10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Performance Comparison of Corona-based Wire-plate and Needle-mesh Aerosol Chargers
XIAOTONG CHEN, Qiaoling Liu, Jingkun Jiang, Da-Ren Chen, Tsinghua University
Abstract Number: 227 Working Group: Instrumentation
Abstract The performance of a wire-plate aerosol charger and a needle-mesh aerosol charger operated at low DC-corona-discharge current was investigated in this study. Both chargers are designed for miniature electrical particle sizers. The wire-plate charger has an aerosol charging channel established between two metal blocks with a corona discharge chamber embedded in one block. A perforated plate was used to partition the chamber and aerosol charging channel. A Tungsten wire of 50 µm in diameter was installed in the corona chamber for unipolar ion generation when a high voltage was applied on the wire while keeping the perforated plate at the electric ground. Generated ions enter the charging channel via the ion diffusion only. No sheath flow or ion driving voltage was featured in the wire-plate charger. An aerosol flow was injected into the aerosol charging channel via the inlet tube and exited from the outlet tube. The needle-mesh aerosol charger was modified from the mini-charger (Qi et al., 2008) by replacing the perforated dome-shaped end of the corona-discharge tube with a flat-meshed end. A tungsten needle was positioned at the center of corona tube for ion generation. An ion-driving voltage of 30 V was applied between the corona tube and charge case to drive ions into the aerosol charging zone established between the corona tube and charger case. Both chargers were operated at their optimal operation conditions except that the corona current was kept at 0.3 μA for the low energy consumption while achieving high charging efficiency.
Both intrinsic and extrinsic charging efficiencies of particles with different mobility diameters achieved by the chargers were measured. The wire-plate charger has lower intrinsic charging efficiencies than the needle-mesh charger. For the wire-plate charger, the extrinsic charging efficiencies at the aerosol flow rate of 0.3 lpm are slightly higher than that at the flow rate of 0.6 lpm. The extrinsic charging efficiency of the wire-plate charger remains approximately 80% for particles larger than 40 nm. For the needle-mesh charger, its extrinsic charging efficiency at the 0.6 lpm aerosol flow rate was higher than that at the flow rate of 0.3 lpm for particles greater than 60 nm. The trend was reversed for particles less than 60 nm. Large particles carry more electrical charges and lose more due to the presence of weak electrical field and the increased residence time in the charging zone at the 0.3 lpm aerosol flow rate. The extrinsic charge distributions of particles with mobility sizes less than 300 nm were also characterized. It is found that multiple charge effect was observed for particles with the sizes larger than 20 nm in the wire-plate charger, and the wire-plate charger attached fewer charges on particles than the needle-mesh one.
The performance of both chargers was also evaluated via the birth-and-death charging model with the ion-particle combination coefficients estimated using the Fuchs limiting sphere theory. Reasonable agreement was achieved between the calculated and measured charging efficiencies. The discrepancy between the experimental and calculated charge distributions could be attributed to the loss of charged particles and the spatial non-uniformity of ions in the charging zone. A Gaussian distribution function was further proposed to fit the measured particle charge distributions.