Influence of Non-Unified Electric Field to the Combustion of Liquid Hydrocarbon Fuels

Ilchenko E.P., Shevchuk V.G.
Institute of Combustion and Advanced Technologies, Odessa National University of I.I. Mechnikov
2 Dvoryanskaya Str., Odessa, 65026, Ukraine
The possibility of influence to the process of combustion of liquid hydrocarbon fuels through applying an electric field is
considered. The investigations were made for typical representatives of fuels depending of soot production rate. It’s
shown that mass combustion rate can be both increased and decreased depending on the polarity of the field.
Keywords: Electric field, electric discharge, liquid hydrocarbon fuel, hexane, benzene, methanol, mass combustion rate
PACS: 52.80.-s
INTRODUCTION
The specific of the electric phenomena during combustion that differs them from classic objects of investigation
of the low-temperature plasma is the next one. When the flame is put in the electric field, the diffusion of oppositely
charged carries of charge leads to the appearance of the charged areas and re-distribution of the potential of the
applied electric field. The spatially distributed chemical reactions are the source of the charged particles in the flame.
These chemoionization reactions are the cause of the main particularities of the ionization structure of the flame
front.
While investigation of the influence of the electric field to the combustion of liquid hydrocarbon fuels one
applies only the unified outer field as well [1, 2, 3], and the main fall of the potential difference takes place in the
burning-out zone so there is no sufficient influence to the mass combustion rate (MCR). The applying of the field
that ’pierces’ the flame, i.e. influences to the preparation zone, is more effective.
As it is revealed in [4], the influence of the pulsing and direct electric field to the reaction zones of the premixed
flames of the propane-air depends on polarity of the applied voltage.
When applying the outer non-unified electric field to the burning drop of the hydrocarbon fuel the directed
motion of the charged soot grains arises. It can be presented as blowing of the burning drop with the outer flow
(analogous to [5]). As shown at [6], the convection arises combustion rate for 1+U∞R/2D∞ times. Here U∞ is the
blowing flow velocity, D∞ is the diffusion coefficient for the mixture fraction evaluated in the oxidizing gas far from
the drop of radius R. It’s shown also that specific MCR (K/K0)-1 can be approximated as bε/2, where bε is the
Reynolds number, and the experimental data for the heptane and methyl alcohol satisfies this correlation exactly
enough [6].
In order to determine the influence of non-unified electric field to the diffusion flame characteristics in the
present work we consider the influence of non-unified electric field to the all zones of the flame (preparation zone,
combustion zone) for both pre-breakdown and breakdown values of the field. The investigated objects are the flames
of the liquid hydrocarbon fuels (methyl alcohol, hexane, benzene) formed on the stationary drop [7]. The choice of
the fuels is caused by their smoking rates during combustion.
EXPERIMENTAL TECHNIQUE AND METHOD OF INVESTIGATIONS
The investigations were carried out within the stationary drop scheme [7]. A stationary drop of liquid fuel from 3
to 10 mm in diameter was formed on a porous spherical particle made of a fine metal mesh. Fuel was supplied
evenly through a thin (1 mm in diameter) needle with the use of an electromechanical supplier. During the process
of a stationary combustion, it was necessary to achieve such a consumption of a fuel when the drop is covered with a
thin liquid layer of constant thickness. The control over the thickness was performed with a visual tube. Such a
procedure allows achieving the maximal stationarity of combustion.
The direct high voltage was applied to the particle and the outer electrode of the cylindrical capacitor having 80
mm in diameter made of fine metal mesh (non-unified field). The metal ring having 80 mm in diameter was also
used as the outer electrode, it was put on the various heights relatively to the drop. The information about current
that runs through the capacitor was obtained by using an oscilloscope, photo- and video-registration of the burning
drop was made. Every time data about combustion in the field were matched with those without field. Such
differential method allows noticing the influence of the field to the MCR with a precision ≤3%.
Non-Unified Field
In case when drop was charged positively, and the mesh – negatively, for the reasons same that for the unified
field (i.e. due to ion wind mechanism influence) the flame bends to the outer electrode. During this process its form
liked a bell turned upside down because positively charged soot grains bend to the negatively charged mesh and they
carry the gas ambience. While increasing the field, the flame top opens more and more (the flame doesn’t exist on
the great part of the surface). As the result an average distance from the drop to the combustion zone increases and
the part of the surface where reaction doesn’t take place increases, both the heat flux from the combustion zone to
the drop and, consequently, MCR decreases (fig. 1) according completely with the assumption that the ion wind
mechanism influences to the combustion process. In this picture

kg/m2·s – the same when the field is off.
FIGURE 1. The influence of the non-unified cylindrical field to the combustion rate of the hexane
In the case of opposite polarity («–» on the drop, «+» on the cylindrical electrode), the flame covers the drop, as
soon as the positively charged soot grains now are attracted by the negatively charged drop that intensifies the heatmass-
transfer to the drop surface that leads to the increasing of the MCR.
The video recording of the burning particles of magnesium 6 mm in diameter shown that the main features are
the same as for the hydrocarbon fuels. The differences are caused by higher conductivity of the products of burning
and the lesser height of the flame. That resulted in the stronger influence of the field to the combustion process. The
role of the natural convection is lesser than that for the hydrocarbon fuel flames.
There are the follows particularities that were experimentally revealed for the methyl alcohol. For the first, the
non-unified electric field, unlike the unified one, influences to the MCR (fig. 2). Next, possibly due to weak
smoking, the applying the negative potential to the drop doesn’t result in turbulization FIGURE 2. Dependence of the MCR on the intensity of a non-unified field for the methyl alcohol for the various polarities of the
drop (1 – negative, 2 – positive potential)
When the intensivity of the field achieves the breakdown value (independently of the form of outer electrode that
can be a needle, a plate, a ring or an electrode of any other form), there chaotic set of spark discharges between drop
and the electrode appears. During this a strong turbulization appears. MCR increases up to 2÷4 times. The cause of
this may be conditioned both the kinetic mechanism (since spark channels, ‘sewing’ the combustion zone and the
preparing zone of the fuel are a powerful sources of supplying of chemical active particles in the reaction zone), and
the turbulization of combustion zone by itself due to gasodynamic influence upon zone of flame.
Non-Stationary Electric Field and Phenomenon of Hysteresis
The experiments described earlier were carried out in the stationary, i.e. unchangeable in time, non-unified field.
The experiments carried out in the non-unified, but non-stationary field have shown that while increasing the field
the flame begins to vary in the chaotic way as whole. While further increasing the field the form of the flame
transforms to the strongly elongated ‘tail’ that extends from the drop to the metallic cylinder. While increasing the
voltage the top of the flame covers the drop tighter and tighter and then the blow-off of the flame and its sharp
transition to the drop trace takes place. The following decreasing of the voltage led to the whole covering the drop
with the flame. This process took place as sharp as the blowout, but, unlike this, the covering took place under lower
voltage. Let’s define z – the distance between the drop center and the lower border of the flame, dd – drop diameter.
Figs. 3, 4, 5 show corresponding dependencies z/dd(U) for the non-unified cylindrical field (dd=5,6 mm) for hexane,
methyl alcohol and benzene. MCR with the field off
Let’s consider the investigated process more detail. As it can be seen from fig. 3, 4, 5, under the voltage U<Ucr
bo
(Ucr
bo – critical voltage of the blowoff) the flame coordinate relatively the drop center z0 doesn’t change practically.
While achieving U=Ucr
bo and not a big increasing of the voltage the flame coordinate changes sharply and the
blowoff of the flame to the drop trace takes place.
During the following decreasing of the voltage and the achieving U<Ucr
co (Ucr
co – critical voltage of the covering)
the recovering of the flame on a frontal point under its motion from the drop trace takes place.
Critical voltages of the blowout-covering, and the area of the hysteresis loop change, as it is seen from fig. 3, 4,
5, depending on the sort of the used fuel.
The analogous phenomenon is investigated in [5], that describes the hysteresis behavior of the flame under the
blowing the drop with the convective flow. The drops of acetone and alcohol were investigated. It’s shown [8] that
critical blow velocity of the burning drop Vbo, under which the blowoff of the flame takes place is 27–58 cm/s. Let’s
suppose that the blowoff takes place when the velocity of the electric wind caused by the corona discharge between
outer electrode and the drop (flame) is equal to velocity of Stephan motion of combustion products moving from the
combustion zone towards outside. Then we can bring forth a hypothesis that the velocity of the electric wind under
the flame blowoff should be equal to critical velocity of the hydrodynamic flow blowing the drop (Vbo).
Let’s estimate character velocity of the Stephan flow for the lower part of the flame, since the blowoff begins
from the frontal point. It can be shown that Stephan velocity of the products
VSt
prod = (((

0 m /ρgas)·rd
2/rf
2)·Tpr/T0)·(npr/n0)/(Tpr/T0), where rf – combustion zone radius, n0 and npr – mole number of
the initial matter and the combustion products, T0 and Tпр – their temperatures, consequently. For hexane
VSt
prod = 33·10-2 m/s.
Under the velocity of the convective flow blowing the drop greater than Stephan flow velocity, i.e. Vbo>VSt
prod,
the blowoff of the flame will take place.
The computations show that the velocity of the electric wind generated by the corona discharge is
commensurable with the critical velocity of the convective flow Vbo that leads to the blowoff: Vbo
ew>VSt
prod.
Thereby, the blowoff of the flame on the drop, burning in electric field, has the hydrodynamic nature, however,
its nature, unlike the convective blowoff, is conditioned by the mechanism of the electric wind, generated by the
corona discharge.
CONCLUSIONS
The main results derived in the work may be summarized as follows.
1. As proved experimentally, it is possible to both increase and decrease a mass combustion rate for liquid
hydrocarbon fuels (benzine, benzene, hexane etc.) up to 15% by the applying of the external electric field of the prebreakdown
intensity and configuration. It’s stated that such an influence is caused by the mechanism of the ion wind
as the result of moving of charged soot grains in the field. While achieving the breakdown values the MCR can be
increased up to 2÷4 times.
2. It’s shown that in the case of non-unified field of pre-breakdown intensivity for any type of hydrocarbon fuel
under the critical combustion velocity less than stationary one the process of blowoff (covering) of the drop with the
flame has the hysteresis character. The nature of such phenomenon, unlike the convective blowoff, is conditioned by
the mechanism of the electric wind, generated by the corona discharge. The velocity of the electric wind in the nonunified
electric field is commensurable with the critical velocity of the hydrodynamic flow blowing the burning
drop.
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