A Study of Mechanism of Fuel Droplet Combustion using Laser Diagnostics in Microgravity

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Laser induced fluorescence method (EXCIPLEX) has been employed for remote, non-intrusive measurements of the temperature of a fuel droplet burning under microgravity. A fuel droplet doped with naphthalene and TMPD was allowed to burn in a quiescent gaseous environments. The fluorescence emission spectra from a droplet subjected to the nitrogen laser excitation were measured with an image intensified optical multichannel analyzer. The results showed that a newly developed diagnostic system was found to be applicable successfully for droplet thermometry under microgravity. The ratio of fluorescence emission intensities at two different wavelengths was an appropriate criterion for in-situ determination of droplet temperature. Two-dimensional visualization of the soot concentration profile in a flame formed around a suspended droplet was carried out under microgravity using the planar laser light scattering technique. The soot concentration and the instantaneous amount of soot were estimated approximately from the intensity of the scattered light using the image analysis system. The fuels employed were water-in-oil emulsions composed of base fuel, water and sufactant. The base fuel was n-dodecane. The water content was varied from 0 to 0.3 in volume. The sooting region can be observed spherically around the droplet by using the apparatus developed in the present work. The results showed the unsteadiness of the sooting behavior as well as the flame behavior. The maximum of the soot concentration is located in the vicinity of the inner edge of the sooting region, which corresponds to the soot shell observed previously. The time history of the instantaneous amount of soot is similar to that of the instantaneous flame radius. The maximum of the soot concentration does not vary with the water content, while the amount of soot decreases significantly with the water emulsification.