A Comparative Assessment of Two Optical Fibre Techniques for Measuring the Speed of the Electric Wind in a Negative Polarity, Atmospheric Corona Discharge

Two optical fibre anemometers, one based on a Mach–Zehnder interferometer (OFIA) and another a novel laser-optical fibre Bragg grating sensor (FBGA) have been constructed to measure the speed of the convective air flow generated in the point-plane gap of a negative corona discharge in atmospheric air. In both configurations the sensing section of the optical fibre anemometers is subjected to controlled, repetitive bursts of infrared radiation from a CO2 laser and the combination of localized heating and convective cooling by the corona wind results in an optical signal that is directly calibrated to the speed of the wind. Both the OFIA and FBGA successfully measured the speed of the corona wind in the discharge gap, with wind speeds ranging from 0 to 0.7 m s-1 observed at different locations in the discharge gap. However, the FBGA, due primarily to the ability to average sensor response over many measurements, exhibited an order of magnitude higher sensitivity than the OFIA (Δv 4 × 10-3 m s-1 compared to 3 × 10-2 m s-1). Both sensors yielded measurements of wind speed that were an order of magnitude lower than similar measurements using laser Doppler anemometry (LDA), indicating that the seed particles introduced as part of the LDA system are providing false reading of wind speed due to acceleration by the gap electric field.