Quantitative Optical Analysis and Modelling of Short Circuits and Blow-Outs of Spark Channels under High-Velocity Flow Conditions 2018-01-1728
This study models short circuits and blow-outs of spark channels. The short circuit model assumes that a spark channel is short-circuited between two arbitrary locations when the electric potential difference between the two locations exceeds the voltage which enables electrical insulation breakage in-between. The threshold voltage can be raised by increasing the distance between the two locations and decreasing the discharge current. Discharge current, in this model, represents the influence of both the spread and the number of electrically charged particles, i.e., electrons and positive ions, distributed near the two locations. Meanwhile, the blow-out model assumes that a strong flow diffuses electrons and positive ions in the spark channel, and consequently the discharge blows out. As the number of electrons emitted from the cathode decreases, i.e., as the discharge current drops, the probability of all electrons or/and positive ions in a spark channel being diffused before reaching the electrode becomes progressively higher. Therefore, the model provides a lower limit to the discharge current for maintaining the discharge. As the length of a spark channel increases, electrons and positive ions are exposed to the flow for longer duration. Therefore, the lower limit increases with the length of the spark channel. Both developed models were confirmed to consist with experimental results obtained under high-velocity flow conditions. Additionally, this study evaluates the accuracy of the equation developed by Kim et al., which is commonly employed in ignition models for predicting the electrical resistance of spark channels, under high velocity conditions. While the equation agreed well with the experimental results at a discharge current greater than 70 mA, it predicted an excessively high electrical resistance at a discharge current less than 70 mA. Hence, this study suggests changing the discharge current exponent with adjusting the coefficient when applying the equation to high-velocity flow conditions.