Effect of hydrogen addition on soot formation in a shock tube

Despite the efforts spent in the last years, the understanding of the mechanisms of soot formation in combustion systems is still a challenging task. The aim is the reduction of soot particle emissions both to increase the combustion efficiency and for environmental concern. In this direction, hydrogen-hydrocarbon hybrid fuels have received increasing attention, thanks to the improvement in the ignitability and in the flame stability. As few of these issues are clearly known, an important research field consists of the identification and the understanding of the chemical effects of additives, like hydrogen, added to the fuel, concerning, for example, their role in the soot formation. The development and the validation of soot formation modeling requires more experimental data obtained in different conditions of fuel type and composition, pressure and temperature. Many optical diagnostic techniques have been developed to this purpose. Important contributions come from shock tube experiments, allowing to work under controlled conditions, in a temperature and pressure range similar to those of practical systems. A large amount of studies is focused on the application of light extinction technique (usually performed with an He-Ne laser at 632.8 nm) to the measurements of the soot volume fraction [1-2], and, consequently, to the evaluation of the induction delay time, the soot yield and the rate of soot formation. Anyway, for a deeper investigation of soot formation, the knowledge of the soot particle sizing and growth is required. As the sampling technique and TEM analysis [3] give information about soot particle only at the end of the shock wave, not allowing to follow all the stages of the soot growth, it is mandatory the application of an optical non-intrusive diagnostic technique, as the scattering/extinction. Although this technique is well developed in stationary flames, few works are present in the literature on the application in shock tube experiments [4-5]. In this work, the scattering/extinction technique is implemented and applied in a shock tube in order to study the effect of hydrogen addition in the soot formation mechanisms. The global soot growth parameters, as the soot yield and the induction decay time, as well as the temporal behavior of the soot structure can be investigated. Measurements are carried out on ethylene (2% diluted in Argon), and mixtures where 1% and 0.5% H2 are added, at relatively low pressure (about 500 kPa).