Molecular beam/Wien filter application to the study of charged soot in flames: Methodology and mass distributions of particles in butadiene flames

Abstract

Charged soot particles during their initial growth have been investigated at 27 mbar in premixed butadiene oxygen and ethyne oxygen flames. They were withdrawn in a skimmed nozzle beam and analyzed for their mass distribution by use of a Wien filter (velocity selector) combined with an energy discriminator. The use of this novel device and the method of determining mass distributions are described in detail. Simultaneously, velocity distributions of the relatively heavy soot particles in the “seeded” gas beam were obtained. Computer simulations showing the validity of the method are presented, together with investigations of the influence of the nozzle diameter on the particles' velocity and mass distributions in the beam. Distributions of mass, mainly of positively charged particles between m = 2 ∗ 10 3 and 30 ∗ 10 3 u , and their change with height in the flame, are reported for sooting butadiene flames with 0.88 < C O < 0.92 and 38 < velocity of unburned gas, ν u < 46 cm/s. The influence of a pressure change and the distribution of masses for negatively charged particles are also presented. The change in the masses and the increase of the average mass with time can be explained through surface growth by hydrocarbons, such as ethyne, being proportional to the surface area of the soot particles. In the ranges of flame conditions investigated the growth rate does not depend on C O , but is sensitive to flame temperature, which was dependent on ν u and the burning pressure.