Transient temperature measurements in an ideal gas by laser-induced Rayleigh light scattering

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Abstract
A laser-induced Rayleigh light-scattering (RLS) system was assembled and used to noninvasively measure the transient molecular number density in an ideal gas. This information was used to find the transient gas temperature when operating at known pressure. The laser was a 4 W argon ion operating on all lines at a total power of about 2.5 W. The theoretically predicted photon arrival rate at the photomultiplier tube detector was calculated and compared well with the observed photon rates. These rates were high enough that sampling could be averaged over a 1 s period with theoretical uncertainty due to electronic shot noise below 0.1%, and below 2% for a 0.001 s averaging time. A propagated error analysis showed uncertainty in the transient temperature due to all sources was 2–4 K. The RLS system was used to record transient air temperature at several locations above a flat plate during heating from room temperature to 475 K. Results showed buoyancy-induced fluctuations of about 3 Hz, and instabilities in temperature in addition to the overall temperature rise due to plate heating. Excellent transient temperature records were obtained, substantiating the predicted 2–4 K uncertainty.