Experimental study on deposition enhancement of ultrafine particles in a duct flow by riblets

High filtration for ultrafine particles (UFPs) has many potential energy-saving and environmental applications. In this study, riblet surfaces with different geometries were investigated for their enhanced UFPs deposition in duct flow conditions for the first time. Polydisperse sodium chloride (NaCl) particles as UFPs were used and tested in a wide range of Reynolds numbers, ranging from laminar to fully turbulent flow. The upstream and downstream size-resolved particle concentrations of the NaCl particles were measured. Experimental results show that deposition velocity increased with Reynolds numbers and decreased with particle size for both empty duct and duct furnished with riblet surfaces. Duct with riblets surface had the highest enhanced deposition velocity ratio for UFPs in the laminar regime, which could reach about 7.0 times for particles less than 20 nm, and about 4.8 times for particle size between 20 nm and 50 nm. The experimental results reveal that the friction factors of the duct with riblet surfaces were close to that of the empty duct in the turbulent flow, while higher pressure drop penalty was observed for riblet surfaces in the laminar flow. The performance index of riblet surfaces by combining enhanced UFPs deposition and high frictional drag was also evaluated. The results show that the highest performance ratio of riblets could be up to 4.0. In addition, the riblet surface with largest protrusion height generally give the best performance.