Development of a Technique to Determine the Effects of Temperature and Composition on the Refractive Indices of Soot

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Soot formed in hydrocarbon flames varies in size and composition with stoichiometry and residence time. To study the fundamental processes associated with soot formation and oxidation, laser probes are used for in situ measurement of the particle size and number density. Critical to the interpretation of the scattered and absorbed laser light is the particle index of refraction. The proposed research will determine the index of refraction as a function of the soot C/H ratio, the particle temperature, and the amount of iron carbonyl added to the fuel. Measurements will be limited to the lower regions of premixed hydrocarbon-air flat flames where the soot particles are much smaller than the wavelength of the laser probe. Reflectivity measurements will also be made on extracted samples heated up to 1100 K. Particle temperature in the flame will be controlled through high power pulsed IR laser illumination synchronized with a visible laser probe. If the research is successful, the ability to quantify with less uncertainty the number density and size distribution of soot particles will enhance investigations into the formation of soot in hydrocarbon flames, and increase the accuracy of radiative heat transfer calculations. This study will also provide the first ever assessment of the effect of a metal additive on sub- 100 nm soot particle optical properties, and could lead to a model experiment for investigating other heavy metal and chlorinated additives.