Laser Measurements of Vaporization in a Burning Spray of Methanol

Abstract

A linear measurement technique based on simultaneous planar imaging of laser induced dye fluorescence and Raman scattering in the liquid phase is reported. Calibrations in a stream of monosized droplets doped with weak concentrations of rhodamin show that the intensities on the droplet images are proportional to the actual droplet volume for Raman scattering and to the initial volume of the droplet for fluorescence, as the mass of dissolved dye does not vaporize. Thus, the mass fraction of liquid fuel that has vaporized before the probing event can be derived from these si mu I taneo us measu re men ts . Experiments are performed in the early development of a burning spray to derive cumulative information on the vaporization dynamics in terms of mass fraction or evaporation constant. Size distributions from conjoined phase-Doppler measurements are also used to derive the rate of droplet consumption along the axis of the burning spray. The present paper reports on development and application of a different diagnostic in which a laser technique based on Raman and fluorescence interactions provides the cumulative mass fraction of vaporized fuel over the liquid course up to the measurement point. The liquid fuel (methanol) is doped with a small amount of an organic dye (rhodamin 6 G ) which does not vaporize with the fuel molecules. A s ingle laser pulse induces simultaneously Raman scattering from methanol and fluorescence emission from the dye. The comparison between the two signals provides information about the vaporization dynamics in so far as the dye fluorescence signal is not reduced during the evaporation. The measurement is insensitive to the thermochemical environment (collisional quenching) as both the dye and the fuel liquid molecules are laser probed in the same condensed phase.