Surface Microwave Discharge as Means of Ignition of

Hydrocarbons in Air Streams

A.F.Aleksandrov1, V.M.Shibkov2, L.V.Shibkova3

Department of Physics, Moscow State University, 119992, Moscow, Russia

Influence of non-equilibrium plasma of a surface microwave discharge on processesof ignition of a supersonic propane-air stream with Mach number М=2 is considered.Alcohol, gasoline and kerosene ignition under conditions of subsonic and supersonicair streams is investigated too.

Advances in aviation technology call for research and development aimed at creating new

efficient means for reducing the ignition time and controlling the combustion of supersonic fuel

flow. A new solution to these problems is application of gas discharge. In the paper a surface

microwave discharge was used for ignition of a supersonic propane-air stream and for ignition of

liquid hydrocarbon films under conditions of a subsonic and a supersonic air stream.

For investigation of the possibility of the ignition of high-speed hydrocarbon flows using

a surface microwave discharge an experimental design, which including a vacuum chamber, a

high pressure air receiver, a high pressure propane receiver, a system for propane-air mixing, a

system for supersonic flow creation, a rectangular aerodynamic channel, a high voltage power

supply source, a magnetron generator, a system for microwave energy input to the chamber, a

synchronization unit, and diagnostic equipment, was used [1-9]. A pulsed magnetron generator

of the centimeter wavelength range serves as the microwave power source. The characteristics of

the magnetron generator are: the wavelength λ = 2.4 cm, pulsed microwave power W < 200 kW,

and a pulse duration τ = 5-200 µs.

Ignition of propane-air supersonic flow was studied depending on the pulsed power

(W = 30-70 kW), microwave pulse duration (τ = 5-200 µs), and air mass consumption

(dmair/dt = 25-120 g/s) propane mass consumption (dmpropane/dt = 1-7 g/s); in this case, the

equivalent ratio for propane varied from 0.3 to 2.Experiments were carried out on the installation

consisting of a vacuum chamber, a receiver of a high pressure of air, a receiver of a high pressure

of propane, a system for mixing propane with air, a system for producing a gas flow, a

magnetron, a system for delivering microwave power into the chamber, an aerodynamic channel,

1 Professor, Department of Physics, Moscow State University, 119992, Moscow, Russia, alex@ph-elec.phys.msu.ru.2 Professor, Department of Physics, Moscow State University, 119992, Moscow, Russia, shibkov@phys.msu.ru3 Professor, Department of Physics, Moscow State University, 119992, Moscow, Russia, shibkov@ph-elec.phys.msu.ru

47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition5 - 8 January 2009, Orlando, Florida

AIAA 2009-690

Copyright © 2009 by Moscow State University. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

a synchronization unit, and a diagnostic system [1-9]. The scheme of installation is submitted in

Fig. 1.

Fig. 1. Block-scheme of the experimental installation. 1 – vacuum chamber, 2 – synchronizationunits, 3 – magnetrons, 4 – directional coupler, 6 – load, 7 – crystal microwave detector, 8 –waveguide, 9 – dielectric antenna, 10 – Laval nozzles, 11 – electric hydraulic valves, 12 – airhigh pressure receiver, 13 – propane high pressure receiver, 14 – high-pressure cylinder withpropane; 15 – valves, 16 – block of probe measurements, 17 – power supply system, 18 –computers, 19 – digital spectrograph, 20 – video and photo digital cameras, 21 – pulsed powersupply, 22 – pulsed lamp of flash, 23 – lens, 24 − entrance diaphragm, 25 – objective lens, 26 –lens, 27 – exit diaphragm, 28 – digital photo camera, 29 – digital oscilloscope, 30 – pump out.

Fig. 2 shows the integral photo of the burning of the supersonic propane-air flow and

Fig. 3 shows the instantaneous shadowgram of this process. The front boundary of the burning

region, coinciding in configuration with the boundary of the region of supersonic burning

observed in the integral photo, is fixed in the shadowgram. This spatial distribution of the

burning front of the supersonic flow for the propane-air mixture proves our hypothesis on the

possible contribution of ultraviolet radiation of the surface microwave discharge on hydrocarbon

fuel ignition kinetics [2, 11]. It should be noted that gas dynamic perturbations arising in the

course of development of the surface microwave discharge can also strongly influence the

ignition of the supersonic propane-air flow. It was shown experimentally that ignition already

takes place for a pulsed power of 35 kW at a distance of 1.5 cm from the front edge of the

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antenna. With increasing supplied energy the intensity of supersonic propane-air flow burning

increases; in this case, for a power of 70 kW burning begins approximately at a distance of

0.5 cm from the front edge of the antenna.

Fig. 2. General view of ignition of a supersonic stoichiometric propane-air stream with a help ofa surface microwave discharge created on a quartz antenna at dmair/dt = 55.5 g/s,dmpropane/dt = 3.6 g/s, τ = 120 µs, Wp = 70 kW.

Fig. 3. Instant shadow picture of ignition of a supersonic stoihiometric propane-air stream underconditions of a surface microwave discharge created on the quartz antenna at dmair/dt = 55.5 g/s,dmpropane/dt = 3.6 g/s, τ = 120 µs, Wp = 70 kW. The flashlamp pulse with a duration of 4 µs isdelayed with respect to the microwave pulse front by 120 µs.

The important parameter during the ignition and burning of the air-hydrocarbon fuel is

the mixture composition. In experiments the equivalent ratio for propane was controlled by

either variation of air consumption at a constant propane consumption per second, or variation of

propane consumption per second at a constant air consumption per second. It was established in

the studies performed that the surface microwave discharge results in ignition of both poor and

rich mixtures. The ignition delay for the supersonic propane-air flow changes depending on the

experimental conditions from 5 to 20 µs. The transverse burning front propagation rate was

determined by the known supersonic flow speed and the measured tangent of the inclination

angle of the sharp front boundary of the characteristic glow. It was obtained that the rate depends

on the supplied power and equivalent ratio. It is maximal in the stoichiometric mixture, reaching

about 200 m/s. The fact of ignition was also shown using a double probe placed in the burning

region; in this case, the signal intensity at ignition of the propane-air flow exceeded the intensity

of the signal corresponding to the surface microwave discharge in an air flow by one order of

magnitude. The flame temperature upon burning of the supersonic stoichiometric propane-air

flow for the surface microwave discharge was of the order of 3000 K.

When a liquid is placed in a vacuum chamber, it evaporates intensely, which prevents

investigation of the ignition of hydrocarbon fuel in the liquid phase at air pressures in a pressure

chamber lower than atmospheric pressure. The system of supply of electromagnetic energy to the

antenna developed in [10-13] allows one to create the surface microwave discharge in a wide

range of air pressures up to atmospheric pressure and to study the ignition of liquid hydrocarbons

using the surface microwave discharge under the conditions of high-speed air flow. Liquid

hydrocarbon ignition was studied using alcohol, benzene, and kerosene applied in the form of a

thin layer to the upper and lower surfaces of the quartz antenna as the examples. Air to a pressure

of six atmospheres can be pumped to the high pressure receiver using a compressor; this allows

us to create high-speed air flows with the given speed at atmospheric pressure in the pressure

chamber.

Fig. 4. Alcohol ignition using surface microwave discharge for the microwave pulse duration

τ = 120 µs, is, initial air pressure in the pressure chamber p0 = 1 atm, pulsed microwave power

Wp = 65 kW, and air flow velocities: (a) υblast = 190 m/s, (b) υblast = 390 m/s.

Fig. 4 shows the photos of alcohol burning in the subsonic (upper photo) and supersonic

(lower photo) flows. It was obtained that for a pulsed microwave power of 70 kW the induction

period in kerosene is equal to approximately 10 µs, and the propagation rate of the front

boundary of the burning region in these conditions reaches 100 m/s. The temperature evolution

measured during the microwave pulse demonstrates that kerosene was ignited at 1200 K, and the

temperature increased to 3200 К promptly (during 10 µs). A regime close to detonation burning

developed near the antenna surface (Fig. 5). However, since the active medium in this

experiment is near the antenna only, the detonation regime damped rapidly.

Fig. 5. Transverse alcohol ignition rate under the conditions of surface microwave discharge

(τ = 120 µs, Wp = 65 kW) created in air flow (υblast = 190 m/s).

The obtained results show that a surface microwave discharge can be used to fast ignition

of gaseous and liquid hydrocarbon fuel under condition of high speed air flow.

ACKNOWLEDGMENTS

This work was partially supported by the Russian Foundation for Basic Research, project

# 08-02-01251 and Russian Academy of Science (P-09 program).

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