We report a study of the formation and growth of soot aerosols in incident shock flows of benzene, toluene, ethylbenzene and indene, highly diluted with argon at temperatures in the range 1600-2300 K at a total carbon concentration in the shock-heated gases of 2.0× 1017atoms/cm3. The extent of conversion of hydrocarbon into soot, i.e. the soot yield, during the shock flow was determined from extinction measurements at two different visible wavelengths, while the rate of growth of the soot particles was obtained from the variation in the intensity of Rayleigh scattering of a 1 W argon-ion laser beam by the soot particles. Of particular interest is our finding that the soot yield some 2.5 ms after the onset of shock heating falls rapidly with increasing temperature from near 100% at 1750-1800 K to less than 5% at 2300 K. We show how this surprising result leads to the conclusion that aromatic hydrocarbons can form soot on pyrolysis by two very different routes. One route is direct in that the aromatic molecules undergo condensation reactions without ever losing their aromatic structure whereas in the second, indirect route the molecules decompose initially into small non-aromatic species before any soot is formed. We have tested this interpretation in several different ways and in every case the results are in conformity with it. Of particular interest in this respect is our finding that the aliphatic hydrocarbon trans-decalin (perhydronaphthalene) with a H:C atom ratio of nearly 2:1 forms soot on pyrolysis more rapidly and completely under some conditions than does acetylene, with an H:C ratio of 1:1, at the same temperature and carbon atom concentration. For shock temperatures of ca. 1750-1850 K, where the shock records were of a particularly simple form, we were able to confirm that the main feature predicted for free-molecule coagulation, namely that the rate of increase of the mean particle volume dV̄(t)/dt varies as V̄1/6(t), is indeed observed experimentally. In addition we were able to determine from the scattered and transmitted light intensities (i) the absolute mean size of the soot particles as function of time, and (ii) the absolute coagulation rate of the aerosol. We interpret our measurements of coagulation rate in terms of a model aerosol of spherical sticky particles which have the self-preserving size distribution characteristic of free molecule coagulation and find good agreement between our observations and the predictions of this model.
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