The Temperature Measurement Research For High-speed Flow Based On Tunable Diode Laser Absorption With Wavelength Modulation Spectroscopy

Due to the particularity of the high-speed flow, in order to accurately obtain the static temperature, the measurement system should has some characteristics of not interfereing with the flow, non-contact measurement and high time resolution. According to the traditional measurement method, tunable diode laser absorption spectroscopy (TDLAS) technology can meet the requirements for high-speed flow.TDLAS has two main methods:direct absorption spectroscopy(DAS) and wavelength modulation spectroscopy(WMS). Direct absorption spectroscopy is simple and the calibration is not needed. However, it is dependent on the absolute level of absorption and is very sensitive to the baseline fitting error, so it can only be used for such cases as the absorption lines is isolated, the absorbance of spectral line is moderate and the environment pressure is not very high. For WMS the laser injection current driver signal consists of a high-frequency (f) sinusoid superimposed on a lower frequency (fslow) wavelength scan. The detected signal is then input to a lock-in amplifier which isolates the harmonics at the frequencies of interest, namely WMS-1f and WMS-2f. The detection signal frequency is higher, so the wavelength modulation spectroscopy can reduce1/f noise and obtain high signal-to-noise ratio. With the development of TDLAS in recent years, calibration-free wavelength modulation spectroscopy is gradually mature, it laid a theoretical foundation for the widely application of wavelength modulation spectroscopy.In this paper, the TDLAS system is established with the water vapor as the absorbing component and is applied to the temperature measurement of the shock tube to get the time-resolved temperature in State5flow under the high pressure. The main research results are as below:1、The principles of DAS and WMS are introduced, then the basic methods of line selection is described. The hardware and software of the TDLAS system, especially the harmonic signal extraction method are given.2、The method of calibration-free wavelength modulation spectroscopy are summarized and the procedure flow charts of iterative algorithm for data processing is given.3、Through the simulation with Matlab the influence of optical depth and laser modulation on the absorption lineshape is investigated, and this analysis can be used to optimize the lineshape for temperature measurements. It is important for choosing the suitable modulation parameters in the subsequent experiments.4Through Hitran2008, two water absorption lines of1409nm and1395nm are chosen to establish a WMS measurement system. The room temperature test shows that the harmonic signal results through Lab VIEW program are consistent with that of the lock-in amplifier and the temperature error is about5%. For high pressure situation(the State5in the shock tube), another line pair(7168.43cm-1and7185.597cm-1)was selected using a scanned-wavelength technique with wavelength modulation and2f/1f detection. The scanned rate is1kHz and the wavelength is modulated at250kHz. The results indicates that State5shock tube flow can be monitored through this system.