Modelling ultrafine/nano particle dispersion in two differential mobility analyzers (M-DMA and L-DMA)

In the present work, the dispersion of ultrafine/nano particles in two differential mobility analyzers (DMA) namely, a medium DMA (M-DMA) and a long DMA (L-DMA) is numerically analyzed using the Lagrangian tracking method. Simplified geometries of the two DMA’s (M-DMA and L-DMA) that are truly representative of a wide class of DMAs have been considered for the present analysis. The exact profiles of velocity and electric field are used for conducting the present investigation. The Langevin equation is numerically solved to track the particles inside the DMAs. The Brownian force has been modelled as a Gaussian white noise random process. The effect of Brownian force on the dispersion of ultrafine/nano particles is clearly evident from the present investigation. The performance evaluation of both the DMAs have been carried out by comparing the transfer functions obtained using the present methodology with the widely accepted transfer functions of Knutson & Whitby and Stolzenburg. The numerical results obtained using the present methodology compare quite well with the experimental data available. It is also shown that DMAs with smaller effective electrode lengths have higher collection efficiencies for real nano-sized particles.