Velocity Measurement of Sub-Millimeter-Scale Gas Flow by Spark Tracing Method

A molecular tagging technique using the spark tracing method has been applied to measure velocity distributions in sub-millimeter-scale gas flows, formed as air jet flows through a sub-millimeter channel. Spark lines are generated by air ionization when applying high voltage due to the electrical discharge phenomena. The velocities measured using the displacement of spark lines were 10-30% smaller than those using the theoretical equation in a rectangular channel. In order to identify the cause of the measurement error, the relationship between the ionized air regions and the gas flow velocities was investigated by numerical simulation. The simulation revealed that a spark line goes through the pathway with the minimum electric resistance, and that the velocities from the theoretical equation agreed with the spark line velocities when the spark line width is assumed to be zero. Using this result, we propose a new velocity correction technique using the relationship between the spark line width and the measured velocity. The velocities from the experiments with the suggested correction agreed well with those from the theoretical equation. Furthermore, the corrected spark tracing method was applied to a mixing air jet flow field with different temperatures through two channels.