Combustion of Two-Component Miscible Droplets in Reduced Gravity

This paper presents recent theoretical and experimental results from an ongoing research program that deals with reduced-gravity combustion of bi-component droplets initially in the mm size range or larger. The primary objectives of the research are to study the effects of droplet internal flows, thermal and solutal Marangoni stresses, and species volatility differences on liquid species transport and overall combustion phenomena (e.g., gas-phase unsteadiness, burning rates, sooting, radiation, and extinction). The research program utilizes a reduced-gravity environment so that buoyancy effects are rendered negligible. Use of large droplets also facilitates visualization of droplet internal flows, which is important to this research. This program is a continuation of extensive ground based experimental and theoretical research on bi-component droplet combustion which has been ongoing for several years. The focal point of this research program is a flight experiment (Bi-Component Droplet Combustion Experiment, BCDCE). This flight experiment is still under development. However, supporting ground studies have been performed, and preliminary data have been obtained from flight experiments (Fiber Supported Droplet Combustion Experiment, FSDC-1 and FSDC-2). These flight experiments were performed during the STS-73/USML-2 and STS-94/MSL- I missions. In the experiments, droplets composed of low- and high-volatility species are burned. The low-volatility components are initially present in small amounts. As combustion of a droplet proceeds, the liquid surface mass fraction of the low-volatility component will increase with time, resulting in a sudden and temporary decrease in droplet burning rates as the droplet rapidly heats to temperatures close to the boiling point of the low-volatility component. This decrease in burning rates causes a sudden and temporary contraction of the flame. The decrease in burning rates and the flame contraction can be observed experimentally. Measurements of burning rates as well as the onset time for flame contraction allow effective liquid-phase species diffusivities to be calculated, e.g., using asymptotic theory. A goal of the research is to relate effective liquid species diffusivities to droplet internal flow characteristics. Droplet internal flows will be visualized in future flight and ground-based experiments.