Experiments were performed to investigate the UV spectral absorption characteristics and evaporation characteristics of monopropellant droplets composed of hydroxylammonium nitrate (HAN), methanol, and water. Fiber-supported droplets initially in the millimeter size range were investigated, and the droplets evaporated in room air with an ambient temperature of about 23 +- 2 °C and a relative humidity of about 44% +- 4%. The experiments employed 50-μm optical fibers to carry UV light into and out of individual droplets, and absorption spectra of the liquid between the fibers were measured using a spectrometer coupled to one of the fibers. It was found that transient variations in NO_3~- levels depended significantly on the initial droplet composition. Evaporation rates were observed to vary significantly with the composition of a droplet, and the presence of methanol significantly increased droplet vaporization rates. Droplets exhibited two-stage vaporization histories with measurable evaporation rates at early times and essentially no evaporation at sufficiently long times. The experiments also indicate that methanol was preferentially evaporated during early stages of evaporation. Experiments showed that at sufficiently high HAN loadings, vaporization rates could be essentially zero. Theoretical estimates suggest that this behavior is caused by reductions in vapor pressure as the mass fraction of HAN in the liquid is increased.
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