Mass Flux Sensing via Tunable Diode Laser Absorption of Water Vapor

A high-speed mass flux sensor based on diode laser absorption of water vapor is designed, constructed and tested in a low-speed wind tunnel at Stanford and a supersonic ground test facility at NASA Langley. Water vapor absorption is monitored, capitalizing on the presence of its large mole fraction in combustion-driven facilities and the availability of fiber-coupled diode lasers that access the 2ν1, 2ν3, and ν1+ν3 absorption bands near 1.4 microns. Mass flux is determined from the product of velocity and density measured using a pair of absorption transitions: velocity is obtained from the relative Doppler shift of the absorption line centers for beams directed upstream and downstream in the flow. Temperature is determined from the ratio of absorption signals and coupled with a facility pressure measurement to obtain density. The sensor exploits wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f) for large signal-to-noise ratios. The optimization of the modulation for 1f-normalized WMS-2f (WMS-2f/1f) signals for velocity sensing is presented for the first time. Criteria to select the absorption transitions are presented and the spectroscopic parameters of the needed database are determined with laboratory measurements. Using this database, the measurement of temperature is validated against thermocouple measurements in a heated cell. The velocity measurements are validated at low velocity to ±0.5 m/s in a high-uniformity wind tunnel. Finally, the measurements of temperature and velocity are demonstrated in supersonic flows in the direct-coupled supersonic combustion test facility (DCSCTF) at NASA Langley.