CONVECTIVE AND DIFFUSIVE DISPERSION OF PARTICLES IN LAMINAR TUBE FLOW- EFFECTS ON TIME-DEPENDENT CONCENTRATION MEASUREMENTS

In the measurement of transient particle concentrations, the effect of convective and diffusive dispersion in the sampling tube should be co nsidered. Such dispersion, arising from the axial velocity distributio n in the tube flow and the transverse diffusive particle flux, will pr oduce a concentration at the outlet of the sampling line that has a mo dified temporal dependence from that of the inlet concentration. We ha ve solved numerically the unsteady concentration equation for the full y-developed Poiseuille flow, yielding the temporal and spatial concent ration distribution. The model was validated by comparison with the an alytical solution for the case of convection-only, and through experim ents performed for the combined convection diffusion situation. Result s are presented, for the case following a step change in the inlet con centration, in terms of the dimensionless parameter xi (defined by LD/ ur0(2)BAR where L is the length of the sampling tube, D is the particl e molecular diffusivity, uBAR is the average flow velocity and r0 is t he internal radius of the sampling tube) and the dimensionless time t (defined by tuBAR/L). Typically, the time for the outlet concentratio n to decay to 1% of its initial value varies from 2.8 to 5 times the c onvection transit time (defined by L/muBAR) for xi values between 2 x 10(-3) and 0, respectively. The procedure is highly relevant to the pe rformance of aerosol measurement systems to rapidly-varying particle c oncentrations.