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

A laser-induced Rayleigh light-scattering (RLS) system was assembled and us ed to noninvasively measure the transient molecular number density in an id eal gas. This information was used to find the transient gas temperature wh en 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 phot on arrival rate at the photomultiplier tube detector was calculated and com pared well with the observed photon rates. These rates were high enough tha t 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 averag ing time. A propagated error analysis showed uncertainty in the transient t emperature 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 he ating from room temperature to 475 K. Results showed buoyancy-induced fluct uations of about 3 Hz, and instabilities in temperature in addition to the overall temperature rise due to plate heating. Excellent transient temperat ure records were obtained, substantiating the predicted 2-4 K uncertainty. (C) 1999 American Institute of Physics. [S0034-6748(99)04608-0].