Lev P. Grachev

Experiments on Propane Ignition in High-Speed Airflow Using a Deeply Undercritical Microwave Discharge

Experiments with deeply undercritical microwave discharges in high-speed airflow and the effects of the discharge on propane-air ignition are described in the present paper. The discharge was excited in the quasioptical beam of a linearly polarized microwave radiation with power 1 kW at a wavelength 12.5 cm, which was generated using a magnetron-type generator of continuous radiation. The discharge was initiated by an electric vibrator located within a submerged air stream at velocities varying from zero to supersonic speeds. The propane jet was injected into the discharge region through a hole in the base of the vibrator. These experiments demonstrate the possibility of propane ignition and combustion at supersonic speeds following propane injection into a discharge region.

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.

Efficiency of Propane-Air Mixture Combustion Assisted by Deeply Undercritical MW Discharge in Cold High-Speed Airflow *

†† Results are presented from an experimental investigation of propane-air combustion in a stationary microwave discharge at different conditions. In these experiments, a deeply undercritical discharge is initiated in the base of an electromagnetic vibrator, which is immersed in cold supersonic airflow. Two schemes for mixing propane with air are considered. In the first scheme, a propane-air mixture is delivered through the vibrator and is ignited at the outlet from the vibrator. In the second scheme, propane delivered through the vibrator mixes with air captured by an inlet hole in the vibrator nose prior to being also ignited by the MW discharge at the base of the vibrator. Propane combustion with efficiency of approximately 60 % was demonstrated at a mixture velocity up to 200 m/s in the combustion region. The heat release reaches approximately 1 kW at a propane mass flow rate of 2·10 -2 g/s and discharge power of approximately 200 W.

Surface Discharge in a Microwave Beam

A review of new experimental results on the characteristics of pulsed microwave (MW) surface plasmas on a dielectric plate is presented. The plate is located in a quasi-optical linearly polarized electromagnetic (EM) wave beam with an electric field lower than the critical breakdown field. Experiments were realized with two wavelengths of radiation. The plate was located both in a plane perpendicular to the pointing vector of this radiation and in a plane containing the pointing vector and a vector of the electric component of radiation. Preliminary experimental researches have shown that the corresponding transversally and longitudinally initiated surface MW discharges were realized in a surface-developed streamer type for a certain range of air pressure. The growth rate of streamer channels forming the discharge exceeds the sound speed in air. Separate areas of these channels, which have lengths comparable with the half wavelength of the field, interact energetically with the EM field producing the discharge in an efficient manner. The opportunity offered by the realization of these discharges attracts attention to questions on their practical application and further directions for experimental research.

Domains of Existence of Various Types of Microwave Discharge in Quasi-Optical Electromagnetic Beams

High-and medium-pressure discharges excited by quasi-optical microwave beams in air are classified using experimental data. Diffuse, streamer, overcritical, undercritical, and deeply undercritical discharges are distinguished. The domains of existence of each type of discharge are determined as functions of the air pressure and field strength for two particular electromagnetic wavelengths.

Initiated Surface Microwave Discharge as an Efficient Active Boundary-Layer Control Method

The first-step result of experimental works on verification of advanced method of boundary-layer control based on localized plasma generation is presented. Boundary-layer control is executed by means of vortex structure creation near streamlined body surface. Experiment has been performed in specially designed wind tunnel. The microwave radiation remotely feeds a resonant distributed system, initiating and sustaining the point discharges, generating the vortex structure in the stream. Measurement data together with results of modeling confirms the feasibility of initial idea.

Microwave breakdown of air initiated by a short electromagnetic vibrator

The electrical breakdown of air (under a pressure from several tens of Torr to several hundreds of Torr) is initiated in a linearly polarized quasi-optical microwave beam using a rounded-end cylindrical metallic electromagnetic vibrator placed parallel to the electric component of the microwave field. The vibrator is much shorter than the wavelength of the field. The field strength at the top of the vibrator as a function of its length is determined. An empiric expression relating the field strength at the top with the vibrator length-to-diameter ratio is found for the vibrator length range studied. The practicability of locally measuring the field in a quasioptical microwave beam is substantiated. The idea is to determine the maximal air pressure at which electrical breakdown of air initiated by a short vibrator placed at a given point of the beam takes place.

A Microwave Discharge Initiated by Loop-Shaped Electromagnetic Vibrator on a Surface of Radio-Transparent Plate in Airflow

A review of new experimental results on realization of initiated microwave streamer discharge in deeply subcritical form on a surface of radio-transparent dielectric plate is presented. Experiments were carried out in the beam of electromagnetic radiation created by a generator of average power no greater than 1.5 kW with the wavelength 12.3 cm. A new type of an initiator has been applied in the experiment for microwave discharge realization. It represents a streamlined EM vibrator rolled in an open ring. Such loop-shaped initiator generates two-electrode discharge. Initiator of this type possesses some advantages with respect to known and applied before. It has been specially developed and investigated for executing of the given experimental program. The microwave discharge of deeply subcritical type was realized on the dielectric surface in the airflow with the velocity up to 25 m/s at atmospheric pressure. We have obtained data on power level necessary for this discharge realization in various electrodynamic and gasdynamic conditions. We have investigated discharges over various radiotransparent dielectric plates at different levels of delivered microwave power in a wide range of subsonic airflow velocity. We have investigated spatial and temporal structures of this discharge and have measured parameters of a discharge trace in the flow. We have obtained first experimental results on creation of a line of these discharges with fixed step between them in the system of the open-loop initiators. Such discharge structure will be applied in experimental investigations on analysis of new ways of boundary layer control.

Surface streamer microwave discharge

An atmospheric-pressure pulsed microwave discharge on the surface of an insulating plate and between two insulating fllms is considered. The discharge is initiated in a quasi-optical microwave beam away from the beam-forming elements. The TEM field of the beam is less than the critical breakdown value. The initiator is a metallic dipole placed either directly on the plate surface facing away from the radiation source or between the films. The discharge has the form of branching streamer channels adjacent to the surface and filling up the cross section of the microwave beam with a field level higher than the value separating the subcritical and highly subcritical forms of the microwave streamer discharge.

Air ionization in a near-critical electric field

A set of experimental dependences of the air ionization effective rate on the electric field strength is presented. The concept of the critical breakdown field is discussed. It is indicated that experimental data are quantitatively inconsistent with analytical results based on this concept. This inconsistency is eliminated if the ionization balance takes into account not only dissociative adhesion of electrons to oxygen molecules but also their detachment from the molecules that gained a charge during the charge exchange process. Based on the results obtained, a new physical interpretation of the critical field is suggested. A formula for the effective rate of air ionization in near-critical fields is derived.