Non-Thermal Plasmas for Combustion Applications


Carmen Guerra Garcia (Fulbright student), Manuel Martinez-Sanchez (Professor)


Collaborators:


Choon Sooi Tan (Senior Research Engineer) Michaël Lupori, Sébastien Mannaï (visiting students) Giulia Pantalone (UROP)

Motivation 


In non-equilibrium or non-thermal plasmas, electrons are typically much more energetic than heavy species (ions and neutrals) and their energy distribution function may not follow a Maxwell-Boltzmann function. These electrons multiply through cascade ionization and can create large pools of radicals (atoms or molecules with unpaired electrons and a high affinity to react) through electron impact dissociation or electronic excitation followed by dissociative quenching. The share of the energy deposited in the gas between the different electron impact reactions is a strong function of the reduced electric field in the discharge gap and by modifying this parameter different electron impact processes can be favored.
Many situations can benefit from this ‘selective’ creation of reactive species, amongst them challenges arising from combustion applications [1]. Non-thermal plasmas can be used as an artificial injection of radicals to accelerate the chain branching reactions of the combustion process in order to stabilize flames, extend lean flammability limits or reduce ignition delay times [1-5].
At the Space Propulsion Laboratory at MIT we have been exploring high voltage (kV), pulsed nanosecond discharges at high repetition frequencies (kHz) as a means of creating non-thermal plasmas. Future work will try to extend the experiments performed to a practical combustion application.


Approach / Tools 


High voltage nanosecond duration discharges can be used in a repetitive manner to create a sustained pool of short-lived excited species and ions and longer-lived radicals in a gas [6]. Experiments with nanosecond discharges have been performed in different gases at pressures ranging from 0.1-1atm and temperatures up to 1000K. The electrical pulses used had voltages up to 10kV, pulse width of 10ns and maximum repetition frequencies of 30kHz and were triggered in 5-10mm long discharge gaps. The study tried to evaluate the energy share between thermal (gas heating and shock waves generated by the rapid energy deposition) and non-thermal (chemical species produced) processes taking place in the plasma. Measurements taken included: electrical, optical emission spectroscopy and Schlieren photography.


Results


Scaling of the different discharge regimes observed at atmospheric pressure to sub-atmospheric conditions, following the classification in [6], was achieved.

 

Figure 1 - Energy deposition by incident pulse as a function of peak voltage seen by load. Bullets are measured data and the continuous line is the estimated maximum energy that can be delivered to the gas.

Namely, a filamentary-like regime, a more diffuse discharge and a localized discharge in the immediacies of the anode were observed. Typical current and voltage waveforms for the filamentary discharge are shown in Figure 2.

 

Figure 2- Typical current and voltage pulse situation for a filamentary discharge.


Different experiments have been performed to evaluate the effect of the reduced electric field, the applied voltage and the frequency on the energy delivered to the gas and the share of this energy that goes towards thermal effects (Figures 3 and 4) and ionization (Figure 5).

Figure 3- Temperature increase in the gas as a function of cumulative energy deposition (from optical emission spectroscopy measurements).
Figure 4- Shock wave generated by the rapid energy deposition observed through Schlieren photography (S. Mannaï).
Figure 5- Electron number density as a function of energy deposited in the plasma (from optical emission spectroscopy measurements).
A photograph representing the accumulation of many discharges (diffuse mode) as seen by the naked eye is shown in Figure 6.

Figure 6- Diffuse mode as captured by digital camera (exposure time 1s).

  

Acknowledgements


This work has been partially supported by the International Fulbright Science and Technology Award.

References


[1] S. M. Starikovskaia, “Plasma assisted ignition and combustion”, J. Phys. D: Appl. Phys. 39: R265-299 (2006).
[2] W. Kim, M. G. Mungal, M. A. Cappelli, “The role of In situ reforming in plasma enhanced ultra lean premixed methane/air flames”, Combustion and Flame, v 157, n2, 374-383 (2010).
[3] S. A. Bozhenkov, S. M. Starikovskaia and A. Y. Starikovskii, “Nanosecond gas discharge ignition of H2 and CH4 containing mixtures”, Combustion and Flame, 133 (1-2): 133-146 (2003).
[4] G. Pilla, “Etude Expérimentale de la Stabilisation de Flammes Propane-Air de Prémélange par Décharges Nanosecondes Impulsionnelles Répétitives”, PhD. Thesis, École Centrale Paris (2008). 
[5] S. V. Pancheshnyi, D. A. Lacoste, A. Bourdon and C. O. Laux, “Ignition of Propane-Air Mixtures by a Repetitively Pulsed Nanosecond Discharge”, IEEE Transactions on Plasma Sc. V 34, n 6: 2478-2487 (2006).
[6] D. Pai, “Nanosecond Repetitively Pulsed Plasmas in Preheated Air at Atmospheric Pressure”, PhD. Thesis, École Centrale Paris (2008).