Theoretical and experimental studies on evaporation and combustion of multicomponent droplets in reduced gravity

Results from two distinct investigations are reported in this dissertation. In the first investigation, linearized stability analyses were performed to investigate effects of Marangoni stresses on hydrodynamic stability of bi-component droplets slowly evaporating in a spherically-symmetrical manner in a hot environment. It was found that thermal and solutal Marangoni effects may both be important in determining stability. For most common liquid mixtures that approximate ideal solution behavior, thermal Marangoni effects were predicted to be stabilizing while solutal Marangoni effects were predicted to be destabilizing; a competition between these two effects determines the overall stability characteristics for a bi-component droplet. Results from the model suggest that spherically-symmetrical gasification may be hydrodynamically unstable for alcohol/water mixture droplets and hydrodynamically stable far alkane/alkane mixture droplets.;In the second investigation, experiments were performed on reduced-gravity combustion of single-component, bi-component, and tri-component droplets initially in the mm size range. Droplets were composed of alkane/alkane, alcohol/alcohol, and alkane/alcohol mixtures. Reduced-gravity was used to strongly promote spherical symmetry. The experiments were performed at pressures ranging from 0.03-0.3 MPa, and various inerts (helium, argon, nitrogen, and carbon dioxide) were used in the ambient. The experiments provided validation for existing theory on influences of liquid-phase species diffusion on transient combustion characteristics, and allowed estimation of effective liquid species diffusivities for heptane/hexadecane droplets to be made over the pressure range 0.03-3 MPa when high-pressure data available from the literature was combined with the new data generated here. Heptane/hexadecane liquid species diffusivities were found to increase substantially with ambient pressure; this was attributed to increases in droplet surface temperatures as the ambient pressure is increased. New phenomena were also observed in the experiments. Examples of these new phenomena are: sooting thresholds associated with gas-phase and liquid-phase compositions as well as the ambient pressure; transient soot shell behaviors; gas-phase soot-shell instabilities; effects of convective flows on burning rates and sooting; disruption of heptane droplets under apparently nonsooting conditions; burning-rate minima of alkane mixture droplets at sub-atmospheric pressure; and effects of the ambient composition and pressure on sooting and burning rates.