Precise Velocity Measurement by Using Even-Symmetric 2f/1f Harmonics Extracted from Up- and Down-Scanning WMS Signal

Wavelength modulation spectroscopy (WMS) is recognized as a good noise immunity method in tunable diode laser absorption spectroscopy (TDLAS). It can obtain high signal-to-noise ratio harmonic intensity signals for precise gas parameter measurement, such as temperature and concentration. However, the TDLAS-based velocity is sensitive to the Doppler shift in the laser frequency caused by the high-speed flow, which usually requires complex wavelength calibration and precise signal alignment. This work proposed a novel method for the precise velocity measurement directly by extracting the time interval of the even-symmetric 2f/1f harmonics signal with a self-convolution algorithm. The even-symmetric 2f/1f harmonics signal is obtained from the up-and-down laser optical frequency scanning WMS with lock-in amplification technology. A portable measurement system has been developed to achieve simultaneous velocity and temperature measurement of the gas in turbojet engine exhaust with different operation states. The maximum standard deviation of the velocity derived by the proposed method is 15.6 m/s, while that by traditional direct absorption spectroscopy (DAS) and WMS are 69.2 and 26.4 m/s, respectively. Experimental results have verified the improvement of velocity precision by the proposed method. It can provide reliable experimental data for improving the design of combustors and optimizing numerical combustion simulations.