1 American Institute of Aeronautics and Astronautics

Nonequilibrium Plasma Relaxation and Thermalization in Hydrocarbon-Air Mixtures

Andrey Starikovskiy1

Princeton University, Princeton, NJ 08544

Nikolay Aleksahdrov2, Moscow Institute of Physics and Technology, Dolgoprudny, RUSSIA

Burning of highly diluted homogeneous mixtures such as ultra-lean mixtures or mixtures involving heavy exhaust gas re-circulation (EGR) can lead to development of very efficient and low pollution engines. The difficulty is to overcome the slow flame propagation and ignition problems for these mixtures. One way of doing it is to use stratified charge operation another by homogeneous charge compression ignition. Both systems suffer however from high unburned hydrocarbon emissions and difficulties in controlling the combustion timing and rates. The approach based on non-equilibrium plasma control of the mixture reactivity and this method utilization for small-scale IC engines has been proposed. Nonequilibrium plasma thermalization rate in hydrocarbon-air mixtures was experimentally investigated. A kinetic model was developed to describe the processes that contribute towards the fast transfer of electron energy into thermal energy under the conditions considered. Calculations based on the developed model agree qualitatively with analyses of high-voltage nanosecond discharge observations.

I.Introduction

Now de-facto the only technology for fuel-air mixtures ignition in IC engines exists. It is a spark discharge of millisecond duration in a short discharge gap. The reason for such a small variety of methods of ignition initiation is very specific conditions of the engine operation. First, it is very high-pressure of fuel-air mixture – from 5-7 atmospheres in old-type engines and up to 40-50 atmospheres on the operating mode of HCCI. Second, it is a very wide range of variation of the oxidizer/fuel ratio in the mixture – from almost stoichiometric (0.8-0.9) at full load to very lean ( = 0.3-0.5) mixtures at idle and/or economical cruising mode. Third, the high velocity of the gas in the combustion chamber (up to 30-50 m/s) resulting in a rapid compression of swirling inlet flow.

Let’s analyze briefly the effect of each parameter on the ignition of the mixture in order to determine the optimal control strategy for the ignition of ultra-economical high-speed engine. Engine efficiency is determined by completeness and the rate of combustion of the fuel-air mixture. The most efficient mode of operation is the mode in which complete combustion occurs in particular point of the phase diagram of the engine (crank angle). In this mode the engine provides the cleaner exhaust (without impurities of CO and CH). Where are the limitations of the possibility of organizing such an optimal control?

The first limitation is the speed of flame propagation – even in pre-heated and compressed gas. Typical flame propagation velocity is about tens of meters per second. Thus, the minimum time of flame propagation from the point of ignition to the walls of the combustion chamber - and, thus, time of energy release in the system even in the stoichiometric mixture is a few milliseconds. That is, the rate of flame propagation in the combustion chamber is a critical parameter, limiting the engine speed. Reduction of the time of flame propagation can be achieved in two ways: 1) flame front velocity increase in the air-fuel mixture (using pre-treatment); 2) travelling distance reduction (using spatially-distributed ignition). Both problems are quite complex in terms of technical implementation.

The first task - increasing the flame propagation speed - is complicated by the wide range of fuel-air ratio in different engine’s modes of operation. Lean mixtures will always produce slower flames, which slow down the energy release rate simultaneously with decreasing of its absolute value. On the other hand, the fast flames in stoichiometric mixtures provoke the additional acceleration of the flame front. This acceleration can lead to the detonation combustion modes with unacceptably large pulsed loads on structural elements. That is why we need an adaptive control of the process, depending on load and engine speed.

Multi-point ignition systems allow to reduce the distance of the flame propagation. Unfortunately such an approach is faced with technical problems. The most serious limitation is lack of space on the surface of the combustion

1 Research Specialist, Mechanichal and Aerospace Engineering Department 2 Professor, Department of Problems of Physics and Power Engineering

49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition4 - 7 January 2011, Orlando, Florida

AIAA 2011-1213

Copyright © 2011 by authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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chamber. Most of the space is occupied by inlet and outlet valves, and the dimensions of the ignition system, as a rule, should be limited to the standard spark size. Another limitation is the presence of conductive surfaces (cylinder head and piston). Near the top dead center (TDC) distance between spark and the piston surface is only a few millimeters and typically almost an order of magnitude smaller than the distance from the location of the spark plug to the cylinder wall. This considerably limits the possibilities of a volume discharge generation in the combustion chamber. The discharge usually develops in the direction of the electric field lines which almost coincide to the shortest path between the electrodes.

The only exception is the barrier discharge. In this case the discharge spreading along a dielectric surface can significantly increase the size of the zone of excitation. Unfortunately, in an IC engine it is practically impossible to provide a significant dielectric surface because of the low thermal conductivity of the latter. Heat flux decrease inevitably leads to higher surface temperature and spontaneous ignition of the mixture well before the optimum conditions.

Electrical discharge provides two mechanisms of the ignition acceleration: heating and radicals production. We are trying to analyze dynamics of nonequilibrium plasma thermalization in hydrocarbon-air mixtures.

II. Streamer Plasma Thermalization

The experimental installation is shown in Figure 1. It consisted of a reaction chamber, a discharge system, a diagnostic system and a signal generator that synchronized the plasma and laser.

The reaction chamber included a ceramic burner, the electrode system and the gas (fuel and air) supply system, as shown in the dotted square in Figure 1. Gas flows were preheated in a thermally insulated region from 300 K up to 800 K. The mixing chamber and plasma nozzle were made of alumina ceramics for additional thermal and electrical insulation. The discharge gap was set directly above the nozzle along the axis of the premixed flow. The diameters of the anode (high-voltage electrode) and cathode (grounded electrode) were 200 μm and 500 μm, respectively. The inter-electrode distance was 8 mm. The plasma was generated by about 20 kV positive pulses of about 10 ns in duration and a less than 1 ns rise time at a repetition rate of 10 Hz. The reactant flow rate was set at about 20 cm/s to allow each discharge pulse to occur in a fresh gas mixture. The optical diagnostic system consisted of an optical emission spectroscope (OES), and a picosecond-gated ICCD camera.

Figure 1. Experimental installation for nonequilibrium plasma relaxation analysis [1].

Temperature measurements were performed via OES together with a thermocouple. OES recorded the rotational

temperature in the discharge zone by capturing the emission of the second positive system of the N2 molecule at 337.1 nm. The gas temperature was stable to within ± 10 K in the discharge gap. Thus, all temperatures shown here were the initial gas temperatures measured by the thermocouple.

Figure 2 shows the temperature measurements results from optical emission spectroscopy (OES) and the thermocouple. By recording the emission of the second positive system of nitrogen molecules at 337.1 nm, the OES gives similar results to those obtained from the thermocouple data. At 300 K, the temperature measured by OES is approximately 342 K, which is roughly 40 K higher than the initial temperature.

3 American Institute of Aeronautics and Astronautics

Figure 2. Rotational temperature measurement using emission of second positive system of

N2 molecules at 337.1 nm, T = 300 K. tdis = 10 ns (discharge phase) [1].

III. Numerical modeling

We simulated numerically the evolution in time of active species during the discharge and in its afterglow and calculated the energy transferred into heating through various channels by the instant t = 1 μs. The initial value of Te was assumed to be in the range 1-10 eV corresponding to the mean electron energy in the discharge. The calculated results are not sensitive to this magnitude because the rate of electron energy exchange is higher for higher energies. Indeed, hot electrons rapidly lose their energy in the inelastic collisions with excitation of electronic states of N2 and O2 molecules; however, when the electron energy decreases below the energy threshold of these processes, the effective frequency of electron energy exchange drops drastically. The reason is that in this case electrons can lose their energy only in elastic collisions and in the inelastic collisions with excitation of vibrational and rotational levels of N2 and O2.

Figure 3 compares the calculated ratio of this energy to the total energy deposited in the discharge phase with the results by [1] and with the ratio calculated under the assumption that 28 % of the energy spent on the excitation of electronic N2 and O2 states is quickly transferred into gas heating (see [2]). We carried out calculations for dry air and for air with 1 % H2O. At E/N = 100 – 200 Td the results obtained agree with the other calculations, which were made for dry air. Here, the fast gas heating is dominated by electron impact dissociation of O2, by quenching of N2(A3Σu

+), N2(B3Πg) and N2(a’1Σu

-) states in collisions with O2 and by quenching of O(1D) states in collisions with N2 [2]. Our calculations showed that the percentage of the energy transferred quickly to gas heating increases with E/N and reaches 44 - 49 % at E/N = 1000 Td, depending slightly on the electron density at t = τ, nef, and humidity. At high E/N, the deposited energy is first spent on ionization and fast heating is controlled by the processes of plasma decay.

Under the conditions considered, the plasma decays due to electron-ion recombination and electron attachment to O2 molecules followed by ion-ion recombination. Electron-ion recombination includes dissociative recombination and three-body recombination with a third body M = e or M = H2O in humid air. The rate of reaction (e +AB+ +e →A + e ) depends strongly on Te. On the assumption that the relaxation of Te is much faster than the plasma decay, we obtained that this reaction is the dominant mechanism of the plasma decay at nef = 1014 or 1015 cm-3. The contribution of this process into the plasma decay decreased when taking into account the evolution in time of Te. Our calculation showed that the rate of this reaction was noticeably less than the rates of dissociative electron-ion recombination when considering the effect of so-called “recombination heating” [3] and the effect of nonideality [4,5]. In this case, the role of this reaction in the plasma decay is relatively small and is reduced to decreasing the rate of electron energy relaxation in the discharge afterglow (see figure 4).

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100 10000

10

20

30

40

50

60

70

28% of energy spent on N2(el) + O2(el)(Popov (2001))

Hea

ting

perc

enta

ge, %

E/N, Td

ne0=1014 cm-3; dry air

ne0=1015 cm-3; dry air

ne0=1014 cm-3; 1 % H2O

ne0=1015 cm-3; 1 % H2O

SDBD, 1 atm, Air FIW-SW, 20 Torr, Air

Figure 3. The percentage of the input energy transferred into gas heating in dry and humid (with 1 % H2O) for 1 μs as a function of the reduced electric field at which the energy was deposited in a high-voltage nanosecond discharge. The calculations were carried out for various values of electron density at the end of the discharge, nef. Closed circles correspond to calculations by [1] and red curve corresponds to the calculations assuming that 28 % of the energy spent on the excitation of electronical N2 and O2 states is quickly transferred into gas heating (see [2]).

10-4 10-3 10-2 10-1

103

104

nef=1014

nef=1014

nef=1015

nef=1015

T e, K

time, s

Figure 4. The evolution in time of the effective electron temperature in the afterglow of the high-voltage nanosecond

discharge sustained in dry air at E/N = 103 Td for nef = (a) 1014 and (b) 1015 cm-3. The solid curves correspond to calculations taking into account three-body electron-ion recombination and the dash curves correspond to calculations

neglecting this process. Figure 5 shows the evolution in time of the densities of charged particles in the afterglow of a discharge sustained

at E/N = 1000 Td in dry air for nef = 1014 or 1015 cm-3. Our calculations showed that in this case the dominant positive-ion species evolves in time in the following way:

N2

+ → N4+ → O2

+ → O4+

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Here, the electron density decreases by an order of magnitude for ~ 15 ns when the dominant positive ion is O2+,

whereas the densities of other ion species are around an order of magnitude lower. However, the rate of dissociative recombination for N4

+ and O4+ ions are an order of magnitude higher than that for O2

+ ions [6,7]. Therefore, the contributions of these processes into electron loss are comparable and the analysis of our results showed that these processes are also important to the plasma decay.

10-4 10-3 10-2 10-11012

1013

1014

O2-

N2+

O4+

N4+

O2+

e

E/n = 1000 Td, nef=1014cm3

Dens

ity, c

m-3

time, s

10-4 10-3 10-2 10-11012

1013

1014

1015

O2-

O4+N2

+

N4+

O2+

e

E/n = 1000 Td, nef=1015cm3

Den

sity

, cm

-3

time, s

Figure 5. The evolution in time of the densities of charged particles in the afterglow of the high-voltage nanosecond

discharge sustained in dry air at E/N = 103 Td for nef = (a) 1014 and (b) 1015 cm-3.

Although the initial electron density was high, the role of electron attachment followed by ion-ion recombination was important to the plasma decay for the following two reasons. Firstly, the rate of electron-ion recombination was relatively low because of the predominance of simple positive ions. Secondly, due to the effect of “recombination heating”, the electron density decayed at the electron temperatures Te ~ 0.1 eV (see figures 16 and 17) at which the rate of three-body electron attachment to O2 has a wide peak [8,9]. The role of the processes with negative ions turned out to be even more important to fast heating because, in dissociative electron-ion recombination, only around one-half the released energy is transferred to gas heating, whereas another half the energy is spent on dissociation of molecular ions and excitation of the products. As opposed to electron-ion recombination, there is reason to believe that almost all ionization energy released in three-body recombination of positive and negative ions, the dominant mechanism of ion-ion recombination under the conditions considered, is transferred into gas heating. Indeed, three-body ion-ion recombination occurs in the following way [9]. One ion loses a fraction of its energy in a collision with a neutral particle in the vicinity of an ion of opposite charge and hence becomes captured by this ion. Then, the charge exchange occurs between the ions and the ionization energy is expected to be transferred to the translation degree of freedom of the particles because excitation of internal states of the particles is inefficient in this process [9].

The analysis of our calculated results showed that, under the conditions studied, ion-ion and dissociative electron-ion recombination made the major contribution into fast gas heating; ion recombination provided a 24 % contribution at nef = 1014 cm3 and a 14 % contribution at nef = 1015 cm-3, whereas the contribution of electron ion recombination was, respectively, 5 and 12 % in these cases. The 1 % addition of H2O does not affect noticeably the percentage of fast heating (see figure 1). In this case, the dominant positive-ion species evolves in a more complicated way (see figure 6):

N2

+ → N4+ → O2

+ → H2O+ → H3O+ → H3O+H2O → H3O+(H2O)2 → H3O+(H2O)3

However, the plasma decays when the dominant positive ions are simple molecular ions (O2+, H2O+ and H3O+) with

relatively low rates of dissociative electron-ion recombination and the situation is similar to that in dry air.

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10-4 10-3 10-2 10-1 1001010

1011

1012

1013

1014

N4+

H9O4+

H7O3+

H5O2+

O4+

O2+

H3O+

N2+

O2+H2O

H2O+

e

O2-

Den

sity

, cm

-3

time, s

10-4 10-3 10-2 10-1 1001010

1011

1012

1013

1014

1015

H9O4+

H7O3+

O4+

H5O2+

H2O+

O2+

N4+

N2+

O2+H2O

O2-H3O

+

e

Den

sity

, cm

-3

time, s

Figure 6. The evolution in time of the densities of charged particles in the afterglow of the high-voltage nanosecond

discharge sustained in humid (with 1 % H2O) air at E/N = 103 Td for nef = (a) 1014 and (b) 1015 cm-3.

IV.Acknowledgements The work was partially supported by Russian Foundation for Basic Research under the project “Nonequilibrium plasma thermalization”, AFOSR under the project “Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion”, and NEQLab Research Company under the program “Reforming of Hydrocarbon Fuels”. References [1] L. Wu, J. Lane, N. Cernansky, D. Miller, A. Fridman, A. Starikovskiy. Plasma Enhanced Combustion in Methane-, Ethane-, Propane- and Butane-Air Mixtures Below Self-Ignition Threshold. The Combustion Symposium. 2010. [2] Moreau E 2007 Airflow control by non-thermal plasma actuators J. Phys. D: Appl. Phys. 40 605 [3] Anikin N B, Starikovskaia S M and Starikovskiy A Yu 2002 Polarity effect of applied pulse voltage on the development of uniform nanosecond gas breakdown J. Phys. D: Appl. Phys. 35 2785 [4] Starikovskaia S M, Anikin N B, Pancheshnyi S V and Starikovskii A Yu 2002 Time resolved emission spectroscopy and its applications to study of pulsed nanosecond high-voltage discharge Proc. SPIE 4460 63 [5] Shcherbakov Yu V and Sigmond R S 2007 Subnanosecond spectral diagnostics of streamer discharges: I Basic experimental results J. Phys. D: Appl. Phys. 40 460 [6] Soloviev V R, Konchakov A M, Krivtsov V M and Aleksandrov N L 2008 Numerical simulation of a surface barrier discharge in air Plasma Pphys. Rep. 34 594 [7] Soloviev V R and Krivtsov V M 2009 Surface barrier discharge modeling for aerodynamic applications J. Phys. D: Appl. Phys. 42 125208 (13pp) [8] Aleksandrov N L, Kindysheva S V, Kirpichnikov A A, Kosarev I N, Starikovskaia S M and Starikovskii A Yu 2007 Plasma decay in N2, CO2 and H2O excited by high-voltage nanosecond discharge J. Phys. D: Appl. Phys. 40 4493 [9] Magne L, Pasquiers S, Gadonna K, Jeanney P, Blin-Simiand N, Jorand F and Postel C 2009 OH kinetic in high-pressure plasmas of atmospheric gases containing C2H6 studied by absolute measurement of the radical density in a pulsed homogeneous discharge J. Phys. D: Appl. Phys. 42 165203 (17pp)