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The distribution of excess charges in the diffusion
flame of hydrocarbons
To cite this article: S M Reshetnikov et al 2016 J. Phys.: Conf. Ser. 669 012040
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The distribution of excess charges in the diffusion flame of
hydrocarbons
S M Reshetnikov, I A Zyryanov, A P Pozolotin and A G Budin
Physics Department, Vyatka State University, 610000, Kirov, Moskowskaya Street,
36, Russia
b185@mail.ru
Abstract. The paper discusses the experimental investigation results of the potentials
distribution (probing) and the excess charges in diffusion flames of propane and polymers -
PMMA (polymethyl methacrylate) and divinyl rubber. The study of the excess charges
distribution was made by analysis of flame deformation in the presence of an external lateral
electrostatic field. The existence and the accumulation of excess charges in the flame are
generally explained on the basis of consideration of the interaction processes of condensed
phase particles and charges (electrons) generated during combustion in the presence of a dusty
plasma. However, this mechanism does not give the opportunity to consider the sign and
magnitude of the excess charges distribution in the flame, depending on the products
composition and the value of the oxidant excess. It is unclear how oppositely charged
immiscible streams of flame are formed and coexist.
The study of the chemo-ionization phenomenon - the formation of charged particles during the
combustion, is critical and fundamental problem. Since the combustion process is accompanied by
electrical phenomena, it is very tempting to use electrical phenomena to control the combustion
process (ionization). Therefore that problem of consideration of mechanism and using electrical
properties during combustion is actual. The complexity of solving the chemo-ionization problem is
caused by the fact that before the ionization an electron is a quantum object, and after ionization it
becomes a classic one. Equally urgent is the problem of existence and value of the local excess
charges in various locations of the flame. The existence of its own electric field in the flame torch was
discovered long ago by Michael Faraday. However, until now the mechanism of appearance and the
accumulation of excess charges in the combustion zone is unclear. The process of charges
accumulating in the reaction zone is explained by the ability of dispersed particles of dusty plasma to
absorb and desorb with different intensity ions and electrons.
Dusty plasma is an ionized gas containing charged particles of condensed matter [1]. The condensed
phase is formed in the combustion zone during the conversion of hydrocarbon fuel. The growth rate of
the particles is comparable to the rate of chemical reactions [2]. The sizes of particles produced during
the conversion of propane, are between tens of angstroms to tens of micrometers. The presence of
dispersed particles of different sizes requires taking into account the characteristics of their interaction
with the charges.
The accumulation of charge on the condensed phase particle is investigated in the study [3]. It is
shown that the dispersed particle can accumulate on its surface charge, reaching 104 elementary,
VII Conference on Low Temperature Plasma in the Processes of Functional Coating Preparation IOP Publishing
Journal of Physics: Conference Series 669 (2016) 012040 doi:10.1088/1742-6596/669/1/012040
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
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creating around itself the potential of tens millivolts. In the study [4] the dynamics of charging carbon
black particles in the flame over time is analyzed. It is shown that at the initial stage of carbon black
growth (10-8-10-3s since the formation) mostly negative charge is accumulated on the particles.
Negative charge accumulating process is caused by the interaction of carbon black particles and
electrons having high mobility [4]. Further, due to the interaction of negative carbon black particles
and positive ions, concentrations of negative and positive carbon black particles are equalized.
Through 10-1s since the formation carbon black particles begin to accumulate positive charge [4].
The authors investigated the potential distribution and the excess charge in the diffusion flames of
hydrocarbon fuels, propane and polymers [5, 6]. Photo of propane flame with marked equipotential
lines are shown in Fig. 1 and 2. Diffusion flames were organized on the burner consisting of two
coaxial pipes: first with propane, diameter 8 mm at the center, and second, outside - with air, diameter
20 mm [5]. Fuel consumption varies between 0,5 - 2,7 cm3/s, oxidant - 13-16 cm3/s, which allows to
vary the oxidized excess ratio α.
The position of the front in diffusion flame is realized in accordance with the solution of Burke-
Schumann equation [8], when the ratio of oxidizer/fuel is equal stoichiometry, and the potential is
zero. In the flames with a lack of oxidant (α < 1) in the vicinity of the combustion front a region of
negative potential (Figure 1) is found, bounded by the surface of zero potential. In the central part of
the flame upstream region of positive potential is detected. The maximum negative potential is 120
mV, positive 80 mV. In the field of negative potential localization is located the area of maximum
temperature. The temperature there is 1150 K.
Figure 1. The potential distribution in the flame of a propane when α <1
The potential distribution in flames when α > 1 is presented in Figure 2. In the bottom part of the
flame a negative potential region is detected, the maximum value of which is detected at R = ± 4 and
H = 1 mm. Upstream in the flame a positive potential region is found. The maximum value of the
positive potential is achieved at a height H = 4 mm. The values of potentials in absolute value are 150
mV. The area of maximum temperature is located in the upper part of the flame in the area of positive
potential. The temperature there is 1200 K.
Figure 2. The potential distribution in the flame of the propane with excess of oxidant
VII Conference on Low Temperature Plasma in the Processes of Functional Coating Preparation IOP Publishing
Journal of Physics: Conference Series 669 (2016) 012040 doi:10.1088/1742-6596/669/1/012040
2
According to the kind of the distribution of excess charge in flame polymers can be divided into
two groups. In the first group of polymers (for example PMMA) flame is deflected to a negative
electrode (Figure 3a). The flames of the second group of polymers (for example divinyl rubber) in the
lateral electric field are divided into two torches, that is shown in Figure 3b. Separation of the flame in
the electric field indicates presence in flame excess positive and negative charges which do not
interact with each other.
Figure 3. Flames of polymers in the lateral electric field
Accumulation of positive excess charge in the flame is usually associated with the difference in
mobility of ions and electrons, for example, [7]. Electrons having higher mobility, leave the reaction
zone, and the flame becomes positively charged. This approach does not explain the accumulation of
negative charge in the flame. The accumulation of excess charge both positive and negative in a flame
can be explained on the basis of the interaction of dispersed particles with ions and electrons.
The charge accumulated in the dispersed particles is determined by the time of its location in the
combustion zone [4]. Formation of carbon black occurs in the front of the reaction, further particles
entrained by gas flow. In authors experiments, carried out with the propane flame gas velocity is about
15 mm/s. In the initial stages of life in front of burning carbon black particles become negatively
charged. During the characteristic time 10-1s particles have time to go from the front of the combustion
a distance about of 1.5 mm to form a region of negative potential, which have good agreement with
the experimental results (Fig. 1). When removed for a distance more than 1.5 mm from the front of the
combustion, charge of the dispersed particles is changed to positive. Thus, with a deficiency of
oxidant, the flame front along its entire surface is negatively charged. The negative charge of the
carbon black particles during the motion upstream is replaced by positive, which results in forming a
region of excess positive charges upstream of the combustion front, (Fig. 1).
In the flames with α > 1 the formation of carbon black begins in the end of the burner in the
bottom part of flame. Here an excess negative charge is found (Fig. 2). Further, at movement upwards
flow there is a growth and recharging the condensed particles. The excess charge in the flame is
formed by the competition of the charges of the particles formed in the front and in the bottom of the
flame. Thus, neutrally charged region is created in the flame (Fig. 2, h = 2 mm). When moving
upstream the positive charge accumulated on the particles formed in the lower part of the flame
becomes much larger than charge of particles formed at a given height in the front of the reaction. This
leads to formation of positively charged area.
The distribution of electric potential in the area of the combustion of propane and polymers has
been experimentally investigated. It was found that if α < 1 in the flame front there is detected a
negative potential and there is realized the maximum temperature. In the diffusion flames when α> 1 a
negative potential is detected in the bottom of the torch. At the top of the flame the area of maximum
temperature is detected, which is located in the area of positive potential. An explanation of the results
obtained on the basis of the process of interaction carbon black particles and charges in flame.
Experimentally found separation of polymers flames in an electric field, which indicates the existence
of excess charges with a different sign in flame, which do not interact with each other.
VII Conference on Low Temperature Plasma in the Processes of Functional Coating Preparation IOP Publishing
Journal of Physics: Conference Series 669 (2016) 012040 doi:10.1088/1742-6596/669/1/012040
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References
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VII Conference on Low Temperature Plasma in the Processes of Functional Coating Preparation IOP Publishing
Journal of Physics: Conference Series 669 (2016) 012040 doi:10.1088/1742-6596/669/1/012040
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