David H. Campbell

Collisional effects on laser-induced fluorescence measurements of hydroxide concentrations in a combustion environment. 1: Effects for v′ = 0 excitation

A full vibrational–rotational level rate equation model has been used to investigate the application of laser-induced fluorescence techniques to number density measurements of hydroxide in a combustion environment. The extent of deviation of the population in the laser-pumped vibrational and rotational levels from the-two-level model prediction, due to collisional exchange mechanisms, was investigated for a range of pressures, temperatures, laser powers, and collisional exchange rates.

Laser-induced fluorescence in high pressure solid propellant flames

The application of laser-induced fluorescence (LIF) to the study of high pressure solid propellant flames is described. The distribution of the OH and CN radicals was determined in several solid propellant flames at pressures up to 3.5 MPa. The greatest difficulty in these measurements was the separation of the desired LIF signals from the large scattering at the laser wavelength from the very optically thick propellant flames. Raman experiments using 308-nm excitation were also attempted in the propellant flames but were unsuccessful due to LIF interferences from OH and NH.

Vibrational level relaxation effects on laser-induced fluorescence measurements of hydroxide number density in a methane–air flame

The laser-induced fluorescence technique was investigated via detailed rate equation modeling of hydroxide in a simulated premixed atmospheric methane-air flame environment. The extent of deviation from the simple two-level model, due to buildup of population in the vibrational bath levels from quenching and vibrational exchange collisions, was addressed as were the effects of variation in the magnitude of the collision -al energy exchange rate constants. Typical results show a breakdown in the two-level model on a nanosecond time scale and indicate that OH number density measurements with accuracies better than an order of magnitude will require (1) better information on detailed quenching rates and (2) laboratory measurements which address the time history of the fluorescent signal on a nanosecond time scale.

High‐pressure combustor for the spectroscopic study of solid propellant combustion chemistry

An apparatus for spectroscopic studies of solid propellants burning at pressures up to 7 MPa (1000 psi) is described. The propellants are burned in a high‐pressure combustor equipped with sapphire windows for optical access and a servomechanism that raises the propellant as it burns, allowing examination of a specific portion of the propellant flame for extended times. The results presented involve the collection of propellant flame emission spectra with a diode array detector over a fixed spectral region.

Detailed temporal behavior of laser-excited sodium tracer in nitrogen and application to nitrogen number density measurements at low densities.

Detailed rate equation modeling of the laser-excited sodium-molecular nitrogen system was conducted to investigate the applicability of the steady-state three-level model to prediction of sodium laser-induced fluorescence intensities. Redistribution of vibrational population in the nitrogen molecules can, under some conditions, produce a transition to a second-state condition not predicted by the simple three-level model. The feasibility of a major species number density measurement using tracer gas impurity laser-induced fluorescence is discussed in light of the sodium-nitrogen modeling results.