Alexander A. Ravaev

Investigations of Subcritical Streamer Microwave Discharge in Reverse-Vortex Combustion Chamber

In this paper, we report on the experimental investigations of two innovative technologies combined in one device, namely, the application of an initiated streamer microwave (MW) discharge for ignition and flame control in a reverse-vortex combustion chamber, also known as a Tornado combustor. The pulsed subcritical streamer MW discharge (SSD) in a quasi-optical wave beam was experimentally investigated in the atmospheric-pressure model of a Tornado combustor with transparent dielectric walls. The possibility of a surface MW streamer discharge formation at new conditions, particularly on the dielectric walls of the cylindrical combustion chamber, has been confirmed. The mechanism of SSD realization, at which the discharge development takes place in a volume of the combustion chamber, has been proposed. It was shown that SSD could be applied for ignition of fuel/air mixtures in an axial area of the reverse-vortex combustion-chamber bottom plate, which is normally used for a fuel feeding.

Control of flow characteristics using localized plasma discharges

The microwave (MW) system was developed for full-scale measurements of aerodynamic characteristics in a wind tunnel. Reliable initiation of plasma discharges was obtained on the airfoil model with 11 initiators over its convex and concave walls at their regular 10-mm spanwise spacing. Experiments showed possibilities to delay flow separation and to raise lift coefficients by 15% together with the drag dropped by 5%. It has proven the developed concept of flow optimization through modification of its turbulent structure and the plasma-control method efficiency.

Electric discharge in air in a deeply subcritical field of a quasi-optical microwave beam

We describe the results of experiments on initiation of an electric discharge in air in a quasi-optical microwave beam by an electromagnetic vibrator fixed above the screen. This method for initiating the electrical breakdown makes it possible to obtain a discharge at a level of the electric field component in the microwave, which two orders of magnitude lower than the minimal critical field of the electrodeless breakdown of air. In our experiments, the threshold value of the air pressure determining the low- and high-temperature forms of the microwave discharge are determined depending on the field level.

Induced field of an electromagnetic vibrator above a conducting screen placed in a microwave beam

The induced field of a cylindrical electromagnetic vibrator with spherically rounded ends is calculated. The vibrator is above a flat screen placed in a linearly polarized quasi-optical microwave beam. The plane of the screen is perpendicular to the Poynting vector of the radiation. The axis of the vibrator is aligned with the vector of the exciting field electrical component. In calculation, the length of the vibrator and the vibrator-screen distance were varied, while its diameter and the initial field were kept constant. It is found that the induced field of the vibrator with a length close to the half-wavelength of the field is maximal at the ends of the vibrator and the field strength resonantly depends on the length of the vibrator. The shortening of the “half-wavelength” vibrator that provides a maximal induced field is determined. The result of numerical simulation is to an extent intriguing. It is revealed that the induced field of a resonance half-wavelength vibrator rises considerably when the vibrator-screen distance becomes shorter than the quarter-wavelength of the field. The Q factor of an equivalent electromagnetic oscillating circuit characterizing the vibrator also grows, and the induced field more and more concentrates between the screen and the surface of the vibrator’s ends facing the screen. Full-scale experiments qualitatively support theoretical predictions. The results allow researchers to considerably extend the application area of vibrators as initiators of breakdown in high-pressure gases to ignite microwave discharges in quasi-optical beams with a low initial field.

Excitation of a system of electromagnetic oscillators in the field of a remote microwave radiation source. Part 2. Low-Q systems

The paper presents the results of simulation and experimental testing of the method of forming a periodic temperature profile on the surface of a dielectric body by a regular multisystem of low-Q resistive electromagnetic (EM) oscillators heated in the field of a quasi-optical beam of external microwave radiation. It is shown that, in a system of low-Q oscillators, excitation of a multimode EM-field structure is less pronounced than in the case of high-Q elements, which facilitates formation of a uniform temperature profile with a required contrast on the model’s surface. The efficiency and drawbacks of systems of EM oscillators based on high-Q and low-Q elements are compared, and recommendations on their practical application in an aerodynamic experiment are given.

Characteristics of the above-screen electromagnetic dipole that initiates the electric breakdown of gas in the quasi-optical microwave beam

The experimental study of cylindrical electromagnetic dipoles with spherical rounded ends that initiate the electric breakdown of gas in the presence of the subcritical field of the quasi-optical microwave beam is presented. It is demonstrated that the efficiency of the breakdown initiation substantially increases when the distance between the dipole and the screen becomes less than one quarter of the wavelength of electromagnetic field. Based on the experimental results for a wide range of initial parameters, the degree of shortening of the half-wavelength dipole that provides the maximum induced field at its vertex (pole) is determined and the field magnitude and the conditions for the accuracy of the resonance length of dipole are estimated.

Electrodynamic Problems of Boundary-Layer Control Method Based on Array of Microwave-Heated Elements

First results of theoretical and experimental investigations being an integral part of the works on development of new boundary-layer control method using regular spanwise array of MW-heated elements on a dielectric surface of airfoil model are considered in this paper. Attention is paid to three basic electrodynamic problems of the project. The first one is to develop and design new electrodynamic system consisting of tens of closely located electromagnetically coupled and, what is very important, homogeneously heated elements. The second problem is to provide high responsivity of such system, i.e. high efficiency of its heating under action of microwave radiation from remotely located irradiating system of the MW generator with limited output power. And the last problem is to create required spanwise temperature profile (contrast) on a model surface during tests in a wind -tunnel. Various types and versions of MW-heated elements, including high and low-Q resistive vibrators based on ceramic resistors and composite materi als, principally new systems of high-Q metallic and graphite vibrators located on a thin foil -laminated substrate, “usual” quarter-wave plate-type MW absorbers and some others have been studied. The results of their numerical simulation and experimental investigations, comparative analysis, advantages and disadvantages are considered in detail. By the moment almost all the specified problems were successfully solved. Physical grounds of creation of regular multisystem of MW-heated vibrators on a dielectric surface were developed. This allowed to design airfoil model and to start joint aerodynamic wind tunnel tests together with specialists from Institute of Hydromechanics and National Aviation University in Kiev.

Regular set of gas discharges on the surface of a dielectric in a quasi-optical microwave beam

Various ways of producing a regular set of plasma objects in the field of a quasi-optical microwave beam using passive vibrators are considered. These objects provide the basis for a periodic vortex structure on the surface of an immersed dielectric body. The properties of a deeply subcritical surface discharge in the flow, as well as its structure and spatial and energy characteristics, are described.

Boundary-Layer Control Based on Localized Plasma Generation: Development of the Microwave System

,The brief review of the results of theoretical and experimental investigations of advanced method of boundary-layer control based on localized plasma generation obtained in frames of the three-year joint project CRDF #UKE2-1508-KV-05 are presented in this paper. Emphasis is placed on the electrodynamic aspects of the developed method in laboratory conditions in Moscow Radiotechnical Institute and at carrying out of final experiments on multidisciplinary wind-tunnel installation in Institute of Hydromechanics and National Aviation University in Kiev. The main problem had fundamental character and consisted in development of unique electrodynamic multisystem of close located initiators of microwave (MW) discharges on a dielectric surface in “free-space” conditions. Herewith it was initially necessary to solve the problem of creation of a uniform field distribution in such system in a field of plane electromagnetic (EM) wave, i.e. to solve the problem of multimode excitation of such arrays of EM vibrators. At the same time already the first experiments have highlighted a number of other engineering problems. First, it was necessary to develop new irradiating electrodynamic system providing quite uniform field distribution on a model surface in real conditions of experiment in the working chamber of the wind tunnel. Secondly, the periodic system of multiple MW discharges had to be generated at a remote irradiation of model and simultaneously at limited power of the MW radiation source. All the specified problems have been successfully solved. Physical grounds of creation of regular system of multiple MW discharges on a dielectric surface of a streamline body were developed. This allowed not only to confirm feasibility and high efficiency of the proposed method but also to determine prospects of its further development.

Electrical discharge excited by the deeply undercritical field of a microwave beam in a high-speed jet of air and air-propane mixture

An electrical gas discharge initiated by a tubular linear electromagnetic vibrator is studied. The discharge is excited by the deeply undercritical linearly polarized field of a quasi-optical microwave beam. Experiments are conducted at a flow velocity of several hundreds of meters per second. The discharge region is photographed, and the stagnation temperature of the flow in the wake of the discharge is measured. It is shown that a deeply undercritical microwave discharge may arise in air even at such high flow velocities. Moreover, it is found that the discharge ignites and stabilizes the discharge region in a “lean” air-propane mixture. In such an experimental design, propane completely burns out when the flow velocity is smaller than some threshold value. When the flow velocity is high and the Mach number of the flow approaches unity, throttling effects and “thermal blocking” of the jet are observed.