Development of a real-time diode-laser mass flux sensor for simultaneous measurement of density and velocity of oxygen

Mass flux is the product of gas velocity and density. For aeroengines, the inlet air mass flux is useful for calculating net engine thrust and characterizing combustor performance. A diode laser-based absorption sensor has been developed to provide a non-intrusive method to directly measure the engine inlet mass flux in real-time at a rate of 1 Hz. The spectroscopic absorption of molecular oxygen in the A-band near 760nm is used to obtain the gas velocity from the Doppler frequency shift between the absorption features of two crossed beams and the density from the integrated absorbance. Wavelength modulation spectroscopy with 2nd harmonic detection (WMS-2f) is utilized for the velocity measurement, while the density is obtained directly from the integrated absorbance of the oxygen line without the use of WMS-2f. The combination of WMS-2f for the velocity measurement and direct absorption for the density measurement led to a novel hybrid mode of operation. Mass flux measurements made in a Pratt & Whitney PW6000 turbofan during a ground test demonstrated that the sensor is capable of obtaining an instantaneous (1 Hz) uncertainty of 1.8% and a time-averaged uncertainty of 0.71% at a velocity of 100 m/s.