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ABSTRACT
Fibrous filters are commonly used for aerosol purification and sampling. The filtration efficiency has been extensively studied using standard aerosol generators, yet the literature on experimental data and theoretical study concerning the filtration of agglomerates from real engines remains scarce. A filtration efficiency test system was developed to determine the filtration efficiency of two types of filters (uncoated and fluorocarbon coated) loaded by particulate matter (PM) emissions from a gasoline direct injection (GDI) engine. The experimental results showed that the filtration efficiency in terms of PM mass and number increased over time for both types of filters. The fractional efficiency (penetration efficiency) curves for the test fibrous filters rendered a U-shaped curve for particle sizes from 70 to 500 nm, and the most penetrating particulate size (MPPS) decreased over time. A small fraction of accumulation mode particles with the size between 70 nm to 500 nm penetrated the filters while almost all nucleation mode particles with the size below 50 nm were captured by the filters. The filtration efficiency derived from an empirical model based on classical single-fiber theory for laden filters generally agreed with the experimental data for the first 500 s, but suffered a significant deviation by approximately one order of magnitude at 948 s. A better estimate of the filtration efficiency trend with the maximum deviation of about 20% (except for large particles at the high end of the measurement spectra) was obtained by using a revised model which incorporates the effects of the increase in filter solidity, local velocity, dynamic shape factor and effective total length of fibers during the filtration process.
© 2017 American Association for Aerosol Research
EDITOR:
Nomenclature
| AFR | = | air fuel ratio |
| ATDC | = | after top dead center |
| BTDC | = | before top dead center |
| CA | = | crank angle |
| CPMA | = | centrifugal particle mass analyzer |
| DPF | = | diesel particulate filter |
| DR | = | dilution ratio |
| EVC | = | exhaust valve close |
| EVO | = | exhaust valve open |
| GDI | = | gasoline direct injection |
| IVC | = | intake valve close |
| IVO | = | intake valve open |
| MFM1 | = | the “dilution” air mass flow rate |
| MFR | = | mass flow rate |
| MPPS | = | most penetrating particulate size |
| PCM | = | particle counting methods |
| PAH | = | polycyclic aromatic hydrocarbon |
| PM | = | particulate matter |
| SF | = | single fiber |
| VOCs | = | volatile organic compounds |
| TWC | = | three way catalyst |
| T | = | the thickness of filters (µm) |
| Cc | = | the Cunningham slip correction factor |
| Cc, dm | = | the Cunningham slip correction factor based on mass equivalent particle diameter |
| Cc, dp | = | the Cunningham slip correction factor based on mobility diameter |
| cm | = | the accumulated mass per filter volume (kg/m3) |
| c(m) | = | the mass concentration of particles of emissions |
| ED | = | Brownian diffusion efficiency (%) |
| ER | = | interception efficiency (%) |
| EI | = | inertial impaction efficiency (%) |
| EDR | = | interception of the diffusing particles (%) |
| E1 | = | total single-fiber efficiency for single-fiber (%) |
| E1(m) | = | total single fiber efficiency with soot loading (%) |
| E | = | the total filtration efficiency of a filter (%) |
| E(m) | = | the total filtration efficiency of a filter with soot loading in the revised model (%) |
| m | = | the particle mass |
| P(m) | = | the overall penetration efficiency of a whole filter for particle laden filters (%) |
| Df | = | fractal dimensions[−] |
| χ | = | dynamic shape factor [−] |
| fd | = | friction factor of mass equivalent sphere for the agglomerate [−] |
| fd* | = | friction factor of the agglomerate [−] |
| Pe | = | Peclet number |
| R | = | interception parameter |
| Stk | = | Stokes number |
| α | = | solidity (%) |
| α0 | = | the solidity of the bare fiber (%) |
| α(m) | = | the effective solidity of filter with soot loading (%) |
| U∞ | = | face velocity of the bare fiber (cm/s) |
| U0(m) | = | the effective local velocity of filter with soot loading (cm/s) |
| l(m) | = | the total effective length of fiber in a unit volume of the particles laden filters (1/m2) |
| L(m) | = | the total effective length of the fibers of a filter (m) |
| mtotal | = | total accumulated particle mass |
| D | = | the revised diffusion coefficient of agglomerates |
| λ0 | = | collection efficiency raising factor |
| ρe | = | the effective density of particles |
| θ | = | the mean free path of the gas molecules |
| dm | = | the mass equivalent particle diameter |
| df | = | the effective fiber mean diameter |
| dp | = | electrical mobility diameter |
| η | = | the gas viscosity |
| = | the relaxation time |
Acknowledgments
The authors would also like to express gratitude to Mr. Jianhua Xiao, a senior engineer in the State Key Laboratory of Automotive Safety and Energy, for his great support in the test bench setup.
Funding
This study was funded by the National Natural Science Foundation of China (Grant No. 91641119 and Grant No. 51306011). Financial support from the Shenzhen Science and Technology Innovation Committee (No. JCYJ20160318094727251) is also acknowledged.