Mixing Intensification by Electrical Discharge · spectroscopy. Pulse filamentary discharge in...
Transcript of Mixing Intensification by Electrical Discharge · spectroscopy. Pulse filamentary discharge in...
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AFOSR Program ReviewFundamental Mechanisms, Predictive Modeling, and Novel
Aerospace Applications of Plasma-Assisted Combustion
November 9 and 10, 2011, Ohio State University
Mixing Intensification by Electrical Discharge
S. B. Leonov, A. A. Firsov, Yu. I. Isaenkov, M.A. Shurupov, D. A. Yarantsev,
Joint Institute for High Temperature RAS, Moscow, Russia
and
M.N. Shneider, Princeton University, NJ, USA
I.V. Kochetov, A.P. Napartovich, TRINITI, Moscow region, Russia
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Outline
• Subject and Motivation: Instability of Pulse Discharge
• Pulse discharge in ambient gas and in high-speed flow
• Mechanism of jets generation
• Mechanism of specific localization of the plasma filament
• Pulse discharge in vicinity of injector in high-speed flow
• Concluding remarks
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Mixing in High-Speed Flow
• MIXING = kinematic stretching + diffusion
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Mixing in High-Speed Flow
Solution of Fick’s equation
Typical conditions for scramjet:
T=1000K, P=1Bar, L=1mXd<1mm
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Problem of mixing measurement
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Problem of mixing measurement
Laser breakdown
fluorescenceProbing discharge
spectroscopy
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Pulse filamentary discharge in gases and in flow
L=30-100mm, I=1-10kA, t=30-300ns, W=10-100MW, T=15-20kK
Main topic:
After-discharge
channel
instability
10us 300us
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Pulse discharge in ambient gas
-1,0x10-7 0,0 1,0x10
-72,0x10
-73,0x10
-7
-40
-20
0
20
40
60
80
100
Vo
lta
ge
, C
urr
en
t, P
ow
er
Time, s
Pulse Discharge
kV, Voltage
20A, Current
MWa, Active Power
Energy release E=1.2J
Main feature of PS: moderate speed of the voltage rise dV/dt>108V/s.
Tesla coil based power supply
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Highlights
1. Post –discharge zone is unstable in the most
gases. Fast lateral cumulative jets generation
2. In vicinity of boundary between molecular gases
the discharge selects breakdown path between
them
3. The time scale of the flow disturbances (2-5µs)
corresponds to spatial scale of discharge excited
disturbances, which is measured as x=1-3mm
4. In average the flow parameters are affected by
the discharge negligibly
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Practical problems related:
oMixing Intensification in high-speed
flow
oLightning modeling and protection
oFast spark gaps
oNetworks’ protection
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Measuring Technique in these Experiments
• High-speed high-res CMOS camera: 1 to 4 directions
• Schlieren technique 100ns, 0.2mm
• Streak camera 1000pix, 6µs/scan
• Schlieren-streak device
• Laser-based schlieren sensors
• Optical spectroscopy
• Filtered imaging: CN, C2, N2, OH, O, etc.
• Probing discharge spectroscopy
+• Pressure measurements
• Electrical measurements
• Etc.
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Important Feature:
Instability Development
1. At t=30-100 µs, the after-spark channel becomes
unstable (RT mechanism)
2. At t=100-300 µs, the lateral gaseous jets generation
3. At t=300-1000 µs, effective mixing due to the
jets/turbulence
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After-spark channel dynamics,
Air vs CO2, No flow.Air
CO2200us 1ms
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Cumulative Mechanism of Jets Generation
3D restoration of plasma filament
using 2-4 2D images Schlieren image
S. B. Leonov, Y. I. Isaenkov, A. A. Firsov, S. L. Nothnagel, S. F.
Gimelshein, and M. N. Shneider, “Jet Regime of the Afterspark
Channel Decay”, PHYSICS OF PLASMAS 17, 1, 2010
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Cumulative Mechanism of Jets Generation
50 μs
150 μs
400 μs
1 ms
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Cumulative Mechanism of
Jets Generation
40μs
540μs
1040μs
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Discharge localization
1. Active guiding
2. Use the natural properties of medium and electrical
discharge
Basic effect – HV long discharge strives for location
between two gases !!!
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Mechanism of specific localization of discharge
1. The first stage of the spark breakdown is the multiple streamers propagation
from the “hot” electrode toward the grounded one.
2. The second stage is the real selection of the discharge path among the multiple
channels with non-zero conductivity.
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Mechanism of the discharge localization;
Ethylene + Air mixtures
Electron drift velocity in C2H4/Air
mixtures
0.1 1 10 10010
5
106
107
C2H
4:Air, ER=1
C2H
4
Ve,
cm
/s
E/N, Td
Air
C2H
4:Air=1:1
Current dynamics in C2H4/Air
mixtures, U=100 kV•sin(t/5e6)
1 2 30.0
0.5
1.0
1.5
2.0 C
2H
4:Air, ER = 1
Air
C2H
4:Air=1:1
C2H
4I, A
Time, s
h = 3.25 cm, d = 1 mm, P = 1 atm, T = 300 K
0
50
100
U,
kV
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Mechanism of the discharge localization;
Selection of the easiest breakdown path
ionization rate in mixture of air and secondary gas
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Mechanism of the discharge localization
Model experiment
Air
CO2 jet in Air
Air jet in CO2
CO2
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After-spark channel dynamics,
CO2 in Air, Spray in Air No flow.
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.Pulse discharge properties in high-speed flow.
Experimental arrangement.
Pt≈1÷2 Bar, flow velocity M=2 and 2.5
Pulse duration t=40-100 ns
Umax=100 kV, Imax=2.5 kA, Wmax=90 MW
CO2, He, and C2H4 jet.
Grounded electrode coincides with
jet nozzle.
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Experiment in high-speed flow
Subsonic vs Supersonic (single pulse)
Typical schlieren image of pulse discharge in M=0.5 and M=2 flow.
Delay time: t=150us and 100us
What is the discharge generation frequency needed to disturb flow continuously?
Do sequential pulses feel each other?
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Multiple pulsing - Pulse repetition rate
The second pulse repeats the
path of the first pulse
if it is too close to the
electrodes position
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Gas disturbance effectiveness estimation:
measurement technique – schlieren streak camera
Secondary
breakdown
on the first
disturbed
zone
Separate
breakdown V / Lmix < F < V / D
V – flow speed,
Lmix – mixing distance,
D – discharge gap
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Fourier spectra of image density:
No discharge vs triple pulse
Spatial irregularity
SFmax= B(510-2mm/pix) / x(mm) x 1.5mm
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Pulse discharge in vicinity of co-flowing jets
Subsonic flow and CO2 jet
Subsonic flow M = 0.3; CO2 jet
Discharge breakdown along the jet.
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Pulse discharge in vicinity of co-flowing jets
Supersonic flow and CO2 jet
Supersonic flow M = 2.5; CO2 jet
Discharge breakdown along the jet.
No variations of the filament shape.
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Pulse discharge in vicinity of co-flowing jetsDischarge interaction with CO2 and He jet.
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Pulse discharge in vicinity of co-flowing jetsDischarge interaction with C2H4 jet.
a. b.
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Summary
1. Fast localized energy deposition to the gas by filamentary
discharge leads to generation of strongly turbulent area
characterized by fast (V≈200m/s) expansion due to
mechanism of lateral jets escaping.
2. Optimal repetition rate allows to provide the turmoil of gas
in a large volume of flowfield. The time scale of the flow
turbulence (3us) corresponds to spatial scale of discharge
excited disturbances, which is measured as dx=1-3mm.
3. In two-component flow the filamentary discharge strives for
the location between two molecular gases, if the
experimental arrangement allows it.
4. The discharge disposition into a mixing layer and the
instability development are favorable for the kinematic
mixing.
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Acknowledgements.
The work is funded through EOARD-ISTC project #3793p
Thank you!
Questions?
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4 m
23
456
20 mm1
60 mm
7
1 – supersonic flow (М = 2); 2 – laser diode module; 3 – optical windows;
4 – photodiode; 5 – oscilloscope; 6 – computer; 7 – electrodes.
measurement technique - schlieren probe
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Dependences of lg(<Amp>) at different
frequencies on time
for the cases of one and three discharges
Statistically averaged scale of disturbances is
y=1-3mm at t=100us