evolution of O Spatial and temporal evolution of Gas temperature Slide 22 Simulation results for CH 4 /air at = 0.55 279 Td peak E/n 10 13 cm -3 initial electron density Spatial and temporal evolution of CH 4 Spatial and temporal evolution of CO 2 Spatial and temporal evolution of H 2 O Spatial and temporal evolution of CO Spatial and temporal evolution of H 2 Spatial and temporal evolution of O Spatial and temporal evolution of Gas temperature Slide 23 Comparison to the measurements Slide 24 Kinetics in the discharge region ( = 0.55) Most of methane is combusted because of the shorter between pulses compared to the species diffusion . The dissociative quenching of N 2 * produces O radicals, leading to the production of other active species such as H, OH and H 2. Slide 25 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 Summary: Experiments/simulations carried out to refine important kinetics Sampled chemistry/compared to preliminary 2D simulations ignition/quenching (LFL) limit captured in the simulations Slide 26 Premixed laminar CH 4 /air combustion by heating coil near LFL Slide 27 Experimental setup Slide 28 Simulation results for CH 4 /air at = 0.55 Power input = 2W