Laser-based measurements of gas-phase chemistry in non-equilibrium pulsed nanosecond discharges (original) (raw)
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Laser Ignition of Methane-Air Mixtures at High Pressures and Diagnostics
Journal of Engineering for Gas Turbines and Power, 2005
Methane-air mixtures at high fill pressures up to 30 bar and high temperatures up to 200°C were ignited in a high-pressure chamber with automated fill control by a 5 ns pulsed Nd:YAG laser at 1064 nm wavelength. Both, the minimum input laser pulse energy for ignition and the transmitted fraction of energy through the generated plasma were measured as a function of the air/fuel-equivalence ratio (). The lean-side ignition limit of methane-air mixtures was found to be ϭ2.2. However, only Ͻ2.1 seems to be practically usable. As a comparison, the limit for conventional spark plug ignition of commercial natural gas engines is ϭ1.8. Only with excessive efforts ϭ2.0 can be spark ignited. The transmitted pulse shape through the laser-generated plasma was determined temporally as well as its dependence on input laser energy and properties of the specific gases interacting. For a first demonstration of the practical applicability of laser ignition, one cylinder of a 1 MW natural gas engine was ignited by a similar 5 ns pulsed Nd:YAG laser at 1064 nm. The engine worked successfully at ϭ1.8 for a first test period of 100 hr without any interruption due to window fouling and other disturbances. Lowest values for NO x emission were achieved at ϭ2.05 ͑NO x ϭ0.22 g/KWh͒. Three parameters obtained from accompanying spectroscopic measurements, namely, water absorbance, flame emission, and the gas inhomogeneity index have proven to be powerful tools to judge laserinduced ignition of methane-air mixtures. The following effects were determined by the absorption spectroscopic technique: formation of water in the vicinity of the laser spark (semi-quantitative); characterization of ignition (ignition delay, incomplete ignition, failed ignition); homogeneity of the gas phase in the vicinity of the ignition; and the progress of combustion.
Proceedings of the Combustion Institute, 2019
This work aims to provide a better understanding of the cumulative effect of successive nanosecond repetitively pulsed (NRP) discharges on the ignition process. Fast chemiluminescence imaging of both the postdischarge and the following flame was used to analyze the ignition of a lean propane/air mixture (φ = 0.7) by a train of 82 NRP discharges at a pulse repetition frequency (PRF) of 30 kHz, in comparison with the traditional spark ignition. The transition from the non-equilibrium plasma to the ignition kernel has been imaged. It has been found that, differently from traditional spark ignition and despite similar experimental conditions, each NRP ignition develops in a unique way. NRP discharges generate both highly reactive species and thermal instabilities in the gap: the multi-pulse strategy produce a gas motion resulting in a jetting phenomenon, reported here for the first time. An algorithm within ImageJ software was used to quantitatively describe the observed jetting phenomenon. The results presented in this work suggest that jetting is the driver of the ignition initiated by NRP discharges, at least in the experimental conditions investigated here.
Paper # 070RK-0008 Topic: Reaction Kinetics 8 th U. S. National Combustion Meeting
2016
Ignition delay times for five low-vapor-pressure biodiesel surrogates were measured behind reflected shock waves, using an aerosol shock tube. These fuels included methyl decanoate (C 11 H 22 O 2 , CAS: 110-42-9), methyl laurate (C 13 H 26 O 2 , CAS: 111-82-0), methyl myristate (C 15 H 30 O 2 , CAS: 124-10-7), and methyl palmitate (C 17 H 34 O 2 , CAS: 112-39-0), all of which have a fully saturated alkane chemical structure. This study also examined a methyl oleate (C 19 H 36 O 2 , CAS: 112-62-9) / Fatty Acid Methyl Ester (FAME) blend. Experiments were conducted in 4% oxygen/argon mixtures with the exception of methyl decanoate which was studied in 1% and 21% oxygen/argon blends. Reflected shock conditions covered temperatures from 1026 to 1388 K, at pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.3 to 1.4. Arrhenius expressions describing the experimental ignition delay time data are given and compared to those derived from applicable mechanisms available in the literature. Graphical comparisons between experimental data and mechanism predictions are also provided. Experiments of methyl laurate, methyl myristate, and methyl palmitate represent the first shock tube ignition delay time measurements for these fuels. Finally, experiments with methyl palmitate represent, to the authors' knowledge, the first neat fuel/oxidizer/diluent gas-phase experiments involving a fuel which is a waxy solid at room temperature.
Theory of laser-enhanced ionization in flames - Comparison with experiments
JOURNAL DE PHYSIQUE, 1983
A b s t r a c t -Opto-galvanic o r laser-enhanced i o n i z a t i o n ( L E I ) spectroscopy has been performed on t r a c e elements i n a flame u s i n g one-and two-step l a s e r e x c i t a t i o n s . The s e n s i t i v i t y , d e f i n e d as t h e LE1 s i g n a l d i v i d e d by t h e l a s e r p u l s e energy and t h e c o n c e n t r a t i o n o f t h e t r a c e element i n t h e water s o l u t i o n a s p i r a t e d i n t o t h e flame, has been measured f o r a number o f elements. A t h e o r e t i c a l model f o r t h e LE1 process has been developed and t e s t e d . The importance o f enerny l e v e l s , n o t i n v o l v e d i n t h e l a s e r t r a n s i t i o n s , i s emphasized. The agreement between t h e o r y and experiment i s s a t i s f a c t o r y . P o s s i b l e reasons f o r d i s c r e p a n c i e s which a r i s e a r e discussed. The i n c r e a s e o f i o n i z a t i o n r a t e due t o l a s e r i r r a d i a t i o n o f atoms i n a flame causes a charge increase, which can be measured b y a p p l y i n g a v o l t a g e o v e r t h e r e g i o n o f i n t e r a c t i o n . T h i s i s t h e b a s i c p r i n c i p l e o f o p t o g a l v a n i c spectroscopy (OGS) o r l a s e renhanced i o n i z a t i o n spectroscopy (LE1 ). Laser-enhanced i o n i z a t i o n s i g n a l s have been observed f o r a wide range o f elements s t u d i e d i n flames and discharges /l/. D u r i n g r e c e n t years LE1 spectroscopy has e s t a b l i s h e d i t s e l f as a powerful technique f o r t r a c e element a n a l y s i s i n flames /2/. Many elements have been i n v e s t i g a t e d w i t h t h i s technique u s i n g b o t h one-and two-step l a s e r e x c i t a t i o n s . D e t e c t i o n l i m i t s o f sub-parts p e r b i l l i o n (sub-ppb) have been reached /2/, w i t h o u t s a c r i f i c i n g t h e convenience o f t h e a n a l y t i c a l flame as an atomizer. A s t u d y o f t h e v a r i o u s processes t h a t c o n t r i b u t e t o t h e o p t o g a l v a n i c e f f e c t has been undertaken by Turk e t a l . /3/. Here the t h e o r e t i c a l emphasis was on t h e v e r y simple t h r e e -l e v e l model f o r t h e LE1 s i g n a l . I n o u r l a b o r a t o r y we have performed LE1 experiments i n flames u s i n g one-and two-step l a s e r e x c i t a t i o n s . F i g u r e 1 shows s c h e m a t i c a l l y o u r experimental set-up. Two-step e x c i t a t i o n was achieved by u s i n g two separate t u n a b l e dye l a s e r s pumped by t h e same l a s e r . An a i r -a c e t y l e n e flame a t atmospheric pressure u t i l i z i n g a s i n g l e -s l o t b u r n e r was used. A w a t e r s o l u t i o n w i t h t h e a n a l y t e element i s a s p i r a t e d i n t o t h e flame. The temperature o f t h e h o t t e s t p a r t o f t h e flame i s assumed t o be approximately 2200 K.
Impact of nanosecond repetitively pulsed electric discharges on the ignition of methane-air mixture
2015
This article presents an analytical model to describe plasma discharges effects on gas temperature and species dissociation that control autoignition in reactive systems. The model is constructed based on experimental and numerical results of Nanosecond Repetitively Pulsed (NRP) discharges in air and evaluated against the existing experimental data. The model is fully coupled with multi-dimensional flow balance equations where detailed transport coefficients and chemical kinetic mechanism are considered. Sequence of discharge pulses in air and methane-air mixture are computed by means of Direct Numerical Simulations in quiescent and turbulent flow configurations. Gas temperature, pressure and O atoms evolution during NRP discharges in air are in good agreement with experimental results. Ignition phenomenon through NRP discharges of a methane-air mixture is analyzed. The results show an accumulation of vibrational energy in the vicinity of the discharge zone due to the former dischar...
2013
Nanosecond repetitively pulsed (NRP) discharges are being increasingly used in various applications, in particular in plasma-assisted combustion and aerodynamic flow control. First, we studied the thermal and hydrodynamic effects of NRP discharges using quantitative Schlieren measurements and numerical analyses in atmospheric pressure air. The time resolved images show the expansion of the heated gas channel starting from as early as 50 ns after the discharge and the shock-wave propagation from about 500 ns. Gas density profiles simulated in 1-D cylindrical coordinates are used to reconstruct numerical Schlieren images for comparison with experimental ones. We propose an original method to determine the initial gas temperature and the fraction of energy transferred into fast gas heating, using a comparison of the contrast profiles obtained from experimental and numerical Schlieren images. The results show that a significant fraction of the electric energy is converted into gas heati...
Cavity Flow Ignition and Flameholdingin Ethylene-Air by a Repetively Pulsed Nanosecond Discharge
47th AIAA Aerospace Sciences Meeting including The New Horizons Forum and Aerospace Exposition, 2009
The results demonstrate that repetitive nanosecond pulse plasma cavity ignition of ethylene-air flows at moderate discharge pressures, P=0.2-0.25 atm, occurs via formation of multiple arc filaments in the fuel-air plasma. Plasma filamentation occurs in the nanosecond pulse discharge in a fuel-air mixture, although air plasma remains diffuse and low-temperature until the fuel was added. At these conditions, it is difficult to isolate thermal ignition in high-temperature filaments from nonequilibrium plasma chemical fuel oxidation and ignition due to radical generation in the plasma. After ignition occurs in the cavity, the flame couples out to the main flow, extinguishes, and is reignited within a few tens of milliseconds. Main flow ignition frequency at P=150 torr decreases from 30 Hz at u=35 m/sec to approximately 10 Hz at u=65 m/sec. It is demonstrated that that repetitive ignition occurs due to a slow rate of mixing between the main flow and the cavity, which results in combustion products remaining in the cavity for extended periods of time. To counter this effect, a small fraction of the main premixed fuel-air flow was injected into the cavity. At higher flow velocities, u=50-70 m/sec, this approach considerably reduced delay time between ignition events and increased burned fuel fraction. However, no continuous flame was sustained either in the cavity or in the main flow. Comparison of repetitive nanosecond pulse discharge ignition with DC arc discharge ignition, with both discharges sustained in the cavity of the same geometry demonstrated that the use of a DC discharge results in longer delay time between ignition events, much lower burned fuel fraction, and significantly lower flow velocity at which ignition is produced.
Laser-induced particle formation and coalescence in a methane discharge
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1999
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Non-Equilibrium Kinetic Studies of Plasma-Assisted Combustion using Laser-Based Diagnostics
Zeitschrift für Physikalische Chemie, 2011
Experimental investigation of nanosecond pulsed discharge in premixed CH4/air mixtures at atmospheric pressure has been carried out using laser diagnostics. Electron temperature and number density are measured using laser Thomson scattering. Temperature of neutral molecules is measured by CARS. Finally, OH, CH and CH2O are probed using PLIF to identify their role in the reduction of ignition delay and in the improvement of lean burn capability relative to conventional spark ignition. Measurements are compared with numerical simulations performed using CHEMKIN-based code.
Ignition of Propane–Air Mixtures by a Repetitively Pulsed Nanosecond Discharge
IEEE Transactions on Plasma Science, 2000
Results of an experimental study of the efficiency of the ignition of propane-air mixtures by a high voltage repetitively pulsed nanosecond gas discharge (10 kV, 10 ns, 30 kHz) are presented for the pressure range 0.35-2.0 bar. The measured minimal energy for ignition is found to decrease with the pressure. A significant reduction of the ignition delay and a decrease of the overall combustion duration were obtained by using a train of high-voltage pulses. Spectroscopic measurements in a 1-bar air just after a 10-pulse train (300 µs) of about 10 mJ in total energy show the presence of N, N + ,O,andO + atomic species and a gas temperature increase up to 3000 K.