Design and Testing of an Interstitial Particle Sampler

An air sample system has been designed for use on high speed aircraft platforms. The goal is to obtain representative samples of aerosol particles in clear air and of interstitial (or non-activated) particles in clouds. Computational fluid dynamics (CFD) studies, combined with numerical particle trajectory calculations, have been utilized in the design process. The sampler has a blunt body that acts to deflect flow and create a cloud particle shadow zone downstream of the body's widest area, where an interstitial inlet is located. The modeling studies show that at aircraft speeds, particles with different sizes follow different trajectories around the blunt body because of inertial effects and the interstitial inlet at the aft of the blunt-body only samples particles below a critical size. Inside clouds, the sampling is complicated by the formation of splatter particles by impaction of activated droplets on the housing. In addition any formation of a liquid film on the sampler surface and its subsequent interaction with flow around the sampler body will result in further contamination of the interstitial sample. The newly designed Clarkson Interstitial Inlet (CII) has a sampler housing that has been designed to avoid flow separation around the body's curvature and provide for a location of the interstitial inlet that can selectively sample particles over a size range representative of non-activated particles in cloud systems. The contamination of the interstitial aerosol sample by particles formed from droplet splashing and liquid film breakup are avoided by appropriate shaping of the blunt body and controlling the boundary layer flows over the sampler. A full scale model was fabricated and flown on the NCAR C-130 aircraft during the PLOWS campaign. The inlet design and its performance under different cloud/precipitation conditions will be presented. A comparison of the CII measurements and that from the other standard aerosol inlets flown during this campaign will also be presented. Figures show the principle of operation of the interstitial inlet and the manufactured model installed on the C-130 aircraft.