Experimental study of particle losses close to the entry of thin-walled sampling probes at varying angles to the wind

This article summarizes the results of an extensive experimental study of sampling losses in thin-walled probes at various values of velocity ratio R and the probe orientation with respect to the freestream. The purpose of this study was to gain insights into the complex interaction of various parameters that influence sampling losses and the consequent effect on the overall sampling efficiency. A 0.635 cm diameter sharp-edged tube was mounted in a small wind tunnel where the freestream velocity could be varied over a wide range of values. Polydispersed spherical glass beads were used as the test aerosol. The number concentration and the particle size distribution were measured using the aerodynamic particle sizer (APS 3310). The sampling efficiency was determined as a function of orientation for a range of particle sizes (or Stokes number). By using an existing model to predict the aspiration efficiency for thin-walled probes, the sampling losses could be isolated from the sampling efficiency. In this manner a new empirical model was developed to predict the losses as a complex function of Stokes number, sampler orientation, and velocity ratio. The losses appear to be influenced by particle inertia, impaction, gravitational settling in the boundary layer developing inside the thin-walled probe, and vena contracta or flow recirculation loss near the entry. It was evident from the results that these losses are strongly influenced by the Stokes number and sampler orientation. The losses also increased strongly with increasing value of velocity ratio for all orientations.