Formation of charged soot aggregates by combustion and pyrolysis:charge distribution and photophoresis

摘要

Soot aggregates formed by propane combustion in a diffusion flame and benzene pyrolysis in a flow reactor are studied by a microscopic video system(at standard temperature and pressure)and transmission electron microscope.The radius of soot aggregates is in the range 0.1-1.0 mum.The size of primary particles in aggregates is 10-30 nm for the combustion aggregates and 40-200 nm for the pyrolysis aggregates.By video observations of aggregate movement in the electric field it is determined that soot aggregates are charged.The typical aggregate charge is a few elementary units.The soot charge distribution is roughly bipolar and symmetric.This charge distribution is governed by the Boltzmann law with the charging equivalent diameter d_D being larger than the aggregate mean mobility diameter by a factor of 1.4,3.0 and 1.8 for soots sampled from the flame axis,region over the flame and from the flow reactor,respectively.Photophoretic movement of soot aggregates driven by a helium-neon laser beam(3 W/cm~2)is observed by the video system.The aggregate photophoretic velocity is determined to be increasing from 15 to 160 mum/s with the equivalent radius increasing from 0.1 to 2.9 urn.Estimations of the sun photophoretic velocity V_Ph~Sun(in the Earth's atmosphere at ground level)indicate that it is about 20% of the settling velocity for the range of aggregate radii studied in this work.The velocities of soot aggregate photophoresis in the Earth thermal irradiation were estimated to be negligible with regard to the settling velocity.