Development of a Diode Laser Sensor for Measurement of Mass Flux in Supersonic Flow PDF Download

Author: Leyen S. Chang
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Languages : en
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Book Description
Mass flux is one of the most critical parameters in the calculation of engine thrust and assessment of aeroengine performance. Conventional mass-flux measurements are facilitated with invasive temperature and pressure probes which tend to disturb the flow, generate shock structures, have limited lifetimes, and require frequent maintenance. In response, there is a growing opportunity for tunable diode laser (TDL) diagnostics, which can be deployed noninvasively with fast time response and high accuracy. The focus of this work is the development of a TDL mass-flux sensor based on water vapor absorption for deployment in a combustion-driven Mach 2.7 wind tunnel at NASA Langley. The 1f-normalized wavelength-modulation spectroscopy with second harmonic detection (WMS-2f/1f) technique was employed for its unique noise-rejection capability and increase in signal-to-noise ratio; this method was used to measure velocity with improved precision while simultaneously determining temperature. Density was inferred from an independent pressure measurement and the ideal gas law. The sensor temperature measurements were validated against thermocouple readings in a heated cell at Stanford to within 1%. Measurements of velocity were made in a low-speed wind tunnel with accuracy within 0.5m/s of a pitot probe reading; a reduction of 50% in the standard deviation of the velocity measurement was also observed by using optimized WMS-2f/1f. In addition, the influence of flow nonuniformity on line-of-sight (LOS) measurements is investigated by simulating path-integrated lineshapes from computational fluid dynamics (CFD) solutions. The results are quantified in order to develop a correction to the path-integrated measurements obtained in this work. The capstone mass-flux measurements were performed in the NASA Langley Direct-Connect Supersonic Combustion Test Facility isolator. Temporally resolved velocity data was corrected according to the nonuniformity analysis of path-integrated lineshapes, bringing the sensor velocity measurement within 0.25% of the value predicted by the facility code (4m/s in a 1630m/s flow). Temperature measurements were made with high precision (10K standard deviation in a 990K flow), and agreement with the predicted value was also within 1%. Mass-flux measurements had similar precision (standard deviation less than 1% of full scale) and accuracy (within 1% of predicted value). Finally, spatially resolved velocity and mass-flux data taken along both the height and width of the isolator were found to be in close agreement with the CFD solution. These results show that TDL mass-flux sensing based on WMS-2f/1f can produce temporally and spatially resolved measurements with high precision and accuracy in a supersonic flow, thus demonstrating the sensor's potential for future deployment in unknown mass-capture environments such as inlet models and flight tests.