Over the past several years, numerous studies have linked ambient concentrations of particulate matter (PM) to adverse health effects, and more recent studies have identified PM size and surface area as important factors in determining the health effects of PM. This study contributes to a better understanding of the evolution of particle size distributions in exhaust plumes with unconfined dilution by ambient air. It combines computational fluid dynamics (CFD) with an aerosol dynamics model to examine the effects of different streamlines in an exhaust plume, ambient particle size distributions, and vehicle and wind speed on the particle size distribution in an exhaust plume. CFD was used to calculate the flow field and gas mixing for unconfined dilution of a vehicle exhaust plume, and the calculated dilution ratios were then used as input to the aerosol dynamics simulation. The results of the study show that vehicle speed affected the particle size distribution of an exhaust plume because increasing vehicle speed caused more rapid dilution and inhibited coagulation. Ambient particle size distributions had an effect on the smaller sized particles (ϳ10 nm range under some conditions) and larger sized particles (Ͼ2 m) of the particle size distribution. The ambient air particle size distribution affects the larger sizes of the exhaust plume because vehicle exhaust typically contains few particles larger than 2 m. Finally, the location of a streamline in the exhaust plume had little effect on the particle size distribution; the particle size distribution along any streamline at a distance x differed by less than 5% from the particle size distributions along any other streamline at distance x.
INTRODUCTIONParticulate matter (PM) emissions from internal combustion engines have been studied extensively because of their health effects and contribution to ambient PM levels. 1 Studies over the last 10 years have linked ambient PM concentrations to increased death rates, an increased incidence of asthma, and adverse cardiac effects. 2-5 Accumulating evidence suggests that particle composition, size, and surface area are also important factors in determining the heath effects associated with PM. 6,7 The PM generated from internal combustion engines is typically small, Ͻ0.1 m in diameter, and the efficiency of particle deposition in the respiratory tract is a function of particle size, with smaller particles depositing more efficiently. 6,8 Understanding the evolution of particle size distributions in an exhaust plume could therefore have implications for human health.Particle size distributions from vehicle exhaust have been measured by a variety of sampling and dilution systems: a bag sampler, 9 dilution tunnel, 10-17 and ejector diluter. [12][13][14][15][16]18 Accurate measurement of the exhaust particle size distribution at various locations downstream of the exhaust plume is a very difficult task because it involves simultaneous coagulation, nucleation, and dilution by ambient air, which also contains particles. ...
We propose and demonstrate the improvement of conventional Galilean refractive beam shaping system for accurately generating near-diffraction-limited flattop beam with arbitrary beam size. Based on the detailed study of the refractive beam shaping system, we found that the conventional Galilean beam shaper can only work well for the magnifying beam shaping. Taking the transformation of input beam with Gaussian irradiance distribution into target beam with high order Fermi-Dirac flattop profile as an example, the shaper can only work well at the condition that the size of input and target beam meets R(0) ≥ 1.3 w(0). For the improvement, the shaper is regarded as the combination of magnifying and demagnifying beam shaping system. The surface and phase distributions of the improved Galilean beam shaping system are derived based on Geometric and Fourier Optics. By using the improved Galilean beam shaper, the accurate transformation of input beam with Gaussian irradiance distribution into target beam with flattop irradiance distribution is realized. The irradiance distribution of the output beam is coincident with that of the target beam and the corresponding phase distribution is maintained. The propagation performance of the output beam is greatly improved. Studies of the influences of beam size and beam order on the improved Galilean beam shaping system show that restriction of beam size has been greatly reduced. This improvement can also be used to redistribute the input beam with complicated irradiance distribution into output beam with complicated irradiance distribution.
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