A Characterization Of Alcohol Fuel Vapor For Wavelength Modulation Spectroscopy Applied To Microgravity Flame Spread

A diode laser diagnostic is being developed for use in an ongoing investigation of flame spread in microgravity at NASA Glenn Research Center. Flame spread rates through non-homogenous gas mixtures are significantly different in a microgravity environment because of buoyancy and possibly hydrostatic pressure effects. These effects contribute to the fuel vapor concentration ahead of the flame being altered so that flame spread is more rapid in microgravity. This paper describes spectral transmission measurements made through mixtures of alcohol, water vapor, and nitrogen in a gas cell that was designed and built to allow measurements at temperatures up to 500 C. The alcohols considered are methanol, ethanol, and n-propanol. The basic technique of wavelength modulation spectroscopy for gas species measurements in microgravity was developed by Silver et al. For this technique to be applicable, one must carefully choose the spectral features over which the diode laser is modulated to provide good sensitivity and minimize interference from other molecular lines such as those in water. Because the methanol spectrum was not known with sufficient resolution in the wavelength region of interest, our first task was to perform high-resolution transmission measurements with an FTIR spectrometer for methanol vapor in nitrogen, followed recently by ethanol and n-propanol. A computer program was written to generate synthesized data to mimic that expected from the experiment using the laser diode, and results from that simulation are also presented.