Robust gas flow metering under extreme industrial conditions

Abstract: In this thesis, a different method for creating acoustic pulses that can be used in the ultrasonic spectrum is proposed. The aim is to develop a robust and reliable system that uses ultrasonic techniques, such as transit time or sing-around, to measure gas flows in extreme environments. Extreme environments involve high temperatures, contaminating dust and sometimes high moisture content. The investigated method in this work utilizes electric spark discharges to generate acoustic pulses. Studies of the gap discharge acoustic emitter were performed in two parts: environmental tests and studies of the transmitted sound. Environmental tests were performed at industrial sites to test the gap discharge emitter when exposed to heavy surface contamination and moderate temperatures. Studies of the transmitted sound were performed with a primary focus on the time stability of the emitted sound. Due to the nature of the spark discharge phenomena, there are inconsistencies in the transmitted acoustic pulse. When pulses are transmitted and received consecutively, their measured travel times will contain a time jitter relative to each other. This jitter is investigated and put into the perspective of a gas flow measurement situation. Acoustic pulses from the gas discharge emitter are shown to be strong enough to be used in large geometries of several meters. Additionally tests were performed in the industrial environments to determine if the acoustic pulses can be sent through large gas flow ducts and detected at the opposite side. The tests show that the gap discharge transducer at the prototype stage performs well in a real industrial environment. The emitter continues to work when subjected to heavy contamination. The emitted sound is loud enough to be detected using standard piezo ceramic ultrasonic transducers when sent through large gas flows (air). If used in measurement situations that involve acoustic travel paths longer than around 1.5 meters and gas flows in the range of a few m/s or larger, this emitter can deliver sufficient accuracy.