L.V. Shibkova

Freely localized microwave discharge in a supersonic gas flow

A discharge produced by a focused microwave beam in a supersonic gas flow has been investigated experimentally. It is shown that the degree of ionization and the gas temperature in the discharge are fairly high and that the main properties of the discharge plasma are only slightly affected by the supersonic air flow. Discharges produced by focused microwave beams can find application in supersonic plasma aerodynamics.

Influence of surface microwave discharge on ignition of high-speed propane-air flows

Ignition of supersonic propane-air flow under conditions of low-temperature plasma of surface microwave discharge is experimentally studied. We show that both rich and poor mixtures ignite, and the combustion intensity is maximal for the stoichiometric mixture. The dependence of the supersonic propaneair flow ignition time on the reduced electric field, E/n, under conditions of nonequilibrium gas-discharge plasma is experimentally obtained. The induction period is shown to decrease from 1 ms to 5 μs with the increase in E/n from 40 to 200 Td. The propagation velocity of the combustion front boundary (depending on the equivalent mixture ratio and the input microwave power) is maximal for the stoichiometric mixture and reaches 160 m/s at E/n = 150 Td. Under these conditions, the combustion temperature is about 3000 K.

Microwave and Direct-Current Discharges in High-Speed Flow: Fundamentals and Application to Ignition

DOI: 10.2514/1.24803 The main parameters and properties of an electrodeless freely localized microwave discharge, a surface microwave discharge, transversal in relation to the gas flow direct-current and pulse-periodic electrode discharges, andacombined microwavedirect-current dischargeareexperimentally investigated.It isshown thatalltypesof the discharges result in a reliable ignition of hydrocarbon fuel. Combustion of a propane–air supersonic stream is realized under condition of the combined surface microwave and direct-current discharge. To find out the influence ofdifferent channels ofenergy transfer onignition ofcombustible mixtures in asupersonic flow,the kinetic modelof ignition of hydrocarbons–air mixtures, and taking into account the influence of electric field on processes of dissociation molecules and creation of the active radicals, excited and charged (electrons, positive and negative ions) particlesunderconditionsofnonequilibriumplasmaofthegasdischargewasdeveloped.Mathematicalmodelinghas revealed a strong influence of the reduced electric field on induction period. The microwave discharges may find applications in different fields of supersonic plasma aerodynamics and in development of new-generation plasma sources for plasma chemistry, nano- and microelectronics purposes (plasma treatment of surfaces, etching, and film deposition), and so on.

Stabilization of liquid hydrocarbon fuel combustion by using a programmable microwave discharge in a subsonic airflow

Under conditions of a programmable discharge (a surface microwave discharge combined with a dc discharge), plasma-enhanced combustion of alcohol injected into a subsonic (M = 0.3–0.9) airflow in the drop (spray) phase is stabilized. It is shown that the appearance of the discharge, its current-voltage characteristic, the emission spectrum, the total emission intensity, the heat flux, the electron density, the hydroxyl emission intensity, and the time dependences of the discharge current and especially discharge voltage change substantially during the transition from the airflow discharge to stabilized combustion of the liquid hydrocarbon fuel. After combustion stabilization, more than 80% of liquid alcohol can burn out, depending on the input power, and the flame temperature reaches ∼2000 K.

Surface microwave discharges in air

A microwave discharge excited on the outer surface of a dielectric antenna has been investigated. The transverse and longitudinal dimensions and propagation velocities of the discharge have been measured as functions of the air pressure and the power and duration of the exciting microwave pulse. The spatial distributions and time evolution of the gas temperature, electron density, and radiation intensity of the discharge have been determined. It is shown that the degree of ionization of the discharge plasma can exceed 10%. The spatial distribution of the electron density is found to depend strongly on the air pressure.

Microwave Discharge on the Surface of a Dielectric Antenna

A microwave discharge initiated by a surface wave on a dielectric body placed in a supersonic air flow is studied. The discharge is shown to represent a thin plasma layer that uniformly covers the antenna surface. In experiments, the discharge propagation velocity may be as high as 100 km/s, which is several orders of magnitude higher than the velocity of sound in air. The peak pulse power necessary to excite the discharge in a wide range of air pressures (from 10−3 to 103 Torr) is no higher than 100 kW. It is shown that the gas temperature may rise to 1000–2000 K, rapidly increasing (with a rate of ≈50 K/μs) at the early stage of discharge evolution. The discharge of this type may find applications in super-and hypersonic plasma aerodynamics (such as control of the flow near the surface of a body moving in a dense atmosphere, reduction of surface friction, optimization of ignition and combustion conditions for supersonic flows of gaseous fuel, etc.). It may also be used to advantage in development of new-generation plasma sources for micro-and nanoelectronics purposes (plasma treatment of surfaces, etching, and film deposition).

Mechanisms of microwave surface discharge propagation

A microwave discharge propagating over the surface of a dielectric antenna is studied. It is experimentally shown that the velocity of discharge propagation over the surface is maximal early in microwave pulse application and grows with the applied power. The breakdown wave defines the velocity of the discharge at its early stages (t = 1–3 μs). Ambipolar diffusion governs the discharge propagation at the stage of its evolution (t= 3–100 μs), and, finally, slow surface combustion is possible only at the stationary stage of the discharge (t > 100 μs). The electric field is localized in a thin (∼1 mm) surface layer. High values of the reduced electric field, E/n = 100–500 Td, provide efficient energy deposition to the plasma, i.e., favor the rapid heating of the gas and the efficient generation of charged particles. This makes the discharge promising for hypersonic aerodynamics.

Inflammation dynamics of thin alcohol films under the action of a surface microwave discharge initiated in atmospheric-pressure air

Rapid nonthermal plasma-stimulated inflammation of liquid hydrocarbon (alcohol) films is realized under the conditions of a surface microwave discharge initiated in quiescent atmospheric-pressure air. The induction period is found, and the velocity of the intense combustion front is determined. Combustion is initiated by the low-temperature plasma of the surface microwave discharge that exists at high values of the reduced electric field. It is shown that the induction period varies between 10 and 100 µs depending on the input power, the plasma-stimulated inflammation occurs on the antenna at the site where the surface microwave discharge burns, and the velocity of the intense combustion front near the antenna reaches 300–350 m/s.

Ignition of hydrocarbon films under the conditions of surface microwave discharge

In the present work, rapid plasma-stimulated ignition of liquid hydrocarbons was carried out in still air under conditions of surface microwave discharge. It was shown that, depending on the microwave input power, the breakdown time changed in a range from 5 to 30μs, ignition occurred on an antenna in the area of the surface microwave discharge at a temperature not exceeding 1000 K, and the speed of the front boundary propagation of the intense ignition region near the antenna was 300 m s−1.

Parameters of the flame due to surface-microwave discharge-initiated inflammation of thin alcohol films

Nonthermal plasma-stimulated inflammation of thin alcohol films under the conditions of a microwave surface discharge initiated in quiescent air under atmospheric pressure is realized. The main parameters and properties of the flame due to alcohol inflammation and combustion are studied. It is shown that inflammation occurs when the gas temperature near the antenna is no higher than 1000 K. When the reduced electric field is high, the flame temperature near the antenna reaches 3300 K and the electron concentration equals 2 × 1012 cm−3. The electron temperature during alcohol combustion varies from 0.8 eV at distance y = 10 mm from the antenna surface to 0.3 eV for y = 40 mm.