EXPERIMENTAL STUDY OF SHOCK WAVE INTERACTING PLANE GAS-PLASMA BOUNDARY

Transcription

1 ISTP-16, 2005, PRAGUE 16 TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA EXPERIMENTAL STUDY OF SHOCK WAVE INTERACTING PLANE GAS-PLASMA BOUNDARY Znamenskaya I.A., Koroteev D.А., Popov N.A. Moscow State University, Russia Corresponding author: Keywords: shock wave, temperature evolution, gas-plasma boundary, instant ionization Abstract Shock wave interaction with plane gas-plasma boundary was realized experimentally: nanosecond volume discharge was initiated in test camera channel while shock wave was moving along the electrodes. Discharge glow near shock front and energy balance was studied. 1 Introdution Shock wave interaction with zone of instant (nanosecond-lasting) ionization is a complicated non-stationary process. It includes pulse energy release in gas flow, followed by formation and evolution of discontinuities, disturbed flow s drift of energy input area; energy redistribution in gas-plasma medium on macroscopic and kinetic level. The research of quick energy release influence on parameters of gas flow is a very actual problem. This problem is closely linked with applied problems of laser physics, plasma aerodynamics [1], plasma chemistry. In the presented work special pulse volume discharge was initiated in test camera channel while shock wave was moving in the channel along the horizontal plane electrodes. The purpose of the work is analysis of nonstationary process of flat shock wave interaction with area of instant volume energy input in gas at presence of transversal disturbances. 2 Description of the experiment Experimental researches of pulse energy input were realized on shock wave tube ( Fig. 1 ) with a special discharge section ( Fig. 2 ). Pulse volume discharge with pre-ionization by ultraviolet radiation from plasma electrodes [2-5] was used. The tube and attached discharge section have rectangular profile 48x24 mm. All experiments were made in the air. Helium was used as a pumping gas. In the discharge section (which has length of 100 mm) volume discharge was realized after O 2 and N 2 molecules had been pre-ionized by UV lightning of plasma sheets. Integral lighting of discharge was recorded by photo cameras through the windows of discharge section in the direction which is perpendicular to the vector of flow s velocity. Fig. 1. Experimental setup: 1- low pressure section, 2- diaphragm section, 3- piezosensors, 4- oscillograph, 5- discharge section, 6- photo cameras, 7- high voltage setup, 8- vacuum pump On the first stage of flow ionization plasma sheets (or electrodes) were switched. 1

2 Irina Znamenskaya, Dmitry Koroteev, Nikolay Popov Just after ns the electric current of main (volume) discharge is initiated. Total discharge time was no more than ns. Quick processes in gas flow are determined by non-stationary interactions of discontinuities and inhomogeneities moving with subsonic (~10 2 m/s) up to hypersonic (~ 10 4 m/s) velocities. The range of characteristic gasdynamic time interval t g is ~ s. Energy input process are realized in time interval t e << t g and thus energy input can be considered as instant. Shock wave s velocity in gas-plasma medium is ~10 3 m/s, (shock wave Mach number M = 2-6). At the nanosecond-scale time of flow ionization the shock wave M ~ 6 moves on distance of 1 mm. If ionization time is bigger it should be necessary to take into account gas flow configuration changes and thermal heating during time of energy input t e. Thus, for adequate experimental simulation of pulse energy input in non-stationary gas flow with shock waves it is necessary to realize homogenous ionization with nanosecond duration. Pulse volume discharge with preionization should be considered as instant energy input in gas flow. Fig. 2. Discharge section with shock wave: 1- shock surface, 2- direction of wave s motion, 3- low pressure area, 4- surface discharge of plasma sheets, 5- high pressure area, 6- metallic electrodes, 7- dielectric layer In homogeneous gas UV radiation from surface discharge forms plasma background in the area between plasma electrodes (main discharge gap). This provides homogeneity of ionization (energy input) of the volume discharge ( Fig. 3 ). The definition of energy input s capacity on the volume stage was based on the measurement of discharge s voltage and current. Average capacity of specific energy input was equaled ev/mol. The part of energy input in plasma sheets was about 12%. Thus, the setup allows experimental modeling of the process of creating plasma area in homogenous supersonic flow with the shock wave. In the wide range of parameters (air pressure was between 20 and 600 Torr) spatial structure of volume discharge in such flow is quite homogenous. Side-walls influence on the structure of ionization area is not too large. The time of discharge s influence is much less than characteristic gas-dynamics time. Fig. 3. Discharge image (without shock wave) Pulse volume discharge was burnt on the different stages of shock wave's motion through the area between plasma electrodes. Piezosensors synchronized the beginning of discharge with any phase of gas-dynamics process. Let s consider motion of a plane shock wave in a channel of a shock tube with constant speed. Parameters of gas at rear side of shock wave are constant and they are defined by Rankine-Hugoniot relations. At some moment T=0 of shock wave passage through the test section channel there is a pulse ionization of this section. Duration of ionization is about 200 ns, t e << t g i.e., as shown above, it is possible to consider energy input as instant. Being homogeneous ( Fig. 3 ), the discharge is redistributed in a non-uniform gas flow according to its structure. Integrated recording of the discharge electroluminescence in a stream represents the image with an 2

3 EXPERIMENTAL STUDY OF SHOCK WAVE INTERACTING PLANE GAS-PLASMA BOUNDARY exposition of the discharge flash. Fig. 4 and Fig. 5 are the images of discharge s area with plate shock wave M=2.3. Experiments showed that discharge concentrated in the low pressure area in front of shock wave. Also we can see that lighting is not homogeneous. It is more Fig. 4. Shock wave in discharge s area which passed about 30% of the electrode s length intensive near the shock surface. Fig. 4 is luminescence image of the discharge gap crossed by a shock wave (M = 2.3) which has passed about 30 % of electrodes' length. Both discharges (surface and volume) burn in zone of the low gas' density in front of shock wave. Thus, at initiation of the discharge in the channel with a shock wave, gas-plasma plane border is created instantly (during 200 nanoseconds) on shock wave front. Gas flow Rankine-Hugoniot relations on shock surface were violated [2] pressure in discharge zone raised quickly. 3 Calculations and results discussing It is known that uniform (in space and time) deposition of energy W into the perfect gas leads to the pressure growth p = (γ-1)w/v. There is the discontinuity breakdown on each border of this domain (Riemann problem). The subsequent flow is characterized by the complicated 2D process of interaction and evolution of these secondary discontinuities. The main 1D structure elements are presented on Fig. 6 : AB: - energy input domain, BC - shock wave, BD - tangential discontinuity, BEF - rarefaction wave, AG - shock wave, AH - tangential discontinuity, IA - shock wave. Fig.6. 1D flow configuration after instant energy input Fig. 5. Shock wave in discharge s area which passed about 75% of the electrode s length ionization leads to volume discharge's current redistribution in the area of the low density due to local medium conductivity dependence on the value of Taunsend parameter E/N (E-electric field, N-molecules concentration). Low density area became a source of the raised intensity of a luminescence. At the moment of instant discharge energy release in front of moving shock Experiments have shown [3, 4], that at cutback of energy input area value of a discharge's current does not vary essentially. The maximal specific energy input value in these areas can be estimated as: e=e 0 V 0 /V (1) Where (e 0 - energy input value in a homogeneous gap, V/V 0 - a fraction of ionization area in the discharge gap volume ( mm). At some small values of V this relation loses sense; the current of the discharge is redistributed in the discharge chamber. 3

4 Irina Znamenskaya, Dmitry Koroteev, Nikolay Popov int ,01 0,02 0,03 0,04 0,05 0,06 0,07 q,mj/mm 3 Fig.7. Discharge luminescence intensity dependence on specific energy Integrted recording of nanosecond discharge image in non-stationary gas flow in an expecting mode was used for analyzing of discharge redistribution in flow. Experimental researches of plasma glow map of nanosecond discharge in front of shock wave were made. Images of the discharge glow reveal that plasma is not absolutely homogenous along shock movement direction in the area between shock wave and discharge gap perimeter. Glow intensity increases towards the shock front - especially at the images where V/V 0 ~ 0,8-0,3. Average discharge glow intensity dependence on q is in Fig. 7. The analysis of a discharge glow field at small values of V allows to make the assumption, that at least at V/V 0 >0,1 relation (1) is valid. Calculated specific energy depending on V 0 /V in low pressure area at ρ/ρ 0 = 0.33; Р = 20 Torr, is on Fig. 8. Calculations were made basing on the model [6,7], including plasma chemical and fast heating processes in air excited by gas discharges. The gas heating is investigated with allowance for the reactions of predissociation of highly excited electronic states of oxygen molecules (which are populated via electron impact or the quenching of the excited states of N 2 molecules), the reactions of quenching of the excited atoms O( 1 D) by nitrogen molecules, the VT relaxation reactions, etc. Dashed line marks calculation by ratio (1). If discharge area is compressed by shock to 1/10, discharge energy released in gas is accumulated mostly in vibration and translation modes. Calculations show ( Fig. 9 ), that in 1 µs after pulse discharge gas temperature in front of shock wave is close to temperature behind the shock M=2 (on Fig. 9 dashed line is temperature of shock heating). In that case discharge heat should strongly influence shock wave and gas flow parameters. Energy input, ev/mol. 1,2 1,0 0,8 0,6 0,4 0,2 0,0 P 0 = 20 Torr ρ / ρ 0 = V 0 / V Fig.8. Specific energy growth on discharge volume withdrawing. Temperature, K M = 2. V 0 / V = Time, ns 10 3 Fig. 9. Temperature evolution in compressed discharge area Conclusions. Research of non-stationary process of flat shock wave interaction with area of instant volume energy input in gas was conducted. Shock wave interaction with plane gas-plasma boundary was realized experimentally: nanosecond volume discharge was switched in test camera channel while shock wave was moving in channel along the flat horizontal plasma electrodes zone. Experiments revealed that volume nanosecond discharge plasma area was compressed in low 4

5 EXPERIMENTAL STUDY OF SHOCK WAVE INTERACTING PLANE GAS-PLASMA BOUNDARY density area between shock wave and discharge gap perimeter. Average glow intensity in that area is proportional to V 0 /V up to V 0 /V~10. The discharge energy was converted to increase the gas temperature in zone, bounded by two tangential discontinuities. Experiments and analysis demonstrated possibility of flow correction using nanosecond energy input in flow with shock wave in channel. Acknowledgments This study was supported by the Program of the Russian Academy of Science «Interaction of Plasma with High-Speed Gas Flows». References [1] Chernyi G.G. The Impact of Electromagnetic Energy Addition to Air near the Flying Body on its Aerodynamics Characteristics. (Russian contribution). // AAIA Proceedings 2nd Weakly Ionized Gases Workshop. Norfolk, USA, P [2] Gulu-zade T.A., Mursenkova I.V., Znamenskaya I.A. Pulse Ionization of Shock Wave Surface. Proc. 22th Int Symp on Shock Waves. London V2. P [3] Znamenskaya I.A., Gulu-zade T.A., Ivanov I.E. Pulse discharge with UV pre-ionization by sliding sheets in 2D supersonic flow. Proc. XV International Conference on Gas Discharges and their Applications GD Toulouse P [4] Lutsky A.E., Znamenskaya I.A. Localization of Pulse Discharge Plasma in Gas Flow. Proceedings of the 4th Workshop on Magneto-Plasma Aerodynamics in Aerospace Applications. Moscow, IVTAN, P [5] Znamenskaya I.A., Lutsky A.E. and Mursenkova I.V. The Surface Energy Deposited into Gas during Initianion of a Pulsed Plasma Sheet Discharge. Technical Physics Letters, Vol. 30, No. 12, pp , [6]. Popov N.A. Investigation of the mechanism for fast air heating in gas discharges Proc. of the 4th Workshop on Magneto-Plasma Aerodynamics in Aerospace Applications. Ed. V.A.Bityurin, Moscow, IVTAN, 9-11 April, P [7]. Popov N.A. Formation and development of a leader canal in air Plasma Phys. Report, Vol. 29. No. 8. P