Function and Performance of a Low Turbulence Inlet for Sampling Supermicron Particles from Aircraft Platforms

A low-turbulence, aerosol sampling inlet (LTI) has been developed for use on aircraft. The inlet makes use of boundary layer suction in a porous diffuser to slow the sample flow from aircraft air speeds near 150 m/s to velocities near 5 m/s without generating turbulence. The reduction of turbulence reduces losses of supermicron particles by turbulent deposition and permits the use of laminar flow calculations and well-understood drag formulations to accurately predict particle motion. Large particles are enhanced in the sample flow due to inertia. These enhancements are predicted with numerical analysis of fluid flow and integration of the equations of motion for the particles. The diffuser discussed in this article has been used in a number of field experiments, and the enhancement factors have been provided to the experimenters measuring aerosol downstream of the inlet. Some particles are doubtless lost in transport from the LTI to the aircraft fuselage. Estimates of those losses have also been made and provided to experimenters. The enhancement factors (EF) and losses are a function of particle Stokes number. They should be applied to measurements that segregate the sample into size bins that are narrow enough to assign a well-defined EF to each bin. Uncertainties in EF have been estimated. The uncertainties in EF and in the efficiency of transport to the aircraft are small compared to the uncertainties reported in the literature for sampling supermicron particles through solid conical diffusers. Thus the LTI contributes to our ability to quantitatively characterize supermicron particles from aircraft platforms.