Experimental study of solid particle deposition in 90 ventilated bends of rectangular cross section with turbulent flow

Nowadays, ventilation ducts, especially duct bends, play a significant role in filtering aerosol contaminant for the built environment. This article experimentally investigates practical averaged aerosol deposition in 90 ventilated bends of rectangular cross section by measuring particle concentration changes at bend inlet and outlet in steady state. The measured penetrations at Reynolds number Re= 17,900 and 35,600 are in good agreement with previously published data at small Stokes number, which justifies the effectiveness of the measurement process. Particle penetration decreases from approximately 100% for St = 5.2 104particles to 64% for St= 0.55 particles in acrylic glass bends. For the particles of Stokes number larger than 0.089, particle penetration is much higher than that in previous studies. This behavior is believed to be caused by the consideration of particle rebounce from bend wall in the presence of centrifugal force. The changes in Reynolds number do not significantly alter the trend of deposition velocity and penetration. Deposition velocities are compared and verified with available droplet data and current numerical and analytical analysis. For the high deposition velocity increase compared to that in straight ducts, the major mechanism is attributed to the flow impaction onto outer bend wall. In this type of bend, secondary flows and streamwise eddies provide a further reason for distinct deposition increase of small particles. In a word, the deposition mechanisms are analyzed analytically and systematically, and then classified in order clearly. In addition, two empirical models were proposed in this article, which are valid for Stokes number St = 5.2 104-0.55 under the experimental conditions.