To develop a non-equilibrium plasma-assisted combustion method using microwave, we have performed the experiment of spray combustion by the superposition of non-equilibrium plasma using microwave and investigated the characteristics of combustion and reformation of plasma-assisted combustion. Various radicals generate under the superposition of non-equilibrium plasma using microwave and increase with increasing the microwave input power. The emission intensity of O radicals indicates an approximately constant value, and the emission intensity of Hα radicals increases with decreasing the oxygen ratio. Because the feed rate of oxygen is a constant and the oxygen ratio is decreased by increasing the fuel feed rate in this experiments. The emission intensity of OH radicals gradually increases with decreasing the oxygen ratio, because the formulation of OH radicals is affected by O radicals and Hα radicals. According to an increase of various radicals, the combustion reaction is promoted under the oxygen ratio of more than 1.0 and the reformulation reaction is enhanced under the oxygen ratio of less than 1.0. The combustion temperature is measured at the observation window, which is 25 mm downstream of the nozzle, by the Boltzmann plot method using emission spectroscopy of CH rotational bands. It is shown that the combustion temperature increases with increasing the microwave input power. The causes for the temperature increase are that unburned fuel decreases and the high-temperature region of the flame is shifted to the nozzle side with increasing the microwave input power. The exhaust gas volume increases with increasing the microwave input power, because unburned fuel decreases. The cold gas efficiencies under microwave input power of 350 and 750 W in the case of plasma-assisted conditions are 1.6 and 2.1 times higher than the cold gas efficiency in the case of normal conditions, because the produced gas volume increases and the mole fractions of combustible gas increase with increasing the microwave input power.
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