Plasma jet ignition studies

Abstract

The governing parameters of plasma jet ignition are investigated using CH4-air at an equivalence ratio of 0.7 in a cylindrical combustion bomb, 10cm in diameter by 3.75 cm deep. Three plasma chambers of cavity sizes of 82, 150, and 260 mm3, all having 2 mm orifices are used. In addition, for each size chamber, three different aspect ratios (depth ot diameter of the cavity) are tested. For most of the tests, the spark duration is varied from 0.3 to 2.5 ms, while its energy is maintained at 1.2J. In, addition several plasma media are tested. The results are assessed in terms of a combustion index which is a function of the peak pressure and the time the peak is reached. It is found that besides the plasma cavity and orifice size, the spark duration has a considerable influence on the combustion index and there exists an optimum duration for each plama chamber. There is a wide difference in performance for different aspect ratios for the same cavity size even when the same surface area to volume ratio is used. Thus, the aspect ratio seems to be an important parameter whose value must be chosen with care for best results. For the plasma cavity of 260 mm3 the optimum aspect ratio is found to be equal to 0.46. In this configuration, tests in which the spark energy is varied from 1 to 3J, while its duration is maintained at 2 ms revealed an optimum energy near 2J. The effect of radicals is investigated using different plasma media of CH4, (CH4+6O2+NO2) and (CH4+7O2). In general, CH4 as a plasma medium gives better performance, in agreement with earlier investigators who suggested a possible influence of the H radicals. In our tests, however, further improvement in the combustion index is obtained when the (CH4+6O2+NO2) mixture is used, and this is being attributed to the generation of OH radicals.