Conference Paper Measurement of Vibrational and Rotational Temperature in Spark Discharge Plasma by Optical Emission Spectroscopy


We measured the vibrational and rotational temperatures in a spark ignition plasma by using optical emission spectroscopy, and investigated the influence of the air flow and atmospheric pressure on these temperatures. The optical emissions from the plasma were led to an imaging spectroscope through an optical fiber. The temperature was regulated by fitting a theoretically calculated spectrum to an experimentally measured spectrum, which was formed by nitrogen molecule emission from 372 nm to 382 nm. We examined the spark discharge plasma with a flow of atmospheric air at a discharge energy of 80 mJ. The air flow caused the spark discharge channel to elongate downstream. At the center of the spark plug gap, the vibrational temperature in the plasma was 4000 K, while the rotational temperature was 2000 K. This plasma can be regarded as being in non-thermal equilibrium, because the vibrational temperature was higher than the rotational temperature. Near a position 3 mm downstream from the spark plug gap, the vibrational and rotational temperatures increased to 4500 K and 4000 K, respectively, while approaching each other. Both temperatures reached a maximum value. These results show that the plasma transits from non-thermal equilibrium to thermal equilibrium as it is elongated by the air flow. Ignition efficiency improvements can be expected if the time required to transition from non-thermal to thermal equilibrium can be shortened.

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