Sampling Submicrometer Particles Suspended in Near Sonic and Supersonic Free Jets

This paper is concerned with sampling submicrometer particles in perisonic flows. The study employed a high volume (30-45 L/min) condensation aerosol generator to produce stearic acid particles having a mean diameter of 0.8μm and a geometric standard deviation of 1.28. The aerosol was diluted with dry air and accelerated to Mach 0.6, 0.8, 1.26, or 1.47 through a flow nozzle. Aerosol mass concentrations were determined using a small bore probe in the jet and by a large bore probe sampling isokinetically upstream of the jet nozzle. The results of both samples were compared to compute the sampling error associated with I ho high spood jot sample. The mass of stearic acid colloctod on polycarbonate mombrono flltors was determined by gravimoirlc and chromalogrophlc mothods. Studies at Mach 0.8 with four sampling probes having Inlet wall to bore area ratios ranging from 3.8 to 0.28 (a knife edge) demonstrated that probe wall thickness effects are not significant when the sample is extracted isokinetically. Subisokinetic experiments using the knife edged probe showed relative errors of 124 ± 12% when sampling at 2 0% of the isokinetic condition. The subisokinetic results are compared favorably with the extended empirical results of other authors. For the supersonic cases it is shown that the subsonic velocity downstream of the sampling probe bow shock can be used in estimating the sampling error.