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Diesel Exhaust Aerosol Measurements Using Air-Ejector and Porous Wall Dilution Techniques
Abstract:
The objective of this work is to improve the understanding of
variables like dilution and sampling conditions that contribute to
particle-based emission measurements by assessing and comparing the
nucleation tendency of diesel aerosols when diluted with a porous
wall dilutor or an air ejector in a laboratory setting. An
air-ejector dilutor and typical dilution conditions were used to
establish the baseline sensitivity to dilution conditions for the
given engine operating condition. A porous tube dilutor was
designed and special attention was given to integrating the dilutor
with the exhaust pipe and residence time chamber. Results from this
system were compared with the ejector dilutor. Exhaust aerosols
were generated by a Deere 4045 diesel engine running at low speed
(1400 rpm) and low load (50 Nm, ~10% of rated). Primary dilution
parameters that were varied included dilution air temperature (25
and 47°C) and dilution ratio (5, 14, and 55). Particle measurements
were made at 0.3, 0.75, and 1.0 s to evaluate particle growth in
the residence time chamber.
Exhaust size distribution measurements made using the ejector
dilutor were bimodal with high concentrations of nucleation mode
particles. Varying the dilution ratio from 5 to 55:1 (with a
dilution air temperature of 25°C and residence time of 1 s) led to
the greatest change in the particle number concentration (4 x 10⁸
to 4 x 10₁⁰ particles/cm₃) compared to changes in the other
variables. Particle concentration was lower with higher dilution
air temperatures and particles were larger in size. Size
distributions downstream of the porous tube and ejector dilutor
were qualitatively similar in shape. Using a simple dilution model
and equations for particle growth in the free molecular regime,
particle growth in the two residence time chambers was compared.
Model results suggest that dilution in the porous tube dilution
system occurs more slowly than in the ejector dilutor. This is
consistent with the findings that the particle number
concentrations were consistently higher and the geometric mean
diameter was generally 1 to 5 mm larger downstream of the porous
tube dilutor.