Ultrasonic capacitance transducers for flow and temperature tomography in air

Abstract

Capacitance transducers have been constructed that can operate at frequencies of up to 2 MHz in air, over a wide bandwidth, and with good efficiency. These have been used to propagate signals in air across regions containing variations in air flow and temperature. A source and receiver pair were scanned along a set of projections under computer control, allowing various ray paths across a chosen cross section to be selected. For each location, the received waveform was digitized and recorded, and the propagation time determined from cross-correlation techniques. The data were then used to form tomographic images, which corresponded to variations in either temperature or flow speed, both of which affect the local acoustic velocity. In one experiment, an air-jet was imaged at various distances from the nozzle. In another, the temperature variations in air above a hot metal tip were determined. Images will be shown for both sets of experiments, and the usefulness of the technique discussed.