Research On Key Technology Of Wavelength Modulation Spectroscopy

Tunable Diode Laser Absorption Spectroscopy?TDLAS?is a non-contact measurement technology of gas characteristic parameters with high sensitivity and fast fast time response.Wavelength Modulation Spectroscopy?WMS?in TDLAS has been widely used in the pollutants gas emissions monitoring,combustion diagnosis of coal-fired boilers and aerospace engine.At present,it has the following problems in practical application of WMS:?1?The measurement results of fixed-wavelength WMS often need to be calibrated with a known gas mixture.However,it is difficult to ensure the consistency between the gas conditions of interest and the calibration conditions,resulting in inaccurate measurement results.On the other hand,The variations of external conditions,such as ambient temperature,drift of driving current and aging of circuit will cause the difference between laser center and line center,which affects the accuracy of gas concentration measurement of fixed-wavelength WMS.?2?In the complex measurement environment,wavelength-scanned WMS realizes the calibration-free measurement of gas parameters by the harmonic fitting method.In application of calibration-free wavelength-scanned WMS fitting method,it is necessary to ensure that the starting point of sampling period of zero-absorption background signal,transmission intensity signal and elaton signal is consistent.However,uncertainty of triggering characteristics of squarewave rising edge makes it difficult to ensure the same starting point of each signal sampling period,which results in poor harmonic fitting accuracy and thus affects the measurement accuracy.?3?In the process of constructing the combustion diagnosis system,the detector and transmitter are generally placed at both ends of the object to be measured,and the laser is accurately incident on the center of the detector by adjusting the laser emission angle.However,it is difficult to adjust the multi-channel laser emission angle because the structure of the conventional temperature field measurement system is complex.Furthermore,when the area of interest is small,the effective optical path is short,resulting in less absorbance.In view of the above problems,the research work of this paper is as follows:1.The wavenumber drift correction algorithm based on calibration-free fixed-wavelength modulation spectroscopy is proposed and the gas concentration calibration-free measurement is realized.The gas concentration is corrected by real-time calculation of the laser center,which reduces the effects of the above problems on measurement.The transitions of CO₂ near 4958.97cm^-1 are selected to validate the accuracy of this algorithm.2.The signal synchronization correction algorithm based on harmonic fitting strategy is proposed to improve the measurement of gas parameters on the basis of improving the effect of harmonic fitting accuracy.The effects of different starting points of sampling period of zero-absorption background signal,transmission intensity signal and elaton signal on measurement accuracy is studied.The transitions of CH₄ near 6046.95cm^-1 are selected,and the concentration measurement system is built to verify the validity of the signal synchronization correction algorithm.A temperature measurement system of high temperature tube furnace is set up to verify the temperature measurement accuracy of the algorithm.3.The integrated optical machine structure with transmitter and receiver is developed to solve the problem in the construction of measurement system,and the measurement system was set up to reconstruct the two-dimensional temperature distribution.The CH₄ concentration measurement system is set up to verify the accuracy of the device.Then,the integrated temperature measurement system with transmitter and receiver is set up to reconstruct the two-dimensional temperature distribution of the diffusion flame combustion furnace by combining the improved Landweber reconstruction algorithm.