Fundamental study of a nanoparticle virtual impactor

To study the application feasibility, a virtual impactor has been constructed and its performance under different operational conditions has been studied. Our experimental result shows that the studied virtual impactor has the 50% cutoff size ranging from 15 to 60 nm with varying the operation condition. Further, the cutoff size of 60 nm can be obtained at the impactor chamber pressure of 3.0 × 104 Pa. The pressure is much higher than those with single-nozzle solid-stage impactors. Design parameters have also been investigated experimentally. The experimental data was scaled with the Stokes number in this study. The performance of the virtual impactor can then be estimated with the measured operation variables. A significant finding in the parametric study is that the optimal diameter ratio of collection probe to nozzle is around 1.8 when a critical orifice is used, which is different from the value of 1.4 recommended by previous studies when using virtual impactors for submicron particle applications.; A modification based on studies of the prototype virtual impactor mentioned previously has been performed. The replaceable cone-shape nozzle, interchangeable collection probe tip, and different outlet positions were implemented into this modification. Results show that the internal loss particle inside the chamber can be greatly reduced, and by using different throat sizes of cone shape nozzle, collection efficiencies can be higher than in the previous design. Different collection probe designs with porous metal as the collection surface were also studied.; A numerical model was also established to predict the performance characteristics of this virtual impactor. The model consists of (1) calculation of the flow field by solving the compressible Navier-Stokes equations with non-structured grid schemes and (2) limiting particle tracking by solving the trajectory equation with the inclusion of compressible flow drag force. Results show good agreement between the model prediction and experiment data. The model has been used for the optimization of this nanoparticle virtual impactor, and assists in interpreting the experimental data.