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

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

Mixing in High-Speed Flow

• MIXING = kinematic stretching + diffusion

Mixing in High-Speed Flow

Solution of Fick’s equation

Typical conditions for scramjet:

T=1000K, P=1Bar, L=1mXd<1mm

Problem of mixing measurement

Problem of mixing measurement

Laser breakdown

fluorescenceProbing discharge

spectroscopy

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

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

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

Practical problems related:

oMixing Intensification in high-speed

flow

oLightning modeling and protection

oFast spark gaps

oNetworks’ protection

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.

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

After-spark channel dynamics,

Air vs CO2, No flow.Air

CO2200us 1ms

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

Cumulative Mechanism of Jets Generation

50 μs

150 μs

400 μs

1 ms

Cumulative Mechanism of

Jets Generation

40μs

540μs

1040μs

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 !!!

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.

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

Mechanism of the discharge localization;

Selection of the easiest breakdown path

ionization rate in mixture of air and secondary gas

Mechanism of the discharge localization

Model experiment

Air

CO2 jet in Air

Air jet in CO2

CO2

After-spark channel dynamics,

CO2 in Air, Spray in Air No flow.

.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.

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?

Multiple pulsing - Pulse repetition rate

The second pulse repeats the

path of the first pulse

if it is too close to the

electrodes position

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

Fourier spectra of image density:

No discharge vs triple pulse

Spatial irregularity

SFmax= B(510-2mm/pix) / x(mm) x 1.5mm

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.

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.

Pulse discharge in vicinity of co-flowing jetsDischarge interaction with CO2 and He jet.

Pulse discharge in vicinity of co-flowing jetsDischarge interaction with C2H4 jet.

a. b.

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.

Acknowledgements.

The work is funded through EOARD-ISTC project #3793p

Thank you!

Questions?

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

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