Wenting Sun, Joseph Lefkowitz, Jay Uddi and Yiguang Ju. Timothy Ombrello, Fred Schauer, John Hoke and Campbell Carter

Transcription

1 New combustion regimes and kinetic studies of plasma assisted combustion Wenting Sun, Joseph Lefkowitz, Jay Uddi and Yiguang Ju Department of Mechanical and Aerospace Engineering, Princeton University Princeton, NJ 08544, USA Timothy Ombrello, Fred Schauer, John Hoke and Campbell Carter U.S. Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson AFB, OH, Nov.6-7, 2012 MURI Plasma 3 rd Yr Review Meeting MURI Topic #11: Chemical Energy Enhancement by Nonequilibrium Plasma Species

2 Report Documentation Page Form Approved OMB No Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE NOV REPORT TYPE 3. DATES COVERED to TITLE AND SUBTITLE New combustion regimes and kinetic studies of plasma assisted combustion 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Princeton University,Department of Mechanical and Aerospace Engineering,Princeton,NJ, PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR S ACRONYM(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES U.S. Government or Federal Rights License 14. ABSTRACT 11. SPONSOR/MONITOR S REPORT NUMBER(S) 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Same as Report (SAR) 18. NUMBER OF PAGES 50 19a. NAME OF RESPONSIBLE PERSON Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18

3 MURI Facility Summary and collaborative team structure 3000K Shock Tube (Starikovskiy) 1000K 300K Flame chemistry (Ju, Sutton) Flow Reactors ( Yetter, Adamovich) JSR/Flow reactor Species and kinetics (Ju) MW+laser (Miles) RCM (Starikovskiy) 0.01atm 1atm 100atm (*All facilities designed and fabricated specifically for this program.)

4 Today s Presentation 1. New combustion regimes and kinetic studies of in situ plasma discharge in counterflow flames (Tasks 8 and 9: Kinetic model validation) 2. Multispecies diagnostics in a flow reactor with Mid-IR and molecular beam mass spectroscopy (MBMS) (Task 3: Multispecies measurements) 3. Ignition enhancement and minimum ignition energy by plasma discharge (Task 6: Ignition, Flame Initiation and the Minimum Ignition Energy )

5 1. New flame and ignition regimes with in situ nano-second pulsed discharge Temperature Technical questions:. Can plasma assisted combustion enhances sublimit combustion so that the ignition and extinction limit disappear on the classical S-curve?. What happens when JP-8 has low temperature ignition chemistry? How does PAC interact with low temperature chemistry? relevant or not? Disappear of the S-curve The new monotonic ignition curve Extinction Plasma generated species: O, H, O 2 (a g ) Ignition the classical S-curve Residence time Most combustors Scramjet, afterburner 4

6 Experimental method (in-situ plasma discharge) P = 72 Torr f = 24 khz Laser beam 25.4 mm Peak Voltage = 7.8 KV E = 7500 V/cm, E/N ~ 900 Td Power ~ 17 W (repetitive pulses) 5

7 OH PLIF measurement (CH 4 /O 2 sublimit flames) OH number density (cm -3 ) OH density (molecule/cm 3 ) a= 400 1/s, X o = 55%, X f = 20%, f = 24 khz, P=72 Torr, UV power = 2 mj/pulse OH fluorescence at Q 1 (6) Top burner Direct image 7x x x10 15 Enhancement of OH formation Experiment (w. Plasma) Simulation (w. Plasma) Experiment (w.o Plasma) Simulation (w.o Plasma) X f =0.2 plasma/flame chemistry 4x x10 15 Good agreement for flame 2x x10 15 Bottom burner (fuel) Position (mm) S-shaped ignition/extinction curve measurement: OH PLIF 7x x x x x x10 15 O2 =34% O2 =62% Smooth Transition Extinction 1x10 15 Ignition Fuel mole fraction 6

8 Concentration (ppm) Temperature (K) Numerical modeling of PAC and path flux analysis X O2 = 0.34, X CH4 = 0.16, P = 72 Torr, f = 24 khz, a = 400 1/s fuel H O OH (simulation) OH (experiment) T (simulation) T (experiment) Before ignition LTO oxidizer Distance (mm) e + CH 4 CH 3 + H + e e + O 2 O+O(D) + e no flame, but reaction zone was built up by radicals generated from plasma Electron and ion impact dissociation are the key in PAC 7.

9 Temperature OH number density (cm -3 ) New ignition transition curve with plasma assisted combustion Hypothesis Disappear of the S-curve Extinction Plasma generated species: O, H, O 2 (a g ) the classical S-curve Ignition Residence time Most combustors Scramjet, afterburner 7x x x x x x x10 15 Experimental observation A new combustion regime The S-curve transition O2 =34% O2 =62% Smooth Transition Extinction Ignition Fuel mole fraction Extended flammable regine No extinction limit What if a fuel (JP-8) has low temperature chemistry?

10 How does low temperature chemistry make a difference? From CH 4 to jet fuel, using DME (LTI and gas phase) as example CH 4 + radicals (O/OH/H) CH 3 O/HCO Plasma CH 4 + electrons, radicals CH 3 O/HCO Same chemiluminescence before CH 4 plasma assisted ignition + radicals + radicals Slow chain branching fast chain branching High temperature chemistry only Large hydrocarbons R + O 2 Slow chain branching + radicals RO 2 slow Large hydrocarbons Plasma Low Temperature Chemistry (LTC)? + electrons, radicals R + O 2 fast chain branching RO 2 faster Different chemiluminescence before DME ignition How does LTC affect 9 ignition and extinction?

11 burners Nd:YAG laser 355 nm CH 2 O PLIF at 355 nm from Nd:YAG laser Band pass filter, 400~450 nm ICCD no LIF from cold flow Pros: convenient Cons: low absorption coefficient Further identification of CH 2 O LIF LIF close to fuel side CH 2 O PLIF 1 A A2 X band Before ignition with plasma 1 A 1 Top burner Bottom burner switch fuel/oxidizer side CH 2 O LIF on the fuel side Strong? cold flow w. laser flame w.o laser flame w. laser flame w. laser Clean background chemiluminescence CH 2 O LIF CH 2 O LIF weak without discharge 10

12 Temperature (K) CH 2 O formation in CH 4 and DME ignition Mole fraction Temperature (K) Mole fraction Same chemiluminescence before CH 4 plasma assisted ignition Different chemiluminescence before DME ignition CH 4 /O 2 /He (0.15/0.55/0.3) P = 72 Torr, T 0 = 900 K DME/O 2 /He (0.1/0.55/0.35) P = 72 Torr, T 0 = 600 K T OH CH 2 O Low CH 2 O Time (sec) T OH CH 2 O Formation of CH 2 O, H 2 O 2, HO Time (sec) High T ignition Marked by OH Low T ignition CH 2 O can be used as a marker High T ignition H 2 O 2 2OH

13 CH 2 O PLIF (a.u.) CH 2 O measurements: ignition and extinction CH 2 O PLIF (a.u.) P = 72 Torr, a= 250 1/s, f = 24 khz X O2 =40%, varying X f P = 72 Torr, a= 250 1/s, f = 34 khz, X O2 =60%, varying X f 6x10 5 5x10 5 4x10 5 Extinction LTC 6x10 5 5x10 5 4x10 5 LTC HTC increase decrease 3x10 5 2x10 5 1x10 5 increase decrease HTC Hot Ignition Fuel mole fraction Competition between S-Curve 3x10 5 2x10 5 1x10 5 New ignition/extinction curve without extinction limit low T RO 2 kinetics high T chain branching reactions Fuel mole fraction 12

14 CH 2 O mole fraction CH 2 O PLIF (a.u.) Kinetics of plasma assisted low temperature combustion D modeling of DME/O 2 /He (0.03/0.1/0.896) ignition, P = 72 Torr, T 0 = 650 K No O addition 1000 ppm O addition T=650 K R+O 2 CH 2 O+radicals 90% of total reaction flux Sensitive to O 2 concentration? 6x10 5 5x10 5 4x10 5 3x10 5 5% DME on fuel side times faster! Residence time (ms) Implication Plasma assisted combustion dramatically changed the low temperature chemistry Faster LTC Plasma Slow LTC Turbulent transport LTC in Plasma assisted combustion LTC in turbulent combustion at engine time scales 2x10 5 1x10 5 Fixed X f on fuel side, while changing X O2 on the oxidizer side O 2 mole fraction Prompt radical production from plasma Fast H abstraction (formation of R) Fast LTC (RO 2 reactions) Results can be extended to other large fuels 13

15 2. Multispecies diagnostics in a flow reactor (Task 3: Multispecies measurements) In situ intermediate species diagnostics beyond radicals

16 2a Multispecies Diagnostics in Repetitively-pulsed Nanosecond Discharge in a Laminar Flow Reactor Experimental setup Diluents Detector Electrode Nanosecond- Pulsed Power Supply Oxidizer Fuel Diluents Observation Window Detector Germanium Etalon Vacuum Chamber Vacuum Pump Pulsed Signal Generator Digital Delay Generator Mini-Herriott cell showing 24 pass configuration QCL Laser Oscilloscope Function Generator Reactor/Diagnostics Reactor size: 58.2 x 14 x 152 mm 3 Fuel: C 2 H 4 Pressure: 60 Torr Flow speed in the reactor: ~40 cm/s Mid-IR QCL laser: 1296 cm cm -1 Multi-pass Mini-Herriott cell (12.7 mm OD) Plasma Properties Electrode (40 x 45 mm 2 ) Repetitively -pulsed nanosecond DBD discharge 0-40 khz pulse repetition rate 12 nanosecond pulse duration 5-20 kv peak voltage

17 Direct and ICCD Images of Plasma Discharge in a Reactor Stoichiometric mixtures: C 2 H 4 /O 2 with 75% AR, 60 Torr, V max = 6 kv Direct Image: 1 khz, 3.6 mj/pulse, 2 s exposure time. ICCD images: Gate time = 100 ns, Gain = 250 Direct ICCD 1000 Hz 1000 Hz Cathode Anode 2000 Hz 3000 Hz

18 Absorption Spectroscopy Beer-Lambert Law I I 0 exp, P, T NL exp Si T gv i NL i I ν = Transmitted Signal I 0ν = Laser Signal α = Absorption coefficient i = Denotes absorption line with center frequency ν i ν = Light wavelength S = Line strength of absorption line T = Temperature L= Path length of light N = Number density of absorbers g = Voigt profile line broadening function Multispecies diagnostics: Line strengths from HITRAN database for H 2 O, C 2 H 2, CH 4, C 2 H 4, C 2 H 6, CO 2, CO, O 3, OH, HO 2, H 2 O 2, CH 2 O, NO, N 2 O, NO 2 Temperature measurements: Line strength on S i (T) for temperature measurements Species sensitivity: Multipass and Wavelength modulations

19 Absorption spectrum and wavelength scan Signal vs. laser scan time and etalon fringes Calibrated wavelength vs. time 0.6 Absorption Background Etalon Laser Scan C2H2 CH4 300 Scan Rate [cm-1/s] Signal [Arb. Units] Etalon Data 0.1 Polynomial Fit Time [s] Time [s] 0.003

20 Signal Strength Transmittance Etalon noise /signal, % Multipass Mini Herriott Cell Signal and Noise Properties H 2 O: cm -1 26, 59.57mm 36, 61.23mm 10, 64.65mm 44, 66.67mm 34, 67.86mm 24, 69.19mm E E E E-03 Time, s 12.7 mm in cell diameter Number of passes Noise to Signal Ratio Absorption Depth Increase of pass number increase the sensitivity but a very high pass number causes large etalon noise Number of Passes

21 Measurement of CH 4 : Direct Absorption vs. Wavelength Modulation Absorbance 1 st harmonic signal ( t) 0 asin(2 ft) ppm,Expt 5ppm,Expt CH 4 /N2 60 torr Wavenumber, cm -1 Direct absorption measurement of CH f=50 khz 1 MHz Time, s 30ms,1ppm 30ms,5ppm 30ms,10ppm Wavelength modulated absorption measurement of CH 4

22 Mole Fraction [ppm] Mole Fraction [ppm] CH 4 /C 2 H 2 production by plasma: pyrolysis vs. oxidation Ar/C 2 H 4, fuel mole fraction of , 60 torr Ar/O 2 /C 2 H 4 mixtures, 25% reactants and φ=1. Same fuel concentration [C2H2] with O2 100 [CH4] with O2 0 [C2H2] w/o O Plasma Frequency [Hz] 0 [CH4] w/o O Plasma Frequency [Hz] 4000

23 Energy per Pulse [mj] Effects of plasma frequency on temperature and species Ar/O 2 /C 2 H 4 mixtures with 25% reactants and φ=1, 60 torr Power [W] Mole Fraction [ppm] Temperature [K] Total Energy [mj] Power [W] Plasma Frequency [Hz] [CH4] with O2 [C2H2] with O2 [H2O] Temperature Plasma Frequency [Hz]

24 Reaction products Plasma Reactor exit 2b. Measurements of H 2 O 2 and Intermediate Species in Low Temperature Dimethyl Ether (DME) Oxidation Task 3. Species Measurements by molecular beam mass spectrometry (MBMS) 1 st Turbo pump 2 nd Turbo pump Skimmer Mass analyzer Quartz nozzle atm 10-4 Torr Molecular beam Charged ion separation 10-6 Torr 23

25 The role of intermediate species, HO 2, H 2 O 2 in low/high temperature kinetics Transition from low T to Low T ignition high T ignition H 2 O 2 /H 2 O 2 formation H 2 O 2 2OH Low temperature Intermediate temperature High temperature Schematic of low temperature ignition process However, measurements of H 2 O 2 and HO 2 is difficult Indirect measurement of H 2 O 2 : Sensitive H 2 O absorption at 2.5 um (Hong et al, 2009). Direct measurement: UV Photo fragmentation-oh LIF, (Li, et al, PCI, 2012). Difficult to separate H 2 O 2 from HO 2, and other large hydrocarbons

26 Experimental setup Cross validation with MBMS mgc P = 1 atm, τ = 1.7 s 25

27 Mass spectrum and calibration DME/O 2 /He (0.02/0.1/0.88) at T = 590 K and P = 1 atm (HCOOH) Calibration: H 2 O 2 : H 2 O 2 /H 2 O (30.8% wt) + He Corrected H 2 O 2 concentration via 2H 2 O 2 2H 2 O + O 2 and subtraction of 34 O 2 signal CH 2 O, CH 3 OCHO: Measured D, σ from Ref.[1] Fragmentation: Constant ratio, can be removed from post processing S S i He D D i He i He i He Mass overlap: DME & HCOOH, N 2 & CO S : signal intensity D : mass discrimination factor : cross sections : mole fractions

28 Major species measurements Good agreement between micro-gc and MBMS quantification validation of MBMS techniques 2-D simulations give good agreement with data; 0-D simulations are semi-quantitative II. loti It--t-~ g ~ ~ ~ I I u GC (c:m I),, / ONfl.()72 l\ l RI\ 1S (l' 1Ul~ I) MBMS (<:ISC 2) \..-.-./ --Zh:tu etfl/{l::tsc I) - -Yasunall:\ ~:1 a/ ~ ca~c J I Z b:to "',-,/ { Cli~t' 2) * 20 m odr:ling (7.hao ei OJ!. i :a~" 1) ~50~0--~5~5~0----6~00--~-65~0~~7~0-0~~7~50 Temperature (K) J8000~D~G~' C~' (~<:~IS~c ~l) , MIIMS (<a>< I ) e MBMS (<a><l) Zhao rl rtl (ras<" 1) / J 4000 Yusunn~ cr 111 (rase/ 1) 5 Zhau er ttl (case 2) :: * 2 D mod d;u ~ /,,.,.._.. (Zhao el,,,, ca:;e I ) 1,. ~ "';!::: = = '"' 0 :.I 0 u / / * \ n!i t 0 I \ \ o... ~~~:.... J~ _...J...,.----~..._~ \ \ Temperature (K) (a ) (c) = I -~ <J I D "l ~ " D " E ~ I ~ I ~ GC (rase l ) O. OOO ~50 :..: :.:...=:.;;;55~.::.0=,;_; 6--"0L :... 6~5-0. 7_.0_ ,50 r- T~e ~n~e ~ a~t~u~re~(~k~' )~ , 4000 o GC (rase I ) 3600 MOMS (Ca.jC I) --Zh:\0 el nl (case 1) --. E Ya"""'~ad"l(rasc IY * 20 modeli ng ~ :: 2800 IZh oo ct 11l, <>SCI)! --. ~ E 2ooo... ~ 1600 <J = o" soo u 4011 o ~~~~~~~;s~~ 51lll High CO 2 production 0-D Models show reasonably good agreement for LTC temperature window and peak reactivity Yasunaga et al model has overall higher reactivity and wider LTC temperature window \ \ Temperature (K) (ll) 27

29 H 2 O 2 concentration (ppm) H 2 O 2 measurement case 1 MBMS Zhao et al Yasunaga et al 2D modeling (Zhao et al) Temperature (K) Good agreement for H 2 O 2 formation Different predictions from different models 28

30 2c. Development of a Mid-IR Faraday Rotational Spectroscopy Method to quantify HO2 29

31 Quantitative HO 2 Measurement (very challenging!): Mid infra-red Faraday Rotation Spectroscopy (FRS), 1396 cm -1 2L + 1 Polarization rotation detection paramagnetic species 610 Hz oscillating magnetic field 125 Gauss rms 7.1 μm Linearly-polarized laser light Brian Brumfield, Wenting Sun, Gerard Wysock, and Yinguang Ju, submitted to JACS,

32 Sub-ppm level HO 2 measurement in DME/air flow reactor (1atm, 748K) H+O2(+M) = HO2(+M), HCO+O2= CO+HO2 HCO+(M)=CO+H

33 Temperature Dependence of HO2 Signal in a flame reactor Weighted observations Raw Signal Observation Game changer?!

34 3. Ignition Enhancement and the critical ignition energy by Pulsed Nanosecond Discharge - Pulse Detonation Combustor/Engine (with Timothy Ombrello, Fred Schauer, and John Hoke of the AFRL) Thrust 1 Task 6. Ignition Initiation Time and Minimum Ignition Energy

35 Energy per pulse [mj] Motivation: Demonstrate non-equilibrium plasma enhances ignition in a real PDE vs. a spark plug. Proof-of-concept studies have shown decrease in ignition time for propane/air mixtures in a quiescent environment and atmospheric pressure using repetitively pulsed nanosecond discharges 1 Depositing more energy faster has potential benefits for short residence-time, highly turbulent environments present in a range of propulsion devices Power Supply: Nanosecond power supply delivers 12-ns pulses up to 40 kv (peak) & 40 khz 1-5 mj/pulse deposited into gas Experiment: Spark plug machined into point-to-point electrode geometry with a 1.4 mm gap Nanosecond discharge compared with lab standard Multiple Spark Discharge (MSD) Consumes 115 mj/pulse but deposits only 4-8 mj/pulse into gas Gives multiple sparks of the same energy each. Number of sparks cannot be controlled Ion probes used to quantify wavespeed Ignition is determined when pressure trace reaches a slope of 5 V/s on PCB trace Schlieren imaging performed at 100,000 fps 1. S. V. Pancheshnyi, D. A. Lacoste, A. Bourdon, C. O. Laux, IEEE Trans. On Plasma Science, vol. 34 (2006) ,000 1,500 Time [ns] Sample Voltage Trace

36 Ignition Time [ms] Ignition Time [ms] Ignition Time [ms] % Decrease in Ignition Time Aviation gasoline/air Mixtures Equivalence ratio is varied along with number of pulses at fixed plasma energy/pulse and plasma frequency Nanosecond pulser decreases ignition time up to 25% compared to MSD Pulsed discharge allows more energy to be coupled into gas in a shorter time period than MSD ignition system. Advantageous for the turbulent, small residencetime flows in the PDE Plasma properties: Plasma energy: 2.8 mj/pulse on average Plasma frequency: 40 khz MSD spark system currently in use: Spark energy: 5.7 mj/spark Multiple sparks (1-12 possible) Spark frequency: 0.87 khz Pulse energy and plasma frequency are varied at fixed equivalence ratio Total energy = energy/pulse x number of pulses Ignition time decreases with total energy for nspulser case ns pulser lean limit Eq. Ratio= Total Energy [mj] 2 Pulses 20 Pulses MSD MSD Nanosecond Pulser MSD lean limit MSD rich limit Equivalence Ratio % 25% 20% 15% 10% 5% 0% -5% -10% Eq. Ratio= Total Energy [mj] Equivalence Ratio ns pulser rich limit MSD Nanosecond Pulser 2 Pulses 20 Pulses

37 Schlieren Imaging ns pulser, 3 mj/pulse, 40 khz Comparison with conventional ignition Φ=1 Ethylene/Air Top: ns pulser, 20 pulses at 40 khz Bottom: MSD, 3 sparks at 0.87 khz Time shown is 3 ms after first discharge MSD, 6 mj/spark ns pulser, 3 mj/pulse, 40 khz Effect of high frequency Φ=1 Methane/Air Top: ns pulser, 5 pulses at 40 khz Bottom: ns pulser, 5 pulses at 1 khz Time shown is 7 ms after first discharge ns pulser, 3 mj/pulse, 1 khz ns pulser, 3mJ/pulse, 40 khz, 6 pulses MSD, 6 mj/spark, 0.87 khz, 3 sparks Lean equivalence ratio, equal energy Φ=0.8 Methane/Air Top: ns pulser, 6 pulses at 40 khz Bottom: MSD, 3 sparks at 0.87 khz Time shown is 7 ms after first discharge

38 Flame speed, S b [cm/s] Ignition/ Flame initiation/critical radius Three distinct flame regimes Regime I Spark assisted ignition kernel Regime II Transition from ignition kernel to normal flame Weak flame regime Regime III Self-sustained stable propagating flame Consistent with previous study 2 Ignition failure vs. Critical radius (a) 5.7 ms Ignition Regime I Linear extrapolation Rapid rise 2 ms Regime II Regime II Critical radius Regime III Flame radius, R f [cm] Ignition Stretch rate K [s -1 ] < n-decane/air at ϕ 0.7, 1 atm, 400 K > (b) Kim et al., Proc. Combust. Institute, Vol. 34,

39 Measurements of critical flame radius for ignition vs. pressure Critical radius, R c [mm] TC Heated tube Liquid fuel injection N 2 /O 2 / Vacuum pump Fan P TC electrodes Oven Pressure release tank Vaporization chamber TC: Thermocouple P: Pressure gauge Heater Heater n-decane/air 0.7 What is the effect of plasma discharge volume? What is the effect of turbulence? 8 6 Present Chaos et al. model (2007) Pressure [atm]

40 Conclusions 1. In situ discharge can significantly increases the kinetic effect of plasma and achieve sublimit combustion. 2. A new monotonic ignition transition regime was observed with PAC. 3. PAC enhances low temperature chemistry and may change combustion kinetics in engine conditions with very short residence time. 4. PAC shortens ignition delay time in turbulent PDE combustion environment. Large volume discharge helps to drive the ignition kernel to overcome the critical flame radius at reduced pressure. 5. A reactor coupled mid-infra red absorption spectroscopy and MBMS system are developed and successfully measured H2O2 and other intermediate species. 6. A mid-infrared Faraday rotation spectroscopy method is developed and successfully measured HO2 in a flow reactor.

41 Acknowledgement This work was supported by the plasma research grant from the Air Force Office of Scientific Research (Drs. Chiping Li, Julian Tishkoff). 40

42

43 Voltage [kv] Current [A] Total Energy [mj] Measurement Technique Voltage [kv] Current [A] Time [ns] Current and voltage are measured for each condition Voltage probe: LeCroy high voltage probe (PPE20KV) Current probe: Pearson Coil (Model 6585) Peak voltage for all experiments 6 kv The total energy is computed by integrating the power over a long enough time scale for all reflections to be included Time [ns]

44 1 st harmonic signal of H 2 O 1 st harmonic signal of H 2 O H 2 O and temperature measurements with plasma discharge H 2 O lines at cm -1 and cm -1 Laser scan: 100 Hz, f=1 MHz, t RC = 7.5 μs Voigt profile fitting HITRAN for number density and temperature 6.E-03 4.E-03 2.E-03 2kHz, I 3kHz, I 4.E-18-2.E-03-4.E-03-6.E Wavenumber, cm -1 6.E-03 4.E-03 2.E-03 2kHz, I 3kHz, I 4.E-18-2.E-03-4.E-03-6.E Wavenumber, cm -1 HITRAN: J. Quant. Spectrosc. Radiat. Transfer, 111, (2010).

45 1 st harmonic signal 2 nd harmonic signal Wavelength modulated absorption measurement of CH 4 f=50 khz 1 MHz CH 4 /N2 60 torr Time, s ( t) 0 asin(2 ft) 30ms,1ppm 30ms,5ppm 30ms,10ppm CH 4 /N2 60 torr Laser was scanned at 0.1Hz and modulation at along with using lock in amplifier Time, s 30ms,1ppm 30ms,5ppm 30ms,10ppm

46 Energy per Pulse [mj] Power [W] Temperature [K] Results for Continuous Plasma Total Energy [mj] Power [W] Plasma Frequency [Hz] Plasma Frequency [Hz] Results are for Ar/O 2 /C 2 H 4 mixtures with 25% reactants and φ=1 The flow speed is 40 cm/s and the pressure is 60 Torr Per pulse energy is dependent on plasma repetition frequency Seed electrons and ions left over from previous pulse provide for easier breakdown This effect levels off after about 1000 Hz At high pulse repetition frequency, temperature scales linearly with plasma power

47 OH density (cm -3 ) Maximum temperature (K) Classical S-curve hysteresis between ignition and extinction: S curve 5x10 15 OH density O2 =34% Temperature x Extinction x x x Ignition Fuel mole fraction Rayleigh Scattering [1,2] method for T measurement at 532 nm from Nd:YAG laser Relationship between OH density, local maximum temperature and fuel mole fraction, T o =650 K, T f =600 K He/O 2 = 0.66:0.34, P = 72 Torr, f = 24 khz, a = 400 1/s [1] R.B. Miles, W.R. Lempert, J.N. Forkey, Meas. Sci. Technol [2]J.A. Sutton, J.F. Driscoll, Exp Fluids

48 OH density (cm -3 ) Maximum temperature (K) OH density (cm -3 ) Maximum Temperature (K) Maximum temperature (K) S-curve transition Relationship between OH density, local maximum temperature and fuel mole fraction, P = 72 Torr, f = 24 khz, a = 400 1/s He/O 2 = 0.45:0.55 He/O 2 = 0.38:0.62 6x x x x x10 15 O2 =55% x10 15 w. plasma 900 w.o. plasma 800 Temperature Fuel mole fraction ignition and extinction points were pushed to lower fuel concentrations 7x x x K 1300 Can the hysteresis be removed? 1250 K 4x x10 15 O2 =62% K x10 15 Extinction x10 15 Fuel mole fraction OH density 800 Temperature Fuel mole fraction monotonic ignition and extinction curve (monotonic S-curve) 47

49 1. New flame and ignition regimes with in situ nano-second pulsed discharge OH number density (cm -3 ) a= 400 1/s, X o = 55%, X f = 20%, f = 24 khz, P=72 Torr, UV power = 2 mj/pulse 7x x10 15 O2 =34% O2 =62% Coupling effect Radical generation 5x x x x10 15 Monotonic S-curve Extinction 1x10 15 Ignition Fuel mole fraction Radicals produced by in situ discharge : Dramatically increased the reactivity of CH 4 (no extinction limit) Same chemiluminescence before CH 4 ignition CH 4 + radicals (O/OH/H ) CH 3 O/HCO chain branching + radicals Jet fuels Large hydrocarbons R chain branching + radicals + O 2 RO 2 Low Temperature Chemistry (LTC) Different chemiluminescence before DME ignition Ignition How does LTC affect ignition and extinction? CH 2 O

50 CH 2 O mole fraction CH 2 O PLIF (a.u.) CH 2 O PLIF (a.u.) Kinetic effect of plasma assisted low temperature combustion for CH 3 OCH 3 ignition CH 2 O PLIF measurements at 355 nm to characterize LTC P = 72 Torr, a= 250 1/s, f = 24 khz, X O2 =40%, varying X f P = 72 Torr, a= 250 1/s, f = 34 khz, X O2 =60%, varying X f 6x10 5 5x10 5 4x10 5 3x10 5 2x10 5 1x increase decrease Extinction LTC HTC Hot Ignition Fuel mole fraction Plasma assisted low temperature chemistry No O addition 1000 ppm O addition T=650 K R+O 2 CH 2 O+radicals 90% of total reaction flux 1000 times faster! Residence time (ms) Smooth transition between LTC to HTC Increased radical production 6x10 5 5x10 5 4x10 5 3x10 5 2x10 5 1x10 5 LTC HTC increase decrease Fuel mole fraction Plasma assisted combustion dramatically changed the SPEED of low temperature chemistry Slow LTC Important for PAC Turbulent combustion at small time scales Kinetic studies

51 Importance of LTC and the critical role of H 2 O 2 Low T ignition H 2 O 2 is stable Transition from low T to high T ignition H 2 O 2 2OH H 2 O 2 : low T chemistry indicator How to detect? Indirect measurement: Sensitive H 2 O absorption at 2.5 um (Hong et al, 2009) Direct measurement: Laser absorption at 7.8 um at low pressure non-reactive flow (Aul, et al, PCI, 2011) H 2 O 2 /H 2 O/Ar mixture in shock tube Photofragmentation-LIF (Li, et al, PCI, 2012) HCCI H 2 O 2 LTC DME MBMS Low Pressure MBMS for flame Different masses Mass spectrometry In-situ and high pressure? Interference with HO 2 and H 2 O Calibration (H 2 O 2 decomposes > 55 o C) 1 Challenging for combustible mixtures 1. Ludwig, et al, JPC. A