Fundamental design and application of an acoustic temperature measurement system.

Temperature measurement is critical in virtually every industrial heating application in terms of product quality, process efficiency and environmental emission control. Acoustic pyrometry is a non-contact measurement technique based on the well-known compressible flow relationship between the speed of an isentropic sound wave in a gas and its internal temperature. The objective of the work was to investigate the fundamental components of an acoustic pyrometer both experimentally and through numerical analysis with the aim of designing, testing and commissioning a ruggedised version and applying it to a combustion system. A key issue that the study addressed was the performance of the technique in the presence of invasive combustion and background noise levels present at most industrial plant. The noise levels were recorded at various kilns and furnaces in order that a measurement strategy was identified. A factor, which had also not been fully quantified to date, was the absolute accuracy of the acoustic technique given a typical combustion application where the exact gas composition along the line of flight is unknown. A comprehensive analysis indicated that the acoustic constant has a strong dependence on temperature but this can easily be taken account of in the measurement by using correction factors. A number of transmitted acoustic signal types and processing methods were examined and the error associated with each technique quantified. Transmitted signals included pulses, pure frequencies, chirps, and pseudo random binary sequences (prbs), while processing involved edge detection and correlation. The operation of an instrument based on a prbs transmitted signal is described and test results under controlled noise conditions are presented. These confirm the value of the strategy and demonstrate that measurements can be made with signal to noise amplitude ratios down to 0.5.