Three-line fluorescence thermometry of optically thick shock-tunnel flow.

Two-line fluorescence thermometry measurements based on excitation of the A-X (0, 0) band of nitric oxide were performed in the free-stream flow of a large-scale shock tunnel, the Arnold Engineering Development Center Impulse Facility. Because the flow is optically thick, effects of laser absorption must be taken into account. This can be accomplished by extending the two-line thermometry concept to three lines. We illustrate the three-line technique by combining two-line planar laser-induced fluorescence measurements from separate facility runs. The resulting free-stream temperature T(0) = 468 K ? 65 K is insensitive to assumptions regarding the laser propagation distance through the flow, the vibrational temperature of the flow, and the precise nature of the absorption line shape.

Correction of self-illumination and spectral-leakage effects in luminescent-paint measurements.

An analytical model is presented that accounts for the effects of self-illumination of a diffusely reflecting test article in luminescent-paint measurements. Contributions from multiple reflections of excitation and emission light are summed to infinite order in closed form. Also included are the effects of spectral leakage resulting from an imperfectly filtered source and imperfectly rejected reflected excitation light. It is shown that the conventional method of determining flow variables from the ratio of flow-on and flow-off signals is adversely affected by self-illumination and spectral leakage. An explicit solution is derived for the associated inversion problem. Sample calculations are presented.

Digital imaging technique for optical emission spectroscopy of a hydrogen arcjet plume.

A digital imaging technique has been developed for optical emission spectroscopy measurements of a 1.6-kW hydrogen arcjet plume. Emissions from the Balmer α and ß transitions of excited atomic hydrogen were measured with a computer-controlled red-green-blue color CCD detector with and without line-centered bandpass interference filters. A method for extending the effective dynamic range of the detector was developed, whereby images obtained with a wide range of exposure times are combined to form a single composite nonsaturated map of the plume emission structure. The line-of-sight measurements were deconvoluted to obtain the true radial intensity distribution with an inverse Abel transformation. Analysis of the inverted measurements indicates that the upper levels of the Balmer α and ß transitions are not thermalized with the electrons in the plasma. The local thermodynamic equilibrium assumption fails for this plasma, and the electron temperature is not equivalent to the apparent excitation temperature obtained when a Boltzmann energy distribution is assumed for the atomic hydrogen excited states.

Multiphoton resonances in the modulated fluorescence from an inhomogeneously broadened medium.

We consider the interaction of a fully amplitude-modulated, narrow-band laser with a strongly inhomogeneously broadened two-level medium and show that harmonics of the fluorescence of susceptibility, unlike the time-averaged fluorescence, retain the characteristic multiphoton resonance structure that is found in the absence of inhomogeneous broadening. This behavior correlates with the existence of double branches of the corresponding Bloch-Siegert resonance curves. For strong fields, the depth of modulation and the phase shift of the lowest fluorescence harmonic may deviate significantly from the results for weak, broadband excitation.