ISPC 20
Comparative numerical analysis of influence of the pulse DBD
discharge on the low temperature oxidation of paraffines and
aromatics.
1
Deminsky M.,2Slavinskaia N., 1Konistapina I.,1Podlesnuk E., 2Riedel U., and 1Potapkin B.
1
RRC « Kurchatov Institute »
123182, Kurchatov sq.,1, Moscow, Russia
2
Institute of Combustion Technology, German Aerospace Center ( DLR ),
70569, Pfaffenwaldring 38-40, Stuttgart,Germany
Abstract
In the presented paper theoretical investigation of effect of the initiation and activation of exothermic
chemical process by applying the DBD plasma have been investigated. The non-equilibrium pulseperiodic discharge was used as a source of the plasma. The detailed mechanisms of the n-butane and
methyl naphthalene combustion have been extended with plasma-chemical reactions. The cross
sections of unknown elementary processes of the plasma electron collisions with hydrocarbons were
calculated basing on results of the extended review of literature data, semi-empirical approaches and
Khimera code for calculation of e-molecule collision characteristics. The special attention has been
paid to the low temperatures (below 1100 K) plasma assisted oxidation reactions, where conventional
combustion mechanism was not further valid. The specific plasma chemistry oxidation path ways
have been included in the model, based on the model of pulse-periodic barrier discharge in framework
of Chemical Workbench computational environment. On this approach the new low temperature
mechanisms of oxidation of n-butane and methyl naphthalene has been constructed to analyze the
possible influence of the plasma on their ignition and combustion under the nanosecond discharge
conditions.
Two main aspects have been investigated numerically: efficiency of the plasma energy for the
initiation of hydrocarbon combustion at low temperatures (T<1000 K) and kinetic features associated
with the plasma assisted combustion of linear and aromatic hydrocarbons. It was shown that the
plasma treatment for an acceleration of the low temperature oxidation is more effective for paraffines
than for aromatics. That can be explained with the typical for paraffines and naphtenes negative
temperature behaviour of ignition at low temperature followed with formation of the large amount of
hydroperoxy radicals isomers. The optimal conditions for the effective plasma energy application for
an initiating of the branched chain mechanism were determined. The plasma energy input below ~10-2
eV/mol corresponds to maximal efficiency of the plasma application. Further increasing of the input
plasma energy accelerates simultaneously the competing chain termination reactions (radical and other
active particles recombination).
Comparative analysis of n-butane and methyl naphthalene combustion in DBD
discharge.
Experiments in the plasma assisted initiation of the small hydrocarbons oxidation performed at
low temperatures [1, 2] indicate preliminary growth of the mixture temperature and the active
formation of intermediate species, like peroxy radicals and molecules, below self-ignition regions.
That is in the good agreement with simulation results [3] which shown that the plasma application is
almost effective on the initial stages of the low temperature ignition: acceleration of the branching
reactions. In the present work, the comparative theoretical analysis of plasma assisted ignition for large
hydrocarbons – n-butane and methyl naphthalene was done. As the plasma assisted combustion is
more interesting for practical applications, these hydrocarbons have been selected as popular
SESSION NUMBER & NAME
Cross section, A
2
components of the practical fuel blends. They can successfully present the typical combustion
properties of n – paraffines and aromatics.
On the first stage of the mechanisms development the existing reaction mechanisms of the nbutane and methyl naphthalene combustion [3, 4] have been extended with plasma-chemical reactions.
The cross sections of unknown elementary processes of the plasma electron collisions with
hydrocarbons were calculated basing on results of the extended review of literature data, semiempirical approaches [5] and Khimera code for calculation of e-molecule collision characteristics [6]
(Fig.1).
Shortly, compilation of data for HC
total cross section, K.Floeder, J.Phys.B,1985
momentum transfer I P Sanches,J. Phys. B: ( 41 (2008) 185202)
(C1-C6) permits to derive semi
Total ioniz cross section Jiao C Q, (2007 J. Phys. D. 40 409)
empirical expression of total cross
Total non-elastique
ethane elastic, A2
section of the hydrocarbons in the
ethane diss.
ethane ion
form:
ethane ion.in C2H4(+)
Qt (CnHy) = [(0.28Ny)+(0.83Nn)]E0.81
10
.
This expression gives estimation with
accuracy about 10-20%. Transport
cross section of the hydrocarbons can
be estimated basing on approach of
1
effective radius at large energies and
estimation collision length at low
energies of the electron. BEB
approach was used for calculation
0.1
n π e4 ⎛ E ⎞
0.1
1
10
100
1000
σ ion = ∑ i 2 f ⎜ ⎟
Energy, eV
3
Cross section, A
2
10
2
10
i
total cross section, app.
ionization, BEB
dissociation, dH=3.75 eV
dissociation, dH=4.63 eV
dissociation, dH=4.27 eV
total dissociation
Transport cross. sec.,L approach
Ii
⎝ Ii ⎠
of unknown data for ionization cross
section. Accuracy of that calculation
is about 40%.
Method of similarity functions of
Janev and Reiter used for estimation
of cross section of HC dissociation by
electron impact:
α
A ⎛ 1⎞
1−
ln ( e + aε ) x
σ ( x, y ) = F ( x, y )
ε Et ⎜⎝ ε ⎟⎠
Typical error of such approximation
is about 70%. It should be noted that
cross section of the methyl
1
10
naphthalene is essentially large than
0.1
1
10
100
1000
cross section of the n-butane at low
E, eV
energies that is the consequence of
Fig.1 Cross sections for n-butane (upper picture) and methyl the large quadrupole moment of the
naphthalene. Cross sections of ethane are shown for molecule.
comparison.
The special attention has been paid to the low temperatures (below 1100 K) plasma assisted oxidation
reactions, where conventional combustion mechanism was not further valid. The specific plasma
chemistry oxidation path ways have been included in the model, based on the model of pulse-periodic
barrier discharge in framework of Chemical Workbench computational environment [6]. On this
approach the new low temperature mechanisms of oxidation of n-butane and methyl naphthalene has
been constructed to analyze the possible influence of the plasma on their ignition and combustion
under the nanosecond discharge conditions.
2
First Author, Second Author. INSTRUCTIONS FOR THE PREPARATION OF PAPERS
Induction time,s
Concentration, 1/cm
3
Simulation of the ignition of the stoichiometric mixtures of the n-butane and methyl
naphthalene was carried out for condition of the plasma of the DBD discharge. The frequency was 20
kHz, energy input per pulse 0.04 J/cm3, electric field strength E/N=200 Td. The time dependence of
the main stable species and radicals for methyl naphthalene case is shown on the Fig.2. One can see
that main active produced by plasma are products of primary plasma processes: O,OH, A2CH2 (A2
means two condensed aromatic rings) and secondary ones: A2CHO, A2OH, A2O,A2, HO2,H2O2,CH2O.
In the case of plasma interaction with n-butane, the main primary active species are O,OH,C4H9 and
secondary ones: C4H9O2H,C4H8OOO2H,C4H7OO2H, CH3CHO,CH2O,HO2,H2O2. Rate products and
sensitivity analysis rates indicates that in the case of the n-butane oxidation the process goes through
very effective channel of the low temperature oxidation. That is principal difference with plasma
assisted initiation of oxidation of methyl naphthalene.
Fig.3 shows dependence of
1E17
induction time of the n-butane and
methyl-naphthalene-air
A2CH2OH
stoichiometric mixtures upon
H2O2
CH2O
energy input in the system. Two
H2
1E16
cases are regarded: initiation of
A2CHO
A2CH2
ignition by pulsed plasma with
OH
electric field strength E/N=200 Td
O
and initiation of combustion by
1E15
equivalent heating of the mixtures.
One can see that due to nonequilibrium character of chain
initiation the plasma has obvious
1E-6
1E-5
1E-4
1E-3
Time,s
advantage in comparison pure
Fig.2 Concentration of the main species in the sequence of DBD thermal effect. It was shown also
that
large
hydrocarbons
pulses. Stoichiometric mixture air- methyl naphthalene.
demonstrate
higher
chemical
Equivalent heating ΔT,K
activity
than
small
ones
in the
-2
-1
0
1
2
3
10
10
10
10
10
10
0
10
plasma of nano-second discharge.
The analysis of reaction pathways
-1
indicates that the elevated activity
10
of the large hydrocarbons concerns
heating
with variety of the isomers of the
-2
10
T =900 K
intermediate partially oxidized
species which are responsible for
-3
10
the chain propagation channels.
Plasma
Figure 4 demonstrates character of
T =900 K
-4
Tini=900 K, E/N=200 Td, n-butane
10
the oxidation initiation process by
T=900+dT, n-butane
T=900 K, E/N=200 Td,methyl-napthalene
plasma as a dependence of
T=900+dT,methyl-napthalene
-5
branching value of the chain
10
-5
-4
-3
-2
-1
10
10
10
10
10
imitation of the oxidation vs
Energy Input, eV/molec
process time. One can see that
Fig. 3 Induction time vs energy input. Solid lines – pulsed there are at least two stages. First
plasma effect at E/N=200 Td. Dashed lines – effect of the gas stage is stage of branched chain
heating. Squares – n-butane-air stoichiometric mixture. Stars – oxidation and second of the
methyl-napthalene- air stoichiometric mixture. Initial T=900 K, degenerated chain. The reason of
pressure P=1 atm.
the transfer from branched chain to
degenerated is accumulation of the radicals in the beginning of oxidation and recombination of the
radicals at the second stage. Since ratio of rate of chain propagation for n-butane oxidation to
ini
ini
3
SESSION NUMBER & NAME
recombination rate is large then one ratio for
methyl naphthalene it leads to largest plasma
effect in the first case (see Fig. 3).
The comparison of the plasma effect upon
light (experiment with methane, ethylene and
propane) and heavy hydrocarbons (n-butane) is
10
shown on the Fig.5 as the plasma efficiency (α)
versus the plasma energy input (eV/molec) or
d[Products
production]/dt
equivalent of the thermal energy (K). Plasma
Branching=
d[OH production]/dt
efficiency α=ΔTpl/ΔT is defined as a ratio
between temperature rise due to plasma effect in
the zone of incomplete oxidation and temperature
10
rise in the hypothetical case when all plasma
1E-3
0.01
0.1
1
10
100
energy goes to the simple heating of the mixture.
Time,s
Hence that value shows the advantage of the
Fig.4 Branching value of the chain for n-butane energy using in the non-equilibrium form in
oxidation at initial T=700 K, P=1 atm
comparison with simple heating of the gas by the
same quantity of the energy.
Optimal conditions for effective
Eqvivalent of ΔT,K
plasma energy consumption,
2.8
27.6
276.0
7
when plasma initiates the
methyl-napthalene T=900 K P=1 atm
methyl-napthalene T=600 K P=1 atm
branched chain mechanism were
6
n-butane T=900 K P=1 atm
determined in term of specific
n-butane T=600 K P=1 atm
n-butane T=900 K P=10 atm
energy input. The maximal
5
n-butane T=600 K P=1 atm
efficiency corresponds to area of
Starikovskii, C3H8, ER=1
Adamovich CH4, ER=1, P=70 Torr
plasma energy input below ~10-2
4
Adamovich C2H4, ER=1, P=70 Torr
eV/molec. Further increasing of
energy input leads to the parallel
3
acceleration of the chain
2
termination reactions (active
particles recombination).
1
Branching value
10
0
α= ΔTpl / (plasma equivalent ΔT)
-1
1
1E-3
0.01
0.1
Work is supported by RFBR
grant 09-03-12323-ofi-m
Energy input, eV/molec
Fig.5 Plasma effect upon temperature change due to low
temperature oxidation. Calculations – lines. Experiments – stars
and rhombs.
References
[1]. S.M.Starikovskaia, Journal of Physics D - Applied Physics, 39, 16, 2006, R265,
[2]. E. Mintusov et.al, 46th AIAA Aerospace Sciences Meeting , 7 - 10 January 2008, Reno, Nevada,
7, 26, (2009).
[3]. M. Deminsky et al, ISPC 18, 2009, Bochum, Mechanism of influence of the pulse-periodic
discharge on low temperature oxidation of hydrocarbons.
[4]. N.A.Slavinskaya, J.H. Starke, U. Riedel,” To practical fuel modelling”, GT 2011 – 45198, ASME
2011.
[5] Janev R.K., Reiter D. J.Nuclear Materials 313-316 1202 (2003)
[6] M.Deminsky et al, Computational Materials Science 28 (2003) 169–178
4