There have been consistent efforts in developing more fast and efficient combustion. Microwave discharges are widely used for plasma-assisted ignition of air/fuel mixtures. Among the various types of discharges studied, the microwave discharge has recently demonstrated promising characteristics for ignition at low initial temperatures. Possibilities of the use of a sub-critical streamer discharge to ignite air/fuel mixture are analysed numerically. The effect of ignition area on the propagation of a premixed flame in subsonic and supersonic flows is obtained with the numerical models. The results of numerical simulations are compared with the experimentally measured quantities.
The successful ignition of a combustible mixture not only initiates the combustion but also influences the subsequent combustion process. The ignition system has always posed problems in commercial applications. Many experimental, theoretical and numerical studies have been performed for the past years and various ignition methods (e.g., electric discharge, corona discharge, radio-frequency resonant discharge, microwave discharge, laser radiation) have been tested to achieve simultaneous space ignition via multiple ignition points throughout the combustion chamber to establish a high-performance ignition method.
Plasma-assisted combustion is a promising technique to improve engine efficiency, reduce emissions and enhance fuel reforming. Ignition and combustion control using cold and non-thermal plasmas appearing in microwave discharges has become a major topic of interest. Microwave discharges are widely used for generation of quasi-equilibrium and non-equilibrium plasma. Among the various types of discharges studied, the microwave streamer discharge has recently demonstrated promising characteristics for ignition at low initial temperatures. The streamer discharge looks as a chaotic structure of plasma channels (filaments). Their characteristic diameter is about a fraction of millimetre, and a characteristic distance between the channels is a fraction of wavelength. A streamer filament divides itself into several branches that connect to each other forming a net of thin plasma filaments, whose characteristic length is related to electro-dynamic resonance effects. A local initiation of such a discharge is provided by special facilities.
Streamers understanding involves many scales from the microscopic scale of collisions of electrons with neutral molecules to macroscopic scales ranging from thin space charge layers within each streamer finger up to the streamer tree with possibly thousands of branches. One of the main properties of a developed streamer discharge is that it absorbs almost all the electromagnetic energy incident on it. This is a significant difference between streamer discharges and spatially uniform discharges. Another important feature of streamer discharge is that the streamer spreads in a considerable volume, and the strength of electrical field is much smaller than the critical one (sub-critical microwave discharge).
In this project, possibilities of the use of microwave radiation to initiate combustion of air/fuel mixtures are investigated. The streamer discharge is formed by a electromagnetic beam with the electrical field strength which is smaller than the minimum pulse intensity leading to the air breakdown. The discharge is ignited in a space far away elements forming electromagnetic beam. Premixed flame propagation of air/propane mixture is investigated through a numerical simulation using Navier-Stokes equations coupled with chemical reactions and Maxwell equations. The results of computational studies of combustion of air/propane mixture are compared with the available experimental data. A major cause of the flame acceleration related to an increase in the flame surface area and thereby the total heat release rate, is one of the potential methods in improving combustion efficiency by reducing the burning time in the propulsion systems.
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