Study Of Method For Measurement Of Acoustic Sound Transit Time Under Combustion Noise Environment In Furnace

Technique for measuring the temperature distribution in furnace using acoustic method is a non-touch method for high temperature measurement, and accurate measurement of transit time for acoustic wave is the mainly key to implement this technique. This thesis has deeply studied the methods for measuring the transit time of acoustic wave under strong combustion noise in furnace. During the studying, works have been done as follows:1. This thesis has reviewed and summarized the development and current status of domestic and foreign research about the technique for measuring the transit time of acoustic wave in furnace.2. Combustion noise acquisition has been done in power plant coal-fired boiler. This thesis has analyzed the characteristic of collected combustion noise, and studied the amplitude frequency characteristic and probability distribution of it. Findings of the research show us that, combustion noise predominates below 400 Hz, therefore, we can make 500 Hz as the lower limit for acoustic sound source frequency so as to fulfill the filter processing with high performance. In addition, attenuation losses in such high temperature environment set the upper limit for frequency, so normal useful frequencies of acoustic wave for boiler application is about from 500 Hz to 2,000 Hz. In the analysis of probability distribution for all band combustion noise sampled from coal-fired boiler, many samples of the noise present obvious Gaussian probability distribution. In the analysis of sub-band combustion noise whose band is from 500 Hz to 2000 Hz, the samples of combustion noise whose band is overlapped with the band of acoustic sound source signal also obey Gaussian probability distribution.3. Selection of the acoustic sound source when measuring the transit time of acoustic wave using Cross correlation method has been studied in this thesis. Simulation experiment results show that, when measuring the transit time of acoustic wave based on cross correlation function, the ability to resist noise interference of acoustic sound source is not only correlative with the frequency range, but also mainly depends on the effectivebandwidth relative to noise. When comparing the performance to resist noise of Pseudo random signal, Linear sweep signal, Quadratic sweep signal and Logarithmic sweep signal under same experimental conditions, it can come to a conclusion that, Linear sweep signal has stronger ability to resist noise than the others.4. The time delay estimation methods of Generalized Cross Correlation and Wavelet based Cross Correlation have been studied and tried in measurement of transit time for acoustic wave in boiler combustion noise environment, and the performance to resist noise interference of this two methods have been investigated by simulation experiments using Gaussian white noise produced by computer and real combustion noise sampled from coal-fired boiler. The research results show us that, the ability to resist noise of Generalized Cross Correlation method is stronger than that of Direct Cross Correlation method and Wavelet based Cross Correlation method. From the experiment results and by theoretic analysis, we can come to conclusion that, de-noise method based on Wavelet nonlinear threshold is unsuitable for de-noising the Linear sweep signal.5. A new method for measuring the acoustic wave transit time in boiler combustion noise environment has been proposed in this thesis, which is based on active acoustic sound source. The ability of resist noise interference of this new method has also been studied by simulation experiments which are based on Gaussian white noise generated from computer and real combustion noise sampled from coal-fired boiler. The experiment research results show us that, this new method possesses stronger ability to resist noise interference than Generalized Cross Correlation method, it can not only accurately measure the transit time for acoustic wave in lower signal to noise ratio, but also avoid the influence of correlated noise on measurement of acoustic wave transit time in theory, it is particularly suitable for measuring transit time of acoustic wave under stronger combustion noise in large boiler.