Measurement of NO density in a free-piston shock tunnel using PLIF

We investigate the feasibility of using a combined absorption - planar laser-induced fluorescence (PLIF) method for measuring nitric oxide (NO) number density in the freestream of a free-piston shock tunnel. The absorption coefficient is determined from the exponential decay in PUF intensity through a uniform region of flow. The NO density is determined from the measured absorption coefficient, assuming a known pressure and temperature, determined independently. The measured NO density is insensitive to the pressure and temperature, however. The current measurements are contaminated by the variation of the spectral profile of the laser as it propagates through the flow resulting in a large systematic error. We show that this systematic error could be nearly eliminated by using a spectrally narrower laser.