Browsing by Author "Li, Zi-Yi"

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A detailed numerical study on the evolution of droplet size distribution of dibutyl phthalate in a laminar flow diffusion chamber
(2020-03-01)
Jia, Feng
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Li, Zi-Yi
;
Pui, David Y. H.
;
Tsai, Chuen-Jinn
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環境工程研究所
A numerical model was used to study the homogeneous nucleation process of dibutyl phthalate (DBP) vapor in a laminar flow diffusion chamber (LFDC); the spatial and temporal evolution of DBP droplet size distribution was governed by the population balance equation (PBE). In the PBE, the nucleation rate was calculated by the self-consistent correction nucleation theory (SCCNT), droplet coagulation, vapor and droplet deposition losses were considered. The simulation results showed that the nucleation rate predicted by the SCCNT improved the underestimation of that predicted by the classical nucleation theory. Due to vapor deposition before nucleation and droplet deposition after nucleation on the wall, the DBP mass loss was severe, accounting for about 86.3% of the total inlet vapor mass, and the droplet size distribution shifted towards larger diameters. The simulation results agreed well with the experimental data in terms of the droplet size distribution and average number concentration at the outlet of the LFDC because of the detailed droplet dynamic, transport and deposition mechanisms treated in this model. Based on this model, the number of molecules in the critical cluster was calculated using the first nucleation theorem and found to be larger about 50% than that calculated using the Gibbs-Thompson equation.
The performance of the PM2.5 VSCC and oil-wetted M-WINS in long-term field sampling studies
(2020-10-15)
Thi-Cuc Le
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Fu, Chang-Xing
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Sung, Jung-Che
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Li, Zi-Yi
;
Pui, David Y. H.
Most of PM2.5 FRM samplers and FEM monitors use a well impactor ninety-six (WINS) or very sharp cut cyclone (VSCC) to classify particles smaller than 2.5 mu m in diameter. However, the WINS has the particle loading effect and needs to be cleaned after 3 to 5 sampling days. The VSCC was claimed not to be affected by the particle loading effect in a laboratory study but without the support of field studies. In this field comparison study using daily cleaned VSCC as the reference, the uncleaned VSCC showed overall good performance in three long-term testing periods: period #1 (21 days), period #2 (124 days), and period #3 (30 days). However the fluctuation in PM2.5 concentrations of the uncleaned VSCC occurred in day 14 (D14) and D16 of period #1 due to particle bounce and re-entrainment effects; and also in D19, D31 and D44 of period #2, and in D28 and D29 of period #3 due to particle loading effect. After the sampling periods longer than 21 days in period #2 and #3, the particle loading effect resulted in persistently lower PM2.5 concentrations due to smaller cut-sizes and smoother penetration curves. In comparison, the oil-wetted M-WINS designed by our group showed a better performance with less than +/- 5% bias and +/- 2 mu g m(-3) concentration difference during two long-term tests: periods #2 and #3. This study proved once again that the M-WINS is a viable PM2.5 sampling inlet that meets the long-term PM2.5 sampling or monitoring need.