High-precision low-velocity gas measurement via synchronous baseline fitting and flow inhomogeneity analysis in TDLAS

Accurate measurement of gas flow velocities in low-speed flow fields (0-30 m/s) is critical for applications such as vacuum drying, semiconductor manufacturing, and pharmaceutical/food industries. Based on the laser Doppler effect, non-contact high-precision measurement of gas flow velocity can be achieved by capturing the Doppler shift in absorption spectral lines to quantify the gas flow speed. However, under low-speed flow field conditions, weak Doppler shifts (on the order of 10^-5 cm^-1 per m/s) and spectral line overlapping pose significant challenges to traditional direct absorption spectroscopy (DAS). To address these issues, this study employs a dual-light-path system based on tunable diode laser absorption spectroscopy (TDLAS) and proposes a synchronous iterative fitting algorithm with an L2-norm constraint. This algorithm simultaneously optimizes the baseline and absorption line parameters, effectively eliminating baseline drift and spectral line interference inherent in traditional DAS. Additionally, we analyze the non-uniformity of turbulent flow fields in pipes and model the radial velocity profile using a power-law distribution to elucidate its impact on TDLAS path-averaged velocity measurements. We ensure the reliability of the results by analyzing the effects of turbulence, spectral line distortion caused by stagnant zones, and zero-point deviation during measurements. Using the H_2O absorption line at 7181.1557 cm^-1, the proposed method measures velocities in the range of 0-25.42 m/s with a relative error of less than 1%. The measurement results exhibit high linearity against reference results under the same conditions, indicating excellent system consistency. This research provides theoretical guidance and technical support for the engineering application of TDLAS in low-speed flow field measurements.