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172
23. PLASMA-INDUCED IGNITION AND PLASMA-ASSISTED COMBUSTION
IN HIGH-SPEED FLOW
Sergey B. Leonov, Valentin A. Bityurin, Konstantin V. Savelkin, Dmitry A Yarantsev Institute of High Temperature RAS, Moscow
Abstract. The paper are dedicated to the experimental demonstration of plasma technology abilities in the field of high-
speed combustion. It is doing in three principal directions: control of the structure and the parameters of the duct-driven
flows; the ignition of air-fuel composition at low mean gas temperature; and the mixing intensification inflow. The work
has been fulfilled in Institute of High Temperature RAS (IVTAN).
1. Introduction
The analysis of supersonic combustors
performance shows that several principal problems
related to the supersonic combustion and the flame
stabilization, especially in the case of hydrocarbon
fuels are to be solved for the practical
implementation of such a technology. The plasma-
based methods of the combustion management
under scramjet conditions are considered now as
one of the most promising technologies in this field
[1-3].
An electrical discharge’s properties
strongly depend on the conditions of excitation,
flow parameters and characteristics of supplying
electromagnetic power. The analysis of applicable
discharge types can be done from the viewpoint of
plasma-assisted combustion concept, which
consists of three important items: duct-driven flow
control, plasma-induced ignition/plasma-assisted
combustion due to combustion chemistry
enhancement, and inflow mixing intensification.
The electrical discharges, which are
generated under the conditions of high-speed flow,
possess several specific properties. These features
might be important for the discharges’ applications
in a field of flow control and plasma-assisted
combustion. There can be different kinds of plasma
instabilities, for example, longitudinal-transversal
instability of plasma filament, which has been
found out recently (see section 4). It leads to
intensive small scale mixing inflow. Extra method
of combustion intensification is plasma jets
blowing out to main flow.
It is clear that to manage the combustion
process fully under any conditions a large level of
additional energy deposition is required, in a range
10% from flow enthalpy. The combustor must
operate properly under the conditions, which has
been designed for. So the idea is not in a strong
effect of energy release but in a gentle control of
chemical reactions rate and local multi-ignition.
The second direction is to give the gear to force
combustor to work under the off-design conditions.
It can be a temporal mode and the level of required
electric energy is not vitally important. Such off-
design conditions are: low temperature (probably,
due to undesirably high speed of flow), relatively
low pressure, lean composition, bad mixing, etc.
Our experiments are going to simulate off-design
regimes of the model combustor.
Unfortunately, specific information
available now is not quite sufficient for proper
choice of the discharge type. Our understanding
now is that there is no universal decision in plasma
assistance design and the method of application.
Presence of even a small amount of free radicals
(for example O, OH, H, ON) or vibrationally
excited molecules can effectively improve ignition
conditions but require a not small amount of the
electric power. Each specific situation has to be
considered separately. Under these conditions the
experimental tests and verification of some
analytical predictions are needed urgently. This
work is one of such efforts.
2. Duct-Driven Flow Control
The unconventional methods to improve a
supersonic/hypersonic combustor performance
using electric discharge’s plasma are discussed
widely [1-11]. Two main ideas stimulate efforts in
this field: the control of the inlet/diffuser
parameters and a control of the combustion
chemistry under supersonic flow. In both cases the
electrical discharge changes the structure of flow,
and the thermo-chemical and electro-magnetic
properties of the medium. The analysis shows that
the influence of the plasma generation in high-
enthalpy flows leads to consequences that are not
immediately evident. It is clear that addition of
large amount of the thermal energy might lead to
modification of the wave structure in duct-driven
flows. From the other side such an addition can
change the parameters of whole flow significantly
and not to the desired direction. Thus, the
efficiency of the plasma influence on flow structure
is very important at the diminishing of the possible
penalties.
173
Duct-driven flow control.
The scheme of the flow modification by
plasma method is shown in Fig.1. An idea of the
method is that the surface-generated plasma
provides the energy release under the predefined
location and creates a “smooth” plasma layer near
the duct surface. In dependence on the input power
such a layer generation an lead to boundary layer
(BL) modification [12-14], local BL separation or
extensive (global) separation. Shock waves
structure modification accompanies these
processes. Flow parameters such as Mach number,
pressure value and shocks position can be changed
controllably, as well as their distribution in cross-
section. The effect of instabilities damping and
obstacles’ screening has been described in our
papers [6-9]. Moreover the scheme of the model
experiments is corresponded with Fig.1 directly
(see the next sections).
Inlet’s shocks control.
The scheme of the flow structure
modification in hypersonic inlet has been proposed
in [7]. The idea is described in Fig.2. Later the
Princeton University team has proposed the idea of
“virtual lip” [10]. In the first case it is suggested
that the speed of a vehicle is less than the inlet
design Mach number Mdes. The most dangerous
here is the spillage regime when the third shock
from the cowl lip falls upstream the edge of inlet.
The generation of the plasma layer just upstream
the inlet edge can prevent undesired shock
reflection (dashed lines are old shocks). At the
opposite case when M>Mdes the surface plasma
generation could be useful to change the position of
the second shock upstream and stabilize the
location of the third shock reflection near the edge
of the inlet. The scheme of the model experiments
is shown in Fig.3. The change of the shock position
and angle on the artificial wedge and the duct bulk
parameters are studied under the surface plasma
effect.
Physical model of surface plasma – flow interaction.
The model of the phenomena includes
three important items: (1) discharge structure and
parameters in airflow near the surface; (2) plasma
layer generation process; (3) interaction of the
plasma layer under the supersonic flow.
The discharge structure in airflow depends
on the type of discharge sufficiently. The
transversal quasi-DC discharge has been described
in recent publications [2,7-9,11] on
phenomenological manner. Plasma generation
occurs by means of surface multi-electrode
distributive electric discharge at two different
modes: longitudinal and transversal.
The experimental data on plasma
generation near the body’s surface and influence on
parameters and volume of stabilized separation
zone downstream of wall step have been reported
recently [5, 7, 11]. Some data on the surface
discharge in free stream was presented in paper [7].
Fig.1. Scheme of the flow modification by the surface
plasma.
Fig.2. Idea of the inlet’s shock structure adjustment.
Fig.3. Model experiment arrangement on inlet control.
174
In the case of transversal discharge a relaxation
type of the plasma generation process took place.
An initial plasma filament is being blow down,
breaking and starting again in about 10-20us. In
our case the transversal discharge is characterized
by large level of modulation of the main
parameters, including gap voltage, resistance,
radiation and the position of downstream visible
edge of the plasma. The appropriate oscillograms
are shown in Fig.4.
Fig.4. Detailed correlation between voltage and plasma
radiation at instability.
Two regimes of the discharge renovation
are possible: when the plasma is distinguished (low
voltage mode) and repetitive mode with sequential
runs of individual plasma filaments. The frequency
of such a relaxation process is defined by the flow
local velocity and can be tuned. In our specific
conditions the maximal amplitude of the Fourier’s
spectrum occurs near the frequency 30-50kHz.
Under the conditions of the experiment such a
value is well correlated with the characteristic
length of the plasma channel (xmax) about 2cm. The
process is drawn in Fig.5. An initial breakdown
between the flush-mounted electrodes takes place
under the flow conditions (Pst=100Torr) at the
electric field strength about E=3kV/cm. The first
and sequential breakdowns happen, as a rule, inside
the boundary layer due to a lower value of the gas
density there. The gas temperature inside the
plasma channel starts to rise and the channel swells
up itself volumetrically. This time the plasma
filament comes into the main flow-field and blew
down with the flow speed. The filament’s shape
looks like an increasing loop. The gap’s voltage
increases in accordance with the filament’s length
up to the level when the new breakdown is realized
in the position much closer to the electrodes. Such
a relaxation type of process is repeated with the
frequency, which can be referred to a gas-dynamic
time (see chart in Figs.4, 5). The external shape of
the generated on such manner plasma layer looks
like a near-surface wedge, which is streamlined by
the main flow, as it can be seen in photo and
Schlieren photo.
For the process consideration three
assumptions are done: (1) the gas is weakly
ionized, equilibrium and ideal; (2) Plasma
channel expands isobarically (2-D volumetric
expansion); (3) the local energy input is
proportional to the length of the plasma channel at
the constant electrical current. Energy release to the
plasma filament increase the temperature of the
gas:
TRGTctmW p
11 ,
where W- is the local power input, m- is the gas mass in the plasma channel, G1- is the mass flow rate through the plasma layer, T- is the temperature increase. The isobaric conditions give
the relation between the temperature and the
filament’s volume:
VpTRm ,
where
yyV ×z/2,
Fig.5. Scheme of the model consideration.
175
where y- is the plasma layer thickness (filament’s diameter). Simple transformations give the
following expression for the plasma wedge angle :
yW
pzxytg 111)( ,
where -is the flow velocity, z-is the plasma layer depth. The relation between local power input and
the coordinate y can be found from the expression
on the average power Wav , where - is the period of the plasma filaments oscillations and W(x)- is the local power input, L0- is the inter-electrodes gap.
max
0
)(1
x
avxxWW ,
)2()( 0 xLIExW .
Utilizing these expressions it is possible to
find out the plasma wedge expansion angle through
the integral (bulk) characteristics:
pW
xzxytg av
max
22 211
.
Remarkably, that the angle of the plasma
wedge doesn’t depend on the initial mass flow rate
through the plasma area (thickness of an initial
plasma channel y0). In frames of the model the
temperature of the gas could be estimated as
following:
22
0
max0max tgy
xTT st .
Using the typical values of the
experimental parameters (namely: Wav=1kW, z=20mm, xmax=20mm, =550m/s, Tst0=200K,y0=ybl=0.5mm, p=100Torr) the estimations give the following results: 14 and T 2000K.
The direct measurements give very close
result for the plasma wedge angle and a bit more
value of the gas temperature that can be explained
by the specific method of the measurements and by
plasma non-homogeneity. Some difference in value
of the angle (12 experimental and 14 calculated)
could be explained by the energy loses. Well
known that the plasma of molecular gases has got a
large vibration excitation and relaxation time is
rather large in comparison with gasdynamic time.
The interaction of the plasma near-surface
layer with the main flow occurs on the following
manner (Fig.6). We will consider that the plasma
effective “wedge” acts as a solid “wedge”, although
the actual process of the interaction is much more
complex. Nevertheless the effective angle of
oblique shock wave ( ) associated with the plasma
generation can be related to the effective angle of
the plasma wedge ( ), i.e., finally, to input power.
Fig.6.Scheme of oblique shock generation.
In accordance with such an approach the
angle of the oblique shock can be expressed in
terms of:
cossin)1(
sin)1(22
22
MMtg .
If the effective angle of the plasma wedge
is quite small, the expression can be simplified and
increase of the static pressure can be calculated
through the following formulae:
1
1)(
2Mtg ,
2
2 1
1)(
MtgPst .
As the result we can bind the angle of the
oblique shock with the input power to the surface
plasma. It has to be considered that such a simple
model can be applied successfully for the
prediction of the surface plasma effect, including
the thermal chocking.
Experimental setup description.
The experiments have been conducted at
supersonic speeds in short duration blow-down test
installation PWT-10 of IVTAN. The air used in the
blowdown operation is provided to a high-pressure
reservoir, which has a working pressure of 1 to
5Bar. A fast-acting electromagnetic valve with
diameter 50mm and response time less than 10ms
is connected to the nozzle block and the test
section. Following passage through the test section,
the flow is ducted to either the atmosphere or to a
vacuum tank. Two different test sections are used.
176
Both of them have a circular window for
spectroscopic and natural observations. The two-
dimensional test sections are equipped by a special
flush-mounted insert with electrodes. The operating
mode can be characterized by the following
parameters: Mach number M = 1.1-1.99, static pressure 50-300Torr, Reynolds number of
undisturbed flow Re=(4-10) 106/m, boundary layer
thickness =0.5-2mm, bulk enthalpy of the flow at
100Torr is about 20kW, duration of steady-stage
operation 0.2-0.8sec, and typical air mass flow rate
through the duct about G 0.1kg/sec. The surface discharge can be characterized roughly (see below)
by the following parameters: type of plasma –
quasi-DC multi-electrode surface discharge, typical
electric current through an individual electrode Id1-3A, electric field up to E/n=40Td, typical total
input electric power W = 1-10kW, duration of plasma pulse was between 50 and 100ms.
The first test section was designed for the
experiments on plasma ignition at condition of
fixed separation zone. It has a rectangular cross-
section with dimensions 20x100mm, depth of
rearward-facing step 15mm. The electrodes
insertion is installed at the TS brink, upstream the
wall step. The second TS is made from the
dielectric materials. It has rectangular cross-section
with dimensions 20(h) 50(v)mm and two windows
for the Schlieren observations. The photographs of
the test sections and appropriate schemes of
experiments are presented in Fig.7.
The test sections are equipped by the
following diagnostics: 16 channels pressure
measurement system with response time 0.5ms, fast
CCD camera up to 240fr/sec frame rate and
electronic shutter, Schlieren device with the frame
duration down to 1us, spectroscopic CCD camera,
sensors of radiation, voltage, current and magnetic
field. Plasma temperatures within the discharge
were measured using optic spectroscopy, although
it should be noted that the measurements of plasma
parameters is difficult and presents uncertainties
due to the strong non-homogeneity of the discharge
structure.
Fig.8. Typical Voltage-Current record (bandwidth of the
voltage channel is limited).
Fig.7. Test section 1 with fixed separation zone (a). Test section 2 for duct-driven flow control (b).
177
Typical record of gap voltage and
discharge current is shown in Fig.8. The current-
power characteristic of the transversal surface
discharge is presented in Fig.9. The edges in power
input, when the BL separation without
reattachment took place, are pictured in the last
figure for two values of static pressure (see below).
Fig.9. Transversal surface discharge characteristics at
two anodes, M=1.9, inter-electrodes gap 7mm.
Plasma temperatures within the discharge
were measured using optic spectroscopy by second
positive system of molecular nitrogen and ion of
molecular nitrogen, violet system of cyan and
molecular band of CH. A small addition of CO2was used for the CN generation. The method of processing is found on accurate fitting of
experimental and calculated spectra at rotational
and vibration temperatures variation. The plasma
temperature was measured in the region of the
discharge cords using spectroscopic techniques and
was: 2-3.5kK by second positive system of
nitrogen, 3-4.5kK by CH and up to 6.5kK by CN in
depending on the experimental conditions.
Increasing the input power leads to slow rise of the
maximal temperature. The proper interpretation of
spectroscopic data is still a large problem.
Results of observation.
The plasma generation near the surface of
the duct creates the layer of hot air downstream the
electrodes area. In dependence on conditions such a
hot layer appears in different configurations and is
a cause of different sequences.
Mainly two methods have been exploited
for the observations of the flow structure: namely,
the Schlieren shadow method and measuring of the
pressure distribution. The typical images of the
surface plasma-airflow interaction are shown in
Fig.10. The plasma overlayer and an appropriate
oblique shock wave are seen well. The essentially
subsonic area or huge separation zone is generated
near the surface downstream the plasma area.
Fig.10. Standard Schlieren photo of surface plasma
interaction with duct-driven supersonic flow. Exposure
30us.
Three different situations can be
described: low-power plasma (input power below
1kW for pressure about 100Torr) leads to subsonic
layer generation; medium-power plasma (input
power 1-2kW) leads to subsonic layer generation
with local separation; high-power plasma
deposition (more then 2kW or 10% of the flow
enthalpy) leads to global separation processes,
which is presented in Fig.10. All intermediate
regimes of the interaction have been obtained
experimentally. At input power more than 4-5kW
the chocking of the duct takes place under the
standard conditions of the experiment. The weak
shock waves are generated due to wall irregularities
and local separation areas between nozzle and the
duct. A new oblique shock wave generation is
observed in all cases. New shock has practically the
Mach angle at low power of the plasma and
changes the Mach number in whole duct negligibly.
There is possible to change the Mach number of the
flow downstream and the angle of the oblique
shock by means increasing the input power. The
position of the shock wave can be accurate
localized by the place of plasma generation. The
stagnation pressure downstream the plasma area
decreases that is become apparent in increasing of
Po` downstream, which is measured by the Pitot
gage.
At higher level of the power input to the
plasma the result can be characterized by two
important features. The amplitude of the plasma-
induced shock wave increases sufficiently, up to
direct shock generation and the thermal chocking
of the duct. The second peculiarity is that the
boundary layer is separated without sequential
reattachment. Mach number in whole duct can be
178
changed significantly. Accurate analysis of the
pressure measurements data leads to conclusion
that it is a typical situation when the full pressure
near the wall is decrease and the static pressure is
constant practically or increases, when the global
separation takes place. At the same time the
stagnation pressure at the axes increases slightly.
The difference between the cases is well
recognized: in case of large input power the
“stagnation” pressure near the bottom wall occurs
lower than “static” pressure. It means that a
circulative flow near the wall with reverse direction
of the flow velocity vector is observed. The
situations with and without the global separation
are presented in Fig.11. In the first case the mean
power input was W=1kW, in the second case it was
W=1.7kW correspondingly.
The angle of the oblique shock wave due
to plasma generation depends on the level of the
energy release. We can compare this angle with the
calculated data obtained on the base of proposed
above model. The result of such a comparison is
presented in Table 1. The angles should be referred
to Fig.7, “exp” means experimental, “calc” means
calculated.
Table 1. Dependence of oblique shock angle on the input
power of the electric discharge (in degrees).
Power
, kW
Solid
Wedge
,
0 1 1,7 2,4
, exp 14 0 12
, calc 14 0 14 18 21
, exp 45 31 43 46 50
, calc 45 31 45 49 52
Well seen that the oblique shock angle
grows with the plasma power and that the
experimental values a bit less than calculated ones.
Now we are explaining this difference by the
power loses due to vibration reservoir of molecular
gas.
Fig.12. Mach number modification under the surface
plasma generation.
It is clear that the presence of shock wave
with non-Mach angle has to be reflected in Mach
number modification in whole duct. The result of
measurements is shown in Fig.12. The Mach
number has been recalculated on base of pressure
measurements in three sequential sections of the
duct: upstream and downstream the plasma
generator place. It has to be noted that the plasma
effect doesn’t entail any other harmful sequences
like a turbulence or instabilities generation. Quite
the contrary, the gasdynamic instabilities damping
occurs [9].
Fig.11. Pressure redistribution at surface plasma generation.
179
Shocks position control.
The experimental scheme presented in
Fig.3 has been applied to verify the plasma layer
ability for the oblique shocks control (change the
position and angle). Small model obstacles with
10% and 17% of thickness (3 and 5mm
correspondingly) have been installed into the duct
to simulate the inlet’s configuration. The second
model was blunter. They were positioned 16mm
downstream the electrodes area on the bottom wall
of the duct. The oblique shock falls on the top wall
of the duct in about 28 and 52mm downstream the
obstacle fore-edge zone correspondingly. The
typical Schlieren photos of the fore-generated
plasma effect on the shocks position are shown in
Fig.13 for the different level of the power input.
Fig.13. Schlieren images of plasma –flow interaction
under the different power input.
The Fig.14 presents the result of the shock
position control experiment for two different
obstacles. As it could be noted the plasma effect is
relatively more intensive for a small obstacle.
Under the large level of the input power the shocks
position and configuration are defined only by the
plasma. At such power input the global separation
in the duct occurs.
Fig.14. Shock wave position on the top wall of the duct
vs power input.
Fig.15. Mach number modification under the plasma
effect.
At the same time the plasma generation
influence on the flow Mach number in whole duct
is not strong in comparison with the case when the
obstacle is installed. It is correct if the power level
is much less then the chocking level. The Mach
number dependence on the input power is
demonstrated in Fig.15. The difference with a free
discharge is well visible (see Fig.10 for the
comparison). Actually in this conditions the
pressure loses increase with the power rising.
Detail measurements of pressure
distribution downstream the plasma show that in
180
some important cases instead of braking of the flow
we can observe the flow enhancement in terms of
total pressure recovery factor. The shock changes
its position and the visible amplitude is decreased.
It is easy to see how the plasma excitation
transforms the airflow structure not only near the
surface but also in whole duct. The result of the
measured total pressure recovery factor in
dependence on conditions is shown in Table 2. The
measurements have been done in 200mm
downstream the plasma generation place in
comparison with the pressure just upstream the
plasma generation place. The conditions were
chosen when the Mach number fell from about 1.96
down to about 1.44. Seen that the result at plasma
is better than in case of 17% solid profile.
Table 2. Total pressure recovery factor. Po2/Po
(Mo=1.94-1.98, M2=1,42-1.45).
Power,
kW
0 1 1.4 1.7 2.4 2.7
Wedge 0.77
Plasma 0.86
M2=1.7
0.8 0.81 0.8
Wedge+
Plasma
0.71 0.71 0.8 0.81
3. Plasma-induced ignition of hydrocarbon fuel
at low temperature
The mechanisms of the plasma of
electrical discharges influence on chemical
processes in high-speed flow can be considered and
listed as following:
o Fast local heating of the medium.
o Active radicals and particles deposition.
o Shock waves generation.
o Photo-dissociation and ionization.
Local heating of the medium leads to
intensification of the chemical reactions in these
areas. Besides of this the modification of flow
structure can be done by means of controlled
energy deposition. At enough large level of the
input power the artificial separation of the flow can
be realized. It is a method to increase a local
residence time to provide a zone of local
combustion and the real mechanism of the mixing
intensification. Active radicals’ deposition occurs
due to molecules’ dissociation and excitation by
electrons in electric field and by more complex
processes. If the chain chemical reactions are
realized, the deposition of active particles can lead
to large (synergetic) benefit in reactions’ rate as
well as in required power. Very often the first two
mechanisms are inseparable and the active radicals’
generation is equal to hidden heating. Local shock
waves generation promotes the mixing processes in
heterogeneous medium and initiates chemical
reactions due to heating in shock’s front.
Experiments on non-premixed ignition at
low mean temperature have been done in IVTAN
during last 2 years [2,3]. The scheme of the
discharge excitation in case of backward wall-step
is shown in Fig.15.
Fig.15. Test arrangement under the fixed separation
zone.
Two different modes of the discharge
operation have been found in case of separation
zone. At the first mode the discharge is excited
between electrodes. At the second mode the
discharge current has connected to a metallic wall
in separation zone. We are talking about the second
mode in this paper as the most prospective for an
application. Electric energy input to the plasma
volume was up to 10kW at transversal direction of
plate about 0.1m and axial distance between the
electrodes about 10mm. The mean input power and
the gap voltage in dependence on the static
pressure are presented in Fig.16. The discharge
voltage is not proportional to the discharge gap.
The fast video shows that the length of plasma
“cords” is much more in the case of the second
discharge mode, which is reflected in the gap
voltage increasing. Other difference is that the
discharge in separation zone is generally more
stable. The discharge stability in separation zone is
Fig.16. Discharge characteristics vs Pst.
181
deranged at an external influence like magnetic
field or fuel injection. The photo of the discharge
appearance in the separation zone is presented in
Fig.17.
Fig.17. Surface discharge appearance in separation zone.
Fig.18. Pressure redistribution due to plasma effect.
Plasma excitation near and inside the
separation zone effects on static pressure
distribution in this zone. The Fig.18 shows the
diagram of the pressure redistribution in all points
at the different conditions. It can be considered that
the result is the same for explored conditions:
plasma excitation leads to increase the static
pressure near wall step approximately on 15-20%.
Generally, this statement is correct for the standard
and the second mode of the discharge spacing both.
It can be noted that the pressure distribution in a
separation zone is change significantly. The
pressure gradient occurs much less than in the case
without plasma. Such an effect can lead to
increasing of gas exchange between a separation
zone and a main flow.
The test sequence was the following: wind
tunnel start (duration of steady stage 150ms
approximately), 50ms pause, discharge start
(duration 50-70ms), 20ms pause, fuel injection
during 2-20 ms. Pulse type of the fuel injector with
electromagnetic control was used during the test.
The following injector parameters are typical: type
of fuel – liquid hydrocarbon mixture on base of
isooctane’s; type of injection – high pressure
spraying to separation zone; fuel expense -
Gf=10 100mg/pulse=2 5g/sec. Such a portion requires stoichiometric amount of air in a range
100-1500cm3 at the atmospheric pressure. At the
parameters of the test duct this air portion flows
through the separation zone during several
milliseconds in dependence on the conditions.
Fig.19. Sequential images of fuel ignition by plasma
filaments in high-speed airflow.
182
The dynamic of the ignition process was
analyzed on base of temporal behavior of the flame
luminescence and pressure redistribution in the area
of interaction. Typical frames sequence of the fuel
ignition process is shown in Fig.19. It has been
done at 30us of each frame exposure, 4ms of inter-
frames pause and near-IR optical filter application.
The camera sensitivity was decreased in respect of
non-fuel case due to large level of the luminosity.
Fig.20. Image of the flame from the diffuser side.
To define the exact place of the ignition
the video-record from the side of the diffuser
(exhaust direction) has been fulfilled. Appropriate
image is presented in Fig.20. An upper image
shows how the duct is visible without a flame. The
image below shows the duct at the flame presence.
It is easy to recognize that the flame locates inside
of the separation zone. Sometimes a small amount
of the reacting components penetrates to free-
stream. It is should be considered that at the
condition of the test the location of the flame is just
close to the wall step and along the separation
surface due to fuel-free conditions in a main
stream.
The effect of the proper ignition exists at
quite narrow range of time delays between the
plasma and the fuel injection. The good result takes
place only if the discharge plasma and the fuel
injection occur simultaneously. Moreover, the fuel
pulse pre-injection gives the better result in ignition
than standard mode.
The fuel combustion essentially effects on
pressure distribution in the duct, especially in the
separation zone. The chart in Fig.21 shows the
result of pressure measurements. Note, that the
level of the pressure inside of the separation zone at
combustion is equal the static pressure upstream
the backstep. Tree important things should be
considered. The first one is that the flow regime
upstream of the energy release area is not modified
under the ignition (Mach number about M=1.2).
The second statement is that the large additional
energy release to the area took place, much more
than due the plasma generation. The comparison
with the results of CDF simulation allows us to
estimate the power release in about Wcomb=25kW. It means that the combustion completeness lies in a
range =0.2-0.6 in this operation mode. The third
statement is that the combustion process was
stopped at the discharge switching off. It could be
explained by the low gas temperature in flow
without plasma generation.
Graphs in Figs.22 show the plasma/flame
luminosity in wide near-IR and visible spectrum
and in atomic oxygen resonant line. The charts
Fig.21. Pressure redistribution in separation zone due to fuel ignition by plasma.
183
present a case when a double fuel injection took
place. Well seen that the fuel injection and
sequential ignition lead to extreme increase of the
total luminosity as well as in the OH band and O-line.
Fig.22. Integral spectrum plasma luminosity and atomic
oxygen (778nm) radiation from the discharge-flame
occupied area at double fuel injection.
Several important remarks might be posed
in relation with these observations. Analysis of
plasma-flame luminosity spectral distribution
allows marking out two the most intensive zones:
near-IR molecular radiation (not detailed yet) and
very intensive CN band (389nm). At the same time the continual part of the short-wavelength spectra is
increases too at the fuel injection. The integral
radiation reflects the presence of carbons and
carbon-contented radicals in plasma. The radiation
of atomic oxygen is increase dramatically at the
fuel injection (in a range 2-3 orders of magnitude).
The same behavior can be recognized for the OH-band with the hydrogen continuum. The difference
is that the OH radiation maximum occurs in couple milliseconds later. Such an effect is understandable
if taking into account that the OH complex is a product of chemical reactions. The atomic oxygen
behavior is not so clear. From the one side the
atomic oxygen is important radical, which increases
the rate of chemical reactions. From the other side
it is the product of reactions. We recognize this
effect as an important feature of plasma assisted
ignition method.
The reviewing of the experimental data
and results of kinetic calculations allows
considering that the plasma ignition is very
effective in spite of strong non-uniformity of initial
temperature distribution. Dynamics of the ignition
is determined mostly by the maximal gas
temperature in plasma cords. The combustion
process is nonlinear, so the simple averaging
procedure in regards on the temperature is not
applicable. When the combustion is started in
number of points, the gas dynamic mixing takes
place due to large local gradients of temperature
and pressure. Thus the induction time in non-
uniform system might be much less than in
accordance with mean temperature (averaged as
(mi Ti)/ (mi), where “i”- is some part of reacting volume). That is an effect, which is observed.
4. Plasma/MHD Inflow Mixing
The next item of the plasma assisted
combustion concept is the advanced mixing
[16,17]. The test on pulse discharge influence on
mixing processes under supersonic flow has been
carried out in special dielectric test section of
PWT-10 facility. The scheme of the experiment is
shown in Fig.23. Main flow with Mach number
M=2 and static pressure of air about Pst=100Torrcontained a plain jet of another gas (CO2 in this case) with a close actual velocity of co-flow. The
pulse transversal discharge was excited by means
of two flush-mounted electrodes. The discharge
filament crossed the gap (50mm) and plain jet. The
facility has been equipped by Schlieren device with
spatial resolution not worse than 0.2mm and
temporal resolution about 1us.
Fig.23. Draft layout of the experiment on mixing
intensification in high-speed flow.
The discharge can be characterized by the
following parameters: pulse duration more than
50us (actually it has been limited by discharge
channel breakage due to blowout of plasma
filament by flow), maximal discharge current 100-
150A, steady-stage gap voltage U=500-800V,inter-gap distance 50mm, mean E/n parameter
value 30Td, averaged input power up to W=50kW, spectroscopically defined rotational temperature
Tg 3kK. The dynamics of such a discharge plasma filament in ambient air can be considered by means
of observation of Schlieren photos. Sample of such
photos is presented in Fig.24 for different time
delay in respect of discharge breakdown.
Well seen the process of the discharge
channel expansion as well as the discharge-induced
shock wave propagation. The result looks a quite
184
different under the conditions of inflow-generated
plasma filament. Appropriate Schlieren photos are
presented in Fig.25.
Fig.24. Schlieren photos of the discharge dynamics in
ambient air. Pst=100Torr, gap d=50mm.
Fig.25. Schlieren photos of the discharge dynamics in
airflow. M=2, Pst=100Torr.
We suppose to arrow your extra attention
to the second frame. A zone of longitudinal-
transversal instability of the discharge channel is
well seen as an area with strong density
irregularity. This is a zone of intensive mixing
combined with non-homogeneous heating. The
next frames show the process of central jet mixing
with a main flow due to plasma filament
generation. It is important that while the plasma
channel is excited inflow it moves downstream
with a main flow at the same velocity. The mixing
occurs in a gas portion and the energy release
doesn’t lead to a dramatic change of the bulk
parameters of the flow in duct and to thermal
chocking.
5. Preliminary Conclusion.
It is clear that the problem of supersonic
flow control and supersonic combustion
intensification is rather far from the final solving.
But now the extra mechanisms (plasma technology)
are described as the promising contenders of the
mechanical methods. It is quite possible that the
plasma technology can be applied effectively for a
flow/flight control under non-optimal conditions or
off-design regimes. It can give a possibility of fast,
inertia-less control of external and internal flows,
guiding of separation processes, and control of the
high-speed combustion. This paper is presenting
some last results of small-scale experiments in a
field of duct-driven flow control by inflow
generated electrical discharges, plasma-induced
ignition, and plasma-intensified mixing. The last
important understandings could be reviewed as
following:
The structured plasma changes the flow
parameters in duct on a controllable manner. It
occurs due to local heating, shocks generation and
plasma induced separation.
1. Global separation and unsteady local separation
in duct-driven flow due to the surface plasma
generation have been demonstrated
experimentally. It can be used, probably, as the
agent of the flame stabilization.
2. The conception of plasma-assisted combustion
has been formulated. There were considered
that plasma-induced ignition, plasma-intensified
mixing, and flame-holding by plasma
generation are the methods for high-speed
combustion control. Main physical mechanisms
of the plasma effect are the local heating, active
particle deposition, shock waves generation,
local separation, plasma instabilities generation,
and MHD forces.
3. The effect of plasma-induced ignition in non-
premixed high-speed flow has been
demonstrated under the conditions of fixed zone
of separation. The energetic criteria have been
found out.
4. The plasma intensified mixing effects have
been demonstrated by pulse discharge and due
to MHD interaction. Specific plasma instability
in high-speed flow has been considered as
positive agent of the mixing enhancement.
5. Possible penalties are analyzed under an
application of plasma-assisted processes.
185
6. Acknowledgements.
The results, reviewed here, have been
obtained in frame of the work has been performed
partly under the support of EOARD/AFRL/ISTC
(Dr. John Schmisseur), APL/JHU (Dr. David M.
VanWie), Russian Academy of Science (Cor.
Member of RAS Vyacheslav Batenin).
Also the thankfulness should be passed to
Dr. Alexey Bocharov of IVTAN, Prof. Anatoly
Yuriev of Mozhaysky MESA, Dr. Vladimir
Skvortsov and Dr. Yury Kuznetsov of TsAGI for
the useful discussions. Obviously, that the results
would not be possible without excellent work of
IVTAN’s laboratory personnel who participate in
experimental efforts.
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Recommended