Final Report, EU Contract JOU2-CT92-0055 "PEPSI - Pressure effect on pyrolysis and soot inception" , Shock tube experiments up to 50 bar

A shock tube facility has been designed and built at the Istituto Motori to investigate about the pyrolysis of various hydrocarbons, either gaseous or liquid, and the processes of soot formation and agglomeration. To this aim an optical diagnostic system has been developed which allow to perform light scattering/absorption measurements at different wavelengths in the range 200-1300nm with high time resolution (up to 1us). Firstly ethylene pyrolysis was investigated in the temperature and pressure range 1700-2000 K and 9-15 bar. Different phenomenologies have been observed during the soot particle nucleation period varying the temperature conditions. The presence of almost completely transparent structures, that could be considered as soot precursors, has been evidenced. Coupling the scattering and extinction measurements the size and number concentration of soot particles were inferred. The detected minimum diameter of soot was 5 nm. Depending on the temperature and pressure conditions, the soot dimension increases up to 35 nm, and final number densities of about 1012 cm-3 were obtained. Absorption measurements were carried out in the ultraviolet-infrared range. While the absorption in the I.R. is entirely ascribed to soot particles, the light absorption in the U.V. and visible during the inception period can be attributed to intermediate species having higher H/C ratio, such as PAHs and polyacetylenes. The properties of the soot clusters forming during the agglomeration phase were then investigated. A computer code was developed to calculate the scattering properties of agglomerates. The calculated properties were then compared with the measured ones to infer the prevailing morphology of soot aggregates in our experimental conditions. At the end of the process they appear to be structured as chains of Np=5-10 spherules of diameter Dp=25-30nm. Liquid hydrocarbon fuels have been also tested in the shock tube. They were brought to elevated pressures and temperatures (P=9-12 bar; T=1700-2300 K) behind reflected shock waves in argon atmosphere and their sooting tendency was assessed.