On heat conduction between laser-heated nanoparticles and a surrounding gas

摘要

Uncertainties in modeling heat conduction in connection with the application of laser-induced incandescence(LII)to primary particle sizing are discussed.Comparing two models widely used in this context,namely those of Fuchs [(1963).On the stationary charge distribution on aerosol particles in a bipolar ionic atmosphere.Pure and Applied Geophysics 56,185-193] and McCoy/Cha [(1974).Transport phenomena in the rarefied gas transition regime.Chemical Engineering Science 29,381-388],it is demonstrated that arising differences may be accounted for by the choice of a proper"effective"thermal accommodation coefficient alpha_(eff).In experiments on a large number of carbon blacks an overally good agreement between LII results and specified values for particle sizes based on electron-microscopy(EM)is obtained with a choice of alpha_(eff)= 0.25(based on the McCoy/Cha-model).As aggregate size is expected to influence heat transfer from primary particles,the experimental data are analyzed by a model for an effective heat transfer surface of fractal aggregates.Based on values for the average number of primary particles per aggregate as derived from photocentrifuge measurements the data yield an extrapolated value for the physical accommodation coefficient for isolated particles of alpha_1=0.43.