Diagnostic Investigation On Nonequilibrium Plasma-assisted Combustion With Optical Emission Spectroscopy

Nowadays, rapid increase of the requirement of energy sources in the world caneasily give rise to energy crisis. Meanwhile, the environmental issues become moreand more serious with the increment of energy consumption. In the automobileindustry, great energy waste and serious air pollution result from the incompletecombustion of fossil fuels. Not only is the fossil fuel consumption a problem, but theemissions from burning fossil fuels are also of great concern to the world. At present,the products, which are used to enhance the engine combustion efficiency in themarket, such as the fuel economizer made of magnetic or far infrared materials, do notplay a fundamental role in Energy Saving and Emission Reduction due to theirintrinsic technical deficiencies.A combination of fuel efficiency efforts and emission reduction achievements hasbecome an important commercial and sanitary focus for all of us. In view of this, weput forward a non-equilibrium plasma-assisted combustion technique for moreefficient engine. This technique can improve the fuel combustion because of thenon-equilibrium nature of the plasma, and provide a promising way of boostingconstruction of energy conservation and environmental protection society.The plasma-assisted technology consists of an electronic device that can beattached to an existing fuel injector that applies electrical voltage to the atomized fuelstream prior to combustion, generating plasma in the fuel. This effect essentiallybreaks down the long chains of hydrocarbons in the fuel into smaller parts andgenerate free radicals, allowing the fuel to be burned more completely, which resultsin more miles per gallon or harmful emissions reduction. On the way of exploring thistechnology, the experimental study of non-equilibrium plasma-assisted hydrocarbonfuel combustion has been performed using dielectric barrier discharges (DBDs) withoptical emission spectroscopy.First, the plasma physical and chemical characteristics are presented in thisdissertation, an appropriate scheme of structural design of DBD plasma reactors isbrought forward in theory, and several plasma reactors with different arrangements have been developed based on the scheme.Second, the experimental study of non-equilibrium plasma generated withdielectric barrier discharge has been carried out at atmospheric air. An abnormalphenomenon is found that the discharge power or active/average current is diminishedwith the applied voltage across the electrodes in a particular condition. The reason forthis abnormal phenomenon has been elucidated in detail. The discipline of themicrodischarge filament evolvement and charge transfer based on the applied voltageis deduced. In the end, the effective charge loss coefficient is obtained throughtheoretical analysis and calculation.Third, the experimental study of non-equilibrium plasma-assisted propanecombustion has been performed by applying a discharge on either propane or airstream at atmospheric pressure with the same structural plasma reactor. It is found thatthe non-equilibrium plasma can enhance the flame temperature or luminosity underboth activation approaches. The combustion stability is somewhat sensitive to the leanburning conditions and confined to a relatively narrow operating window, which hasbeen elucidated at length. The main components, as well as physical and chemicalreaction mechanisms in the discharge and combustion regions, are explored throughoptical emission spectroscopy with plasma on and off under both activation methods.Blowout experiment results show that the blowout air-flow rate is increased with adischarge on propane or air stream under certain conditions. Comparison ofcombustion enhancement through different activation methods shows that the activespecies produced by activating air benefit more to the plasma-assisted combustionthan those generated by activating propane with the same activation power for lowpropane flow in a low-equivalence-ratio system.Fourth, the experimental study of non-equilibrium plasma-assisted propanecombustion has been performed by applying a discharge on either propane or airstream at atmospheric pressure with two different structural plasma reactors. It isfound that a temperature rise of about30℃is achieved for activation of propane, butabout50℃for activation of air with a30-W plasma on. Combined with the emissionspectroscopy, possible physical and chemical reaction mechanisms in the plasma and flame zones are discussed in detail under both activation ways. Results show thatsome active species like O-atoms, N-atoms, and excited molecular oxygen andnitrogen produced by activating air components play a greater role than those smallerfragments and radicals generated by cracking propane in plasma-assisted combustionin our experimental conditions.Finally, the experimental study of non-equilibrium plasma-assisted diesel oilcombustion has been performed at atmospheric pressure with a new developed plasmareactor. It is found that the physical appearance of the diesel oil combustion flamechanges considerably with the plasma on and off. Exhaust analysis results show thatthe plasma action allows more hydrocarbons to be oxidized to CO, CO2and H2O, andreduces emissions of HC particals and NOx.In summary，theoretical analysis and experimental study have been performed tovalidate the feasibility of plasma-assisted fossil fuel combustion, and lay a solidtheoretical foundation and technical program for the subsequent applied research onthe non-equilibrium plasma-assisted engine combustion.